TECHNOLOGIES FOR REDUCING DIOXIN IN THE MANUFACTURE OF BLEACHED WOOD PULP Background Paper CONGRESS OF THE UNITED STATES OFFICE OF TECHNOLOGY ASSESSMENT Office of Technology Assessment Congressional Board of the 101st Congress EDWARD M. KENNEDY, Massachusetts, Chairman CLARENCE E. MILLER, Ohio, Vice Chairman Senate ERNEST F. HOLLINGS South Carolina CLAIBORNE PELL Rhode Island TED STEVENS Alaska ORRIN G. HATCH Utah CHARLES E. GRASSLEY Iowa DAVID S. POTTER, Chairman General Motors Corp. (Ret.) CHASE N. PETERSON, Vice Chairman University of Utah EARL BEISTLINE Consultant CHARLES A. BOWSHER General Accounting Office H. GIBBONS (Non voting) Advisory Council MICHEL T. HALBOUTY Michel T. Halbouty Energy Co. NEIL E. HARL Iowa State University JAMES C. HUNT University of Tennessee HENRY KOFFLER University of Arizona Director H. GIBBONS House MORRIS K. UDALL Arizona GEORGE E. BROWN, JR. California JOHN D. DINGELL Michigan DON SUNDQUIST Tennessee AMO HOUGHTON New York JOSHUA LEDERBERG Rockefeller University WILLIAM J. PERRY Technology Partners SALLY RIDE Stanford University JOSEPH E. ROSS Congressional Research Service The views expressed in this background paper are not, necessarily those of the Board, OTA Advisory Council, or individual members thereof. TECHNOLOGIES FOR REDUCING DIOXIN IN THE MANUFACTURE OF BLEACHED WOOD PULP Background Paper CONGRESS OF THE UNITED STATES OFFICE OF TECHNOLOGY ASSESSMENT Recommended Citation: U.S. Congress, Of?ce of Technology Assessment, Technofogies for Reducing Dioxin in the Manufacture of Bleached Wood Pulp, OTA-BP-O-54 (Washington, U.S. Government Printing Of?ce, May 1989). Library of Congress Catalog Card Number 89-600719 For sale by the Superintendent of Documents U.S. Government Printing Of?ce, Washington, DC 20402-9325 (order form can be found in the back of this report) I Foreword As analytical technology 1mproves. we are discovering dioxins associated with many products commonly found 1n the home and workplace. Recently, dioxins have been detected in wastes resulting from the manufacture of wood pulp. Paper products made from wood pulp - are used in tremendous volumes for food packaging. hygienic products, printing paper, writing paper paperboard for shipping containers, and' numerous other household items. Between 650 and 700 pounds of paper products are used annually by each American, with domestic and foreign consumption continuing to ris'e at a steady rate. Most of the paper used 1n the United States 15 white paper made from bleached wood pulp. Chlorine is commonly used as a bleach. It has been found that bleaching can result 1n the formation of dioxin in the pulp when chlorine reacts with organic constituents of wood. Although preliminary surveys have detected ldioxin in pulp mill wastes the scepe of the problem is not yet known. Studies currently under way by the Environmental Protection Agency, the Food and Drug Administration, the Consumer Products Safety Commission, the National Institute for Occupational Safety anleealth. and the US. paper industry will shed light on this when completed later in 1989. Alternative technologies using oxygen as a pretreatment to chlorine bleaching and improved deligni?cation that removes more of the potential reactants from the wood can reduce the amount of dioxin in bleached pulp. Substituting other bleaching chemicals for chlorine also shows promise for reducing the' amount of dioxin produced in the bleaching process should regulation be required. This study provides an assessment of these technologies; it does not address the policy issues related to regulating dioxin in paper products and controlling environmental release. JOHN GIBBONS Director Workshop Participants: Technologies for Reducing Dioxin in the Manufacture of Bleached Wood Pulp Michael Babich U.S. Consumer Product Safety Commission Phillip Cook U. S. Environmental Protection Agency Michael Crandall National Institute of Occupational Safety and Health Ronald Estridge James River Corp. David Firestone U.S. Food and Drug Administration Karen Florini Environmental Defense Fund William Gillespie National Council of the Paper Industry for Air and Stream Improvement Michael. Gough Resources for the Future Russell Keenan Envirologic Data Gregory Kramer U. S. Food and Drug Administration William Kraske Boise Cascade Corp. Renata Kroesa Greenpeace Foundation Steve Kroner U.S. Environmental Protection Agency ?Russell Kross Mead Corp. Thomas O?Farrell U.S. Environmental Protection Agency Greg Schweer U. S. Environmental Protection Agency David Stallings U. S. Fish and Wildlife Service Mel Stratmeyer U.S. Food and Drug Administration Clare Sullivan United Paperworkers International Union Dwain Winters U.S. Environmental Protection Agency NOTE: OTA is grateful for the valuable assistance and thoughtful critiques provided by the workshop participants. The view: expressed' 1n this OTA background paper. however, are the sole responsibility of the Of?ce of 'Ibchnology Assessment. iv OTA Project Staff: Technologies for Reducing Dioxin in the Manufacture of Bleached Wood Pulp John Andelin, Assistant Director, OTA Science, Information, and {Vaturai Resources Division Robert W. Niblock, Oceans and Environment Program Manager James W. Curli Senior Associate Administrative Staff Kathleen A. Beil. Administrative Assistant. Sally W. Van Aller. Administrative Secretary w' Contents Chapter Page 1. Introduction, Summary. and Findings 3 2. The Pulp and Paper Making Processes 17 - 3. Environmental Considerations 29 4. Pulp Bleaching 'Ibchnology 41 5. Technologies for Reducing Chlorinated Organics in Pulp Manufacture 55 Chapter 1 Introduction, Summary, and Findings CONTENTS Pag? PURPOSE AND SCOPE 3 THE US. PULP AND PAPER INDUSTRY 4 . . . 5 The Pulp and; Paper Making Process .. . . 5 Environmental COnsiderations .. . . .. - . . . . . . . . . . . . . 6 Pulp Bleaching. Technology . .- - . 10 'for Reducing Dioxi'n 11 Chapter 1 Introduction, Summary, and Findings PURPOSE AND SCOPE ?Dioxin" is the term used in referring to the family of 210 chlorinated chemicals known as chlorinated dibenzo?para?dioxins and chlorinated dibenzofurans. The most toxic member of this large family of compounds is 2,3,7,8-tetrachloro- p-dibenzodioxin or the closely related compound 2,3,7,8-tetrachloro- dibenzofuran or TCDF) is be- lieved to be about one-tenth as potent. Both of these chemicals can be formed during the manufacture of bleached wood pulp if chlorine is used as a bleaching agent. The amount of TCDD and TCDF formed in the bleaching process is measured in parts per trillion (ppt). The relative toxicity of these chemicals is measured in and expressed as ?Toxic Equivalents? (TEQs) using TCDD as the stand- ard. TCDD is considered to be highly toxic based on laboratory animal experiments and has been linked to malignancies, birth defects, and physical deterioration in animals. Evidence of human health effects is less certain and remains a contentious issue among scientists. The U.S. Enviromnental Protection Agency (EPA) classi- fies it as a ?probable human carcinOgen." The release of National Dioxin Study in 1987 focused attention on evidence that showed a pattern of dioxin concentrations in stream bottom sediment and fish below pulp mill waste outfalls. Subsequent studies by the paper indus- tries and government agencies in North America and Europe have confirmed that detectable amounts of TCDD and TCDF are produced and released into the environment at many bleached kraft pulp mills. Measurements of dioxin levels in the three sources of pollution (pulp, ef?uent, and sludge) are being conducted at all U.S. chemical pulp mills using chlorine bleach under the joint sponsorship of EPA and the American paper industry. The results of these studies and others underway in Canada and Europe will I contribute much more to the information base now available, but ?nal results are months away. Bleached wood pulp is used in paper products such as writing and printing papers, rayon, tissues, towels, disposable diapers, food pack- aging, and a myriad of other products. Paper is . used so widely in modern societies that every person comes in contact with bleached paper products almost daily. The knowledge that TCDD and TCDF are produced and released during the manufacture of chlorine-bleached kraft pulp has prompted action by the Federal agencies charged with protecting health and the -. environment and has stimulated research by a paper industry faced with increasing public concern and, as a consequence, possible regula? tory action. Environmental and consumer groups have organized an international action program to inform the public, prompt consumer action, and stimulate political and regulatory action in the pulp-producing countries of North America and Europe. The United Paperworkers International Union, the labor union that represents many of the paper mill employees in North America, and the National Institute for Occupational Health (NIOSH) are monitoring the potential health . impacts of exposure of workers to dioxin in pulp and paper mills. This study reviews what is known about the production and fate of TCDD and TCDF and other associated chlorinated compounds during - the course of pulp bleaching and brightening. The study is specifically aimed at assessing the state of pulping and bleaching technologies that show promise for reducing the amount of those chemicals resulting from the manufacture of bleached kraft pulp, such as extended delignifi- cation, oxygen deli gnification, the use of bleach- ing chemicals that might substitute for some or all of the chlorine commonly used, modi?- cations of the timing, amount, and operating 4 I Technologies for Reducing Dioxin in the Manufacture of Bleached Wood Pulp conditions of the chlorine bleach sequence. and a review of pulping and/or bleaching technolo- gies that are not yet commercially available, but might be in the future. Because this is a technical background report, neither an assessment of policy Options related to the need for regulatory action, nor the form such regulations might take, are included. Further- more, this assessment assumes?based on past trends and industry projections?that a strong demand for high~brightness bleached pulp and paper products will continue in the future. Should the public boycott or avoid the use of bleached paper products in the future because of a perceived health risk, the paper industry would have to adjust to the changing markets within its own constraints. Markets for unbleached hygienic paper prod- ucts, disposable diapers, coffee filters, and milk . cartons, for example, have developed in Sweden where ?environment-graded? or ?environment- positive? lines of unbleached paper products are retailed along with bleached grades. Recently, the Canadian pulp and paper industry has agreed to reduce the amount of dioxin in paper stock used for milk containers after Health and Welfare Canada, a government agency, discov- ered trace amounts of dioxin leaching into milk . from plastic-coated paper cartons. The pulp and paper industry claims that paper products requir- ing high strength and long-term durability generally, must be made with a highly bleached pulp, and industry believe that this will likely continue to be a major proportion of the pulp produced. Market pulp is a commodity traded widely in world markets. The possibility exists that some countries in the future may impose limits on the amount of residual chlorine compounds or dioxin in pulp and paper products that can be imported. If similar collective action were to be taken by a large number of trading partners, such as the European Economic Community or a coalition of Pacific Rim countries, a significant portion of North America?s export market could be affected. If such trade restrictions were broadly adopted, it could, for practical purposes, impose de facto international performance stan- dards on those firms wanting to compete in international markets. OTA convened a workshop in November 1988 to discuss the latest information regarding three aspects of dioxin and bleached wood pulp manufacture: 1. What is known about the distribution and effects of TCDD and TCDF resulting from environmental releases by pulp mills? 2. What is known about the amount of residual TCDD and TCDF in paper prod- ucts, and what is the level of risk to consumers and mill workers? 3. What is the currentlunderstanding of the precursors that react .with chlorine to form TCDD and TCDF, and what is known about the means to reduce their formation? As a result of the workshop, we were impressed by how fast knowledge about TCDD in bleached wood pulp is accumulating. This report is only a ?snapshot?-of the technology and knowledge about the formation of dioxin and possiblemeans to redUce it as of December 1988. THE U.S. PULP AND PAPER INDUSTRY The, United States ,leads the world in per capita paper consumption. In 1986 U.S. con- sumers used over 660 pounds of paper products per person.l This new record of paper consump- tion continues the steady increase in the domes- tic use of paper that saw per capita consumption rise about 16 percent between 1976 and 1986. Paper, which is indirectly correlated. with gen- lAmerican Paper Institute, I987 Statistics of Paper, Paperboard Wood Pulp (New York, Y:Associated Press International. 1987), p. 2. 'i I Chapter I?lntroducrt'on, Summary, and Findings I 5 eral economic activity, is currently being con- sumed at the rate of about 21 ,000 tons per billion dollars of the real gross national product (GNP) generated. For a variety of reasons, including the reduction of solid waste, efforts to recycle scrap paper are expanding, particularly in urban areas. Nearly 73 million tons of paper, board, and construction board were produced in the United States in 1986.2 Although 4.8 million tons of paper products were exported by U.S. produc- ers, almost 12 million tons were imported for domestic consumption. Most of the imports originated in Canada and were newsprint. More than 70 percent of U.S. exports and 80 percent of imports in 1987 were bleached or semi- bleached pulps.3 Pulp products valued at $3.9 billion were shipped from U.S. pulpmills in 1987. The paper and allied products industry (SIC 26) employed over 674,000 persons in 1986, with about one-third of those directly involved in the production of paper and pulp (SIC 261, 262, 266).4 In general, pulp mills have tended to concentrate in the South and Northwest near major sources of wood, while allied industries and conversion facilities are more broadly distributed near primary markets. Future prospects for the pulp and paper industry are bright. In the Pacific Rim countries where industrial expansion is expected to in- crease dramatically, forest-deficient countries like Japan, Korea, and Taiwan are considered large potential markets. The United States and Canada, with their vast forest resources, their established industries, and technological capac- ity, are well positioned to take advantage of new export markets. However, international competi- tion for new markets is sharpening, with over- seas suppliers in Brazil, Portugal, Spain, Mo- rocco, and South Africa expanding their capac- ity. . SUMMARY The Pulp and Paper Making Process The manufacture of pulp and paper involves the separation and purification of cellulosic wood ?bers from which paper is formed. About half of the wood raw material is cellulosic ?ber, and half is Iignin, hemicellulose, and other extractive compounds that cement and strength- en the fibers. The pulping process involves either the use of chemicals, heat, and pressure in a digester to dissolve the lignin and free the fibers from one another or, in the case of mechanical pulp and chemical-mechanical pulp, the abrasion of wood in a grinder or re?ner to physically tear the ?bers apart. Pulping and bleaching technology mus: be matched to the quality and charabreristics of the pulp and paper to be produced. No single pulping bleaching process can produce pulp suitable for all uses. To produce puri?ed cellulose and white pulps, lignin and other colored extractives carried forward from the digester must be.? - removed or brightened in successive bleaching stages using chlorine and/or other oxidative chemicals. his in the bleaching processes where most chlorinated organic contaminants, includ- ing dioxin, originate. A wide range of bleaching sequences and caustic washing stages may be used to bleach and brighten pulp. The choice of which bleaching agents and' what sequence is generally determined by pulp characteristics desired, such as brightness and strength, bal~ anced against capital and operating costs. 2Ihid. 3U. S. Department of Commerce. l983 U. S. Industrial Outlook (Washington. DC. International Trade Administration. 1988). p. 6- 3. ?American Paper Institute. op crL. notcl p. 54. NOTE: refers to Standard lndustnal Codc.? ?aclassi?cation of US industries used bythe U. S. Department of Commerce. 6 Technologies for Reducing Dioxin in the Manufacture of Bleached Wood Pulp Several pulping processes are still at the precommercial- or pilot plant stage. Most are not ?new? in the sense ofiime, they are new because they have not yet been ac- lanthquuit'tfone, organosolv'teChniQues ester pulping), hydrotropic pulping, and closed- cycle. processes. Research is also under- way on biological pulping precesses, but they will likely compete with mechanical . processes rather than chemical pulps. New pulping technologies are emerging, but their acceptance will depend on efficiency, pulp quality, cost effectiveness, or other factors that make them competitive with conventional tech- nology. To the extent'that new pulping systems might reduce the amount of residual lignin carried over to the bleaching process, the amount of chemical bleaching required to attain the desired brightness might be reduced and subsequently the amount of chlorine used as a bleaching agent trimmed. Environmental Considerations Ef?uents from bleached pulp mills contain a variety of substances, some of which are known or suspected of being toxic, genotoxic, muta- genic, or. teratogenic. Chlorinated organics, . including TCDD and TCDF, that appear to be produced in the chlorine bleaching? and de? lignification processes, are of particular con- cem. The composition of bleaching ef?uent is extremely complex and varies widely from mill to mill. The chlorinated components of the waste stream consist of a variety of compounds, including simple phenols, high and?low molecu- lar weight polymers, and neutral and acidic materials from the breakdown of the phenolic rings in lignin. An average North American pulp mill produces between 35 and 50 tons of chlorinated substances daily. Greenpeace Inter- cepted by the industry. Thesei'inclu'de sodal' i national, the Environmental Defense Fund, and other environmental organizations advocate imme- diately minimizing and eventually eliminating the use of chlorine in order to avoid additional releases of these compounds into the environment through effluent, sludge, or via paper products. Small amounts of TCDD and TCDF can be formed, alang withmany other chlorinated organic chemicals, when wood? pulp is bleached with chlorine gas. About 10 percentof the total solids in the waste streams of bleaching plants contain chlo- rinated derivatives, but this varies among mills depending'on the degree of filtration. Some European countries; Sweden and the Federal Republic of Germany in particular, have im- posed restrictions on the amount of total organi- cally bound chlorine that they will allow their pulp and paper industry 'to discharge.5 Sweden has cut the amount of allowable discharge more than half, from the 5 to 6 kilograms per metric ton (kg/t) of pulp normally produced with conventional chlorine bleaching, . to 2 kg/t. Oxygen deligni?cation and chlorine dioxide substitution have been the technology of choice to meet Sweden?s environmental re- quirements. The National SWedish Environ- mental Protection Board hopes to reduce to 0.1 kg/t by 2010 and to 1.5 kg/t by 1992. With further reductions in allowable discharges, the Swedish industry may need to adopt closed- cycle processes to eliminate chlorinated dis- charges entirely. _S_weden and the: Federal Republic. of Ger- many have mo?ved- from regulating pulp mill discharges by limits on. chemical oxygen demand, to limits on the amount of tatal. organic- cthrine released per ton of pulp produced. This will have a forcing 5Betwecn 250 and _300 chemicals have been identi?ed in pulp mill ef?uents. Many of them are chlorinated organic compounds. See Leena R. Suntio. Wan Ying Shiu. and Donald Review of the Nature and Propenies of Chemicals Present in Pulp Mill Ef?uents," Chemsphere, vol:17. No. 1988, pp. 1249-1290. TCDD and TCDF make up a very small fraction of the total organochlorine emissions. Chapter I?lntroduction, Summary, and Findings 0 7 e?ect on the pulping, bleaching, and waste treatment technologies used in; those coun- tries. . In 1988 the Board also considered the overall problem of dioxin in the Swedish environment and concluded that standards established for reducing would suffice to reduce the dioxin levels from bleached pulp as well. Chlorinated organic ef?uents from pulp and paper manufacture have created environmental problems in coastal waters and estuaries of the Baltic Sea, particularly in the enclosed Gulf of Bothnia. Unlike U.S. pulp mills that use second- ary biological sewage treatment technologies to treat waste ef?uents, mills in Sweden, some in Finland, and many in Canada do not use comparable treatment methods before discharging into the environment. Biological treatment can remove large amounts of chlorinated contami- nants (particularly those that adhere to sediments such as TCDD and TCDF) from mill ef?uents when operated properly. Biological treatment transfers the contaminants from the water and concentrates it in the sludge. The sludge must then be disposed of in a safe and acceptable manner. Biological treatment sys- tems require constant attention to ensure that they are working ef?ciently. U.S. environmental ?regulations that lim- ited the release. of ?Conventional? pol- lutants from pulp mills induced the domes- tic industry to install secondary biological waste treatment plants to meet the stand? ards. Most. plants in Europe and many in. Canada do not have secondary waste treatment. The Federal Republic of Germany will begin tightening restrictions on the discharge of chlori- nated organics in 1989. Maximum allowed discharges of organochlorines will be 1.0 kg/t of bleached pulp under the new regulations. Until recently, West German regulations for pulp mill discharges have not been overly strict. Fees levied on discharged amounts of oxygen de- mand, solids, and measures of toxicity lto ?sh have allowed the German industry to ?buy? the right to pollute. Chlorinated organic residues from pulp bleach? ing have caused less c'oncern in North America where little research into adverse impacts on fisheries and aquatic biota has been conducted. There is ample experimental evidence, however, that pulp mill ef?uents, unless adequately treated, can be toxic to fish and some chlorinated compounds may ultimately find their way through the food web. Dioxin has been shown to be extremely toxic to ?sh at very low concentra- tions under controlled laboratory experiments and has been linked to periodic reductions in reproductive rates of some bird species. U.S. mills, unlike their European counterparts, have installed extensive biological treatment systems to reduce the biological and chemical oxygen demand (BOD and COD) of their waste: ef?u~ ents before discharging them into streams. U.S. environmental requirements for the issuance of a discharge permit for conventional pollutants, such as BOD, COD, and suspended solids, and toxicity tests on mill ef?uents 'make biological waste treatment a preferred technology for mills. Dioxin levels whole?sh sampled in the National iD'ioxin Study and in other samples taken below pulp mills exceeded the safe levels prescribed by the FDA. It is known. that fish can accumulate dioxins in their bodies concentrating mostly in the inedible body parts) many thousands of times more than the concentrations of dioxin in the receiving Ingestion by eating contaminated ?sh may be'the source of the public?zs gieatest exposure to dioxin resulting from pulp manufacture. The detection of dioxin in some fish sainples reported in National Dioxin Study has 8 0 Technologies for Reducing Dioxin in the Manufacture of Bleached Wood Pulp raised concern about the possible effects on human health. Less than 20 percent of the randomly selected sites sampled by EPA in the study showed detectable quantities (more than one part per trillion [ppt]) in whole ?sh samples. Other samples collected at over 300 regional sites selected by EPA for high probability of dioxin contamination showed that nearly one- third of the rivers, lakes, coastal waters, and estuaries contained ?sh with detectable amounts of dioxin that ranged up to 85 ppt?signi?cantly higher than the 25 determined to be unsafe by the Food and Drug Administration (con- tained in edible fillets, not in whole ?shy?but it is not certain that pulp mill waste was the source of all contamination. Only 4 of 57 estuarine and coastal sites sampled in the National Dioxin Study had detectable levels of dioxin in ?n fish or shell?sh. Based on labora- tory tests, fish have been shown to accumulate dioxin in their bodies at rates approximately 20,000 to 85,000 times the concentrations to which they are exposed in water. Fishing has been prohibited in several rivers because of dioxin levels, and advisories have been issued by Wisconsin, Maine, and Louisiana warning of the possible risks of eating contaminated ?sh. A screening analysis of pulp waste at five bleached kraft pulp mills, which was conducted jointly by EPA and the U.S. paper industry, and other research in Europe and Canada clearly links the formation of TCDD and TCDF to the chlorine bleaching process. TCDD was detected in seven of nine bleached pulps sampled, some with-levels as high as 51 ppt. TCDF, which is less toxic than TCDD, was found in eight of nine pulps sampled, with levels ranging up to 330 ppt. Dioxin was detected in wastewater at four of the ?ve mills, but concentrations varied greatly among the mills sampled. The highest levels of TCDD and TCDF were associated with waste from the caustic extraction Stage, which is designed to remove the dissolved materials after the chlorine treatment. EPA and the paper industry are currently surveying 104 additional mills to better determine the scope and extent of dioxin and furan formation. Detailed analyses of TCDD levels and bleaching processes at 25 pulp mill bleach lines will be conducted in the c0urse of the study. Based on information available to date, it is not. clear whether, by themselves, re- sidual TCDD or TCDF in pulp and paper products present significant health risks to the public. Potential exposure from paper products shbuld not be. considered trivial, however. Recent. test results showing high- er than expected migration rates of dioxin- from paper milk into milk indicate that further tests of food packaging and products linked to the possible ingestion of - dioxin is warranted. Compared-to other common sources of daily ambient. exposure to dioxin, dermal exposure front paper products seems to be comparatively small. Estimates of the cumulative-exposure from multiple sources must be considered, An Interagency Task Force led by EPA is currently attempting to do this. The detection of TCDD and TCDF in bleached pulp samples raised concern about whether residual dioxin was being carried for- ward into ?nished paper products. The National Council of the Paper Industry for Air and Stream Improvement, Inc. (NCASI), the industry?s environmental research arm, commissioned an assessment of the potential risks to human health from skin exposure to a variety of paper products, including disposable diapers, facial tissue, toilet tissue, sanitary napkins, coffee filters, and paper towels. The assessment con- cluded that TCDD equivalents in all products tested presented a lifetime cancer risk of less than one in a million. Environmentalists note that assess? ment only considered the risks for 2,39,8? TCDD and without examining Chapter I?Introductt'on, Summary, and Findings 0 9 Ithe-ri'sks from the several hundred other chlori- nated byproducts that are also found in paper products, and that NCASI failed to consider the cumulative risk of using a wide range of paper products daily. Environmentalists also fault NCASI for not using appropriate testing proce- dures for evaluating the possible enhanced mobility of dioxin when in the presence of lipids, solvents,'and fats, such as encountered when using paper towels for cooking or when using creams and baby oils with disposable diapers. OTA learned at its dioxin workshop that there is considerable disagreement about the validity of the testing protocols for determining the rate of migration of dioxin from paper products and into the human body. For instance, there are widely differing opinions about how to estimate the effect of urine leaching from diSposable diapers into the skin of babies, and how to simulate the environment of a tampon in determin- ing the movement of dioxin into a woman?s body. Without standard protocols, disagreements over the meaning of dioxin risk data leaves OTA with no means to evaluate the industry?s ?nd- ings and conclusions. Evaluation of these testing techniques is beyond the scope of this as- sessment. Agreement is also lacking on how to express levels of risk. Does one accept a one in a million risk of cancer as did the U.S. paper industry in its study of risks from dermal exposure to dioxins, or should the acceptable risk be one in a thousand? There are no Federal regulatory standards. Arbitrary levels of risk are used for convenience, and experts differ over the most appropriate levels to use. Semantics also contribute to the confusion: Should dioxin exposure levels be expressed as ?virtually safe dose,? which implies that there ?is a level of exposure at which the cancer risk is zero,? or should the term ?risk specific dose? be used since it does not imply that there is any dose- above zero that is ?safe?? Couple the uncertainties over testing 'proto- cols with disagreement over levels of acceptable risk, and add to it the lack of consensus abbut the potency of dioxin to humans, and a confused picture is presented to those attempting to gauge the danger of dioxin from pulp manufacture and paper use. OTA is aware of no published studies that have attempted to de?ne worker exposures to dioxin in paper mills using environmental measure- ments or biological exposure measurements, such as serum or fat concentrations. The National Institute of Occupational Safety and Health (NIOSH), the Federal agency charged with occupational and health research, is currently developing personal exposure moni- toring methods to address pulp and paper workers? exposure to dioxin. EPA is currently coordinating an Interagency Task Force on Dioxin 1n Paper aimed at gauging the cumulative risk of all of the media thaticould contribute to human exposure. Other coop- erating agencies include the Food and Drug Administration and the Consumer Product'Safe- ty Commission (CPSC). The interagency effort . was characterized by an EPA official at the November 1988 OTA dioxin workshop as an effort to ?determine whether there is a risk?not a ?gold plated? risk assessment,? and a ?snapshot in time? as to the risk aimed at determining whether regulation is needed. The interagency study will build on both the EPA/Industry lO4?mill study and estimates of migration rates of dioxin from paper products. It will include an assessment, based on product-use scenarios, of technologies that could reduce dioxin exposure. OTA found that, as with many other Federal activities, the analytical and regulatory authority for dealing with dioxin 1n paper is fragmented among several agencies, none of which have clear responsibility for dealing with the problem in its entirety. While EPA has jurisdiction over water pollution and dioxin in pulp (dealt with in two separate of?ces), FDA has responsibility 10 I Technologies for Reducing Dioxin in the Manufacture of Bleached Wood Pulp for regulating dioxins in coffee filters and sanitary napkins, and CPSC has responsibility for dioxin in disposable diapers, paper towels, facial tissues, and toilet tissues. The lack of published independent Federal research on the risks associated with dioxin in paper products and the absence of government information on paper worker?s environmental exposure to dioxin required OTA to rely almost wholly on industry?sponsored research. The paucity of government information on workers? risk and exposure to dioxin in paper products may change in the future as a result of the Interagency Task Force on Dioxin in Paper mentioned above, but in the meantime we have only industry information available to evaluate the potential risks from dioxin in paper. Pulp Bleaching Technology About 10 percent of the lignin found origi- nally in wood is carried over from the chemical digestion process to the bleaching stages in kraft ?mills. This residual lignin must be substantially reduced if pure cellulose products or bright paper are to be produced. The less lignin retained in the pulp as it leaves the digester, the less bleaching needed. There are limits, how- ever, as to how much digestion wood ?bers can endure without sacri?cing pulp strength and paper durability or signi?cantly reducing pulp yield. Bleaching is a continuation of the deligni?ca? tion process and is used to brighten and purify the wood pulp. The purer the cellulose and brighter the paper, the less lignin retained, and the longer the paper will last without yellowing and becoming brittle. Chlorine gas has become the preferred bleaching agent because of its relatively low cost and high effectiveness. However, a number of other bleaching chemi- cals are also used, including hypochlorite, chlorine dioxide, oxygen, and hydrogen perox- ide. Ozone has also been tried under experi- mental conditions, but is not yet commercially available. Each has its advantages and disadvan- tages, and several are usually used in sequence to achieve the final result. Between bleaching stages caustic chemicals are often used to remove the dissolved lignin and wash the ?bers before they are subjected to additional bleach- ing. It is apparently during this extraction that much of the dioxin created by chlorine bleach- ing ?nds its way into the waste stream. The tendency has been to bleach more pulp as the demand for products using bleached paper increases. Most of the bleached pulp has been produced by the kraft chemical process. Very little mechanical pulp has been bleached in the past, but now it is increasing. at twice the rate of chemical pulp bleaching worldwide. Mechanical? pulp is not normally bleached with chlo- rine gas but with hydrogen peroxide or sodium hydrosnl?'de. .By reducing 0r eliminating the use of chlorine gas in the bleaching process, the amount of chlorinated organic waste prod- ucts and dioxin can be reduced. The reduction or elimination of organic pre- cursors that form dioxin can also reduce the amount of dioxin produced. Little is currently known. about these precursors? -although lozow?ledge is accumulating rapidly? and more research is needed in this area. It has been shown that by reducing the use of chlorine gas in the bleaching process, the amount of chlorinated organic residues and contaminates can be reduced. Oxygen deligni?ca? tion shows some promise for reducing the amount of lignin carried with the pulp to the bleach process, thus reducing the amount of chlorine gas needed to bleach the pulp to the desired brightness. Current commercial oxygen delignification technology can reduce the use of chlorine gas by 40 to 50 percent. Further 1 1 Chapter I?Jntroduction, and Findings 0 11 reductions in the use of chlorine by more intense oxygen deligni?cation is limited by severe losses in pulp yield and strength properties. Pretreatment of the pulp with nitrogen dioxide before oxygen treatment shoWs promise for increased delignification before bleaching. The cost advantage that chlorine gas once had over many other bleaching agents does not apply . to oxygen. If the US. pulp and paper industry is required to reduce the amount of chlorinated organics and dioxin produced in bleaching pulp, oxygen may become a partial substitute for chlorine because of its low operating cost. Capital cost for oxygen treatment is very high, however, and installation of oxygen deligni?ca? tion in existing mills 'Can reduce production capacity as much. as 4 or 5 percent. Any eval- uation- of the comparative costs of oxygen technology with other deligni?cation systems should consider both capital costs, life-cycle costs based on operating costs and depreciation, and gains and losses in productivity and quality. Such analyses were not made 'by OTA in this study. Technologies for Reducing Dioxin There are several ways to reduce the amount of pollution contributed by bleach plants. These include? 0 further deligni?cation of pulp before bleach- ing; I improved washing of the unbleached pulp (brownstock); substitute nonchlorinated bleach agents; 0 substitute chlorine dioxide for some or all chlorine gas; 0 apply multiple additions of chlorine in split charges instead of using a single, massive charge (low chlorine multiple); 0 improve chemical mixing with the pulp; I adjust the acidity of the unbleached stock before adding chlorine; 1. 0 use c?1?eaner oil- base defoamers that do not contain dioxin precursors, . 0 remove diOxin precursors prior to? treat? ment of the pulp with chlorine; and: 0 improve secondary waste treatment and waste-disposal practices. Pretreatment before bieaching can. reduce the amount of chlorine or other bleaching chemical tested. Technologies for; prebieach delignification "include. 1) extended de- ligni?cation, 2 oxygen 3 pretreatment With nitrogen dioxide before oxygen deligni?cation (PRENQX aliows 5 - more iignin to be ?removed with?m dam- . raging pretreatment With other . Chemicals sashes-chlorine dioxid?gond 5) extraction with Sodium hydroxide: suppie- mented with oxygen and hydrogen perox- ide. Only extended delignification and . oxygen deligni?cation are currently tired commerczaiiy - Other technologies used to conserve'. water' and reduce energy consumption may also -_help reduce the amount of chlorinated products in the waste stream: 0 recycle process water from the chlorination stage (although this may actually com- pound the dioxin problem), 0 use countercurrent washing systems after chlorination, and 1- reduce water use by using higher ratios of ?ber pulp to water (higher consistency). Elimination of chlorine in the bleach se- quence combined with internal recycling of process water aimed at developing a ?pollutioni free? pulping system seems to' offer a good strategy over the long term, but has not yet been adepted for commercial use. 12 0 Technologies for Reducing Dioxin in the Manufacture of Bleached Wood Pulp There are several possible ways to reduce the use of chlorine gas in bleaching to high pulp brightness: l) substittite chlorine dioxide for part or-all of the chlorine, 2) use hydrogen peroxide or ozone (experimental- only), and 3) extraction between bleach - stages with sodium hydroxide, supple- mented with oxygen and hydrogen perox- - ide. It is believed that if the use of chlorine gas is reduced or eliminated in the bleaching process, the amount of TCDD and TCDF formed will be lowered or eliminated along with other chlori- nated organics. However, the relationship is not linear, and other factors, such as mixing ef?- . ciency, have significant effects on the relation- ship. Oxygen is one of the most promising bleaching chemicals for displacing some of the chlorine used in the bleaching cycle. In no case thus far has oxygen been able to completely eliminate the need for chlorine. Chlorine dioxide, a more ef?cient oxidant than chlorine, can be substituted for substantial amounts of chlorine gas. The use of chlorine dioxide in conjunction with chlorine gas is increasing at U.S. mills. It has been demon- strated that substitution of chlorine dioxide for some optimum portion of chlorine gas can signi?cantly reduce the formation of TCDD and TCDF. However, the effects of chlorine dioxide substitution on the formation of other chlori- nated compounds are not known. It may also be possible to reduce the amount of TCDD and TCDF formed by chlorine bleaching if the acidity pH of the reaction is controlled closely and/or- if the chlorine is? applied in smaller, multiple charges instead of one large charge. ur- ther experimentation is needed "to Verify the potential of these process modi?cations. Recent research has also shown that the amount of dioxin can be reduced by lowering the acidity (raising the pH) of the pulp before chlorination, by applying the chlorine charge in three parts instead of one?shot, and by using chlorine dioxide after an initial charge of chlorine gas. A very recent discovery by scien- tists at the Pulp and Paper Institute of Canada has identi?ed oil-based defoamers as one possi- ble source of the precursors that form dioxin when chlorinated. As a consequence, the U.S. industry and chemical suppliers are currently seeking cleaner oil-based defoamers. Precursors must be present for TCDD and .TCDF to be'formed by reaction with chlorine. There? are a number of possible sources: 1) natural constituents in the wood, 2) contaminants in the pipes and machinery of the pulp mill, and 3 additives? chemicals added to the pulp to improve the pulping and bleaching processes. Some defoamers added to the pulp may be Contaminated with precursors that can form dioxin when chlorinated: Chemical suppliers and the pulp and paper industry are searching for clean defeamers, but . none are currently available that can be used e?iectively for brownstock washing. Defoamers are added to the unbleached pulp in small amounts to improve washing. If a defoamer is made from used oil contaminated with unchlorinated dioxins (DBD) .and furans (DBF), these precursors convert to their chlori- nated forms as TCDD and TCDF when exposed to chlorine gas. The United States regulates the use of contaminated recycled oil, so further investigation is needed to determine whether defoamers are a problem for U.S. mills using domestically produced products. Recent in- vestigations by American scientists using test- ing protocols more sensitive than those used in Canada have raised doubts as to whether any oil-based defoamers??whether made from ei- Chapter I?lntrodaction, Summary, and Findings I 13 ther virgin, hydro?treated oil, or used oil?are free of contaminating precursors. Water-based defoamers are also available, and tests show them to be free of DBD and DBF precursors. Unfortunately, while water-based defoamers can be used at other washing stages in the mill, they are not effective for washing brownstock. The use of ?cleaner? defoamers is another option for reducing some of the dioxin produced in the bleaching process, but more research and deveIOpment may be needed to develop suitable products. Oxygen deli ghi?cation technology was discov- ered in 1952, and the ?rst commercial unit was installed in the 1960s.' Since then, oxygen technology has advancedjsteadily until it is now considered a mature and proven process. In 1988, world installed capacity using oxygen deligni?cation IS expected to exceed 10 million metric tons per year. Several manufacturers of pulp and paper equipment market oxygen delig- ni?cation systems. Over 50 oxygen units have been installed worldwide. About half the oxy- gen capacity is in Scandinavia and Europe, one-?fth is in Japan, and one-?fth is in North America. About 92 percent of the oxygen bleaching capacity is installed in kr'aft mills, but a number of sul?te mills?mostly in the Federal Republic of Germany?have also installed oxy~ gen delignification units. Oxygen can also be used in combination with extended digestion (modi?ed cooking). By modify- ing the chemical addition and allowing it to cook longer, the amount of residual lignin in the pulp can ?be reduced prior to bleaching. Further pre-bleaching with oxygen can produce pulp with even lower amounts of lignin. With pre- bleached pulps of low lignin content, it may be technically possible to eliminate the . use of chlorine gas in the bleaching process if chlorine dioxide is used as a substitute. No reliable data'exist that directly link the reduction of TCDD and TCDF wholly to oxygen delignification, and in some cases oxy- gen pulps have been shown to contain some TCDD and TCDF. There is, however, substan? tial evidence that oxygen deligni?cation after pulping and before the ?rst bleaching sta'ge can reduce the amount of chlorinated organics produced in proportion to the reduction in residual lignin. At this time, too few compara- tive analyses have been 'made of dioxin in_ oxygen-treated pulps v. that in conventional pulp to absolutely support the hypothesis that- oxygen bleaching substantially reduces TCDD and TCDF in bleached pulps, although unpub- lished EPA data based on three mills sampled in 1988 seem to support this hypothesis. There is no ?siiver butter? for reducing the TCDD and TCDF in bleached puZp. Sev- 1 era! technologtes and/or process modi?ca- tions may be Med individua?y or in combinatfolrz. The choice of technologies and their effectiveness depends on the existing mill catt?guratt'on and the quality of the puip produced. New mills have more ?exibility. Modem mills are designed to match the capacity of the chemical recovery plant with the planned production capacity of the mill. The addition of ?an oxygen delignification stage increases the volume of ef?uent that must be handled by the chemical recovery plant. In new mills recovery plants can be designed to handle the additional load, but retrofitting an existing mill with an oxygen deligni?cation system can result in overloading the chemical recovery capacity. If a larger size recovery furnace or evaporators are required, the additional capital expense may make oxygen delignification a less attractive alternative for economic reasons. It is estimated that adding an oxygen stage to a mill whose chemical recovery plant is operating at fullcapacity will reduce the productivity of that mill 4 to 5 percent Given a range of equally effective technolo- gies to reduce the use of chlorine gas, capital and l4 0 T?chnologt'es for Reducing Dioxin in the Manufacture of Bleached Wood Pulp operating costs may determine the most cost- effective strategy for reducing the amount of chlorinated organic chemicals produced by pulp mills. Although operating costs may be lower if oxygen delignification is used to replace some of the chlorine gas now used in the bleach cycle, the capital cost of-oxygen systems is large. Both capital costs and operating costs must be consid- _ered in a balanced assessment. Furthermore, -- cost factors will differ from mill-to-mill, mak- ing generalizations dif?cult. OTA did not at- tempt to assess these costs. On the other hand, by using ?cleaner" defoamers, coupled with . substitution of chlorine dioxide for chlorine, it may be possible to achieve much lower levels of TCDD and TCDF in pulp. Whether or not this would also lower the level of other chlorinated compounds is uncertain. Additional control of suspended solids that bind TCDD and TCDF by adding clart'?ers 0r sorbents to the biologiCal waste treat? ment systems, and/or declil?orination of bleach plant waste might prove e?'ectt've in removing dioxin from e??lttent at some mills. Dioxin-beart'ng sludge must then be dis- posed ofin a safe and appropriate manner. Further reductions in environmental releases from pulp mill waste outfalls may result from improving the control of suspended solids in the secondary waste treatment plant. TCDD and TCDF are relatively insoluble in water, but they adhere to fine colloidal material and suspended solids. A well-designed, properly operated secondary treatment plant is capable of removing up to 90 percent of the TCDD and TCDF released. Dioxins are retained in treatment plant sludge. The sludge is normally disposed of in land?lls, where limited studies show that it remains isolated and immobile. Some sludge is retained in sludge lagoons or is incinerated, but a. portion is used for soil conditioners. The remaining 10 percent of the dioxin that remains in suspension can find its way in to the water coarse and remain as sediment in streambeds. The use of clari?ers, chemicals, and settling basins to improve the ef?ciency of waste treatnient, coupled with chlorine dioxide substitution and/or perhaps other delignification technology might prove to be the Optimum solution for some existing pulp mills. More questions remain than do answers as to what risk dioxin from pulp and paper manufac- ture presents to humans and the environment: will additional regulations be needed to reduce the risk of human exposure from dioxin and other chlorinated compounds produced in pulp mills? and which technologies or mix of tech- nologies are best suited for reducing the produc- tion of dioxin in the pulp bleaching process? The pulp and paper industry has a number of technical options available to meet the problem. Other technologies, such as extended delig? ni?cation and the substitution of other oxidants in the bleach process for chlorine gas, may be as effective as oxygen deligni?cation in reducing the use of chlorine. All of these questions require more detailed study before they can be answered with acceptable certainty. Final deci- sions to meet regulatory requirements will have to be made on a case-by-case, mill-by-mill basis. Chapter 2 The Pulp and Paper Making Processes CONTENTS . Page THEPULP AND PAPERMILL 17 Steps in the Pulp and Papermaking Process - . . 18 Pulping Technologies . . 20 Figures Figure Page 2-1. Overall View of Papennaking From Chemical Pulp by the Kraft Process 19 2- 2. Stone Groundwood Pulp Mill Flow 21 2- 3 Re?ner Groundwood Pulp Mill Process2-4 Sul?te Pulp . 25 Table Table Page 2-1 Major Commercial Wood-Pulping 'Ibchnologies - . . . . . . . . 18 Chapter 2 The Pqu and Paper Making Processes The modern manufacture of paper evolved from an ancient art ?rst developed in China, ca. 105 AD. Although the modern product differs considerably from its ancestral materials, papermaking retains distinct similarities to the processes developed by Ts?ai Lun in the Imperial Chinese Court.1 In principle, paper is made by: l) pulping, to separate and clean the ?bers; 2) beating and re?ning the ?bers; 3) diluting, to form a thin ?ber slurry, suspended in solution; 4) forming a web of ?bers on a thin screen; 5) pressing the web to increase the density of the material; 6) drying to remove the remaining moisture; and 7) ?nishing, to provide a suitable surface for the intended end use. Pulp and paper are made from cellulosic ?bers ?bers from trees) and other plant materials, although some materials may be used to impart special qualities to the ?nished product. Most paper is made from wood ?bers, but rags, flax, cotton linters, and bagasse (sugar cane residues) are also used in some papers. Used paper is also recycled, and after purifying and sometimes de- inking, it is often blended with virgin ?bers and reformed again into paper. Other products made from wood pulp (cellulose) include diapers, rayon, cellulose acetate, and cellulose esters, which are used for cloth, packaging ?lms, and explosives. Wood is composed of: 1) cellulose, 2) lignin, 3) hemicellulose, and 4) extractives resins, fats, pectins, etc.). Cellulose, the ?bers of primary interest in papermaking, comprises about 50 percent of wood by ovendry weight. Lignin, which cements the wood ?bers together, is a complex organic chemical the structure and properties of which are not fully understood. It is largely burned for the generation of energy used in pulp and paper mills. As the chemistry of lignin becomes better under- stood, what is now mostly a waste product used for fuel (some is converted to chemical products) could become a valuable feed stock for new chemical products. The pulping process is aimed at removing as much lignin as possible without sacri?cing ?ber strength, thereby freeing the ?bers and removing 1 impurities that cause discoloration and possible future disintegration of the paper. Hemicellulo'se is similar to cellulose in composition and function. It plays an important role in ?ber-to??ber bonding in papermaking. Several extractives oleoresins and waxes) are contained in wood but do not contribute to its strength properties; these too are removed during the pulping process. The ?ber from nearly any plant or tree can be used for paper. However, the strength and quality of ?ber, and other factors that can complicate the pulping process, varies among tree species. In general, the softwoods pines, ?rs, and spruces) yield long and strong ?bers that impart strength to paper and are used for boxes and packaging. Hardwoods, on the other hand, generally have shorter ?bers and therefore produce a weaker paper, but one that is smoother, more opaque, and better suited for print- ing. Both softwoods and hardwoods are used for papermaking and are sometimes mixed to provide both strength and printability to the ?nished product. THE PULP AND PAPER MILL Although there are several chemical and mechani- cal pulping methods used for delignifying wood (table 2-1), separating ?bers, and removing discol- oration, all integrated pulp and paper mills involve the same general steps in the manufacture of pulp and paper. These steps include: 1) raw material preparation debarking and chipping); 2) me- chanical and/or chemical separation of the wood fibers grinding, re?ning, or digestion (cook- ing)] to dissolve the lignin and extractives; 3) removal of coloring agents (primarily residual lig- nin) by bleaching; and 4) paper formation and manufacture. A typical layout of a mill using the kraft chemical pulping process is shown in ?gure 2-1. Mechanical, semichemical, and sul?te pulp mills differ in detail, particularly in wood preparation, ?ber separation, and bleaching, but many of the re?ning, bleaching, and papermaking processes are similar. In addition to the primary steps in pulp and paper manufacture, each mill has extensive facilities to 1A.H. Nissan. Paper, iVoad: Its Structure and Properties, F.F. Wan gaard (University Park. PA: Pennsyivania State University 1981). p. 335. L17- 18 II Technologies for Reducing Dioxr'n in the Manufacture of Bleached Wood Pulp Table 2&1?Ma10r Commercial Wood-Pulping Technologies Pulp grades use Wood type End-product use Chemical pulps.- Sulfite pulp softwoods and hardwoods Fine and printing papers Kraft sulfate pulp Stiltwoods and hardwoods Bleached?printing and writing papers. paperboard Unbleached?heavy-packaging papers, paperboard. 'Dissolving pulp Softwoods and hardwoods Viscose rayon. cellophane. acetate ?bers. and film Semichemicei puips: Cold-caustic process Sottwoods and hardwoods Newsprint and groundwood printing papers Neutral sultite process Hardwoods Newsprint and groundwocd printing papers Mechanical putps: - Stone groundwood Softwoods Re?ner mechanical (RMP) Softwoods Newsprint and groundwood printing papers Thermomechanical (TMP) Softwoods Newsprint and groundwood printing papers SOURCE: Modi?ed lrorn George H. Boyd Ill and Chad Brown. Papar- industry: Outlook for Market Pulp (New York. NY: Kidder; Peabody a 00.. 1981}. p. 5. produce and reclaim chemical agents used in the . pulping process; collect, process, and burn lignin and waste wood to produce energy; and remove and treat wastes from process water for release into the environment. Steps in the Pulp and Papcrmaking Process Raw Material Preparation -. Wood received at a pulp mill may be in several different forms. depending on the pulping process and the origin of the raw material. It may be received as bolts (short logs) of roundwood with the bark still attached, as chips about the size of a half-dollar that may have been produced from sawmill or veneer mill waste or pro-chipped from debarked roundwood elsewhere, or as waste sawdust in the case of some pulping processes. If roundwood is used, it is ?rst debarked. usually by tumbling in large steel drums where wash water may be applied. The debarked wood bolts are then chipped in a chipper if the pulping process calls for chemical digestion or are fed into a grinder in the case of some mechanical pulps. Chips are screened for size, cleaned, and temporarily stored for further processing. Fiber Separation The ?ber separation stage is the point at which the several pulping technologies diverge. In kraft chemi- - cal pulping, the chips are fed into a large pressure cooker (digester), into which is added the appropri- ate chemicals (white liquor). The chips are then cooked (digested) with steam at speci?c tempera- tures long enough to separate the ?bers and partially dissolve? the lignin and other extractives. Some digesters Operate continuously with a con- stant feed of chips (furnish) and liquor, others are charged intermittently and treat a batch at a time. After digestion, the cooked pulp (brown stock) is discharged into a pressure vessel (blow tank) where the steam and volatile materials are siphoned off. The cooking liquor, that by this time has turned dark brown from the dissolved lignin (black liquor), is retumed to the chemical recovery cycle. In the chemical recovery plant, the lignin in the black liquor is burned for the cogeneration of energy, and the chemicals are recovered, puri?ed, reconstituted, and returned to the digester as white liquor for reuse. The brown stock containing the recovered ?bers (having the consistency of cooked oatmeal) is washed with water, screened to remove undigested wood, and cleaned to remove other foreign matter. It is then ready for bleaching and further processing. Fiber separation in mechanical pulping is less dramatic. In the stone groundwood process, de- barked logs are forced against rotating stone grind- ing wheels that are constantly washed by a stream of water. The ground pulp is then screened to remove course debris, thickened, and stored for the paper- making process. Chips are used to produce re?ner pulp and thermomechanical pulp. In both processes the chips are ground by passing them through rapidly rotating Chapter 2?The Pulp and Paper Making Processes I I 9 Figure 2-1?Overall View of Paperrnaking From-Chemical Pulp by the Kratt Process Continuous digester . Gas to Brownsteck Brownstock . BLEACH PLANT FPUI incineration washers decker Wash ?mi digester Unbleached Bleached condensates .0. a I High . Eo? Eigh ensity Debarking EL Lb density storage Wood . Screen Excess tank 50 A releois unbleached ,i :5 3 T0 ill-"P Ch' white 5 a 1: dryer or? Ipper . water i =6 3 3 paper mill Bark. __J_Wa5h . i - to landfill I zone I :3 3:9 - U) or boiler incineration 33 ac Iquor earn - Alkali A id Feed water . EFFECT Rm? (Caustlc) sewer Surface Vapor Sewer condenser Steam 5 El Economizer Foul 1 a I 8 condensates i, :Strong m8 I I I . elm?-4 'black liquor' I Precipitator Chlorin dioxide generator lay-products - White liquor claritier Wash water Sodium makeup Saltcake makeup Siaker Gas to incineration to $2311? Causticizers FUBI Lime mUd [00003) Steam stripper Lime kiln Mill eitluent Steam Weak wash Aerated lagoons Sluge to dewatering and landfill 'l I I l- MAIN PULP LINE BLACK LIQUOR SOURCE: Envirormenl Ontario. Stopping Water Po?uo'on At its Source (Toronto. Ontario: Ministry of the Envirorunent. 1983). 20 0 Technologies for Reducing Dioxiu in the Manufacture of Bleached Wood Pulp disk grinders. Thermomechanical pulp is re?ned (ground) under pressure after the chips are pretreated with steam (chemical thermomechanical pulp uses chemicals and steam for pretreatment). After further re?ning in a second stage, the pulp is screened, cleaned, and most of the process water is removed in preparation for paperrnal-ting. Bleaching or Brightening Since the raw pulp (brown stock) still contains an appreciable amount of lignin and other discolora- tion, it must be bleached .to produce light colored or white papers preferred for many products. Bleaching is normally done in several stages (multistage bleaching). Through chlorination and oxidation the ?bers are further ?deligni?ed? by solubilizing additional lignin from the cellulose. . A number of bleaching agents may be used and are applied in a stepwise fashion within a bleaching sequence. These include chlorine gas, chlorine dioxide, sodium hypochlorite, hydrogen peroxide, and oxygen. Between bleaching treatments, at strong alkali (usually sodium hydroxide) is used to extract the dissolved lignin from the surface of the ?bers. The bleaching agents and the sequence in which they are used depend on a number of factors, such as the relative cost of the bleaching chemicals, type and condition of the pulp, desired brightness of the paper to be produced, and sometimes in response to environmental guidelines and regulations. Bleaching of mechanical pulp is much different than that for chemical pulp. Mechanical pulping leaves the lignin and the cellulose intact. whereas the purpose of chemical pulping is to chemically sepa- rate the lignin from the cellulose ?bers and remove it from the pulp. A major advantage of mechanical? pulping is the high yields of pulp that can be achieved from a given volume of wood. Therefore, bleaching or brightening of mechanical pulps is designed to minimize the removal of the lignin that would reduce ?ber yields. Chemicals used for bleaching mechanical pulps selectively destroy coloring impurities but leave the lignin and cellulosic materials intact. These include sodium bisul?te. sodium or zinc hydrosul?te (no longer used in the United States), calcium or sodium Ihypochlorite, hydrogen or sodium peroxide, and the Sulfur Dioxide-Borol Process (a variation of the sodium hydrosul?te method). Originally, much of the mechanical pulp was not bleached, but the bleaching of groundwood has increased and im- proved technology now enables bleached ground- wood pulp to be used for printing papers, tissues, and towelling. Papermaking The bleached or unbleached pulp may be further beaten and re?ned to cut the ?bers and roughen the surface of the ?bers (?brillate) to improve formation and bonding of the ?bers as they enter the paper machine. Before entering the paper machine, water is added to the pulp slurry to make a thin mixture normally containing less than 1 percent ?ber. The dilute slurry is then cleaned in cyclone cleaners and screened in centrifugal screens before being fed into the. ?wet end" of the paper-forming machine. In the paper making process, the dilute stock passes through a headbox that distributes the ?ber slurry uniformly over the width of the paper sheet to be formed. The ?web? of ?ber that-will make the new paper sheet is formed on a continuously moving bronze or polymer screen (Fourdrinier) or between two such wire screens. Water drains from the slurry through the mesh of the screen, the wet paper-web is consolidated and the paper sheet gains some strength through ?ber bonding. The wet sheet of paper is continuously lifted from the screen (couched) and transferred to a woven felt belt where additional water is squeezed from the paper sheet by pressure rollers. The remaining water is removed on steam-heated cylinders. When the paper is dry it may be treated with stabilizing materials and surface ?nishes to improve durability or printability. Pulping Technologies Mechanical Pulping Processes Thereare six basic mechanical pulping processes: 1) stone groundwood. 2) re?ner, 3) thermomechani- cal pulping, 4) chemical mechanical, 5) defibrated or exploded pulping, and 6) recycled paper.2 Mechani- cal pulping is generally used with softwoods be- 2This section on pulping technologies borrows heavily from a previous OTA assessment: Wood Use: .S . Competitiveness and Techuoiogy, Vol. II: Technicai Report, 0TA-M-224 (Spring?eld. VA: National Technical information Service, 1984). pp. 19-94. Chapter Z?The Pulp and Paper Making Processjes I 21 cause of the added strength imparted by the long ?ber length of softwood species. Some hardwoods require chemical pretreatment (chemical mechancial pulping) to produce a suitable groundwood pulp. Fibers separated mechanically are substantially darn- aged in the process and therefore make weaker paper or paperboard. However, since both lignin and cellulose ?bers remain intact. the yield of paper per unit volume of wood is still greater than that produced by chemical pulping. Pulp yields from all of the mechanical pulping processes typically are near 90 to 95 percent recovery, which is a much higher yield per unit of wood than with the chemical pulping methods because of the retention of lignin. However, paper made from mechanical pulp discol- ors and becomes brittle with age because of its lignin content, which results in a shorter useful life than paper made from chemical pulp. Mechanical pulps are used principally to manu- facture newsprint, printing papers, towelling, tissue, and coated specialty papers that do not require high-strength. Secondary uses include wallpaper and paperboard. Small amounts of chemical pulp are often mixed with groundwood pulp for additional strength. Recycled pulp is used mainly for the manufacture of folding boxboard (grayboard), tis- sue, corrugated board, and newsprint. Paper prod- ucts made from de?brated pulp include hardboards, construction boards, and roo?ng papers. In the stone groundwdod process, debarked short logs (roundwood) are fed whole against wet stone grinders by hydraulic rams. Counter-revolving steel disks are sometimes used in place of abrasive stone in the grinding process. The abrasion of the grinding wheel against the wood physically separates the wood ?bers. The grinding process usually is auto- matic and continuous. The groundwood pulp is then screened, bleached or brightened, treated, and pre- pared for the paper machine (?gure 2-2). Flgure 2-2?Stone Grodndwood Pulp Mill Flow Wood Whlte Overflow . 1 1 water lI 1 water i Coarse Rejects 1 refiner Fine I .l . screens . I i Cleaners Seveall decker 1 1 Stock I chest . 1 i Process waste Pulp Bleaching Sewer SOURCE: Allan M. Springer. Industrial Enw?mamental' Consul: Pulp and Paper industry (New York. NY: John Wiley 2. Sons. 1956}, p. 147. 22 - Technologies for Reducing Dioxr'n in the Manufacture of Bleached Wood Pulp Refiner mechanical pulping (RMP) uses chips in lieu of roundwood and produces paper with higher strength than conventional groundwood because of less damage to the ?bers in the pulping process. The chips arelpassed through a re?ner that has ?xed and rotating disks operating under a stream of water. A wider range of species, including hardwoods, can be processed by the re?ner pulping process. Sawdust and other saw mill wastes can also be used (?gure .- 2-3). Thermomechanical process (TMP) was devel- oped as a modi?cation of the re?ner mechanical pulping process. In TMP, the wood chips are steamed for several minutes under pressure and subsequently re?ned in one or two stages. The 1i gnin is softened by heating the wood chips with pressur- ized steam before they are re?ned blended by passing the ?ber through rapidly rotating disks). The re?ned wood pulp, although still weaker than chemical pulp, makes a stronger paper than ground- wood or re?ner pulp with only a small sacri?ce in yield but with large energy requirements. Some newsprint is now produced wholly from ther- momechanical pulp, thus eliminating the need for the addition of chemical pulp often needed for strengthening paper made from mechanical pulp. The neutral sul?te semichemical (N SSC) pulping process is used at a number of US. mills to produce courser-grade products such as corrugated board, which has a yield of about '75 percent of the wood raw material. In NSSC pulping, wood chips are softened by brie?y cooking them in a neutral sodium or ammonium sul?te solution and then separating the ?bers (de?brating) in a refiner (see also Sul?i?e Pulp-fag below). Recycling can effectively reduce the consumption of both wood raw material and energy when used in conjunction with other mechanical pulping pro- cesses. It does so, however, with some sacri?ce in paper strength. Recycled pulp is manufactured from w?astepaper that is processed into paper stock. A small proportion of the paper stock (5 to 10 percent) is de?inked, usually with caustic soda-based chemi- cals. Most recycled paper, however, is pulped without de-inking. Pulping is accomplished through violent agitation and shearing action performed at high temperatures. The paper produced from recy- cled pulp is generally weaker than papers from Figure 2-3?Fle?ner Groundwood Pulp Process Debarked chips roundwood Chipper Chip storage Chip White washer ?i water 1 Process Primary water re?ner _maketip Feed conveyor Secondary refiner Rejects I refiner I Fine Fiber screens Cleaner 8. Saveall decker Stock chest Bleach or brightening Sewer Alternate facilities Pulp dryer Papermaking SOURCE: Allen M. Springer, inwards: Emil-emote! Control: Pulp andPaperlndusay (New York, NY: John Wiley 8. Sons. 1938), p. 148. virgin materials, because of the breakdown of the used ?bers and loss of ?ber bonding. Chapter 2?The Pulp and Paper Making Processes 23 . Three major developments in mechanical pulping technologies show promise for improving pulp quality: 1) pressurized groundwood pulping, 2) chemical thermomechanical pulping, and 3) hard? wood chemical mechanical pulping. All of these technologies have reached some stage of common cialization. Chemical thermomechanical pulping is currently used at several U.S. mills. Improvements in mechanical pulping show promise for improving the quality (strength characteristics) of paper now produced by mechanical processes. The resulting higher quality mechanical pulps may displace the kraft pulps that are currently mixed with mechanical pulps to improve paper strength. Pressurized GroundwOod Palping?In pressur- ized groundwood pulping, debarked logs are fed to the grinding wheel through a heated, pressurized chamber. The heat and pressure help separate the ?ber, thus breaking fewer ?bers in the grinding process and improving pulp quality. Paper produced from pressurized groundwood pulp is more tear- resistant than paper made from stone-ground pulp, but is inferior to that of thermomechanical pulp. Pressurized groundwood pulping may have the potential for displacing some high-quality chemical pulps in the manufacture of newsprint and other printing papers. Chemical Th ermom echam'cal Pulping? Chem- ical thermomechanical pulping involves treating softwood chips with mild sul?te solutions to modify the lignin and partially delignify the wood prior to grinding in a re?ner. This ?sulfonation? treatment results in paper with higher tear resistance than thermomechanical, re?ner, or stone-ground pulps. Pulp yields decrease to between 85 and 90 percent with chemical thermomechanical pulping, but these yields are still higher than chemical pulping (40 to 56 percent). Hardwood Chemical Mechanical Palping? Me- chanical methods for producing pulp from hardwood species involve pretreating hardwood chips with hydrogen peroxide or sodium hydroxide and proc- essing them like re?ner mechanical pulps. Both hardwoods and softwoods have been successfully pulped by this method, with ?ber recoveries in the 80 to 90 percent range. Pulp produced by hardwood chemical mechanical pulping can be used to produce neWSprint and printing papers. Chemical Pulping Chemical pulping involves treating wood chips with chemicals to remove the lignin and hemicellu- 1636, thus separating and cleaning the ?bers. Delig- nification gives the ?bers greater ?exibility, result? ing in a substantially stronger paper (because of greater contact between the ?bers in the ?nished sheet) than can be manufactured from high-lignin ?bers produced by mechanical pulping. Paper strength and durability is gained at the expense ?of ?ber yield. Chemical processes may yield only half the ?ber that can be recovered by the use of mechanical pulping techniques. Two major chemical pulping processes are cur.- rently in commercial use: 1) kraft (sulfate) pulping, and 2) sul?te pulping. The kraft process dominates the pulp and paper industry, accounting for 76 percent of the pulp produced for paper and paper- board in 1984.3 Paper produced from kraft pulp accounts for most of the bleached boxboard 'and. linerboard used by the packaging industry (which consumes about 58 percent of the paper in the United States). Bleached softwood kraft pulps are often mixed with mechanical pulps to add strength to newsprint and printing papers. Bleached hardwood kraft pulps are added to bleached softwood :pulp to improve printability for specialty paper products like magazine stock and coated papers. Both kraft pulp and sul?te pulp can be used for the production of dissolving pulp, which is used for the production of rayon and acetates. Kraft Pulping?Kraft pulping involves treating wood chips and sawdust with a sodium sul?de and sodidm hydroxide solution (see ?gure 2-1). The highly alkaline chemical and wood mixture is cooked with steam under pressure (digested) for between 1 and 3 hours. Digestion may be either a continuous process or treated in discontinuous ?batches.? Most of the lignin and some of the hemicellulose is dissolved, leaving the remaining cellulose ?bers separated. The cooking liquor containing the dissolved lignin and other extractives (black liquor) is' routed to a chemical recovery plant where the ligitin and Directory of the Paper and Allied Trades 1986 (New York, NY: Vimce Publishing Corp., 1986). 1. 24 0 Rehnologt?es. for Reducing Dioxiu in the Manufacture of Bleached Wood Pulp organic wastes are burned to produce energy needed in the pulping process. Valuable extractives turpentine, tall oil, and resin) are separated for sale as commodity chemicals. Process chemicals are recovered with only a relatively small loss in volume, and after replenishment with sodium salts, they are returned to the digester for reuse. The brown pulp (brown stock) from the digester is washed, screened, and passed through a battery of cleaners. If the pulp is to be bleached, it is ?thickened? by removing excess water and sent through a series of bleach operations. These can vary widely in the type of chemicals used and their sequence. Bleached pulp is then ready for the paper making process. Both softwood and hardwoods can be pulped by the kraft process. Fiber recovery is largely a function of the wood species used, the time and temperature; of cooking, the degree of bleaching, and the paper strength required. Generally, kraft pulp recoveries from softwoods are approximately 47 percent for unbleached pulp and 44 percent for bleached.4 Hardwood recoveries range from 50 to 52 percent for unbleached kraft pulp to 50 percent for bleached. Sul?te Pulping?Lignin can be dissolved by sulfonation with an aqueous solution of sulfur dioxide and calcium, sodium, magnesium, or ammo? nium bisul?te cooked at high temperature and pressure in a digester (see ?gure 2-4). There are four basic sul?te pulping processes currently in commer- cial use: 1) acid sul?te, 2) bisul?te, 3). neutral sul?te, and 4) alkaline sul?te. The major differences be- tween the sul?te processes are the levels of acidity and alkalinity of the sulfite chemical solutions used to break down the wood and remove the lignin. Sul?te pulping processes are suitable only for species with low extractive contents those low in tannins, polyphenols, pigments,.resins, fats, and the like) because . of the interference of these substances with the sul?te pulping process. Al- though calcium is the cheapest sul?te base available, it forms insoluble compounds that cannot be re- claimed economically. Thus, calcium-based pulping is seldom used. Because magnesium- and sodium- based chemicals are recoverable, and ammonium- based chemicals are less expensive and can be burned without harmful environmental effects, they are the most frequently used. Sodium-based sul?te pulping can consist of multistage cooking, successive stages of which differ in acidity. Because one stage optimizes chemical liquor penetration and the other the re- moval of lignin, more lignin may be removed with less ?ber degradation, so that ?ber yields are higher, ?bers are stronger; and a wider range of wood species may be used. Sul?te pulping dissolves some of the hemicellulose as well as the lignin. Neutral sul?te pulping, using sodium and ammonium bases, recovers the largest proportion of ?ber (75 to 90 percent) of all the sul?te, pulping methods. Sulfite pulp is a light color and can sometimes be used without bleaching if. high brightness is not required. Unbleached sul?te pulp is often blended with groundwood and other'high?yield mechanical pulps for strengthening newspaper stock. Sul?te pulp is easily bleached to very bright pulps for writing and printing paper. It is also used for the manufacture of dissolving pulps (through the further removal of hemicellulose) for the -. production of viscose rayon, acetate ?bers and ?lms, plastic ?llers, and cellophanes. Potential for New Pulping Technologies The search for new pulping technologies and process improvements for established commercial technologies continues in the United States, Canada, Sweden, Finland, Japan, Germany, and elsewhere. In the United States, about $815 million is estimated to have been spent on pulp and paper research and development in 1987.5 OTA could not determine what proportion of the was directed at improving pulping technologies. Nearly all is Sponsored by the industry, with only $3 million 4 percent) expended by the Federal Govem- ment. Industry pulping is largely focused on improving established pulping and bleaching proc- esses rather than seeking new pulping technologies. Some of the research and development is driven by Hurley. Comparison of Mills Energy Balance: E?ects of Conventional Hydropyrolysis and Dry Pyrolysis Recovery Systems (Appleton. WI: Institute of Paper Chemistry, 1978). sliiattialle Memorial Institute. Probable Levels ofR&D Expenditures in 1987: Forecast and Analysis (Columbus. OH: Battelle, 1986), p. 11. Chapter 2?-The Pulp and Paper Making ?Processes 0 25 the need for broadening the raw material base. in response to concerns over forest resources. Restric- - tions on water use and pollution control have contributed to the impetus for seeking process improvements. Energy costs as reflected in both energy use by the industry and their impact on the cost of chemicals has led to process improvements in the past, although moderating energy prices have recently reduced these concerns. The emphasis on recycling to reduce the massive problems of solid waste diSposal in metropolitan areas has also been an incentive to using more reclaimed material in paper manufacturing. Finally, the increasing cost of capital to rebuild aging sectors .of the pulp and paper industry have fed the need for more by the industry. There are several reasons why major advance- ments in pulp and paper technology appear to be glacial in comparison to some other more rapidly advancing technologies. First, the pulp and paper industry is mature; the commercial technology, much of which was deve10ped in the late 17005 and 18005, has undergOne evolutionary change, and satisfaction with the basic technology has led to little reason to ?x something that does not appear to be broken. Concerns over future environmental prob- lems and competition from other materials could change this, and to some degree already has. Second, is fragmented by the emphasis on process improvement, therefore few scientists and engineers focus on new pulping processes. In addition, many researchers specialize in one pulping process or another depending on the needs of a speci?c ?rm; few are able to consider all technologi- cal options or innovations for improving pulp yield or overall quality. Third, investment in incremental improve- ment in established processes is easier to sell to corporate directors than risky, long-term, radical changes. Large existing investments in plant equip- ment stretch the amortization period of old equip- ment and slow the acceptance of new processes that require substantial changes and alterations. Finally, the absence of major government invest- ment in long-range, high?risk to seek new, innovative pulping and bleaching technologies may Figure 2-4?6ulilte Pulp Mill Process Fresh water Wood .: I . Wood preparation 1 . . 1 . 1: Liquor preparation Pulping Iv- Powerhouse I i Washing Screening Bleaching 4 I . . I Cleaning 8. reilning . I. I i 2 Pa er iormalion Pressing ?1 00: I Drying Finishing, Product SOURCE: Allan M.Springer. Environmental Control: Polo and Paper industry {New York. NY: John Wiley 5 Sons, 1986], p. 153. limit the advancements that could be made through collective efforts. Individual firms have little incentive to undertake a major, long-term, high- investment program to develop radically new technologies with uncertain payoff in the end, particularly in the current investment climate. Organosolv Pulping (Ester solv pulping?sometimes called ester a two- s-tage process involving hydrolysis (decomposi- 26 I Technologies for Reducing Dioxin in the Manufacture of Bleached Wood Pulp tion of the wood by dilute acids or enzymes) and the removal of lignin with an organic solvent, usually a mixture of alcohol and water. The still experimental process is suitable for both hardwoods and soft- woods. Sawdust as a byproduct from lumber manu- facture can also be pulped.6 Pulp recovery from organosolv pulping ranges between 50 and 60 percent for hardwoods. and 40 and 45 percent for softwoods. Typical hardwood ?ber recoveries com- pare favorably with those from kraft pulping. Fibers produced by the organosolv process are weaker than those recovered by the kraft process. Thus, the papers produced from organosolv pulp are Suitable for uses where strength is not the most important preperty printing papers, ?uff pulps, and dissolving pulp). Little waste is produced by the process, and low alcohols are recovered easily by distillation, thus requiring relatively low capital investment? Commercial viability of this technol- ogy will require that markets be developed for byproducts of the process. . A commercial demonstration plant using the Alcell process developed by Repap Enterprises Corp. of Canada is currently under construction at Newcastle, New Brunswick. The 33-ton-per-day pilot plant will cost $65 million. The Canadian government is underwriting half the cost of the plant. Alcell is an alcohol cellulose organosolv process. ydratropt'c Palping?Hydrotropic solutions are aqueous salt solutions that impart greater solubility to soluble substances lignin) than does water at the same temperature. Sodium xylenesul- fonate, a hydrotropic salt, has been used experimen- tally to delignify wood.8 A hydrotropic pulping process was patented by Ralph H. McKee in 1943.9 Laboratory pulping texts suggested that dissolution of lignin with aqueous sodium xylenesulfonate solutions of 30 to 40 percent had little or no effect on the strength of the pulp and yielded a high alpha cellulose content (important for dissolving pulp). Pulping of peplar was conducted at temperatures of about 150 for 11 to 12 hours. 'Ibsts yielded 52 percent cellulose, compared to 47 percent from comparable sulfite pulp. Unlike sul?te or kraft pulping which uses contaminating inorganic chemi- cals, the lignin recovered through precipitation by hydrotropic pulping is relatively pure and is suitable for conversion to other chemical products. The process is not suited for resinous coniferous species, however, and comparatively little serious. attention has been given to this process by the industry. 6 Theodor N. Keinen. ?Oranosolv l-?ulping With Aqueous Alcohol." TAPPI J.. August 1974. vol. 57, p. 99 et seq. 7N. Sawyer. Status of New Pulpt'ng Processes: Problems and Perspectives (Madison. WI: U.S. Forest Products Laboratory. 1982). p. 10; see also Raymond W. Young and Kenneth W. Baierl. 'Estcr Pulping of Wood: A Revolutionary Process." Southern Page Paper. November 1985. pp. 15-17. aRalph H. McKee. 'Use of Hydrotropic Solutions in Industry," industrial and Engineering Chemistry. vol. 38. No. 4. 1946. p. 382: see also Ralph H. McKee. ?Comparison of Wood Pulping Process." Pulp and Paper Magazine of Canada. vol. 55. No. 2. 1954. pp. 64-66. 9U.S. Patent No. 2303564. Jan. 19. 1943. CONTENTS Page CHLORINATED DERIVATIVES IN THE ENVIRON OVERVIEW. 29 DIOXIN 31 DIOXIN AND PULP AND PAPER MANUFACTURE 32 National Dioxin Study 32 EPAfPaper Industry Joint Five-Mill Study 33 DIOXIN IN PULP AND PAPER PRODUCTS 35 Box Box . Page 3.-A Detection Limits and Levels of Dioxin Contamination 32 Tables Table Page 3-1. Major Chlorinated Derivatives Identi?ed' 1n Pulp and Paper Mill Ef?uents 30 3- 2. Characteristics of the Bleached Kraft Pulp and Paper Mills Used' 1n the Five? Mill EPA/Industq Cooperative Dioxin Screening Study 34 3 -3. Concentration of TCDD and 1n Bleach Plant Wastewater. . . .34 3-4. Mode of Environmental Release of TCDD and TCDF From Pulp and Paper Mills - . 35 3 -5. Safeslevel Concentrations Of 111 Paper. Products . . . . . . . . . . . .. 36 1 Chapter 3 Environmental Considerations stream consist mostly of simple phenols, inhenolic Pulp manufacture, like most chemical processes, and carbohydrate oligomers (high and low 'molecu? results in emissions, ef?uents, and solid residues that must be disposed of. This study focuses on lar weight polymers), and neutral and acidic' materi? chlorinated bleached pulp mill waste ef?uents and - als resulting from the breakdown of the phenolic residual chlorinated compounds in paper products rings in lignin.3 (with emphasis on TCDD and TCDF), therefore air emissions and solid wastes1 will not be considered . It has been estimated that no more than 10 percent here. Technologies for reducing the production of . of the total solids in the waste stream of a pulp chlorinated organics in the pulping process are . bleaching plant contain chlorinated derivatives. discussed in chapter 5. . However, their toxicity to aquatic biota has raised I concerns among biologists.?l Chlorinated mill wastes CHLORINATED DERIVATIVES . have only recently been focused on in the United IN THE ENVIR States, although the toxicity of untreated, undiluted waste to aquatic biota is well documented in the AN OVERVIEW scienti?c literature. A technical committee of Envi- Ef?uents from bleached pulp mills contain a ronment Ontario, the provincial environmental variety of substances, some of which exhibit a agency for Ontario, Canada, recently reviewed the variety of effects in biological tests, such as geno- available information on chlorinated organics and toxicity, mutagenieity, or teratogenicity. These in? dioxin from mill waste. Based on its analysis, the committee recommended that Ontario adopt a long- . term strategy aimed at completely eliminating the elude resin acids and fatty acids, chlorinated phe- nols, and other chlorinated organic substances. The composition of bleaching ef?uent is extremely formation of organochlorines in kraft pulp mills.5 complex and varies from mill to mill depending on The committee also concluded, however, that ?ehlo- the wood Species being pulped, the pulping technol- rinated dioxins do not represent an immediate ogy, bleaching reagents, and waste treatment sys? I danger to human health and welfare,? but itdid note terns used.2 Comparatively little is known about the that heavy ?sh-eaters consuming ?sh caught down- stream of some bleached kraft mills might exceed the acceptable daily intake of TCDD. actual composition of mill waste ef?uents, although substantial scienti?c effort has been spent on re- search. A screening and veri?cation survey of pulp and paper mill ef?uents conducted by the Environ- Sweden and Finland, with pulp and paper mills mental Protection Agency (EPA) tentatively identi- located adjacent to the Gulf of Bethnia and the Baltic tied a number of chlorinated organic chemicals Sea, have experienced environmental damage to (table 3-1). The chlorinated components of the waste marine life from chlorinated organic substances 1Solid waste disposal. particularly the disposal of contaminated sludge from biological treatment plants. is an important factor in the ultimate solution of safely disposing of dioxin. U.S. EPA information on pulp mill sludge disposal provided to OTA by Karen Florini. Environmental Defense Fund. shows that ofthe 104 bleached chemical pulp mills in the United States, 54 dispose of sludge in land?lls, 20 use surface impoundments, 20 incinerate the sludge, 6 convert it to compost or other salable products. 6 apply it to the land as a soil amendment, and 2 dispose of it by other means (total exceeds 104 because some mills use more than one method of disposal). The subject of solid waste disposal has many aspects that range far beyond the focus of this study, including disposal of contaminated municipal sewage sludge. disposal of toxic and hazardous materials. and diSposal of incinerator residues. OTA has published several reports on related topics: Waste in Marine Enviromnems. April 1987; From Pollution to Prevention: A Progress Report on Waste Reduction. June 1987: Technologies and Management Strategies for Hazardous Waste Control. March 1983; Serious Reduction of Hazardous Waste, OTA-ITE-S 18, September 19861 2Between 250 and 300 chemicals have been identi?ed in pulp mill ef?uents. Many of these are chlorinated compounds. Leena R. Suntio. Wan Ying Shui. and Donald Mackay, Review of the Nature and Properties of Chemicals Present in Pulp Mill Effluents." Chemosphere. vol. 17, No. 7. 1988, pp. 1249-1290. - 3Carlton'W. Deuce and Goran E. Annergren, ?Chlorination." The Bleaching of Pulp, Third Edition (Atlanta, GA: TAPPI, 1979). p. ?lbid., p. 71. 5Environment Ontario. Stopping Water Pollution at its Source: Kraft Mill Ef?uent: in Ontario, Report of the 'Ibchnieal Advisory Committee, Pulp' and Paper Sector, Municipal?ndusttial Strategy for Abatement ('lbronto. Ontario: Environment Ontario. 1988), pp.1-2. _29_ 30 I Technologies for Reducing Dioxt'n in the Manufacture of Bleached Wood Pulp Table 3-1?Malor Chlorinated Derivatives Identified in Pulp and Paper Mill E?luents chlorobenzene 1 .1,1-trichloroethane trichlorophenoi 2.4-dichlorophenol dichlorobromomethane chlorodibromomethane trichioroethylene monochlorodehydroabietic acid 1.2-dichloroethane 1,1 ,2,2-tetrachloroethane chloroform methylene chloride tetrachloroethylene 9,10-dichlorostearic acid 2.3.7.8-dibenzofurans' Listed as carcinogens In the Fourth Annual Report on Carcinogens. U.S. Department of Health and Human Services. Public Health Service. National Toxicology Program. 1985. Listed as carcinogens by the National Institute of chpational Satety and Health. see Of?ce oi Teervtoiogy Assessment, and Regulating Carahogens? Background Paper (Chelsea. Ml: Lewis Publishers. 1987], p. 84. SOURCE: US. Environmental Protection Agency. Government Document for Ef?uent Limitations Guidelines andStarxierds for the Pulp, Paper. andPaporboard and the Bu?wrs? Pacer and Board Wis?Poi? Source Categories. EPA (Washington. DC: 1932}. p. 46. released into coastal waters;5 Sweden?s National Environmental Protection Board (Naturvardsverket) estimates that Scandinavian pulp and paper mills contribute between 300,000 and 400,000 tons of chlorinated organic materials to the coastal waters of Sweden, Finland, and Norway annually."f Evidence of environmental harm in the estuaries of the Baltic Sea (including the accumulation of ?dioxin in the ?esh of food where water circulation and exchange are extremely slow, has led the Swedish Government to consider regulations to reduce the amount of chlorinated organic substances produced by bleached sulfate pulp mills by imposing regula- tions that require the use of oxygen bleaching and increased use of chlorine dioxide bleach in place of chlorine gas. Sweden is also considering steps to promote the use of closed-cycle processes that wOuld signi?cantly reduce or even eliminate the release of chlorinated wastes to the environment. The Swedish experience, where biological waste treatment is less prevalent, contrasts with that of the United States where nearly all bleached kraft pulp mills use secondary biological treatment to reduce the biological and chemical oxygen demands of Wastewater. In the course of biological treatment, many potentially toxic substances are removed and concentrated in the treatment sludge.8 Many Swed- ish mills, on the other hand, do not use biological waste treatment, and discharge their ef?uent directly into the environment or use only primary waste treatment. The overall release of chlorinated organic com- pounds from pulp and paper mills has received less attention in the United States up to now. Instead, the major concern arose over TCDD and TCDF that are produced along with other chlorination products during bleaching cycles and are considered to be potentially harmful to human health.9 A similar pattern of concern over dioxin develOped in Canada: A recent report on pollution from kraft mill effluents published by Environment Ontario warned against focusing too closely on dioxins as a result of media publicity, because it ?may divert energies from productive avenues of pollution control into blind alleys of ill-conceived, routine, and expensive sur- veys of ?dioxin' concentrations."lo Products containing TCDD were at one time used extensively as herbicides (agent orange, They are also produced as byproducts . from the . incineration of municipal and industrial waste, the combustion of wood in home furnaces, stoves, and fireplaces, metal smelters, and the incomplete com- bustion of dielectric ?uids (PCBs) in electrical transformers. The use of dioxin-containing materials in industrial processes has since been signi?cantly 6Committee for the Gulf of Bothnia, Water Pollution Problems of Pulp and Paper industries in Finland and Sweden. Report of the Special Working Group. Naturvardsverket Rapport 334 8. in English (Solna. Sweden: Baltic Marine Environment Protection Commission. 1987). app. 3. 7National Swedish Environmental Protection Board, Action Plan for Marine Pollution _(Solna. Sweden: Naturvardsverket. 1987.), p. 27. 3A well-maintained. properly operated biological waste treatment plant can remove 30 to 50 percent of chlorination products and about 85 percent of TCDD and TCDF that is retained on suspended solids. Preliminary. unpublished research by the industry reported at the OTA November 1988 workshop indicates that it may be possible to remove up to 90 percent of the chlorinated organics with supplemental chemical treatment. This work is still experimenal. 9' ?Dioxins" and ?furans' refer generically to chlorinated dibenzo-para-dioxins (CD0) and chlorinated dibenzofurans (CDF), respectively. that have one to eight chlorine substituents. 1?Environment Ontario. op. cit.. note 5. pp. 1-19. Chapter B?Environmentol Considerations 0 3 I curtailed, but their inadvertent production through chemical and industrial processes, and as combus- tion. products continues. DIOXIN11 Dioxin, as generally referred to, is 2,3,7,8- tetrachlorodibenzo?p-dioxin (TCDD). It is the most toxic of 75 chlorinated dioxins and over 135 chlorinated furans Dioxin is a byproduCI of, or a contaminant in, manufactured materials. The chemical reactions and conditions under which they are formed in the pulp mill are not completely 'understood. TCDD was never produced as such intentionally except in small experimental quantities for research. In the United States, TCDD largely earned its reputation as a ?bad actor? in the Agent Orange controversies following the use of herbicides as defoliants during the Vietnam War. Agent Orange was a mixture of the herbicides acid and 2.4- dichlorophenoxyacetic acid These same chemicals were? extensively used in forestry and agriculture with little recognition of the health risks that may be related to the dioxin that contained as an incidental ingredient is no longer manufactured in the United States). The exposure of military? personnel to these chemicals in Vietnam raised the consciousness of the public about the health risks of dioxins when returning veterans blamed a number of their health problems and those of their families on past exposure to Agent Orange while serving in the military. How serious a human health threat is the exposure to dioxins? On purely scientific bases,?the question of human risk has not yet been de?nitively an- swered. Epidemiological data are incomplete and dif?cult to interpret. No study thus far has conclu- sively linked dioxins to the death of a human or to a human disease other than chloracne, or conclu- sively related exposure to dioxin to cancer (although the carcinogenic potential is considered 1 to be ?probable? or to miscarriages.? Abnormal behav- ior, genetic effects, immunological problems, enzy- matic disfunctions, and reproductive problerris asso- ciated with dioxin exposure have been considered, but studies have not confirmed dioxins tolbe the cause. With regard to certain?but not all??1ab'oratory test animals, dioxin has been shown to be extremely toxic and lethal at low levels. Procedures for extrapolating from animal effects to humans are controversial. There is, however, sufficient scien- ti?c evidence to suggest that human exposure to dioxin should be minimized within acceptable levels of risk pending a better understanding of its health implications. The public perceives dioxins to be dangerous and a major health risk as a result of publicity surrounding the Agent Orange contro- versy, the Times Beach incident the Love Canal; and problems related to the disposal of hazardous wastes. The regulatory agencies have opted for a conservative approach to regulating dioxins. Research on rainbow trout, a species often ?used to gauge the toxicity of chemicals, suggests thatiTCDD and TCDF can cause mortality, reduced growth, and - abnormal behavior during the ?shes? early life stages. TCDD was judged to be 10,000 times more toxic than the pesticides endrin or toxaphene, while TCDF was 1,000 times more toxic.? Bioconcentra- tion factors (B CF) for TCDD were found to be much higher than originally estimated. TCDD accumu- lated to about 39,000 times the ambient concentra- tion of the water, and TCDF between 2,640 and 4,449 times (but dioxin seems to concentrate in the gut and inedible parts of It has also been shown that similar preferential bioaccumulation or magni- ?cation of TCDD and TCDF occurs in aquatic birds. 15 However, the effects of TCDD and TCDF on ?This discussion of the human health effects of dioxin' 15 not intended to be de?nitive or analytical with regard to the dangers of dioxin. Rather, it is an encapsulation of other recent surveys of existing knowledge about dioxin and its congeners. '2?Iests on laboratory animals suggest that 1f the toxicity of 2 3 .7. 8-TCDD IS assigned the value of 1.0. the toxicity of 2 3 .7 8- TCDF 15 0. 1. Other dloxms and furans also generally have toxicities that are estimated to range from one-tenth to one- -thousandth that of 2, 3 8-TCDD. '30. S. Environmental Protection Agency. National Dioxin Study, EPAJS 30- SW- 87- 025- (Washington, DC: 1987). pp. 1-8. I?Paul M. Meiirle et aI.. ?Toxicity and Bioconcentration of 2. 3 .7 .8- 'Ibtrachlorodibenzodioxin and 2.3.7.8 ?I?etrachlorodibenzofuran 1nI Rambow Trout. ?Enviromnentol Toxicology and Chemistry.vol. 6. 1988. pp. 47- 62. 15D. L. Stalling ct al.. ?Patterns of PCDD. PCDF. and PCB Contamination In Great Lake Fish and Birds and Their Characterization by! Principal Component Analysis." Chemosphere, vol. 14, No. GR. 1985. pp. 627- 643. 32 0 Technologies for Reducing Dioxin in the Manufacture of Bleached Wood Pulp reproduction, survival, and behavior of bird popula- tidns is uncertain.16 DIOXIN AND PULP AND PAPER MANUFACTURE National Dioxin Study The National Dioxin Study (N DS), a 2-year effort to explore the extent of dioxin contamination in the environment, detected the presence of TCDD and TCDF in ?sh and bottom sediment samples col- lected of several U.S. pulp mills. ?7 Fish samples for the NDS were selected in three ways: 0 90 sites were selected statistically, 0 305 sites near urban and industrial areas were nominated by Regional Of?ces or the Of?ce of Water Regulations and Standards (OWRS), and 57 estuarine or coastal sites were sampled. . Of the 90 sites sampled statistically for ?sh contamination, 17 showed detectable levels of TCDD up to 19 parts per trillion (ppt) in composite whole ?sh samples (see box one-third of the 305 regional samples showed detectable levels of dioxin in whole ?sh samples. These sample sites included rivers, lakes, and some coastal and estuarine waters. TCDD levels in some samples ranged up to 85 ppt. Only 4 of the 57 estuarine and coastal sites sampled had detectable levels of dioxin in ?n ?sh or shell?sh, and these ranged between 1.08 and 3.5 ppt. About 80 percent of the whole ?sh sampled from sites in the Great Lakes were found to have detectable levels of dioxin. A multitude of potential dioxin sources are within the watershed of the Great Lakes and the turnover and flushing of the waters within the Lakes are extremely slow. Outside the Great Lakes, detectable dioxin levels 'were most frequently found in the major river systems that ?ow through industrial and urban areas. Advisories have been issued by Wisconsin, Maine, and Louisiana warning of possible risk of eating contaminated ?sh. Box 3-A?Detecti0n Limits and Levels of Dioxin Contamination Determining the amount of dioxin in the natural environment requires sophisticated analytical pro- cedures and careful statistical sampling and sample preparation. High resolution gas chromatography and mass spectrometry are used to quantify dioxin levels. Gas chromatography separates the dioxins from other compounds by selectively adsorbing the dioxins (based on their speci?c molecular weights) on an adsorbent such as activated charcoal, alu- mina, or silica gel. Mass spectrometry separates the dioxins speci?cally and quantitatively according to their atomic Weights. These technologies can meas- ure dioxins in the range of parts per quadrillion (ppq) in water samples, but this level of detection is still considered experimental for most biological samples. EPA selected a nominal detection limit of one part per trillion (ppt) for ?sh and soil samples collected in the course of the National Dioxin Study. This sensitivity pushed the limits of the state-of-the-art in analytical technology. Compre- hending parts per quadrillion and parts per trillion can boggle the mind. One is equivalent to 1 second in 32,000 years. One is equivalent to 1 second in 32,000,000 years. The potency of dioxin makes measurements at this minute level of resolu- tion important. The Food and Drug Administration (FDA) rec- ommends that consumption of ?sh .be limited if dioxin content exceeds 25 ppt, and consumption is banned when levels reach 51 ppt. In general, EPA found that dioxin levels of ?sh ?llets?the edible portions of the ?sh?had dioxin levels below the detection limits even though whole ?sh samples may be judged to be contaminated. At two-thirds of the sites where dioxin was detected, the maximum values encountered were below 5 ppt. At only four sites did dioxin, levels exceed 25 (the level at which FDA recommends that ?sh consumption be limited). The high-level sites were located in the Androscoggin River at Lewiston, Maine (29 ppt), and the Rainy River at 151.15. Elliot et al., 'Levels of Dibenzodioxins and Dibenzofurans in Eggs of Great Blue Herons (Arden herodias) in British Columbia. 1983-1987: Possible Impacts on Reproductive Success." Progress Notes No. 176. Canadian Wildlife Service, April 1988. p. 7. Environmental Protection Agency. National Dioxin Study. (Washington, DC: 1987), pp. 111-32-33. 18mm" pp. 111-29. International Falls, Minnesota (85 ppt), both of which are located of pulp and paper . mills.? Additional investigations at these sites showed dioxin levels of up to 414 in waste treatment sludges from the mills. As a follow-on to the NDS, EPA is investigating other chemical pollutants that might accumulate in fish; The National Bioaccumulation Study (NBS), which is currently underway, is focusing on a subset of ?priority? pollutants selected from among 400 potential chemical-.s These include non-conven- tional pesticides, semi- -volatile organic chemicals known to accumulate' in human fatty tissue, agricul- tural chemicals, industrial chemicals, and those in pulp mill ef?uents. Four hundred sites are being sampled in targeted industrial, urban, and agricul- tural areas and below pulp mills. Approximately 95 percent of the ?sh samples have been collected. 0f the 75 samples that have been analyzed from ?sh collected below pulp mills, 67 are reported to have dioxin above detectable levels. Samples from 10 mill sites report TCDD and TCDF concentrations in ?sh ?llet tissue above the acceptable FDA limits of 25 ppt.20 Recent data from the NBS based on ?sh sampled from 18 southern rivers receiving mill wastes showed accumulations of dioxin in whole ?sh ranging from about 1 up to 164 ppt. Most whole ?sh samples had dioxin levels between 10 and 40 ppt. Three whole ?sh samples had levels exceeding 100 ppt. The?edible part of the ?sh (?llets) contained much lower dioxin levels and none exceeded 25 acceptable limits, although one ?sh showed a level of 24 of dioxin.21 . EPA/Paper Industry Joint Five-Mill Study The results of the National Dioxin Study indicated that effluent from the manufacture of pulp can . introduce detectable levels of dioxin into the envi- ronment. This prompted the U.S. pulp and paper Chapter 3?Env1'ronmental Considerations II 33 industry, through the National Council of the Paper Industry for Air and Stream Improvement, and EPA to undertake a joint investigation of ?ve bleached kraft pulp and paper mills in 1986. 22 The cooperative screening study focused on three mills known to have dioxin in their waste sludge (all of the mills sampled used the activated sludge waste treatment process) plus two additional mills that were volun- teered by their ?rms to provide broader geographical coverage (table 3-2). The results of the cooperative ?ve-mill study indicated that the bleaching of kraft pulp with chlorine and chlorine derivatives is responsible for the formation of and as byproducts of the pulping process. Dioxin in Bleached Pulps Sensitive gas chromatographic procedures were used to distinguish between the amountsof TCDD and the related isomers of chlorinated furan in the bleached pulp and mill wastes. TCDD was detected in seven of' nine bleached pulps sampled, at levels up to 51 ppt. The median TCDD content was 4.9 ppt, and the mean was 13 ppt.? TCDF was found in eight of nine pulp samples at levels ranging from below detection limits to 330 ppt. The median TCDF content was 50 ppt, and ?the mean was 93 ppt. I Dioxin in Bleach Plant Wastewaters Wastewater from each stage of the pulp bleach sequence was systematically sampled at each mill. TCDD was detected in wastewaters at three of the ?ve mills, and TCDF was detected at four of- the ?ve mills sampled. The greatest discharge of both CDD and TCDF was associated with the caustic extraction stage, which serves to ?ush away the lignin and other coloring agents that are mobilized during the bleaching stages. Lesser amounts of TCDD and TCDF were detected in the wastewaters" of the hypochlorite bleaching stage and the chlorination bleaching stages '(see table 3? 3). pp. 111-30. ?Steven Croner, U.S. Environmental Protection Agency, unpublishedmaterials presented. at OTA dioxin workshop. Washington. DC. Nov. 14-15. 1988. I 21National Bioaccumulation Study data provided to OTA by Karen Florini, Environmental Defense Fund. Washingtou, DC, Feb. 6. 1989. 226. Amendola et al.. ?The Occurrence and Fate of and in Five Bleached Kraft Pulp and Paper Presented at the Seventh International Symposium on Chlorinated Dioxins and Related Compounds. Las Vegas, NV. October 1981' ?Haida p. 8. 34 0 Technologies for Reducing Dioxt'n in the Manufacture of Bleached Wood Pulp Table of the Bleached Kraft Pulp and Paper Mills Used in the Five-Mill EPAllndustry Cooperative Dioxln Screening Study Daily effluent Daily production Furnish (in percent) capacity Bleach sequences (million Mill Hardwood Softwood (tons) Hardwood Softwood gallons) I 85 15 500 CEOH CEOHHP 23 . CEOHHP ll 20 80 W'sit CEHED CEHED 36 Ill 100 NA 1.000 36 IV NA 100 400b CEH 18 30 70 1.200? CDEOHID 41 NA Not applicable a Can also produce 300 tons per day oi re?ner mechanical grourxhvood pulp. Has additional capacity to produce 330 tons per day of groundwood. Also can produce 130 tone of groundmod daily. SOURCE: G. 'Amendola el al.. ?The Occurrence and Fate of and in Five Bleached Kraft Pulp and Paper Mills.'paper presented at the Seventh International Symposium . on Chlorinated Dioxlne and Related Compounds, Las Vegas. NV, October 1987. Table 3-3?Concentration of TCDD and TCDF In Bleach Plant Wastewater TCDD (not) TCDF (opt) Bleaching stage Range Median Mean Flange Median Mean Chlorination 0.01-0.24 0.04 0.07 0.06-3.8 0.24 0.65 Caustic wash 0.01 -3.6 0.24 1.00 QUE-33.0 0.78 7.4 Hypochlorite 0.02-1.9 0.20 0.40 009-92 0.59 2.3 Chlorine dioxide 0.01-0.03 ND ND 0.01-0.13 ND ND ND Not detectable . SOURCE: G. Amends-Ia etal.. 'The Occurrence and Fate of and in Five Bleached Kraft Pulp and Paper Mills.? paper presented at the Seventh International Symposium on Chlorinated Dioxins and Related Compounds, Las Vegas. NV. October 193?. .Dioxin in Wastewaters and Sludges Comparisons among the mills indicated that the ..TCDD and TCDF contents of pulp and wastewater differed greatly from mill to mill. TCDD produced in the pulp bleaching process can be transported to the environment as a residual in ?nished pulp, in the sludge recovered in the wastewater treatment proc- ess, or as treated ef?uent released into streams and pond (table 3-4). TCDD was found in wastewater treatment sludges at each of the ?ve mills sampled.24 Analyses of wastewater from the paper machines showed that some of the dioxin produced in the bleaching process was passed through to the paper making process. Continuing Efforts Although the ?ve?mill cooperative survey con~ ?rmed that chlorinated bleaches can produce diox- ins in the manufacture of bleached wood pulp. the study revealed great variability in dioxin concentra- tions among the mills and within the pulps, waste sludge, and treated ef?uents. The results demon- strated the need for a comprehensive and systematic survey of the receiving waters and biota below the waste outlets of U.S. pulp mills in order to better understand the scape and intensity of the environ- mental loading of TCDD. The survey also indicated that more detailed information was needed about dioxin levels at various steps in the pulping process and in the bleaching sequence. EPA has recently negotiated a cooperative agree- ment with the American Paper Institute (API) and the National Council of the Pulp and Paper'Industry for Air and Stream Improvement (NCASI), both associated with the U.S. pulp and paper industry, to survey all 104 domestic pulp mills that manufacture chemical bleached pulp for-production of dioxin and Environmental,Protection Agency. lndustry'Cooperative Dioxin Screening Study. EPA-44011-83-025 (Washington. DC: 1988). Chapter 3?Envr?ronmental Considerations 0 35 Table Sui?Mode at Environmental Release of TCDD and TCDF From Pulp and Paper (percent) Mill Source I ll Ill Bleached pulp 19 66 30 57 Waste sludge 16 16 100 70 22 Treated effluent 65 18 21 2.3.7.8-TCDF Bleached pulp 19 56 60 31 55 Waste sludge 17 20 36 69 22 Treated effluent 64 24 4 23 SOURCE: G. knot-Idols et el.. 'The Occurrence and Fete ol in Fm Bleached Krelt Pulp and Paper Mille.? paper presented at the Seventh lntemetionel Symposium on criodneted Dioxins and Related Compounds. Les Vegas. NV. October 1987. furan isomers.25 In addition. an industry study will undertake a detailed analysis of dioxin levels and bleaching processes at selected pulp mills. The study will consider the full range of factors that might affect the production and dispersal of dioxins. including annual ef?uent flow, wastewater treatment, sludge disposal pracuces, bleach plant operations, and an analysis of dioxin contents of ef?uents. pulps. and sludges. Detailed analyses of dioxin levels in 25 bleach lines will be included. The study began in the summer of 1988. and is expected to be completed in the summer of 1989. In a related effort. an EPA-led interagency group has undertaken a Multi?Media Risk Study that will utilize data collected in the EPA/Industry lO4-mill study. Under a court?approved consent agreement to consider dioxin in paper, EPA is attempting to estimate the cumulative risk of dioxin from this source in all media?pulp. sludge, and effluent.?5 Data collected in the 104-mill study. and estimates of migration rates of TCDD and TCDF from paper products adjusted by product-use scenarios, will be used to determine whether or not there is a human risk from dioxin in pulp and paper.2T The consent decree also requires EPA to undertake an assessment of environmental risks as well as human risks. The Food and Drug Administration (FDA) and Con- sumer Safety Product Commission (CSPC) have initiated product risk assessments that will become part of the interagency multi-media study. DIOXIN IN PULP AND PAPER PRODUCTS The ?nding of TCDD and TCDF in bleached pulp samples raised questions as to whether residual TCDD might also ?nd its way into ?nished paper products and present a potential health risk to consumers through dermal contact (it did not con- sider other routes of exposure). The NCASI commis- sioned Envirologic Data, Inc. to assess the potential risks? to human health from skin exposure to a variety of bleached pulp products. including dispos- able diapers. facial tissue. toilet tissue. sanitary napkins, and paper towels.29 The results of Envirologic Data?s risk assessment of concentrations of TCDD found in paper products were related to a lifetime cancer risk of one in a million in the general population?a regulatory standard commonly used gauge risk. Based on this measure of risk. a ?virtually safe concentration" of TCDD equiva- lents was calculated for the various products tested (table 3-5). 25Sent: Federal Register. vol. 53. No. 27. Feb. 10. l988. p. 3937; EPA Of?ce of Water Regulations and Standards. and Of?ce of Torrie Substances. U.S. [3?er Industry Cooperative Dioxin Study?Tier 1. Fact Sheet. Feb. 4. I988. 2"See. Enviromnral Defense Fund 5: National Wildlife Federation v. Thomas. No. 85-0973 consent decree entered July 27. 1983). 2"Dwain Winter. US. Environmental Protection Agency. communication at the OTA dioxin workshop. Washington. DC. Nov. 14-15. 1988. 2"Risk assessment is the characterization of the probability of potentially adverse health effects from human exposure to environmental hazards. The risk assessment process consists of four steps: I) hazard identi?cation. 2) dose-response assessment, 3) exposure assessment. and 4) risk characterization. NCASI used EPA's guidelines for Carcinogcn Risk Assessment as a framework for the dioxin dermal exposure study. ?National Cormcil of the Paper Industry for Air and Stream improvement. Assessment of Potential Health Risks From Dermal Exposure to Dioxt'n in Paper Products.1bchnical Bulletin No. 534 (New York. NY: 1987). p. 107. 36 0 Technologies for Reducing Dioxr'n in the Manufacture of Bleached Wood Pulp Table 3-5?Seto-Ievel Concentrations of TCDD in Paper Products Calculated TCDD equivalent (in ppt) Male Female Measured Bleached pulps 5a Communication paper 13" Clerical worker 9.000 9.100 Manager 4.200 4.300 Personal care products Disposable diapers 0? Conventional 540.000 540.000 Superabsorbent 2.000.000 2.000.000 Facial tissue Normal use 66.000000 79.000000 Makeup 230.000 Toilet tissue 27,000,000 65,000,000 Sanitary pads 63,000,000 Paper towels 7.900.000 9.500.000 Composite personal care productsd 160,000 510.000 Combined communication papers and personal care products 4 Clerical worker 8.500 8.900 Manager 4.100 4.200 3 Measured in 7 pulps with levels oi dioxin irom less than 1 to 51 pot. with a median of 4.9 ppt. Measured In bond paper. '3 No TCDD detected In disposable diapers at detection limits at 2.1 and 2.6 ppt. Excluding superebeorbent clsposeble diapers. SOURCE: National Council of the Paper industry for Air and Stream Improvement. Assessment 0! Potential Health Risks From Dermal Exposure to Dioxin in Paper Products. Ted-mica! Bulletin No. 534 (New York. NY: 3987). 107. Calculated safe levels for bond paper, newspaper, and other paper used for communications ranged from 4,200 of TCDD for female managers to 9.100 for male clerks. Safe levels for personal care products were calculated to range from 230,000 of TCDD for female facial tissue used for makeup removal to 79.000.000 for facial tissue by males. Safe levels for paper towels were calcu- lated at 7.900.000 for females and 9,500,000 for males. Toilet tissue safe limits for females were calculated at for females and 65 for males. Actual concentrations of TCDD in samples of bond paper were detemined to be about 13 ppt; in paper towels. 4 ppt; and no detectable TCDD was measured in disposable diapers. NCASI published the results of an assessment of potential exposure to dioxin from coffee brewed using bleached coffee ?lters in May 1988.30 Based on an assumed consumption pro?le for an average and heavy coffee drinker and assuming that 65 to 90 percent of the dioxin migrated from the ?lter, NCASI concluded that the calculated dioxin TEQs for a risk ranging from zero to one in one-million to be between 20 TCDD TEQs for average coffee drinkers to 11 TCDD TEQs for heavy coffee drinkers. The coffee ?lters tested had TEQ dioxin contents ranging from 2.2 to 6.6 ppt. An assessment of dioxin in food packaging paper products has been initiated.3 NCASI commissioned ENVIRON. Inc. to undertake the evaluation. A number of food contact products are scheduled to be undergo risk assessment. such as paper cups and plates. convenience food packaging. paper towels, and pizza boxes. NCASI has put the project on hold pending the development of acceptable test proce- dures to determine the absorption of dioxin into fatty foods. The assessment will resume when test proto- cols are developed Scientists at Health Welfare Canada, a govem- merit agency. reported in August 1988 that they had 30National Council of the Paper Industry for Air and Stream improvement. Assessment of the Risks Associated With Potential Emosure to Dioxin Through the Consumption of Cafe: Brewed Using Bleached Paper Co?ee Filters. Technical Bulletin 546 (New York, NY: 1988), p. 34. 3 National Council of the Paper industry for Air and Stream Improvement. First Progress Report on the Assessment of Potential Health Risks ram Use of Bleached Board and Paper Food Packaging and Food Contact Products, Special Report 87-1 1 (New York. NY: 1987). p. 27. Chapter 3?Environmentai Considerations 0 3 7 detected 0.04 of TCDD and 0.75 of TCDF in packaged in non?bleached paper containers. NCASI whole milk packaged in plasticized bleached paper is currently collaborating with Canadian scientists to cartons-32 They reported no similar levels in milk con?rm these ?ndings. 3'3Jol1n J. Ryan. Food Research Division. Health Protection Branch. Health Welfare Canada. Ottowa. Canada. paper presented at the International Dioxin Symposium. Umca. Sweden. August 1988. Chapter 4 Pulp Bleaching Technoldgy 5? . - . a: .. CONTENTS Page THE BLEACHING PROCESS -. . . . . .. . .Historical Development Of Bleaching TechnologyExtent of Bleaching' 111 the, Industry .. . -. . .- .. . 42 Bleaching Methods.--.. 42 Bleaching Systems30x43 .. Bax . Page 4- A What Is Pulp ?Brightness?? How Is It M?e4?sure'd? .. . 42 4- B.- Shorthand for Describing Bleaching Sequenc45Assessmg Lignin Content. and Pulp Bleach4bilityFigur?s . . Figure I I Page 4-1 Bleaching Sequences in S. and Can4dian Pulp Mills- ._44 4-2, Sequence for the Producnon of Fully Bleached Chemical Pulp.1.-. 47 4- 3. Pulp Brightness at Stages of; the Bleaching Se'quence CEHDEDSchematic Diagram of__;he Chlorination Process - . .. .50 .. Tables - Tabie . . Page; 4- 1. Domestic Bleached Pulp Capacity .. -. . . .. . . . ., 543 4- _2 Bleachmg Chemicals: Faun, Function, Advantages Disadvantages 44 4- 3. Exarrmles of Preblea'ching sequences for Pulp Deligni?catioh. .- . -. . .-..- 48 4-4. CommOn Sequences Used To Bleach Kraft 131111) to Various Degrees of Brightness 49 THE BLEACHING PROCESS Bleaching is the treatment of cellulosic ?ber with chemicals to increase brightness (see box? 4-A). Brightness may be achieved by either lignin removal (deligni?cation) or lignin decolorization. Lignin remains .a major constituent of pulp even after digestion by chemical pulping. For example, kraft pulp may contain up to 6 percent lignin based on its dry weight.1 Unbleached groundwood spruce pulp may contain 27 percent lignin. If chemical pulping removes the lignin from wood ?bers, why then does some lignin remain after the pulping process? The strength of paper is largely due to the chemical bonds (hydrogen bonds) formed . between Cellulose ?bers. Although longer and more severe pulping might remove more of the lignin, thus reducing the amount of bleaching needed, the cellulose molecules might be degraded and their bonding power diminished. Should this happen, the strength of the pulp would be reduced. The removal of lignin by bleaching is regarded as a continuation of the pulping process, albeit somewhat gentler and less destructive, but bleaching too can degrade cellulose if done improperly. Lignin imparts a color to the raw pulp (hence its name ?brown stock?) and unless removed, will continue to darken with age (note the yellowing, darkening, and enbrittlement of newspaper exposed to sunlight). Bleaching by removing the lignin gives higher brightness to the paper than is possible by leaving the lignin in the pulp and brightening by decolorization, and also leads to a more durable and stable paper. In addition to the removal and decolorization of lignin, bleaching serves to clean the pulp of dirt and foreign matter that escaped the digestion process. Bleaching also removes hemicellulose and extrac- tives (hemicellulose is nearly completely removed for the production of dissolved pulps). Bleaching pulp adds signi?cantly to its value as market pulp because the demand for bleached paper is increasing. Chapter 4 Pulp Bleaching Technology Historical Development of Bleaching Technology Bleaching of ?bers for decolon'zation has been practiced since early times. Sunlight was one of the earliest bleaching agents. Japanese paper makers were known to bleach ?bers by soaking them in water from high mountain streams that'contained ozone (the ?rst use of oxygen for bleaching). In 1774, Karl Wilhelm Scheele discovered chlorine and its bleaching action on vegetable ?bers. Several years later in 1799, Charles 'Ibnnantv invented ?bleaching powder" (calcium hypothlorite), thereby converting chlorine to an easily transport- able forrn. For the next 130 years it remained the only available bleaching agent. The ?rst time a US. paper mill used chlorine for bleaching was in 1804. Rapid developments in bleaching teChnology occurred between 1900 and 1930. Multistage bleach- ing using calcium hypochlorite followed by an alkaline extraction stage, then a repeat; of the hypochlorite bleach stage was ?rst adopted by the industry. Later, technologies that allowed the use of gaseous chlorine began to displace hypochlorite 1n the ?rst bleach stage. The use of chlorine' reduced bleaching costs. New equipment developments fur? ther improved bleaching ef?ciency. Improvements in the manufacture of ?chlorine dioxide and dioxide bleaching technology were developed in the 19405. By the 1950's, these developments led to the adoption of the ?ve-stage bleaching sequence that is still used extensively in the industry: (1) alkaline extrac- chlorine dioxide-9(4) alkaline extrac- chlorine dioxide. The ?ve-stage bleaching . sequence allowed very bright pulp to be produced with minor losses in ?ber strength. Oxygen bleaching was discovered. in 1952 by V.M. Nikitin and G.L. Akim in the Soviet Union. In the late 19605, oxygen bleaching was commercial- ized, followed by the installation of the ?rst dis- placement bleach plant' 1n the 19705. This resulted' 1n more rapid bleaching by displacing chemicals through a pulp mat rather than mixing the chemicals 1Douglas W. Reeve, 'The Principles of Bleaching.? 987 Bleach Plant Operations Seminar, Notes (Atlanta, GA: TAPPI Press, 1987). p. 3. ?41? 42 I Technologies for Reducing Dioxin in the Manufacture of Bleached Wood Pulp Box 4-A?What Is Pulp ?Brightness?? How Is It Measured? ?Brightness" is the re?ecting preperties of a sheet of pulp. It is a physical andmeasurable phenomenon. Some mistakenly equate ?whiteness" with brightness, but whiteness is a physiological phenomenon, measured subjectively by the impression and perception of the human eye. For instance. if blue dye is added to a yellow tinted paper, the sheet will look whiter but the sheet will measure less re?ected light (less brightness). Since re?ectance is affected by the nature and the angle of incident light, the surface properties of the pulp sheet, and other factors, the measurement of brightness has been standardized: Brightness is the re?ectance of blue light with a Spectra] peak at 457 millimicrons from an opaque sample of pulp sheets compared to a Specified standard surface. . There are two recognized methods for measuring pulp brightness in North America: I) the TAPPI method (Technical Association of the Pulp and Paper Industry), Standard and 2) the CPPA method (Canadian Pulp .and Paper Association). - .- Method Reported in units of Brightness. Illuminating light is aimed at 45 degrees to the sample and the re?ected - light is measured perpendicular to the sample (90 degrees). Re?ectance is_ compared to magnesium oxide powder (98 to 99 percent absolute re?ectance). Calibrated opal glass standards are used for routine measurements. CPPA Method Reported in units of ISO Brightness. The sample is illuminated with diffused light using a highly re?ecting integrating sphere. Re?ected light measurement is taken at 90 degrees to the sample. Re?ectance is compared to absolute re?ectance from an imaginary perfectly ie?ecting, perfectly diffusing surface. Calibrated opal glass standards are used on a routine basis. A third brightness standard is used throughout the rest of the world: The Zeiss Elrepho standard. It is measured in units termed Elrepho Brightness. GE Brightness is measured with a re?ectance meter manufactured by the General Electric Corp., while Elrepho Brightness is measured by an instrument man ufactured by Zeiss, the German Optical company. Since the two meters have different light geometries, there is no simple relationship between the two measurements. In general, Elrepho Brightness is, on average. 0.5 to 1.0 percent higher than GE Brightness. with the pulp in the conventional way. Since the late 19705 development has taken place in the use of oxygen enrichment in alkaline extraction stages, to further delignify pulp after extended cooking (modi- ?cations of the cooking process to improve deligni- ?cation), and in short bleaching sequences where oxygen is used to supplement chlorine. Extent. of Bleaching in the Industry Nearly 55 percent of the chemical pulp currently produced in the United States is bleached (table By far the greatest proportion of bleached chemical pulp is produced by the kraft process (about 88 percent of the pulp bleached in 1987 was kraft pulp). Very little mechanicalpulp has been bleached in the past, however, this is currently changing. Mechani- cal pulp bleaching is growing at-more than twice the rate of chemical pulp bleaching worldwide and it is likely that this trend will continue in the United 21bid.. p. 2. States as well.2 Overall, the tendency has been to bleach more pulp as the demand for products using bleached paper increases (nearly 39 percent of the' domestic paper and paperboard currently produced is from bleached'pulp). Bleaching Methods Bleaching Agents Pulp cooking can safely dissolve up to about 90 percent of the lignin without degrading the cellulose fiber. Additional delignification is done by bleach- ing. Bleaching of high-yield chemical pulps is achieved by decolorizing with either an oxidizing agent (combines oxygen) or a reducing agent (combines hydrogen). Chlorine gas, sodium hypo- chlorite, chlorine dioxide, oxygen gas, and hydrogen peroxide are oxidants. Sodium hydrosul?te is a reductant. Alkali is used to remove the solubilized Chapter 4?-Pulp Bleaching Technology 0 43 Table 4-1?Domestlc Bleached Pulp Capacity (thoueend metric tons) Grade 1971 Bleached sul?te 1.774 1.256 Bleached kraft 13.364 21.259 (bleached) 1.604 1.455 Total chemical pulp produced 32.779 43.800 Percent oi chemical pulp bleached . . 51 54.7% Eula-eh SOURCEWI w. Reeve. 'The Pmcipiee of lee? ?eech Pleat mam Seminar. MteelAlleniI. GA: TAPPI Press, 1937). p. 2. lignin from the cellulose. Each has its advantages. disadvantages. and limitations (table 4-2). Since the 19303, chlorine gas has been the predominant chemical used for the deligni?cation of pulp. Chlorine dioxide can brighten pulp without damaging the cellulose. Oxygen is comparatively inexpensive and is now coming into its own both for deligni?cation (immediately after digestion and before the bleach cycle) and as a supplement in the ?rst extraction (alkali) stage of the bleach sequence. Hydrogen peroxide is expensive, so it is used much less than other bleaching agents. The effectiveness of a bleaching agent. although a major factor in determining its use in a pulp bleaching sequence, may be offset by the cost of the chemical or the equipment needed to handle it. A critical determinant in choosing a bleaching chemical is the ?selectivity" of the agent. Selectiv- ity refers to the capacity of the chemical to attack lignin while doing minimal damage to the cellulose ?bers. Unbleached pulp (brown stock) contains high levels of lignin, therefore less selective chemicals oxygen and chlorine) can be used in the initial stages of the bleach cycle. With further deligni?ca? tion and lower residual lignin content of the pulp, more chemical is available to react with the cellulose and pulp strength may suffer. Chlorine dioxide and hydrogen peroxide are highly selective, thus they react rapidly with lignin but affect cellulose very little. The highly selec?ve chemicals are generally used in later bleach stages when the lignin content is low and the cellulose is susceptible to degradation. However, both chemi- cals are expensive and are therefore used sparingly. Sodium hydrosul?te. a reducing agent, and hydro- gen peroxide are used for bleaching lignin-rich mechanical pulp. Table Chemicals: Form, Function, Advantages. Disadvantages Chemicals Function Advantages Disadvantages Oxidente Chlorine Oxidize and chlorinate lignin Effective. economical delignilication ?Can cause loss of pulp strength it used irrproperly Hypochlorite Oxidize. brighten and solubilize Easy to make and use Can cause loss of pulp strength if lignin used improperly Chlorine dioxide . . . . 1) Oxidize. brighten and Achieves high brightness without Must be made at the mill site lignin pulp degradation 2) In small amounts with chlorine gas to protect against degradation of pulp Oxygen Oxidize and solubilize lignin Low chemical cost Used in large amounts. Requires Hydrogen peroxide . . Oxidize and brighten lignin in chemical and high?yield pulps Easy to use. Low mpitai cost expensive equipment. Can cause loss of pulp strength Expensive Reduclent Hydrosuifite Reduce and decolorize lignin in Easy to use. Low capital cost Decomposes readily. Limited high-yield pulps brightness gain Alkali Sodium hydroxide . . . Hydrollze chlorolignin and solubilize Effective and economical Darkens pulp lignin SOURCE: Doug.? w. Reeve. 'The Pnnciples of Blead'llng.? lee? Bleach Plant Operations Semrner. Notes (Adana. GA: Press). p. 8. 44 - Echnalogies for Reducing Dioxin in the Manufacture of Bleached Wood Pulp Bleaching Sequences A combination of bleaching and extracting treat- ments is generally used for bleaching chemical pulps (box 4-B). The bleaching chemicals and the order in which they are used make up the ?bleaching sequence.? Bleaching sequences generally contain two phases within each sequence: -1) a deligni?ca- tion segment, whose function is to remove the lignin; and 2) a brightening segment, whose princi- ple function is to increase the brightness of the pulp. Examples of delignifaction stages or segments include oxidation by chlorine (C) followed by Box 4-B?Shorthand for Describing- Bleaching Sequences The pulp and paper industry has developed a series of shorthand descriptors for the multistage bleaching sequences. The following abbreviations are used to designate the bleaching agents: Chlorination . Extraction with sodium hydroxide Hypochlorite (sodium or calcium) Chlorine dioxide Hydrogen peroxide Oxygen Nitrogen dioxide Ozone Bleaching sequences are designated by listing each treatment serially. For example, represents a commonly used five-stage bleaching sequence consisting of a ?rst-stage chlorine treat- ment, followed by a second-stage alkali extraction stage, followed by a third-stagechlorine dioxide - treatment, followed by a fourth-stage alkali extrac- tion treatment, and a ?nal ?fth-stage chlorine dioxide treatment. Washing is conducted between each chemical application. Two bleaching agents may be used in a single stage. For instance, chlorine gas and chlorine dioxide are sometimes combined in an early bleach- ing stage. If chlorine gas is the predominant agent in the mixture, the treatment would be designated as 0n the other hand, if the mixture contains more chlorine dioxide than chlorine gas, the treat- ment would be designated as Do Other commonly encountered oxidative extraction treatments in- elude E0 (or EP, EIH etc. extraction of the dissolved lignin with sodium hydroxide (E). Brightening segments use sodium hypochlorite (H) and/or chlorine dioxide (D). Oxy- gen Can be used for deligni?cation and for reinforc- ing extraction of the dissolved lignin in the alkali stage. Several of the more commonly used bleach sequences in U.S. and Canadian mills are: 1) CEDED, 2) CEDH, 3) CEHDED, 4) CEH, and 5) CED (?gure 4-1). Although these are the most prominent bleaching sequences currently in use, an increasing number of mills are now using oxygen in combination with alkali for extraction (E0), and chlorine dioxide (CD, DC) in the chlorination stages. In addition, there are a number of unique bleaching sequences used by some mills CEHDH, CEI-IEDP, CEDPD, CEDEHD, and CEHCHDED). Factors Affecting the Bleaching Process Process engineers and paper chemists have a wide range of chemicals, processes, equipment, and operating conditions to choose from in optimizing a bleaching sequence. Cost of chemicals, capital cost of equipment, energy requirements, and other oper- ating costs ?gure heavily in bleach plant decisions. While cost control is an important factor, the physical and chemical composition of the wood raw material (furnish) and the desired characteristics (brightness and strength) of the ?nished paper are often more important in selecting bleaching tech- nologies. Bleaching sequences also depend on the pulping process used for initial deligni?cation, some Figure 4-1 ?Bleaching Sequences Used In us. and Canadlan Pulp Mills CEDED 33s. Other 22% CEH, BS CEHDED CEDH, CEHED 13" 19% SOURCE: Data from David Ft. Forbes. ?Upgrading Existing Bleach Plants,? :98? Bleed: Plant Operations Seminar, Notes (Atlanta. GA: TAPPI Press, 1987]. p. 116. Chapter 4?Pnip Bleaching Technology I 45 of which leave higher residual lignin levels remain- ing in the brown stock than do others. The ef?ciency of the bleach cycle (related to its cost effectiveness), also depends on controlling the operating environment within each bleaching stage. Bleaching is achieved through chemical reactions. Operating conditions are related to temperature, time, chemical concentrations, and degree of acidity or alkalinity (pH).3 These factors must be kept in balance to achieve the desired degree of bleaching, while at the same time minimizing damage to the cellulose ?ber. In addition, the ?consistency? (amount of ?ber being bleached in relation to the volume of liquid) of the ?ber slurry being bleached affects chemical penetration and therefore mustlalso be controlled. Computerization and improved sen- sors now allow nearly real- time control over the operating environment in all stages of the pulping and bleaching processes. 4 Lignin Content?Lignin is a large, complex, organic molecule that still holds mysteries for wood chemists. Complete quantitative analysis of lignin from pulp samples would be expensive if performed with precision. This is not necessary from the standpoint of controlling the digestion and bleaching process, however, as simpler methods have been found. Index systems for ranking the lignin content of wood pulp have been devised by the industry (box 44C). The lignin content of unbleached pulp, expressed by its kappa number, determines the amount of bleaching required in the bleach sequence to obtain the brightness desired in the finished pulp. The key is to ?nd the optimum point between cooking and bleaching the best kappa number for un- bleached pulp). For bleachable grades, kappa num- bers of unbleached kraft softwood pulp may range between 20 and 40 and hardwood between 15 and 25 as it leaves the digesters in some mills (kappa number 35 represents approximately 5 percent lignin). Using modern computer-controlled cooking, ap- propriate chip pretreatment, and chip equalizing systems, it is now possible for well-run kraft mills to produce unbleached softwood pulp consistently with kappa numbers between 28 and 32.5 Un- bleached hardwood pulps can be produced with kappa numbers between 20 and 25.6 Pulp can be delignifred to extremely low kappa numbers (2 to 4) by using chlorination followed by a alkali/oxygen (E0) extraction stage.7 Lignin and Brightness?The lignin content, kappa number, and brightness of chemical pulps are . somewhat interrelated. Since pulp brightness is the major objective of bleaching, measurements of brightness expressed as either GE Brightness or as - ISO Brightness (see box 4- A) are often used to track progress through the bleaching sequence. Unbleached kraft pulp has a very low GE Bright- ness (10 to 20 percent) because of the high absorp- tion of re?ected light by the residual lignin. 3 Kraft pulp can be bleached to a very high brightness of 90 percent GE by decreasing its lignin content to near zero without affecting the strength of the pulp. It is possible to bleach kraft pulp to a brightness of 91 to 92 percent GE for special papers. 9 Unbleached acid sul?te pulp with GE brightness of about 60 to 66 percent can also be bleached to 90 percent by removing nearly all of the lignin. Groundwood pulp may have brightness values of about 62 percent GE. 3Rudra P. Singh. ?Principles of Pulp Bleaching," The Bleaching ofPuElp?Third Edition (Atlanta. GA: TAPPI Press. 1979). p. 17. ?Thomas J. Boyle and Carr Smith. ?Bleach Plant Instrumentation and Computer Control." The Bleaching of Pulp?Third Edition (Atlanta. GA: TAPPI Press. 1979). p. 487 et seq. 51ngemar Croon. Alf de Ruvo: 33d Gunnas'l?amvik. 'Bleaching of Kraft Pulps: Oxygen Techniques Today and in the Future.? Svensk Papperstia?ning No 4. reprinted by Sunds De?brator in English (Stockholm, Sweden: 811nm De?brator, 1985). p. 2. 6Kristina Idner, 'Oxygen Bleaching of Kraft Pulp?High Consistency vs. Medium Consistency." 1987 international Oxygen Deiini?carion Conference. Notes (Atlanta. GA: TAPPI Press. 1987), p. 197. 7N. Liebcrgott and B. van Lierop. "Extraction. Part 1:0xidative Extraction," i987 Bleach PiantSeminar. Notes (Atlanta. GA: TAPPI Press). p. 46. 3Reeve. op. cit.. footnote 1. p. 4. 9W. Howard Rapson and Gene B. Stumila, ?Chlorine Dioxide Bleaching.? The Bleaching of Pulp, Third Edition (Atlanta. GA: TAPPI Press, 1979). p. 142. 46 I Technologies for Reducing Di oxin in the Manufacture of Bleached Wood Pulp Box 4-C?Assessing Ligm'n Content and Pulp Bleachabt'h?ty The 'IBChnical Association of the Pulp and Paper Industry (TAPPI) has devised two standardized procedures for determining and reporting the lignin content of pulp: I) Permanganate (K) number (TAPPI Test Method T214). and 2) Kappa number (TAPPI Test Method T236). These indices are used by the industry to control cooking within the digester during pulping and- for determining the bleachability of the pulp. .. Both methods are chlorine demand tests and are based on the amount of permanganate needed to oxidize the contained lignin. The Permanganate. or number. is used for determining the bleachability of chemical pulps having lignin contents below 6 percent (based on weight of oven-dry pulp). The kappa number is applicable to all grades of chemi-- cal and semi-chemical wood pulps, including high- er lignin content pulps with yields as high as 70 percent. Standard procedures have been established for both bleaching indices. Both are based on the amount of potassium permanganate that can react with dry pulp samples. Most modern pulp mills now use automated, continuous oxidation- reduction measurements or optical devices such as brightness meters for on-line measurements to gauge the progress of delignification and the need for additional bleaching. However. permanganate tests are still used in mill laboratories for veri?ca- tion of the instrument reading. Bleaching Systems Bleaching sequences apply various bleaching agents in different orders and combinations. Be? tween each bleaching stage the pulp is generally (but not. always) ?ushed with an alkali extraction solu- tion to remove the dissolved lignin before it is sent to the next bleaching stage (?gure 4-2). The ?rst step of a bleaching sequence is designed to remove the bulk of the residual lignin (deligni?cation) and involves little or no improvement in the brightness of the pulp (?gure 4-3). This step, along with the following extraction stage, is called ?prebleach- ing." The purpose of prebleaching is to remove as much lignin from the pulp as possible to minimize the volume of more expensive bleaching chemicals chlorine dioxide, hypochlorite, and hydrogen peroxide) needed in subsequent bleaching stages. Chlorine gas and sodium hydroxide (CE) have been the preferred chemicals for the prebleach stage of the bleaching process. More recently. mixtures of chlorine and chlorine dioxide have been used in place of (or in addition to) pure chlorine treatment (table Other prebleaching processes are slowly displacing CE as the ?rst stages of the bleach sequence at some mills. Prebleach oxygen deligni?- cation and extended cooking may shorten the bleaching sequence by reducing the amount of lignin carried forward to the bleaching process.? Impetus for considering alternatives to the con? ventional CE stage are based partially on reducing the cost of bleach plant operations and partially on concerns about environmental impacts from dis- charged bleaching ef?uents. These concerns result from detection of chlorinated organic material con; taining chloroform and dioxins in bleached pulp mill sludge. Most of the chlorinated organics contained in bleach plant ef?uent originate from the ?rst chlorine. alkaline1 extraction, and hypochlorite stages. Brightening stages that follow prebleaching re- move less lignin but bring out the brilliance of the pulp through bleaching action. The brighter the pulp desired, the more bleaching stages that must be used (table 4-2). Mill operators have a number of mix-and- match brightening processes to chose from for ?nal bleaching. Choices are largely determined by the relative costs and ef?ciencies of the bleach options and the required brightness of the pulp being produced. The entire bleaching sequence is then a combination of the prebleaching stages and the brightening stages. Prebleach Delignification Chlorination?Chlorine selectively reacts with lignin and under normal bleaching conditions does little harm to cellulose ?bers. Because of its relative 'oRudra P. Singh and Edward S. Atkinson. ?The Alkaline Extraction." The Bleaching of Pulp. Third Edition (Atlanta. GA: TAPPI Press. 1W9). p. ?Johan Gullichsen. ?The Past and Future of Pulp Bleaching." l937 Bleach Plant Operations Seminar. Notes (Atlanta. GA: Press. 1987). P. 19. Chapter Huip Bleaching Technology I 47 Figure I'M?Sequence for the Production of Fully Bleached Chemical Pulp Raw water. Flew water 1 Flewo water Raw water Flaw water condensate or or i or or white water white water I white0 water whlte water white water Uncsreened Screened Bleached Wood pulp pulp pulp pulp pulp pulp Chlor- Alkaline Final 12:) Cookin Washln Screenin :9 i=5 . lnation extraction bleaching Black L033 r. liquor I solids black - liquor Black Black liquor Chlorination Extraction liquor lost in . eiiluenl eilluenl to recovery screening or chlorination SOURCE: Carlton W. Dance and Goran E. Amergren. "Chlorination." The Bleaching of Pulp. Third Edison (Atlanta, GA: TAPPI Press. 1978}. p. 51. cheapness in comparison with other bleach chemi- cals it became widely used for deligni?cation after the pulping process. Chlorination'in the prebleach cycle begins with washed brown stock pulp slurry at low consistency (3 to 5 percent weight of pulp to water) being pumped into a chlorination mixer. Chlorine gas, which is often dispersed in water, is added to the pulp slurry in the mixer and is vigorously mixed (?gure 4-4). The reaction between chlorine and lignin begins immediately in the mixer, and the reaction is completed in a chlorination tower designed to give the proper retention time. If chlorination is con- ducted at low temperatures (5 to 45 retention time may range between 15 and .60 minutes. Higher temperatures reduce the time necessary to complete the chemical reaction. The chlorinated pulp is washed before being sent to the alkali extraction sta?ge. Chlorination is sometimes repeated after extrac- tion if additional deligni?cation is needed, but because of possible cellulose damage a chlorine dioxide Istage is often used. Throughout the process brightness, kappa number, and other indicators of pulp quality are monitored. 12 There is a trend toward modi?cation of the ?rst chlorination stage by including other bleaching agents chlorine dioxide, with the chlorine charge). Inclusion of these chemicals can reduce cellulose degradation, improve pulp strength, and reduce environmental releases. Chlorine dioxide is sometimes used sequentially preceding the chlorine treatment and has been shown to be more effective than when the two chemicals are mixed.13 Chlorine . dioxide can be used to completely replace chlorine in the ?rst deligni?cation stage, but its gains in pulp quality do not offset-the additional expense, and pulp brighmess equivalent to that produced by chlorine can not be achieved. 14 Pretreatment with sodium hypochlorite prior to chlorination has improved the deligni?cation of resinous woods. - Alkaline Extraction?Hot alkaline extraction is the second stage in the pulp bleaching process and completes the prebleaching deligni?cation process. 12Douglas W. Reeve, 'Pulp Chlorination," 1987 Bleach Plant Operations Seminar, Notes (Atlanta. GA: TAPPI Press, 1937), p.87. W. Deuce and Goran E. Annetgren. "Chlorination," The Bleaching of Pulp. Third Edition (Atlanta. GA: TAPPI Press. 1979), p. 65. 1?Douglas C. Pryke. ?Chlorine Dioxide 1n the Chlorination Stage. 1987 Bleach Plant Operations Seminar, TAPPlNotes (Atlanta. GA: TAPPI Press. 198?). p. 55. .. 48 0 Technologies for Reducing Dioxin in the Manufacture of Bleached Wood Pulp Figure 4-3?Pulp Brightness at Stages of the Bleaching Sequence CEHDED 100 80 Brightness eo 4st 20?- 0 Dellgnitlcatlon Bleaching stage SOURCE: N. Uergott and .B. Van Lierop. "Oxidatlve Bleaching A Review: Part I: Deligni?oatlon," Hrip Paper Carmela. vol. 87. No. 8. 1986. p. 58. Moderate temperature or cold temperature alkaline extraction is also used after later bleach stages in the brightening process of multistage bleaching se- quences. Cold alkaline extraction is particularly important in the production of dissolving pulps for the manufacture of rayon and acetate. Alkaline extraction removes the soluble colored components and lignin released in the preceding deligni?cation stage (chlorination or oxygen), therefore reducing the amount of bleaching chemicals needed in subsequent stages and improving the durability of the pulp. Sodium-hydroxide has been shown to be the most ef?cient alkali for decreasing the kappa number of pulp. Ef?cient extraction is nearly as important as prebleach deligni?cation in the ?rst bleaching stage. For instance, after chlorination and washing, but before alkaline extraction, about 30 to 50 percent of the chlorinated lignin in removed. After alkaline extraction, 80 to 90 percent of the lignin is re- moved.15 A second hot alkaline extraction is some- Tabie 4-3?Exarnples of Prebleaching Sequences tor P'ulp Deligni?cation CE Chlorine Alkali extraction CDE . . . (Chlorine Chlorine dioxide) Alkali extraction CEO . . . Chlorine (Alkali Oxygen extraction) OCDE .. Oxygen - (Chlorine Chlorine dioxide) - Alkali extraction DCE . . . Chlorine dioxide - Chlorine - Alkali extraction SOURCE: Douglas W. Reeve. 'The Principles of Bleacl?tlng.? 1987 Bieach Plant Operation-rs Seminar. TAPPI Notes (Atlanta. GA: TAPPI Press. 1987), p. 10. times used later in the bleach sequence after bleaching with chlorine dioxide or sodium hypo- chorite CEHED, or CEDED se- quences) to improve pulp brightness stability and conserve bleach chemicals.16 The pulp is subjected to the alkaline extraction treatment for 60 to 90 minutes at most mills in the ?rst post-chlorination extraction stage, although some operate on a shorter schedule. The second alkaline extraction usually lasts from 30 to 60 minutes. - The ?rst alkaline extraction stage contributes the largest potential pollutant load released from the pulp bleach plant. It may be possible to reduce the pollutant loss from the extraction stage considerably by substituting oxidative extraction, particularly sodium hypochlorite, for the ?rst alkaline extraction stage bleaching sequences)? Sodium hypochlorite added to the sodium hydroxide extrac- tion solution (En) may reduce the color (a rough indicator of pollution load) in waste water by about one-half.18 Even larger reductions in waste water color (about three-quarters) have resulted when hydrogen peroxide is added at the alkaline extraction stage. Oxidative Extraction?Oxygen gas added to sodium hydroxide in the extraction stage (E0) decreases the kappa number, conserves chemicals in subsequent bleaching stages (in some cases it can reduce the number of bleaching stages), and reduces pulp strength loss and coloration in the waste water.?_ Studies have shown that the addition of 153. van Lierop et al.. "Caustic Extraction, Pan 1: Reaction Variables." i987 Plant Operations Seminar. Notes (Atlanta, GA: TAPPI Press. 1987), p. 44. 16Singh and Atkinson, op. cit., footnote 10. p. 91. 17Ibid.. p. 99 It?i.iebergott and van Lierop. op. cit., footnote 7. p. 46. 19Ibid., p. Chapter 4?Pqu Bleaching Technology 0 49 other oxidizers, such as hydrogen peroxide or sodium hypochorite, to an E0 extraction stage might allow for a shortened, three-stage bleaching se? quence capable of bleaching pulp to high brightness (88 to 90 percent ISO). The E0 extraction stage has gained rapid acceptance since only modest addi- tional investment in new equipment is needed. Oxygen has been substituted for alkaline extrac- tion immediately after the chlorination stage.20 If coupled with a following chlorine dioxide bleach . stage, the COD sequence can produce fully bleached pulp with major savings in chemicals. A three-stage sequence using Oxygen in the second stage follow- ing a chlorine dioxide-chlorine deligni?cation stage has been used by the Chesapeake Corp. at West Point, Virginia since 1972. The Chesapeake . mill was the ?rst commercial application of oxygen in the extraction stage. Brightening Stages Chlorine Dioxide?Chlorine dioxide is very se? . lective in attacking lignin without signi?cantly 1 degrading cellulose, while producing high bright- . ness pulp. In the dioxide bleach stage, chlorine dioxide' 15 ge'nerated as a gas at the mill and dissolved in cold water. The aqueous chlorine dioxide solution is mixed with the prebleached pulp, heated to about 70 and is normally held in a reaction vessel for . approximately 3 hours.21 Because of chlorine dioxide's high cost, it is most commonly used at or near the end of bleaching sequences CEHD, CEHED, CEDED, and CEHDED). Sequences using chlorine dioxide in only one bleach stage generally produce lower brightness pulps.22 For instance, the CEHD se- quence on softwood lcraft pulp would probably be limited to 85' percent G.E. brightness. In order to achieve the highest brightness (90+ percent G.E.), two chlorine dioxide stages are generally required CEDED and CEHDED sequences). Chlorine dioxide can also be used in conjunction with a Table 4-4?Common Sequences Used To Bleach Kraft Pulp to Various Degrees of Brightness Flange of brightness Sequence CEH CEHH CHEH 70-80 CEHEH CCHEHH can CEHD '80-85 CHED CEHDD CCHEDH CEDED CEDHED OCDEHD 85-92 SOURCE: Adapted [rem Allen M. Springer. industrial Environnentai Cannot: Pulp and Paper Mostly (New York. NY: Join Wiley 3. Sons. 1936). p. 161. hydrogen peroxide bleach stage (CEHDP or CEDPD) to produce 90+ percent G.E. brightness pulp.23 Peroxide?Hydrogen peroxide is a very effective cellulose?preserving bleach agent and IS well suited for improving the brightness of highly iigni?ed pulps, such as mechanical groundwood or chemi- mechanical pulps, without signi?cantly reducing its yield. Hydrogen peroxide IS an extremely yersatile delignifying chemical and has been proposed for use as a chip pretreatment before kraft pulping and as a deligni?er in the prebleach stage prior to,' or as a substitute for, the C, CD, or Dc prebleaching stages. 24 It' 15 also used' 1n association with sodium hydroxide 1n alkaline extraction Because of' us high cost, hydrogen peroxide is used most often in the later stages of'pulp bleaching. Peroxide is used in the intermediate stages of the bleaching sequence as a replacement for hypochlo- rite or chlorine dioxide. It 'is frequently used as the last stage in the bleach sequence where it can add a few points of brightness to the pulp and improve its brightness stability.? Peroxide alone is a relatively P. Singh and Bjorn C. Dillner. ?Oxygen Bleaching." The Bleaching of Pulp. Third Edition (Atlanta, GA: TAPPI Press. 1W9). p. 181. 21Rapson and Strumila. op. cit. footnote 8. p. 114. ?Douglas W. Reeve. 67. 23Rapson and Suumila. op. cit. p. 144. Chlorine Dioxide Bleaching,? 1987 Bleach Plant Operations eminar. Notes (Atlanta. GA: TAPPI Press. 1987). p. 24]. R. Presley and R. R. Kindron. "Hydrogen Peroxide Bleaching." 1987 Bleach Plant Operations Seminar. TAPPiNates (Atlanta. GA: TAPPI Press. 1937). p. 75. 50 I Technologies for Reducing Dioxin in the Manufacture of Bleached Wood Pulp Figure 4-4?Schematic Diagram of the Chlorination Process Washer Chlorine Chlori- gas Water nation tower Chlorinated ul Butter Chlorine storage disperser -- Mixer?FF. . Chlorination Consistency Flow control control Chlorinei pulp effluent ratio tested by redox potential measurements. polarographic measurements or color measurements SOURCE: Carlton W. Dance and Goran E. Annergren, "Chlorination," The Bleaching of Pulp. Third Edition (Atlanta. GA: TAPPI Press. 1979}. p. 52. ineffective means for bleaching kraft pulp.25 How- "ever, when used in sequences with chlorine?based bleaching agents, peroxide is an ef?cient deligni?er and brightner. Peroxide is also uSed for intermediate bleaching stages of the kraft bleaching sequence or as a ?nal treatment to increase and stabilize the brightness of chemical pulps.26 The peroxide bleach liquor is usually in the range of 1 to 3 percent hydrogen peroxide. An appropriate volume of peroxide liquor, sodium hydroxide. and other chemicals to stabilize the peroxide are mixed with pulp and heated with steam to the reaction temperature (35 to 70 The peroxide-pulp mixture is allowed to react under controlled tem- perature for an optimum time (1 to 5 hours). When the reaction is complete, the pulp is washed and sent to the next bleaching stage or washed and neutral- ized with sulfur dioxide if it is the ?nal bleach stage. Peroxide, coupled with oxygen and/or ozone, shows some promise in research laboratory evalu- ations for formulating chlorine-free bleach sequences to reduce release of chlorinated organics in the waste stream. A three-stage sequence OZP (oxygen, ozone, peroxide) has yielded brightness values of 85 percent GB in euchalyptus kraft pulp.21f However, the pulp suffered a substantial loss in tear strength. The ZP bleaching sequence produced southern pine oxygen pulps of 80 percent GE brightness with good brightness stability. In contrast to peroxide and oxygen bleaching, ozone bleaching has not been developed to the point of commercialization. ypochlon're?The use of hypochlorites for bleach- ing wood pulp began in the early 18805. Although the development of chlorine bleaching technology in the 19005 led to a decrease in the use of sodium and calcium hypochlorite, still about 40 percent of the kraft pulp mills in the United States and Canada use at least one hypochlorite stage in their bleach sequence. Hypochlorites have been used effectively on sul?te pulps where an alkaline extraction stage is interposed with two hypochlorite stages 25DH. Andrews and R.P. Singh. "Peroxide Bleaching." The Bleaching afPulp. Third Edition (Atlanta, GA: TAPPI Press. 1987). p. 237. 26mm. p. 212. 2"fluid.I p. 243. 28Lee E. Larsen and H. deV. Partridge. "Bleaching With Hypochloritcs.? The Bleaching afPulp. Third Editions (Altauta, GA: TAPPI Press. 1979). p. 101. Chapter 4?Pqu Bleaching Technology I 5! Kraft pulps, being more dif?cult to bleach than sul?te pulps, require that a chlorine and alkaline extraction stage be added in the prebleach segment of the bleach sequence. Until chlorine dioxide and peroxides became available in the 19405, kraft pulps of 85 percent GE brightness were the brightest that could be produced with hypochlorite bleaching and still maintain acceptable pulp strength, but these pulps had poor brightness stability. Hypochlorite is nonspeci?c, that is, it attacks cellulose as well as lignin, therefore it requires careful control if a reduction in pulp strength is to be avoided.29 Bleaching sequences such as CEHD, CEHED, and CEHHD are used widely for producing pulps of 86 to 88 percent GE brightness, CEHDED is used for pulps of 88 to 901- percent GE brightness, and CEHDP and CEI-IEDP for pulps of 90 percent GE brightness using peroxides. Hypochlorite is also used in small amounts for oxidative extraction (see above). Some mills use hypochlorite as a replace- ment for the ?rst alkaline extraction stage to reduce the color in bleach plant effluent.30 Retention times and chemical concentrations vary . for hypochlorite bleaching depending on which stage it is being used in the bleaching sequence. Retention times range from a low of 30 minutes at some mills to 3.5 hours for those using hypochlorite in the brightening stage. Reaction temperatures for hypochlorite stages are generally kept low (85 to 110 to minimize cellulose degradation. A ?simpli?ed bleaching" process for hypochlo- rrte has recently been developed. 3? Simpli?ed bleach- ing uses a short (10- minute) bleach cycle at higher . temperatures than normally used (180 The hypochlorite treated pulp is sent without washing to a chlorine dioxide stage. This ostensibly produces pulps of higher brightness at lower cost. Studies have shown that one of the largest contributors to the environmental release of chloro- forms from a bleach plant is the effluent from the hypochlorite stage.32 It is hypothesized that chloro- form is produced under speci?c conditions existing in the hypochlorite stage rather than simply as the result of chlorine-based chemicals. The speci?c conditions and reactions contributing to the produc- tion of chloroform are not well known, however, and more research is needed to establish causation. These early ?ndings of the linkage between hypo? chlorite reactions and chloroform production has led - some to propose that the release of chloroform ?compounds from pulp mills could be reduced by eliminating the large-scale use of hypochlorite in the bleach sequence. Ozone?Ozone is one of the most powerful bleaching and oxidizing agents. It is a special form of oxygen produced by the discharge of an electrical current in oxygen gas. While oxygen atoms nor- mally occur in pairs, the electrical discharge makes three atoms associate with one another, thus giving extraordinary oxidative properties to ozone. Its decomposition to oxygen after bleaching produces. neither a residue, nor undesirable inorganic by- products. Ozone, in a bleaching sequen'ce with hydrogen peroxide, can produce high- brightness pulps. Ozone is not used commercially for pulp bleaching. Some pilot plant studies have been conducted, but additional development work would be needed to permit its widespread commercial use. 1? Ozone, in conjunction with preliminary oxygen deligni?cation, holds promise for reducing the? amount of chlorine and hypochlorite used in the prebleach and brightening stages of the bleaching sequence. Ozone bleaching is particularly well suited to bleaching sul?te pulps because of their low residual lignin content.33 High-brightness, high- quality, hardwood kraft pulps can be produced by using ozone in the ?rst stage of the bleaching 29192.8.AlthonseJ..l-l Bostwick, and D.K. Jain. Bleaching.? TAPPU.. vol. 70. No. 6. June. 1987, p. 113. 3?Larsen and Partridge. op. cit. p. 102. ?Using Hydrogen Peroxide and Oxygen to Replace Sodium Hypochlorite in Chemical Pulp 31R. G. Hise and H. L. Hintz.? Hypochlorite Bleaching. #987 Bleach Plant Operations Seminar. Notes (Atlanta, GA: TAPPI Press. 1987). p. 65. 32Ibid. 33R. Part et "Laboratory and Pilot Plant Bleaching of Various Pulps With Ozone. 1984 Oxygen Deligng?ication Symposium, (Atlanta. GA: TAPPI Press, 1984). p. 33. 52 0 Rehnologies for Reducing Dioxin in the Manufacture of Bleached Wood Pulp sequence ZEP, ZEZP, and Because it is a very powerful and nonselective chemical, its use is usually limited to the early bleaching stages. Kraft softwood pulps must be deligni?ed, prefer- ably with oxygen or through extended cooking to reduce 'the need for chlorination in the prebleach segment. before bleaching with ozone. Low kappa number kraft softwood pulps bleached with the OZEP sequence produced pulp comparable in bright- ness and strength to those produced from the CEHED ?ve~stage sequence.35 Brighter kraft soft- ?wood pulp can be produced?with oxygen-ozone- peroxide and/or chlorine dioxide bleaching se- quences OZEP, OZEPP, OZEPD, OZED, and OZEPD). For the highest brightness. chlorine diox= ide was needed in the ?nal bleaching stage. Ozone is not currently used commercially by the industry. Experimental results and pilot plant opera- tions indicate that ozone might have future promise as an alternative bleaching agent. Further discussion of oxygen-based. nonchlorine bleaching technology. including ozone bleaching. is found in chapter 5. . 3?Steven S.K. Ow and Rudra P. Singh. "Advances in Ozone Bleaching. Pan ll: Bleaching of Softwood Kraft Pulps With Oxygen Ozone Combination." Oxygen Deligni?catz?on Syposiwn. TAPPI Notes (Atlanta. GA: Press. 1984). p. 43. 351bid.. p. 49. educi?g .- .QI CONTENTS . a; -- I: . Page OXYGEN DELIGNIFICATION .. .. . . . . .- . . . .--.- . . . .. . . . . . .- 56 Oxygen Delignificat?ihfi Technology . . . -.. 59 Effects of Oxygen Dehgnl?canon on Pollutant Loads . . . . . . . 1 .- -. . 64 Extended Dehgm?cation . . . 64 Pretreannent of Oxygen Pulps With Nitrogen D1ox1de (PREN OX) . -. .- 66 dea Anthraqumone/Oxygen .Pulping--.67 DISPLACEMENT BLEACHING . . . 67 BLEACHED PULP MILL . . . . .- . . . . .- .. 68 WASTE: OUTLET TREATMENT TECHNOLOGIES -. . . . . . . . . . .- . . .769 Primary Treatment of Suspended Solids .. . . 71 Secondary Biological Treatment .. . . .. . . . .. .. .71 Oxidation Ponds . . . . -. . . . . . .. . 7.1. Achvated Sludge -72 New or Developing. Treatment Thehnolog1es.. . I. .. -.-. I72 Figures . .- Figure . ILPage 5- Chlorinated Waste From Bleaching of Hardwood Pulp . -. . 57 5 -2. World Preduction Capacny of Oxygen-Deligm?ed Pulp . -. .- 58 5- 3 High Consistency Oxygen Deligni?cation Stage .. 62 5- 4-. Oxygen Deligni?ca?iibn System Installed in .a Bleached Kraft Pulp Mill -63 .5 -5. Phenolrc Compounds Formed With and Without Oxygen Bleaching . .65 5 6 Continuous DigeStex 1'61 Extended Deligni?cation . .. 67 5-7. MulUStage Displacement Bleach SyStem Single Tower Bleach Plant Sh6wing aii. EDED Wash Sequence . .. . 69 5- 8. Closed- -Cy'cle Bleached Kraft Pulp MillTable Page 5-1 Oxygen Deligni?catio?n Systems Installed Worldwide - -. . . 60 5-2 Effluent Characteristics of- Oxygen-Treated Bleached Pulp and Pulp Bleached - . by the Common Chlorinated Bleach Processes 65 5- 3. Ef?uent Characteristics for Softwood- Kraft Pulps With and Without Oxygen Deligni?cati'on . . -. ?65 5-4.- of PCDD and. PCDF. (expressed as TEF) 1n Recelvmg Waters From I Swed1sh Pulp Mills and Without Oxygen Deligni?catio'nI =66 1 Chapter 5 Technologies for Reducing Chlorinated :Organics 1n Pulp Manufacture The bleaching plant is the major source of waterborne environmental pollutants .. produced by pulp and paper mills. As much as 40 percent of the biological oxygen demand (BOD), 25 percent of the suspended solids (SS), 70 percent of the color, and nearly all of chlorinated organic materials (T 0C1) produced by pulp mills originate in the bleaching process. The amount and nature of bleach plant pollutants vary considerably among the bleaching sequences used, wood species, and pulping proc- esses. Water is used in large quantities in the bleaching process (averaging 20,000 to 30,000 gallons per ton of pulp produced, although modern mills use much less than this). In contrast to pulping chemicals, the bleaching chemicals are generally not recovered but are discharged as wastes after treatment. Chlorination, often the ?rst bleaching stage after pulping in many bleaching sequences, requires large volumes of water to dilute the unbleached pulp to a low consistency for subsequent bleaching (about 3 percent fiber by weight compared to water), and is therefore a major source of chlorine-contaminated water. Chlorination sometimes uses as much water as all of the subsequent bleaching stages combined. To reduce the amount of fresh water consumed, the chlorination ?ltrate is often recycled in the bleach- ing stage, or paper mill white water is used for dilutiOn. General hypotheses have been advanced to ex- plain the possible occurrence of dioxins in pulp, paper, and mill wastes. If true, these hypotheses may suggest ways to reduce the amount of dioxins produced in pulp and paper manufacture. First, since lignin or wood extracts probably Contain some dioxin precursors, the reduction of the amount of lignin exposed to chlorine in the bleach plant might reduce the volume available for dioxin formation. It 1 . is now believed that lignin may not be a major source of precursors as originally thought. Second, by reducing the amount of chlorine used, or eliminating the use of chlorine bleach altogether, the formation of dioxins might be reduced or even . eliminated. Third, a recent discovery by the Pulp and Paper Institute of Canada that oil-based defoamers made with contaminated used oil may be the source of nonchlorinated precursors of dioxin and furan that become tetrachloro-p?dibenzodioxin (TCDD) and tetrachlorodibenzofuran (TCDF) with chlorination.1 By using ?cleaner? oil- or water-based defoamers (although water-based defoamers may not be effec- . tive in washing brownstock), this source of precur- sors can be eliminated. Fourth, preliminary findings indicate that '-the? formation of TCDD and TCDF can be reduCed by modifying conventional chlorine bleach sequences. For instance, by applying chlorine gas in smaller, successive split charges rather than a single large charge, research has shown reductions in TCDD.2 By carefully controlling the acidity (pH) of the unbleached pulp within an optimum range, TCDD has also been reduced. Applying chlorine bleach before the addition of chlorine dioxide (CD) tends to produce less TCDD than if chlorine and chlorine dioxide are mixed (CD) or if chlorine dioxide is applied separately before the addition of chlorine. . TCDD and TCDF formation is also sensitive to the ratio of chlorine to lignin. The Swedish Pulp and Paper Research Institute has found that if chlorine additions are kept below 15 percent (chlorine to lignin), TCDD and TCDF can be kept to low levels.3 These observations need veri?cation. Fifth, improvements in secondary biological waste - treatment can further reduce the amount of ?ne, colloidal suspended solids on which TCDD and IR. H. Voss et 211.. ?Some New Insights Into the Origins of Dioxins Formed During Chemical Pulp Pulp and PaperAssociatt?on Enviroamenr Conference Proceedings, Vancouver, B. C., Oct. 25- 26, 1988. p. 31. 2Ronald B. Esuidge and William Kraske. American Paper Institute, material presented at the OTA dioxin workshop. Washington. DC. Nov. 14-15, 1988. 3Knut P. Kringstad et 31.. "Bleaching and the Environment.? 1988). _55_ I988Pu1?p liieaching Conference, Orlando, FL, June 5-9. 1988 (Atlanta, GA: TAPPI. 56 0 Technologies for Reducing Dioxr'n in the Manufacture of Bleached Wood Pulp TCDF are transported.4 Supplemental treatment with chemical coagulants, precipitants, adsorbents, and perhaps destruction by ultraviolet light and chemicals could increase the ef?Ciency of the existing waste treatment facilities Further evalu- ation is needed on these options. Should it prove necessary to reduce the amount of dioxins produced from pulp and paper manufacture for health or environmental reasons, one approach to consider is prebleaching technologies that would reduce the amount of lignin passing to the bleach plant. Another approach would Utilize brightening technologies that reduce or eliminate chlorine gas from the bleaching sequence. The several options available to reduce the formation of TCDD, TCDF. and other chlorinated organics are not mutually exclusive and can be linked at stages throughout the pulping, bleaching, and waste disposal processes to achieve low levels of ?discharge. Because of the pollution potential of chlorinated compounds and the volume of wastewater produced- by chlorine bleaching, the Chlorination stage is the focal point for efforts to control pollution resulting from the manufacture of pulp. There are several possible approaches to reducing the amount of dioxin and other chlorinated organics formed during chlorine bleaching: - Delignify pulp to a further degree before bleaching ?extended deligni?cation ?oxygen deligni?cation ?-pretreatment with nitrogen (more lignin can be removed without ?ber damage) 0 Substitute other bleaching chemicals for chlo- rrne ??chlorine dioxide for part or all of the chlorine ?hydrogen peroxide ?alkaline extraction supplemented with oxy- gen and peroxide ??ozone (precomrnercial) 0 Modify chlorination procedures (more research needed) ?-optimize the acidity (pH) of the pulp ?use smaller multiple charges of chlorine instead of one ??apply chlorine ?rst, then bleach with chlo- rine dioxide 0 Remove known sources of precursors contaminated defoamers or other additives, prewash pulp, etc.) 0 Improve waste treatment systems (more re- search needed) ?chemical coagulants ??sorption or precipitation enhancers ?destruction with ultraviolet light or cheati- cals ?anaerobic treatment .. Elimination of chlorine in the bleach sequence combined with internal recycling of process.,water aimed at developing an overall ?pollution'free? pulping system probably offers the best theoretical strategy for reducing, the pollution from bleach plants over the long term. but because of practical limitatiOns, it may not be commercially viable for sometime.5 OXYGEN DELIGNIFICATION Although the introduction of any chlorinated bleaching chemical chlorine gas, sodium, or calcium hypochlorite), can generate some chlori- nated organic compounds, chlorine gas used in the prebleaching stages of the bleaching sequence produces the largest amount (figure 5-1). Chlori- nated organics produced by pulp mills contain ,small amounts of TCDD and TCDF (see ch. 2).6 Their presence in pulp mill wastes is ascribed to chlorina- tion in the bleach sequence althdugh the precise chemical reactions and mechanisms that produce dioxins are not known. Other factors might also contribute to dioxin formation. Ever since the connection was made between dioxin and the use of chlorine, the emphasis of those advocating process changes to reduce the formation of TC DD, TCDF, and other chlorinated organics has focused on oxygen deligni?cation technology. The Swedish example has served as a demonstration of Environmental Protection Agency. USEPA Bench Scale Wastewater Treatabr'lt'ty Satay Putp and Paper Mill Discharges of 2378-TCDD and 2378-TCDF: Proposed interim Control Measures Interim NPDES Permit Strategy (Westlake. OH: EPA Region 5. 1988). p. 17. 5Allan M. Springer. Industrial Environmental Control: Pulp and Paper Industry (New York. NY: John Wiley Sons. 1986). p. 161. 5U.S..Environmental Protection Agency. National Dioxin Study. August 1987. p. Ill-33. Chapter 5?Technologies for Reducing Chlorinated Organics in Pulp Manufacture 0 57 Figure 5-1?Chlorlnated Waste From Bleaching of Hardwood Pulp 7O 0 Bleach stage SOURCE: Allan M. Springer. industrial Environmental and Paper {wuss-y {New York. NY: John Wiley 8. Sons. 1986}. p. 166. one method to reduce the amount of chlorine needed in the bleaching process. Sweden has encouraged the international move toward adopting oxygen deligni- fication to solve the chlorine problem. It should not be overlooked that ?rst, most of the bleached kraft mills in Sweden have already installed oxygen deligni?cation at a considerable capital expense, and second, oxygen deligni?cation is a Swedish technol? ogy largely manufactured in Sweden. The effectiveness of oxygen deligni?cation to reduce the amount of chlorine needed is well documented. It should be noted, however, that other technologies and process modi?cations are also available that can reduce the amount of dioxins produced, but mine will reduce the amount of chlorine-based bleach needed to the degree that oxygen can. Conversion to oxygen deligni?c ation is not always the best solution. However, the substitu- tion of oxygen bleaching for chlorination in prebleach- ing and brightening sequences is considered by some pulp and paper experts to be a technological trend that likely de?nes the future state-.of-the-art in low-chlorine bleach plant design. Ef?uent from oxygen bleaches, such as oxygen gas, ozone, or peroxides, can be recycled internally to destroy harmful byproducts that might be formed. Transition from conventional chlorine bleaching to oxygen deli gni?cation and bleaching has been faster in Scandinavian countries?particularly Sweden? than it has in the United States and Canada.7 In Sweden, oxygen bleaching has been used in place of - biological waste treatment that is commonly used in North America. Although oxygen deligni?cation was developed in the Soviet Union, the process was commercialized in Sweden in the late 19605 and in . the Union of South Africa in the early 19705. Early interest in oxygen deligni?cation stemmed primarily from its ability to reduce pulp; mill pollution. Substantial reductions in BOD, color, and chlorinated organics in the ef?uent can be realized, as well as savings in bleaching agents. O'xygen prebleaching may not signi?cantly alter the kinds of chemicals formed (although this has notI been determined with certainty), but, when properly conducted, it will probably generate smaller quanti- ties of all these compounds.8 Moreover, bleaching operations using oxygen do not normally call for a hypochlorite stage, as a result, little chloroform is released (other non-oxygen bleaching sequences also do not use hypochlorite stages, CEDED). Chlorinated organics?not speci?cally dioxins? are major pollutants in the Baltic Sea. Nowhere in the United States have chlorinated organics pre- sented the problems that have been experienced in the Baltic region. BOD has been the major environ- mental concern in the United States (US. standards for BOD were stricter than Sweden?s); attempts to 7Fifteen Swedish pulp mills manufacture bleached sulfate pulp. Nine mills currently use oxygen in the ?rst prebleaching stage of the bleaching sequence and two others have plans to install oxygen stages in the bleach sequence. Committee for the Gulf of Botlmia. Water Pollution Problems of Pulp and Paper Industries in Finland and Sweden, Naiurvardsverket Rapport 3384. May 1987. p. 55. 3Knut Kringstad and Krister "Spent Liquors From Pulp Bleaching," Enviromnentai Science and Technology. vol. 18. No. fl. 1984. p. 246A. 58 I Technologies for Reducing Dioxin in the Manufacture of Bleached Wood Pulp. address this problem have relied on secondary biological waste treatment and internal changes in pulp mill processes. With the exception of Sweden and Germany (Germany has no kraft mills), where environmental requirements have forced acceptance, economics have been a signi?cant motivating factor in adoption of oxygen deligni?cation technology. For instance, in Japan where oxygen deligni?cation systems have recently gained acceptance, oxygen is cheap because it is produced as a byproduct of nitrogen recovery, which is needed in the manufacture of printed electronic circuits. At North American mills where oxygen deligni?cation has been installed, waste improvements have been accompanied by economic bene?ts from shorter bleaching sequences and smaller and cheaper ef?uent treatment systems. Oxygen deligni?cation can cause degradation of the pulp and reduce paper strength. In 1963, this problem was reduced with the discovery that the addition of magnesium chemicals magnesium carbonate or sulfate) can reduce or prevent degrada- tion of cellulose ?bers.9 Oxygen deligni?cation and bleaching produces pulp that compares favorably with conventional bleached pulps in most ways. Strength properties of oxygen pulps are less than conventional pulps, but may be acceptable for some products if deligni?cation by oxygen does not exceed about 40 to 50 percent of the pretreatment lignin level.10 Experts differ as to the viscosity and strength of oxygen pulp compared to conventional pulp. Brightness stability for oxygen pulps is equal to or better than that of conventional pulps. Since the construction of the ?rst pulp mill to use oxygen delignification in 1970 at Enstra, South Africa, there has been a steady increase in the annual world production of oxygen pulps (?gure 5-2). In 1988, world installed capacity was expected to exceed 10 million metric tons per year. About half the oxygen capacity is in Scandanavia and Europe, one-?fth is in North America, and one-?fth is in Japan. About 92 percent of the installed capacity of oxygen deligni?cation systems is in kraft mills, and 60 percent is in bleached softwoods (table 5-1). The use of oxygen deligni?cation is expected to expand Figure 5-2?World Production Capacityiot Oxygen-Dellgni?ed Pulp so Capacity-~thousand metric tons per day Years SOURCE: Larry Tenoh and Stuart Harper, "Oxygen-Bleaching Praetloos and Benefits: An Overview," Joumal. November 1987. p.57. worldwide as environmental standards are tightened and will likely accelerate even more if savings in the cost of bleach plant operations favor oxygen pulps. Capital cost of oxygen deligni?cation systems is high. Cost estimates based on prior conversions from a conventional chlorine bleaching process to oxygen deligni?cation range between $20 million and $30 million for an existing pulp mill with a capacity of 750 to 1,000 tons per day, depending on the need to modify supporting equipment, such as recovery boilers, evaporators, etc. If the mill re- quired expansion of supporting equipment, such as evaporators and bro'wnstock washers,.the cost would escalate to $40 million to $50 million. If, in addition, the mill did not have suf?cient reserve recovery boiler capacity, the additional liquor treatment required for oxygen delignification would raise the cost to $80 million. However, if the mill did not require recovery modi?cations, costs may go as low as $8 million to $10 million. The American Paper Institute estimates that if the 98 bleached chemical pulp mills in the United States that have not yet installed oxygen deligni?cation equipment (5 have already done so for a combined Croon and D.H. Andrews. ?Advances in Oxygen Bleaching: I. Demonstration of Its Feasibility and Scope," TAPPI Journal. vol. 54. No. 2. p. 1893 at seq. l"Larry 'Ibnch and Stuart Harper. ?Oxygen-Bleaching Practices and Bene?ts: An Overview." Journal, November 1987. p. 55. Chapter 5?Technologies for Reducing Chlorinated Organics in Pulp Mamifacmre 0 59 total of 6.10) tons per day of oxygen pulp capacity). The total capital outlay to the US. industry would then be about $3 billion or $40,000 per daily ton of capacity to ?t out the US. industry with oxygen systems (if capital costs were annualized and changes in operating costs were considered. the costs of oxygen deligni?cation would be lower). For green?eld construction of a new bleached chemical pulp mill using oxygen deli gni?cation. the capital cost of an oxygen system is more attractive. The capital cost of installing an oxygen deligni?ca- tion system in a mill of 1.000 tons per day capacity could be between $25 million to $35 million more than for conventional chlorine bleaching. The higher cost of oxygen deligni?cation compared to conven- tional chlorine bleaching is part due to the cost of the oxygen generating plant ($12 million to $14 mil? lion), and part due to the need for larger recovery boilers.? Oxygen Technology The objective of chemical pulping is to reduce the amount of lignin carried forward with the brown- stock pulp to the brightening stages of the bleach plant. The less lignin that prebleached pulp contains. the less bleaching that is required. Conventional kraft pulping. for example. produces pulp with a kappa number between 32 and 35. By subjecting conventional pulp to oxygen deligni?cation. the kappa number may be reduced to 16 or 17.?2 This allows the use of a short bleaching sequence. since the amount of lignin to be bleached is reduced up to 50 percent. An even higher proportion of lignin can be removed from sul?te pulp. Oxygen deligni?ca- tion is considered to be an extension of the cooking process. With an oxygen bleached pulp it is possible to reduce the amount of chlorine gas bleach. With additional research and development, it may be possible to eliminate it altogether by using ozone, hydrogen peroxide. and/or chlorine dioxide. Ef?uent from the oxygen deligni?cation stage is disposed of by cycling it through the pulp mill chemical recovery cycle. If chlorine is used as a bleaching chemical. the effluents cannot be disposed of in the recovery plant because of the corrosiveness of chlorides and the dif?culty in purging chlorides from a closed recovery cycle. White liquor. the same reagent that is used in the kraft pulping process. can also be used in the oxygen deligni?cation process and then recycled through the recovery plant. Installation of an oxygen deligni?cation unit may require that the capacity of the chemical and energy recovery plant be increased to accommodate the additi3onal dissolved organic and inorganic chemi- cals.l Several commercial oxygen deligni?cation sys- tems have been developed. They differ more in detail than in operating principles. These units are of two general types: 1) high consistency, and 2) medium consistency. Consistency refers to the amount of wood ?ber in relation to the volume of solution in a reactor vessel. The higher the ratio of ?ber to water. the higher the consistency. High consistency oxygen deligni?cation systems use pulp slum'es containing 20 to 32 percent ?bers. Medium consistency is 10 to 15 percent ?bers. At high consistencies, the pulp is more of a ?fluff? than a ?uid. The key to effective oxygen deligni?cation systems is the ability to disperse oxygen ?nely enough to last for the necessary reaction time. This cannot be done ef?ciently with pulp consistencies of less than 6 percent because of energy considerations. Differences among the oxygen deligni?cation systems are mainly in the design of the reaction vessel and associated pumping and gas handling equipment. In general. the capital equipment cost is lower for a medium consistency system than for a high consistency system.14 On the other hand. the consumption of oxygen and alkali is somewhat higher for medium consistency oxygen deligni?ca- tion. Medium consistency systems require a longer retention time than high consistency reactors to achieve the same degree of delignification, therefore capacity for medium consistency units must be larger to maintain the same rate of production. Medium consistency oxygen stages are often used to ?Ronald J. Slinn. vice president. American Paper Institute. personal correspondence. August 1988. Cost data supplied by the industry have not been veri?ed with other sources. ?Crow and Andrews. op. cit. note 9. p. 1896. ?Springer. op. cit. note 5. p. 172. MKarnyr. lnc.. Oxygen Delignt'?carion. bull. No. (Glens Falls. NY: 1987). p. I. Table 5-1?Oxygen Installed Worldwide Capacity Medium or high Hardwood Company Lo'cation Startup Sequence a.d. metric tonslday consistency or softwood KRAFT Scandinavia - Billerud Gruvoen. Sweden 1972 OGDEDED 500 HO SWD Munksjo- Aspa. Sweden 1973 OCDEDED 380 HO SWD Stora Kopparberg Skutskaer. Sweden 1977 OCDEDED 650 HO SWD MoDoCell Husum. Sweden 1977 OCDEDED 1.000 H0 SWD Norrlands Skogsagares Cellulosa Mallvik. Sweden 1978 OCDEDED 600 HO SWD Stora Kopparberg SkulSkaer. Sweden 1978 OCDEDED 650 HO SWD Svenska Cellulosa Oestrand. Sweden 1980 OCDEDED 1.000 H0 HWDISWD Sodra Skogsagarna Monsteras. Sweden 1981 OCDEDED 1.000 H0 HWDISWD Kopparfors Norrsundet. Sweden 1983 OCDEOOD 1.000 H0 SWD OCDEPDEPD MC Fiskeby Skaerblacka. Sweden 1 986a OCDEODD 51 0 MC HWDISWD Sodra Skogsagarna Moerrum. Sweden 1989a 420 MC 8WD 19893 MC HWD 8.410 Europe and Cellulose d?Aquitaine St. Gaudens. France 1973 OCEDED 500 HO HWD ZCP Kwidzyn. Poland 1978 OCDEHD 600 HO 5WD UST Illimsk, USSR 1979 ODCEHDED 800 HO SWD Zellsto? Pols. Austria 1984 ODCEDED 630 MC SWD WO Prommash Svetogorsk. USSR 1985 ODEDED _5_5_5 MC HWD 2.985 Africa and South America Sappi Enstra. S. Africa 1970 ODED 200 . HC SWD Sappi . Enstra. S. Africa 1978 ODED 500 HC HWD Sappi Ngodwana, S. Africa 1985 ODICED 575 HO Suzano de Papel Celulose Suzano. Brazil 1989a 1.8135 - MC HWD 2.640 North America Chesapeake West Point. VA 1972 GOOD 550 HO HWD Eddy Forest Products Espanola, Ontario 1977 OCDEOHD 500 HO SWD Eddy Forest Products Espanola. Ontario 1980 OCDEHD 500 HO HWD Union Camp Corp Franklin. VA 1981 OCED 800 HO HWD Procter 8 Gamble Oglethorpe. GA - 1980 1.000 H0 SWD Union Camp Corp Eastover. SC 1984 OCDED 650 HO HWDISWD Consolidated Paper Wisconsin Rapids. 1980 OCDEOD 459 MC HWD Champion Internat'l Pensacola. FL 1986 OCDEOD 730 MC HWD Champion Internat'l Pensacola. FL 1987 OCDEOD MC 3WD 5.740 p00,? {33119031ng amzavfnuaw 32;: u; ugxagq Suganpag .10} 331301011qu 0 09 Table 5-1?-Oxygen Installed Worldwide?Continued Capacity Medium or high Hardwood Company Location Startup Sequence a.d. metric tonslday consistency or softwood Japan Daishowa Shiraoi 1975 OCEHD 550 HO HWD Jujo Paper Kushiro 1975 OH 600 HO 8WD Taio Seishi Paper . Mishima 1984 ODICEOHED 665 MC HWD Oji Paper Tomakomai 1985 OH 550 MC SWD Chuetsu Pulp Kogyo.K.K. Sendai 1986 OCHPHEPD 550 MC HWDISWD Taio Seishi Paper Mishima 1986 OCEHDD 525 MC 5WD Daishowa Suzukawa 1986 620 MC SWD Oji Paper Ebetsu 1986 ODICEHD 650 MC HWDISWD Daishowa - Shiraoi 1986 400 MC HWD Hokuetsu Paper Niigata 1986 OCEHD 480 MC HWD Kishu Paper . Shingu 1987a OCEHD 5.590 SULFITE Hunsfos Hunsfos. Fed. Flep. of 1979 OCEHH 130 HO HWDISWD Norway Bayrische Zellsto? 2 Kelh?eim?: Fe'd.? Fle'pToi?" 1979 OEDH or OEPH ?160" Germany Flauma-Repola Rauma, Finland 1983 OCEDH 450 8WD Celpak Paskov. Czechoslavakia 1984 OCDED 660 HO SWD Hannoversche Papierfabriken Alfeld-Gronau. Fed. - Flep. of Germany 1986 OCEH 250 8WD PWA Waldhol Mannheim, Fed. Rep. 01 Germany 1986 POS-POA-HHC 185 MC 8WD Flambeau Paper Wisconsin. USA 1987a 0H MC13 HWD 2.085 SPECIALTY Peterson 8. Son -.. Moss. Norway 1975 . 140 MC: Korsnas Marma Korsnas, Sweden 1984 499 MC 24o *Under construction blr?tzrioepherir: reactor acid peroxideloxygen 3 alkaline peroxide oxygen t?I-ligh-yield polysullide 'Screen rejects SOURCE: Larry Teach and Stuart Harper. 'nygen-Bleaching Practices and Bene?ts: An Overview.? Tappr?Jodmal. November 1987. p. 56. [9 ammojnuvpy dprar up ssguvsxo {7330213101213 Sugonpa sag?ozouqoal?g .131qu 3 $110 4 62 0 TechnoIogies for Reducing Dioxin in the Manufacture ofBleached Wood Pulp achieve 35'to. 40 percent deligni?cation.? Both medium and high consistency bleaching systems require ef?cient washing of the oxygen bleached pulp to prevent carry over of the dissolved organics into the bleaching system. Medium consistency oxygen delignification causes less degradation of the wood ?bers than high consistency systems, but because of. the slower rate ,of deligni?cation it is more dif?cult to delignify to very low kappa numbers using?medium consistency. Two-stage medium consistency oxygen systems have been proposed to overcome this problem.15 Medium consistency may have several other advan- tages over high consistency processes, such as: Massive dewatering equipment is not needed. ?Loss in pulp strength is about one-third. Magnesium salt protectors are not needed. Use of oxygen is reduced by one-fourth. Almostno carbon monoxide is produced. Little danger of explosion from gas accumula- tions.16. gm. High Consistency Systems I High consistency oxygen deligni?cation systems can produce high brightness pulps up to 90+ GE with supplemental bleaching. In the high consistency processes well-washed brownstock pulp. that is 'dewatered after discharge from the digester to 28 to 32 percent consistency, is treated with alkali (oxi- dized white liquor) and magnesium salts (a protector or inhibiter) in a mixer, it is then fluffed and fed into an oxygen reactor (?gure 5-3). The pulp is heated with pressurized steam (90 to 120 and oxygen gas is injected into the bottom of the reactor. The atmosphere in the reactor is maintained at about 80 percent oxygen. Gases produced by oxidation of the lignin are purged to avoid combustion. The oxygen- ated pulp is washed after discharge from the reactor before being sent to the bleach plant. Medium Consistency Systems The steps for preparing the brownstock for oxygen treatment that are used in the medium - consistency process are similar to those used for Figure 5-3?ngh Consistency Oxygen Stage atered Dewatered stock Salk Steam Vent Steam mixer mixer ?Thick pulp - flutter J. Thick stock pump (for), Ouench 150-[b shower .1. .1. .1. .L steam Dilution am Dilute .3. zone Oxygen reactor I SOURCE: Adapted [ram Kenneth E. Smith.? Oxygen Bleaching System Operating Wail at Union Camp' 5 Franklin Mill," Page a Paper. October 1982. high consistency oxygen deligni?cation. Pulp con- sistency is adjusted to between 10 and 15 percent. Magnesium salts may or may not be added to the pulp depending on the selected procedure. Oxygen gas is dispersed throughout the pulp mixture, and steam is added to bring the mixture to a temperature of about 100 before it is injected at the base of the oxygen reactor (?gure 5-4). The mixing of the 15mm. p. 12. - ?Michael D. Meredith and Joseph M. Bentvelzen.? Bleach Sequence of the ?808.? 1984 Oxygen canon Symposium (Atlanta. GA: TAPPI Press. 1984). P- 112. Chapter 5?Technologr'esforgReducing Chlorinated Organics Pulp Manufacture - 63 Figure Dellgnitication System Installed in a Bleached Kraft Pulp Mill Continuous digester Excess unbleached I while Gas Brownslock Brownstock water Wash Fqu incineration washers decker water ABLEACH PLANT 4 Wash water dig?? 'i -. Bleached con ensates .0. n?a Wood '50! Inc a 504 Elgh-t . a, 5' i 5501:?; Degamng En i I :3 a: tankg Wood ?3 I .12 a n: Screen a: 13 - 5 re?ects "t t? pup Oxygen 3 a dryeror'" Chi er I . a: paper ml Bark We 3 a E: to landfill Wash . :3 5 a; 5 or holler zone .1 Air I ox izer a; Gas to incineration St'eam . . Feed water to QFOCBSS Alkali Acid 1 MULTIPLE EFFECT EVAPORATUR (Caustic) senior Sudace . lilapor Sewer condenser Steam i Economizer Foul . . condensates I I Ll i?gg?gnquori? Precipitator . . Chlorine dioxide generator by-producls Green $3323; hydromde liquor While liquor Steam stripper Saltcake makeup cia_eri 'claritier 1 Gas to Grits ti. i! Wash water lncinerat on . aus czers - Dlssolvino Fug, to landfill co) tank Drags Lime mud 3 1 to landfill 1 Mill eiliuent Slugs to dewatering and MAIN PULP LINE BLACK LIOUOH Equipment subject to increased load Lime kiln Aerated lagoons Weak wash Submerged Dalian SOURCE: Environn ant Ontario. Stopphg Water Pollub'on At its Source (Toronto, Ont: Mini-Isn'y ol' the Environment. 1938). . oxygen and steam with the pulp prior to transport to the reactor is the major difference in materials handling between the two systems. Reaction begins immediately. and about half the oxygen is consumed in the ?rst 2 minutes. The oxygenated pulp is normally retained in the reactor for 45 to 60 minutes. 64 0 Technologies for Reducing Dioxr'n in the Manufacture of Bleached Wood Pulp Effects of Oxygen Delignr'?cotion on Pollutant Loads The processes and conditions under which diox- ins are formed during pulping, bleaching, and brightening are not well known. Dioxins are usually formed in small quantities, and the conditions leading to their formation are poorly understood. Heat, light, and catalytic action have been shown to stimulate the conversion of chlorinated precursors to dioxins.? Chlorine makes up about 44 percent of a TCDD molecule. Lignin is a complex polymer containing many resin acid, fatty acid, and phenolic derivatives that could serve as precursors for the formation of dioxins in the presence of chlorine under appropriate physiochemical conditions. Other non-lignin components extractives, nonchlori- nated dioxins, and catachols), may also be precur- sors of TCDD or TCDF. Impco Division of Ingersoll-Rand, an equipment manufacturer that produces oxygen deligni?cation systems, claims that oxygen treatment of pre- bleached pulp before a short chlorination brighten- ing sequence signi?cantly reduces the pollution load in kraft mill ef?uents compared to the most common ?ve-stage bleach sequence CEDED (table 5-2). Similar results are reported from prebleach oxygen . deligni?cation using a chlorine-chlorine dioxide ?rst stage bleach sequence (CDEOD) compared with a long bleach sequence (CDEODED) without oxygen pretreatment (table 5-3). Furthermore, the total amount of chlorinated phenolic compounds formed in the bleaching process decrease considerably as a result of oxygen deligni?cation (?gure 5-5). The phenolic compounds released from conventional and oxygen-deligni?ed pulps are lower in quantity -but probably do not differ much in composition compared to those produced from conventional pulp.18 To the extent that phenolic compounds may be linked with the production of TCDD, this may indicate that oxygen deli gni?cation could reduce the amounts of phenolic precursors present in the pulp. Limited data are available that directly relate the reduction of TCDD and TCDF to oxygen deligni?- cation. There is substantial evidence that oxygen deligni?cation can reduce the amount of waste chlorinated organics produced, as well as reduce BOD and COD (chemical oxygen demand), and can signi?cantly reduce ef?uent color. Few analyses have been made of dioxin in ef?uents and pulps from oxygen-treated pulps. The Swedish Pulp and Paper Research Institute recently published some prelimi- nary information on the effects of oxygen deligni?? cation on dioxins and furans. Sweden?s experience with oxygen deligni?cation and appropriate bleaching sequences show that the release of dibenzo-p-dioxins (PCDD) and dibenzofurans in kraft pulp mill ef?uents can be reduced to relatively low levels (table 5-4). Similar reductions in PCDD and PCDF have been recorded in prebleached oxygen pulps. Conventionally bleached softwood pulps contained levels of PCDD and PCDF of between 9 and 29 ppt: oxygen pulps contained between 0.2 and (expressed as TEQ?toxic equivalents).20 A similar sample of bleached pulp from ?ve US. kraft mills without oxygen deligni?- cation averaged 13 and 93 TCDF (TEQ about 22 ppt, within the range of Swedish mills reported above).21 One U.S. mill with a softwood oxygen deligni?cation line and a short bleaching sequence including chlorine dioxide (CD) in the ?rst bleaching stage reports no detectable levels of TCDD in either bleached pulp or effluent.22 Extended Deligni?cation Alkaline digestion (cooking) and oxygen deligni- ?cation are both aimed at reducing the amount of lignin in the wood pulp before it is brightened in the bleach plant. Therefore, oxygen is considered an Environmental Protection Agency, Dioxins, (Cincinnati. OH: EPA Industrial Research Laboratory. 1980). 18U. Genngard el al., ?Oxygen Bleaching and Its Impact on the Environment." [984 Oxygen Deligng'?cation Symposium (All anta. GA: TAPPI Press, 1984). P. 101. and PCDF include TCDD and TCDF as well as other related isomers. ?Kringstad ct al.. op. cit, note 3. Pp. 2-3. 21US. Environmental Protection Agency. U.S. Industry Cooperative Dioxin Screening Study. EPA-44011 -88025 (Washington. DC: 1938). p. 76. 2211.5. Environmental Protection Agency, op. cit, note 4, p. l. Chapter 5?Tecltnologies for Reducing Chlorinated Organics in Pulp Manufacture 0 65 Table 5-2?E?iuent Characteristics of Oxygen-Treated Bleached Pulp and Pulp Bleached by the Common Chlorinated Bleach Processesa (pounds per ton) Bleach sequence BOD5 COD. Color CL- CEDED . 10-39 80-90 180-250 80-70 OCED 10-15 I 40-50 30?50 30-40 Percent Difference 45-55 45-50 80-85 35-55 a Based on 50 percent deligni?cation. SOURCE: ingersoll- -.Rand Oxygen Debgd?cation Technology: An Update (Nashua. NH: lngersoli- -Fland. 1986) p. 4. Table 5-3?Ei?uent Characteristics tor Softwood Kraft Pulps With and Without Oxygen (kgrmetrtc ton) Bleach Sequence BOD5 i COD Color TOCL . CDEODED 15-21 65-75 200-300 5-8 OCDEOD 8-11 30?40 80-120 3-4 Reduction (Percent) 40-50 45-55 80-75 35-50 ?Oxygen stage reduction in kappa number 45- 50 percent. Larry Tench and Stuart Harper. Oxygen-Bleeding Practices and Bene?ts: An Overview: Journal November 1987. 58. . Figure 5-5?Phenollc Compounds Formed With and Without Oxygen Bleaching 0 Chlorinated phenolic compounds?g/t 50 - 10- 01. ygen prenteached 30Kappa number - Extraction Chlorination SOURCE: U. Garmard at al.. 'Oxygen Bleeding and Its impact on the Environment." 1984 Oxygen S?nposium (Atlanta. GA: TAPPI Press.1884]. p. 101. extension of the cooking process. Retaining the pulp in the digester for a longer period and exposing it to a modi?ed time-temperature-alkaline cycle can reduce the amount of lignin retained in the brown? stock. There is a practical limit to the amount of .cooking that can be done without dissolving the desired components of the wood ?ber and reducing the pulp yield. A balance, therefore, between cook- ing and other deligni?cation processes must be used. A combination of extended cooking?sometimes called ?modi?ed? cooking?and oxygen deligni?- . cation has been used to achieve kappa numbers between 10 and 12 for unbleached pulp.23 :Others believe that kappa numbers as low as 7 can be achieved without loss of pulp strength-.21 With prebleached pulps of such low lignin Content,I it may be technically possible to eliminate the 'use of chlorine in the bleaching process altogether. The standard kraft cooking procedure generally yields softwood pulp with kappa numbers between 30 and 35. Oxygen deligni?cation can reduce the standard prebleached pulp to 16 to 20, although commercial practice is typically limited to 20 kappa because of strength considerations. Extended cook- ing plus oxygen treatment can produce pulps with kappa numbers between 7 and 12. By starting the kraft cook at a relatively low concentration of alkali 23Stig Andtbacka. ?Low Kappa Pulping Followed by Oxygen Delignil'lcation Australian Pulp and Paper Industry Technical Association Journal, March 1936. p. 129. 24Karnyr. Inc" Kamyr Continuous Cooking Plus Modi?ed Continuous Cooking Plus Medium Consistency Oxygen (Glens Falls, NY: date unknown). 66 - Technologies for Reducing Dr'oxt'n in the Manufacture ofBleoched Wood Pulp Table 5-4?Emiesions and as in Receiving Waters From Swedish Pulp Mills With and Wlthout Oxygen Delignliicaticn Production TEF Pulp Bleaching process (glyr) Without oxygen delignification 1 Softwood (E0) 300,000 2-5.8 Hardwood (E0) ED 2 Softwood (E0_p) (Ep) 125.000 0.7 Hardwood (C10+Dgo) With oxygen delignification 3 . Softwood 160,000 0.4 Hardwood 160,000 4 Softwood (E0) 75.000 0.7 Hardwood (E0) 5 Softwood (E0) 235,000 0.1 Hardwood 0 (E0) 85,000 a TEF is the 'Toxicity Emission Factor" according to G. Eadon as discussed In J.S. Bellini and DE. Barnes. Toxicologyand industrial Health. vol. 1. No. 4, p. 235. SOURCE: Knut P. Kringstad et al., addendum to ?Bleaching and the Environment.? 1988 international Pulp Bleaching Conference. Orlando. FL, June 5-9. 1988 (Atlanta. GA: TAPPI. 1988), pp. 2-3. and lengthening the retention period, low-lignin pulps are produced with comparable strength to pulps containing more lignin that are produced by stande cooking methods. Extended deligni?cation can be either a batch process or a continuous process. Continuous modi- ?ed cooking digesters have been developed by Kamyr, Inc. and others, which split the white liquor (alkali) charge and introduce it into different points in the impregnation and cooking stages and at the base of the reactor (?gure 5-6). As its name implies, extended deligni?cation requires a longer cooking time than standard pulp- ing. While standard kraft pulp is cooked for 1 to 3 hours, extended deligni?cation, including impreg- nation (steeping) and cooking, may require over 4 hours.25 Initial concentration of alkali in the cooking liquor is about half of that used in the standard cook. By the end of the cook, however, about 2 percent more alin is used in the extended process than in the standard. Pilot tests have shown that pulp lignin concentration at the end of extended deligni?cation may be 40 to 60 percent of that contained in standard pulp. Pretreatment of Oxygen Pulps With Nitrogen Dioxide (PRENOX) . Laboratory experiments and pilot plant studies have demonstrated that treatment of pulp with a combination of nitrogen dioxide and oxygen prior to oxygen delignification can also reduce the lignin in prebleached pulps.26 Nitrogen dioxide has been shown to promote more selective deligni?cation in the oxygen deligni?cation stage, thus resulting in less damage to the ?ber.21f The upper limit of deligni?cation for most softwood kraft pulps is considered to be about 50 to 55 percent for oxygen treatment. Deligni?cation to these limits, howeyer, causes ?ber degradation. To avoid this, oxygen deligni?cation is generally reduced to about 40 to 45 percent. By using PRENOX before the oxygen stage, deligni?cation rates of about 75 percent (kappa number 8-10) may be possible to achieve.23 25Andtbacka, op. cit., note 23, p. 130. 2511011" Brannland et a1. Oxidation ofPuip With Prior to Oxygen Deligni?cation?A Novel Process With Potentially less Pollution.? S-93E: 04187 2000 MarknadsRadet, Reprinted by Sunds De?brator. May 1986. 2'70. Lachenal and C. DE Choudens, ?High Ef?ciency Oxygen and Peroxide Deligni?cation.? Cellulose Chem. Technol., vol. 20, p. 55?. 28Bryan L. Sorensen, 'New Bleach Plants: A Review of Present State of the An,? 1987 Bleach Plant Operations Seminar (Atlanta, GA: TAPPI Press, 1987). p. 183. Chapter 5??Technologies for deducing Chlorinated Organics in Puip Manufacture 0 67 Chips In White liquor Flgure 5-6?Contlnuous Digest I I I for Extended To recovery To dittuser Wash liquor SOURCE: Stig Andtbacka. "Low Kappa Pulping Followed by Oxygen De?gri?ration." Australian Pulp an! Pamr industry Tedmicai Association Jeumal, March 1936, p. 130. Sada-Anthraquinone/Oxygen Pulping Although the kraft pulping process ef?ciently produces pulps with unmatched quality and may be used with any wood species, there are continuing efforts to develop a pulping system that eliminates sulfur chemicals from the pulping process because of the odor and environmental concerns. The most expensive capital investment in a kraft pulp mill is the chemical recovery plant. A soda-anthraquinone plant requires a similar recovery plant to that of a conventional system, however, the odor control . equipment used in conventional mills is not needed. The soda process was a forerunner of the kraft process.29 Its major drawbacks are low pulp yields and inferior pulp quality that result from long cooking times, high temperatures, and the strong solution of sodium hydroxide needed to produce bleachable grade pulps. The addition of small amounts of anthraquinone (AQ) to the pulping liquor are effective in accelerat? ing the soda pulping process and improving pulp yields. Anthraquinones have also been used with kraft pulping, but a larger amount of the expensive . chemical must be used to be effective and residual I amounts of AQ can interfere with the chemical recovery plant. Furthermore, AQ is regulated :by the Food and Drug Administration, and residual amounts are permitted in ?nished products. Laboratory experiments with a two-stage sod'afAQ- oxygen-sodium hydroxide deligni?cation of hard- wood has produced pulp of kappa numbers between 10 to 12 with about 5 percent higher pulp yields than comparable kraft pulp. Lignin content of softwoods pulped (kappa number 20) by the two-stage process did not match those of the hardwood pulp,? how- ever.30 Researchers found that to avoid ?ber degra- dation, the soda cooking must be stopped at a high kappa number and the remainder of deligni?cation done with oxygen. DISPLACEMENT BLEACHING There is no difference between the chemistry of displacement bleaching and that used in conven- tional bleaching, only in the ef?ciency 'of the mass 29Hutch Holton, ?Softwood Pulping: A Major New Process," Pulp Paper Canada, vol. 78, No. 10. October p. 19. 1 Tsai, H-m. Chang. and 1.5. Gratzl, ?Optimization of Soda-AQ/Oxygen Pulping of Southern Pine.? 2934 Oxygen Deiigru?cau?on Syr'nposim (Atlanta. GA: TAPPI Press, 1984). p. 31. 68 0 Technologies for Reducing Dioxin in the Manufacture of Bleached Wood Pulp transfer.31 When bleaching chemicals are displaced through a pulp mat, the ?bers are continually exposed to highly concentrated bleaching chemi- cals, therefore bleaching is very rapid. Displacement (sometimes referred to as ?dy- namic") bleaching is generally conducted in a multistage displacement tower (?gure 5-7). Filtrate withdrawn at each stage is supplemented with makeup chemicals and reused. Unbleached pulp moves sequentially upward, while white liquor ., moves downward .through the bleaching column .i LDisplacement bleaching uses a minimum of water, thus reducing the amount of waste ef?uent and enabling the recycling of bleaching chemicals The system is suited for use in a ?closed-mill" con?gu- ration (see above). Any water soluble bleaching chemical, including chlorine gas, can be used in displacement bleaching. Softwood and hardwood pulps of 86 to 88 GE . brightness have been produced by displacement ?systems 32 Displacement bleaching technology has [produced mixed results. Although it requires more chemicals, and causes corrosion and scaling prob- lems in some instances, displacement bleaching is still considered promising. .. PULP MILL During the past 20 years the concept of a pulp mill that could signi?cantly re- duce the amount of waste ef?uents has received some attention. The closed-cycle concept involves: a reducing the amount of water consumed by using ?ltrate as wash water, 0 cycling the spent bleaching chemicals to the chemical recovery plant, and 0. recovering the salts introduced into the recov- ery system for either re-use in manufacturing the bleaching chemicals or for disposal (?gure 5-8). By closing the water cycle of a bleached kraft mill, the demand for heat is reduced and consider- able energy savings can result. Excess heat produced in the process is disposed of as low-grade waste heat into cooling waters. A closed-cycle mill bleach plant might use only 3,900 gallons per air-dry ton of pulp produced, while an average North American Mill would use 20,000 gallons.33 A mixture of chlorine dioxide and chlorine gas is substituted for pure chlorine in the ?rst bleaching stage to avoid reducing the. pulp strength during the high- tegnperature-chlorination (60 Pulp manufacture and bleaching, being chemical processes, obey the general principles of chemical combination and mass balance. In a mill in which water and all chemicals are recycled, all of the chemicals must be kept in preper balance or either an excess or shortage of bleaching chemicals will occur. An excess of certain chemicals, particularly sodium chloride (common salt), can contribute to corrosion of the recovery boiler. Some of the recovered salt can be reused to produce chlorine dioxide, but the excess must be disposed of. Corrosion-resistant materials and careful chemical control have largely overcome these problems. The Great Lakes Paper Co. at Thunderbay, Ontario, installed the ?rst closed-cycle mill in 1977, but the process has not been successful, and the mill is not now operating in the closed-cycle mode. Severe operating problems have been encountered which have resulted in abandoning the process. If these problems could be overcome by further development, the closed-cycle concept might reduce the cost of biological waste treatment, reduce energy costs; increase ?ber yield, decrease water us age, and reduce chemical costs. It is possible that some variation of the closed- cycle bleached kraft mill concept could be oped to incorporate oxygen deligni?cation, ex? tended deligni?cation, or displacement bleaching, thus avoiding some of the problems encountered with conventional bleaching sequences. 31Johan Gullichsen, ?DiSplacement Bleaching," The Bleacfu'ng Edition (Atlanta. GA: TAPPI Press, 1979), p. 276. 32Springer. op. 011., note 5. p. 170. 33W. Howard Rapson. ?The Closed-Cycle Bleached Kraft Pulp Mill," The Bleaching of Pub?Third Edition (Atlanta. GA: TAPPI Press, 1979). p.415. . 3fSpringer, op. cit.. note 5. p. 129. Chapter 5?Technologies for Reducing Chlorinated Organics in Pulp Manufacture 0 69 Figure-5-7?Multistage Displacement Bleach System Single Tower Bleach Plant Showing an EDED thlash Sequence .. I 43 Distribution i: nozzle I. EXtraction To Cl2 stage Stock in TREATMENT TECHNOLOGIES Several physical and chemical treatments have been developed to cleanse pulp ,and paper ef?uents. Waste treatment processes include resin separation and ion exchange; use of chemicals such as alumi- num oxide, metallic ions, lime, amines. and ozone; adsorption by wood; biological treatments such as enzymes and bacteria; ?ltration systems such as Two-stage filtrate I I I @i gs Wk SOURCE: John Guliichsen, "Displacement Bleaching." The Bleaching of Pulp?third Edition (Atlanta. GA: TAPPI Press. 1979}. p. 288. i Hydauiic . cylinder 'i L, a Dioxide I Caustic Extraction j/ activated charcoal; membrane separation; and ul- tra?ltration, irradiationfand reverse osmosis: It is technically feasible, but extremely costly. to?clean pulp-mill wastes to the purity of drinking water. 'In judging the economics of waste treatment systems. the cost of the exotic waste treatment technologies - is generally compared with the cost of secondary biological treatment, which is the US. industry standard for removing most foreign materials'other'_ than color. .9 70 0 Technologies for Reducing Dioxin in the Manufacture of Bleached Wood Pulp Figure 5-8?Closed-Cycle Bleached Kraft Pulp Mill Unbleached Pulp Water A Condensing Stripping at 9 White Condensa evaporatOr Water . Liquor preparation . Salt Buiping Bleaching Chemical chemical . manufacture Water Furnace Bleaching chemicals . - Wood Fresh Cooking Bleaching water Black ll uor . 1? Water evaporator Washing oxygen Bleached DUID SOURCE: W. Howard Rapeon. he Closed-Cycle Bleached Kraft Pulp Mill." The Bleach?ng c! Pulp?Third Edition (Atlanta. GA: TAPPI Press. 1979). p. 414. The effectiveness of any waste treatment system is related to the internal processes used for delignify- ing and brightening the pulp. The load on the waste treatmentplant can be reduced by using some of the technologies discussed above, oxygen deligni- ?cation, soda-AQ pulping, or displacement bleach- ing. Use of bleaching sequences including oxygen, chlorine dioxide, hypochlorite, peroxide, or ozone can also change the nature of the treatable wastes. Waste treatment and pulping and bleaching tech- nologies, therefore, may be considered as an integral system that can be balanced to achieve the desired performance standards. Theoretically, the optimum way to make waste treatment a less costly venture is to integrate it into the manufacturing process. Bene?ts can then be gained from conservation of raw material, ?bers, additives, energy, and water. Cost analyses of internal controls v. external controls generally show that internal modi?cations may be less capital intensive.35 0n the other hand, external pollution control does not interfere with established manufac- turing processes and provides ?exibility for a range of operating conditions and exigencies. Realisti- cally, successful control probably requires a combi- nation of both. Color and solids are the ?rst targets for cleanup of the pulp waste stream as it leaves the mill outlets. Untreated pulp-mill ef?uents are generally of a deep mahogany color. Most of that color is derived from the bleaching process, and most of that is from the caustic extraction process after the ?rst bleaching stage. Most of the color is attributable to lignin in the waste ef?uent. Lignin is resistant to biodegradation, 35H. Review of Pulp and Paper Waste Management, EPA Series EPA-11243484 (Washington. DC: U.S. Govemmenl Printing Of?ce, 1973). p. 32. Chapter 5?Teciznologies for Reducing Chlorinated Organics in Pulp Manufacture 0 71 and secondary biological treatment plants remove only 30 percent or less of the color component.36 Primary Treatment of Suspended Solids Suspended solids in the waste stream consist of bark, ?ber, ?llers, clay, and coloring agents. These are removed by sedimentation, ?otation, and screen- ing. Grit chambers, screens, and chemicals are used as sediment clari?ers to remove grit. Removal of suspended solids and grit currently constitute pri- mary (?rst stage) treatment of pulp and paper mill waste. Secondary Biological Treatment Removal of suspended solids in the ?rst stage is generally followed by biological waste treatment that is principally aimed at reducing the BOD of treated water. Secondary biological treatment has little effect on ef?uent color, but it may signi?cantly reduce levels of toxic pollutants.37 As its name implies. biological treatment relies on the assimila- tion and conversion of potential pollutants in the waste ef?uent by bacteria, fungi, algae, and other living organisms. Since biological treatment relies on the physiological processes of living plants and animals to reduce the pollution load, the second stage of waste treatment is similar to farming?the plants and animals must be kept healthy, productive, and reproducing. In order to promote biological activity, adequate air and nutrients must be provided to the biota. Biological treatment can remove 80 to 95 percent of the BOD. Research has shown that chlorinated bleach plant derivatives are more dif?c ult to degrade by biological processes than nonchlorinated wastes. Several biological treatment systems are currently used to treat pulp mill waste Oxidation ponds, activated sludge, and aerated basins). Emerging biological treatment technologies include: rotating biological surfaces, ?xed-?lm activated sludge, aerated activated carbon, and deep tank- aeration. Bench-scale wastewater treatability studies con- ducted by EPA indicated that the addition bf alum or i lime can remove more than 95 percent of TCDD and in bleach plant wastewater by improving the recovery of suspended sediments to which they adhere. Use of chemical treatments would probably require additional clari?cation and sludge dewater- ing facilities at most mills. The application of a non-ionic polymer to an oxidation pond reduced TCDD and TCDF to less than detectable levels.38 Oxidation Ponds Oxidation ponds or basins depend primarily on surface exchange with the atmosphere for aeration, although some oxygen may be supplied photosyn- thetically by aquatic plants. Large surface areas are generally needed to provide suf?cient air to main- tain biological activity, therefore oxidation ponds tend to be large and shallow. Since the rate of biological activity increases with temperature, oxi- dation ponds work best in warmer southern climates. Oxidation ponds are relatively inexpensive, require little mechanical equipment, produce little secon- dary waste products that must be disposed of, and in emergencies can serve as temporary impoundments should an accidental discharge of harmful chemicals occur in the mill. Racetrack-shaped oxidation ditches are sometimes used to eliminate the primary clari?cation stage. Oxidation ditches require me- chanical aeration because of the smaller water surface and perform more like an extended aeration activated sludge process than like a conventional oxidation pond system. . Oxidation basins are frequently equipped with aerators to increase the rate of biological activity (aerated stabilization basins). Nitrogen and phos- phorous fertilizers are sometimes added if the waste stream is nutrient de?cient. These supplemental treatments can reduce 'gihe retention time in the oxidation pond to 8 to 10 days in order "to reduce BOD to a low level. The addition of mechanical aerators increases the cost of oxidation ponds, but they are generally cheaper than activated sludge systems. 3?5Springer. 0p. cit.. notes. p. 182. C. Walden and J. C. Mueller. investigation afthe Efect of BOD Reduction Systems on Toxicity. CPAR Rept. No. 150-] (Ottawa. Ont: Canadian Forest Service, 1973). 3liU.S. Environmental Protection Agency, op. cit.. note 4. p. 9. 72 0 Technologies for Reducing Dioxin in the Manufacture of Bleached Wood Pulp Activated Sludge Activated sludge treatment systems are often used where space limits the use of oxidation ponds. An adaptation from sanitary sewage treatment, acti- vated sludge is a high-rate biological process that can reduce waste treatment retention time to 3 to 8 hours. The biological mass that is produced in the aeration stage of the treatment process is separated in a secondary clari?er and the active biological components are returned to the process, thus further accelerating biological activity. Activated sludge systems are ?exible and can be adapted to treat a wide range of wastes. However, these systems cannot withstand shocks from emer- gency mill releases nearly as well as oxidation ponds, and the biological balance of the process is more sensitive to chemical and biological perturba- tions. The capital and operating cost of activated sludge systems are generally twice as great as aerated ponds. The pulp and paper industry is the largest user of pure oxygen-activated sludge (UNOX) technology, with 16 plants in operation in 1986.39 Pure oxygen systems can remove 87 to 97 percent of BOD. The process has several advantages over conventional activated sludge systems such as, smaller aeration tanks required better tolerance to ?shock loading,? and better sludge settling. A buildup of carbon dioxide at some UNOX plants has raised concern about the potential toxicity to ?sh. Retention time in pure oxygen systems is about 3 to -4 hours. Two-stage activated sludge systems, such as the Zurn Attisholz process, using two oxygen levels sequentially with high recycling rates can reduce BOD 95 percent and the process is very stable. Cost of the two-stage process may be less than with conventional activated sludge.40 There is some evidence that the two-stage oxygen activation proc- ess is more ef?cient in reducing toxicity of kraft wastewater.41 New or Developing Treatment Technologies A number of of coagulants that remove color have been tested. Although several are effective, alum, ferric sulfate, sulfuric acid, activated carbon, etc., they are expensive. Lime is the least costly precipitation agent, and can be reclaimed in a kraft mill by oxidizing in the lime kiln. Pretreatment of waste ef?uents with enzymes before precipitation with lime can increase ef?ciency, as can the additions of magnesium sulfate. Fungi have also been used to remove color from wastewater. White rot fungus-(Phonerochoete Sporium bards) can metabolize the lignin responsi? ble for most of the color of wastewaters. It can also eliminate toxic chlorinated and halogenated com- pounds from the waste stream, including dioxins.42 Although the process (MYCOR) is economical, the fungi culture cannot sustain itself and may collapse, thus it must constantly be recultured. Conventional ion exchange resins have not pro- ven technically successful in removing color from pulping and bleaching wastes, but specialized syn- thetic resins seem promising. Resins are sensitive to overloading from SUSpended solids and contenti- nants that reduce their effectiveness, thus they have been used primarily on both small waste streams and isolated output from the chlorination and ?rst- extractive stages of the bleaching sequence. Ion exchange resins have thus far found only limited use in the pulp and paper industry. Activated carbon, because of its large adsorption surface area, is an effective scrubbing agent. The charcoal process has been used commercially, but its high cost and regeneration requirements make it less attractive. The ef?ciency of membrane processes is considered good, but their costs remain high. Rotating Biological Surface A new developing technology, rotating biological surfaces (RBS), involves rotating polyethylene discs alternately through the wastewater and into the air as they rotate on a shaft. The process is analogous to the 39Springer, op. cit., note 5, p. 240. 4?Ibid.. p.242. ?Waste Water 'Ihchnology Center. An Assessment of Kra?? Bieacirery E?luent Toxicity Reduction Using Activated Siudge. EPS 4-WP-77- 3 (Ottawa. Ont: Environment Canada. 1W7). Environmental Protection Agency. National Dioxin Smdy. EPAJS30-SW-87-025 (Washington. DC: 1987). p. VI-2. Chapter i?Techuotogies for Reducing Chlorinated Organics in Pulp Manufacture I 73 simple ?trickle ?lter where wastewater is perco- lated through a porous medium exposed to air in the interstices. RBS usually operates in three stages, and removal of up to 90 percent of the BOD has been achieved with this technology. Ef?ciency of the process is proportional to the surface area of the discs, rotation speed, and. submergence depth of the discs. Application of RBS is probably limited to small- or medium-size pulp mills because of costs in scaling the equipment to large volumes. In smaller operations the cost of RBS is about the same as the activated sludge process. Although pilot-scale test- ing of RBS has shown promise for removing BOD and toxicity, mill-scale studies have encountered operating dif?culty.43 Innovations in Activated Sludge Technology High-Rate, Fixed-Film Activated Sludge? Solid particles clays, sand, or calcium carbon- ate)-added to activated sludge systems can improve sludge settling and prolong solids retention. Cal- cium carbonate has been particularly effective be? cause of its buffering capacity with regard to acid components in the waste ef?uent. The particles develop a ?xed-?lm growth, allowing a high level of biological growth to develop on the expansive particle surfaces. Preliminary studies indicate that .nearly all of the solids can be retained in the system except a small amount that is lost with the treated Water. Most of the solids are lost through the respiratory processes as carbon dioxide and water. ?The oxitron prdcess is a variation of the ?xed-?lm activated sludge system. Ef?uent, free of suspended solids, is fed to a ?uidized bed of sand through which .pure oxygen is injected. Biological growth develops arotmd the sand particles, ?nally causing them to rise in- the ?uidized bed where they are selectively removed. The sand 15 later cleaned of the growth and returned to the ?uidized bed. Tests of the process show it to be ef?cient and compact, but it is unable to tolerate any solids in the wastes to be treated. The oxitron system is best suited for large waste volumes of 2 to 3 million gallons daily. There are no commercial installations in service, but new tech- nologies may utilize elements of the process. Aerated Activated Carbon Filter?Experiments have shown that due to its immense surface area activated charcoal is an ef?cient ?lter medium. Coarse granular charcoal placed in a ?lter bed has "been shown to be effective in removing contami? . nants from waste ef?uent. To keep the system aerobic, air is pumped through the ?lter bed. The system is tolerant of excessive suspended solids, but the capital and operating costs are likely to be high While the system has been used on sanitary wastes, there has been limited operating experience in the pulp and paper industry. Captor System?An English innovation, the cap-? tor System is similar to the conventional diffused- aeration activated sludge process, but it encapsulates the biological growth in small sponges. With the biomass contained in the sponges, the treated ef?uent is discharged through a screen sized to retain the impregnated sponges. Sponges'are cleaned for reuse by squeezing through a wringer. Developers claim that a seCondary clari?er is not- needed, therefore the plant size may be reduced by about 20 percent from that of a conventional activated sludge plant. The Captor System has been used for municipal wastes, but it is not certaiit that this treatment would be suf?cient to meet U. S. .water standards. It may prove useful' 1n upgrading existing overloaded biological treatment systems. Deep Tank Aeration?Deep tank aeration in- creases the amount of time that air is in contact with the waste ef?uent. As its name implies, the process uses a deep tank or deep shaft as an aeration chamber. This eliminates the need for a secondary clari?er but still requires a ?oating clari?er. {Deep aeration occupies less space than conventional activated sludge systems and is capable of handling emergency ?shock? loads from the paper mill. Anaerobic Treatment .1 Anaerobic waste treatment is neither new, nor innovative. However, with reduced water usa'ge by many of the modern pulp mills, there has been a general increase in the potency of the pollution load, and anaerobic treatment is being reconsidered as a means to improve waste treatment. Anaerobic sys- tems are well suited for treating high-strength waste, Environmental Protection Agency, Development Document for E?uent Lum'tatiorts Guidelines and Standards for the Pulp, Perrier, and Paperboard and the But?tders? Paper and Board Mitts Point Source Categories. EPA (Washington DC: 1982), p. 342. 74 0 Technologies for Reducing Diqxin Err-the Manufacture of Bleached Wood Pulp ?1 but with modi?cations such as, attached-?lm and expanded bed reactors, it may alsobe possible to treat low-strength waste anaerobically as well. Anaerobic (absence of oxygen) decomposition is - a microbial process, which' 13 primarily dependent on bacteria. Unlike aerobic processes activated =1 sludge and oxidation ponds to which air is supplied) *anaerobic processes depend on bacterial action that obtains oxygen from sulfate and nitrate ions in the waste stream When applied' 1n the pulp and paper industry, anaerobic treatment is generally used as a waste pretreatment before release to the aerobic stages of the treatment process. Anaerobic systems are sensitive to imbalances in thevarnbient waste, but the process is durable and can be applied to a wide range of waste ef?uents. -- High temperatures are needed to reduce treatment time, long startup periods are required, odor emissions can be signi?- cant, and there is dif?culty in achieving low BOD levels with anaerobic decomposition. Superintendent of Documents Publication Order Form momma. Charge your order. C, .7 6587 It?s easy! - YES please send me the following indicated publications: Technologies for Reducing Dion'n in the Manufacture of Bleached Wood Pulp GPO stock number 052-003-01154-9; price $3.75. .m l. The total cost of my order is 5? (lntemational customers please add an additional All prices include regular domestic postage and handling and are good through 11/89. After this date. please call Order and Information Desk at 202-783?3238 to verify prices. Please Type or Print 3. Please choose method of payment: . (Com an or rsonal name) pe Check payable to the Superintendent of Documents (Additional addressiattention line) GPO Deposit Account (S dd ?3 VISA. CHOICE or MasterCard Account treeta (City, State. ZIP Code) . Thank you for your order! (Credit card expiration date) (Daytime phone including area code) (Signature) 5/89 4. Mail To: Superintendent of Documents. Government Printing Of?ce. Washington. 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