Reducing Dioxin in Bleached Wood Pulp

Technologies for Reducing Dioxin in theManufacture of Bleached Wood Pulp

May 1989

NTIS order #PB89-223291

Recommended Citation:

U.S. Congress, Office of Technology Assessment, Technologies for Reducing Dioxin in theManufacture of Bleached Wood Pulp, OTA-BP-O-54 (Washington, DC: U.S. GovernmentPrinting Office, May 1989),

Library of Congress Catalog Card Number 89-600719

For sale by the Superintendent of DocumentsU.S. Government Printing Office, Washington, DC 20402-9325

(order form can be found in the back of this report)

Foreword

As analytical technology improves, we are discovering dioxins associated with manyproducts commonly found in the home and workplace. Recently, dioxins have been detectedin wastes resulting from the manufacture of wood pulp. Paper products made from wood pulpare used in tremendous volumes for food packaging, hygienic products, printing paper, writingpaper, paperboard for shipping containers, and numerous other household items. Between 650and 700 pounds of paper products are used annually by each American, with domestic andforeign consumption continuing to rise at a steady rate.

Most of the paper used in the United States is white paper made from bleached wood pulp.Chlorine is commonly used as a bleach. It has been found that bleaching can result in theformation of dioxin in the pulp when chlorine reacts with organic constituents of wood.Although preliminary surveys have detected dioxin in pulp mill wastes, the scope of theproblem is not yet known. Studies currently under way by the Environmental ProtectionAgency, the Food and Drug Administration, the Consumer Products Safety Commission, theNational Institute for Occupational Safety and Health, and the U.S. paper industry will shedlight on this when completed later in 1989.

Alternative technologies using oxygen as a pretreatment to chlorine bleaching andimproved delignification that removes more of the potential reactants from the wood canreduce the amount of dioxin in bleached pulp. Substituting other bleaching chemicals forchlorine also shows promise for reducing the amount of dioxin produced in the bleachingprocess should regulation be required. This study provides an assessment of thesetechnologies; it does not address the policy issues related to regulating dioxin in paperproducts and controlling environmental release.

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Workshop Participants: Technologies for Reducing Dioxinin the Manufacture of Bleached Wood Pulp

Michael BabichU.S. Consumer Product Safety Commission

Phillip CookU.S. Environmental Protection Agency

Michael CrandallNational Institute of Occupational Safety and Health

Ronald EstridgeJames River Corp.

David FirestoneU.S. Food and Drug Administration

Karen FloriniEnvironmental Defense Fund

William GillespieNational Council of the Paper Industry for Air and

Stream Improvement

Michael GoughResources for the Future

Russell KeenanEnvirologic Data

Gregory KramerU.S. Food and Drug Administration

William KraskeBoise Cascade Corp.

Renata KroesaGreenpeace Foundation

Steve KronerU.S. Environmental Protection Agency

Russell KrossMead Corp.

Thomas O’FarrellU.S. Environmental Protection Agency

Greg SchweerU.S. Environmental Protection Agency

David StallingsU.S. Fish and Wildlife Service

Mel StratmeyerU.S. Food and Drug Administration

Clare SullivanUnited Paperworkers International Union

Dwain WintersU.S. Environmental Protection Agency

NOTE: OTA is grateful for the valuable assistance and thoughtful critiques provided by the workshop participants. The viewsexpressed in this OTA background paper, however, are the sole responsibility of the Office of Technology Assessment.

iv

OTA Project Staff: Technologies for Reducing Dioxinin the Manufacture of Bleached Wood Pulp

John Andelin, Assistant Director, OTAScience, Information, and Natural Resources Division

Robert W. Niblock, Oceans and Environment Program Manager

James W. Curlin, Senior Associate

Administrative Staff

Kathleen A. Beil, Administrative Assistant

Sally W. Van Aller, Administrative Secretary

ContentsChapter

Page1. Introduction, Summary, and Findings . . . . . . . . . . . . . . . . . . .2. The Pulp and Paper Making Processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

. . . . . . . . . . . . . . . . . . . . 173. Environmental Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . : 2 9 . . . . . . ..

4. Pulp Bleaching Technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . ....... 415. Technologies for Reducing Chlorinated Organics in Pulp Manufacture . . .. . .. . .. ........ 55

vi

Chapter 1

Introduction, Summary,and Findings

CONTENTSPage

PURPOSE AND SCOPE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .THE U.S. PULP AND PAPER INDUSTRY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .SUMMARY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

The Pulp and Paper Making Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Environmental Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

34556

Pulp Bleaching Technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10Technologies for Reducing Dioxin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 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 aschlorinated dibenzo-para-dioxins and chlorinateddibenzofurans. The most toxic member of thislarge family of compounds is 2,3,7,8 -tetrachloro-p-dibenzodioxin (2,3,7,8 -TCDD or TCDD); theclosely related compound 2,3,7,8 -tetrachloro-dibenzofuran (2,3,7,8 -TCDF or TCDF) is be-lieved to be about one-tenth as potent. Both ofthese chemicals can be formed during themanufacture of bleached wood pulp if chlorineis used as a bleaching agent. The amount ofTCDD and TCDF formed in the bleachingprocess is measured in parts per trillion (ppt).The relative toxicity of these chemicals ismeasured in ppt and expressed as “ToxicEquivalents” (TEQs) using TCDD as the stand-ard. TCDD is considered to be highly toxicbased on laboratory animal experiments and hasbeen linked to malignancies, birth defects, andphysical deterioration in animals, Evidence ofhuman health effects is less certain and remainsa contentious issue among scientists. The U.S.Environmental Protection Agency (EPA) classi-fies it as a “probable human carcinogen.”

The release of EPA’s National Dioxin Study in1987 focused attention on evidence that showeda pattern of dioxin concentrations in streambottom sediment and fish below pulp mill wasteoutfalls. Subsequent studies by the paper indus-tries and government agencies in North Americaand Europe have confirmed that detectableamounts of TCDD and TCDF are produced andreleased into the environment at many bleachedkraft pulp mills. Measurements of dioxin levelsin the three sources of pollution (pulp, effluent,and sludge) are being conducted at all U.S.chemical pulp mills using chlorine bleach underthe joint sponsorship of EPA and the Americanpaper industry. The results of these studies andothers underway in Canada and Europe will

- 3 -

contribute much more to the information basenow available, but final results are months away.

Bleached wood pulp is used in paper productssuch as writing and printing papers, rayon,tissues, towels, disposable diapers, food pack-aging, and a myriad of other products. Paper isused so widely in modern societies that everyperson comes in contact with bleached paperproducts almost daily. The knowledge thatTCDD and TCDF are produced and releasedduring the manufacture of chlorine-bleachedkraft pulp has prompted action by the Federalagencies charged with protecting health and theenvironment and has stimulated research by apaper industry faced with increasing publicconcern and, as a consequence, possible regula-tory action.

Environmental and consumer groups haveorganized an international action program toinform the public, prompt consumer action, andstimulate political and regulatory action in thepulp-producing countries of North America andEurope. The United Paperworkers InternationalUnion, the labor union that represents many ofthe paper mill employees in North America, andthe National Institute for Occupational Health(NIOSH) are monitoring the potential healthimpacts of exposure of workers to dioxin in pulpand paper mills.

This study reviews what is known about theproduction and fate of TCDD and TCDF andother associated chlorinated compounds duringthe course of pulp bleaching and brightening.The study is specifically aimed at assessing thestate of pulping and bleaching technologies thatshow promise for reducing the amount of thosechemicals resulting from the manufacture ofbleached kraft pulp, such as extended delignifi-cation, oxygen delignification, the use of bleach-ing chemicals that might substitute for some orall of the chlorine commonly used, modifi-cations of the timing, amount, and operating

4 • Technologies for Reducing Dioxin in the Manufacture of Bleached Wood Pulp

conditions of the chlorine bleach sequence, anda review of pulping and/or bleaching technolo-gies that are not yet commercially available, butmight be in the future.

Because this is a technical background report,neither an assessment of policy options relatedto the need for regulatory action, nor the formsuch regulations might take, are included. Further-more, this assessment assumes—based on pasttrends and industry projections—that a strongdemand for high-brightness bleached pulp andpaper products will continue in the future.Should the public boycott or avoid the use ofbleached paper products in the future because ofa perceived health risk, the paper industry wouldhave to adjust to the changing markets within itsown constraints.

Markets for unbleached hygienic paper prod-ucts, disposable diapers, coffee filters, and milkcartons, for example, have developed in Swedenwhere “environment-graded” or “environment-positive” lines of unbleached paper products areretailed along with bleached grades. Recently,the Canadian pulp and paper industry has agreedto reduce the amount of dioxin in paper stockused for milk containers after Health andWelfare Canada, a government agency, discov-ered trace amounts of dioxin leaching into milkfrom plastic-coated paper cartons. The pulp andpaper industry claims that paper products requir-ing high strength and long-term durabilitygenerally must be made with a highly bleachedpulp, and industry analysts believe that this willlikely continue to be a major proportion of thepulp produced.

Market pulp is a commodity traded widely inworld markets. The possibility exists that somecountries in the future may impose limits on theamount of residual chlorine compounds ordioxin in pulp and paper products that can beimported. If similar collective action were to betaken by a large number of trading partners, such

as the European Economic Community or acoalition of Pacific Rim countries, a significantportion of North America’s export market couldbe affected. If such trade restrictions werebroadly adopted, it could, for practical purposes,impose de facto international performance stan-dards on those firms wanting to compete ininternational markets.

OTA convened a workshop in November1988 to discuss the latest information regardingthree aspects of dioxin and bleached wood pulpmanufacture:

1.

2.

3.

What is known about the distribution andeffects of TCDD and TCDF resulting fromenvironmental releases by pulp mills?What is known about the amount ofresidual TCDD and TCDF in paper prod-ucts, and what is the level of risk toconsumers and mill workers?What is the current understanding of theprecursors that react with chlorine to formTCDD and TCDF, and what is knownabout the means to reduce their formation?

As a result of the workshop, we were impressedby how fast knowledge about TCDD and TCDFin bleached wood pulp is accumulating. Thisreport is on] y a “snapshot” of the technology andknowledge about the formation of dioxin andpossible means to reduce it as of December1988.

THE U.S. PULP AND PAPERINDUSTRY

The United States leads the world in percapita paper consumption. In 1986 U.S. con-sumers used over 660 pounds of paper productsper person.1 This new record of paper consump-tion continues the steady increase in the domes-tic use of paper that saw per capita consumptionrise about 16 percent between 1976 and 1986.Paper, which is indirectly correlated with gen-

l~erlcm Papr In.ql[u[e, ]9/?7 statistics of Paper, Paperboard& Wood Pulp (New York, NY: Associated Press International, 198’7), p. 2.

Chapter 1—Introduction, Summary, and Findings ● 5

eral economic activity, is currently being con-sumed at the rate of about 21,000 tons per billiondollars of the real gross national product (GNP)generated. For a variety of reasons, including thereduction of solid waste, efforts to recycle scrappaper are expanding, particularly in urban areas.

Nearly 73 million tons of paper, board, andconstruction board were produced in the UnitedStates in 1986.2 Although 4.8 million tons ofpaper products were exported by U.S. produc-ers, almost 12 million tons were imported fordomestic consumption. Most of the importsoriginated in Canada and were newsprint. Morethan 70 percent of U.S. exports and 80 percentof imports in 1987 were bleached or semi-bleached pulps.3

Pulp products valued at $3.9 billion wereshipped from U.S. pulpmills in 1987. The paperand allied products industry (SIC 26) employedover 674,000 persons in 1986, with aboutone-third of those directly involved in theproduction of paper and pulp (SIC 261, 262,266).4 In general, pulp mills have tended toconcentrate in the South and Northwest nearmajor sources of wood, while allied industriesand conversion facilities are more broadlydistributed near primary markets.

Future prospects for the pulp and paperindustry are bright. In the Pacific Rim countrieswhere industrial expansion is expected to in-crease dramatically, forest-deficient countrieslike Japan, Korea, and Taiwan are consideredlarge potential markets. The United States andCanada, with their vast forest resources, theirestablished industries, and technological capac-ity, are well positioned to take advantage of newexport 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 involvesthe separation and purification of cellulosicwood fibers from which paper is formed. Abouthalf of the wood raw material is cellulosic fiber,and half is lignin, hemicellulose, and otherextractive compounds that cement and strength-en the fibers. The pulping process involveseither the use of chemicals, heat, and pressure ina digester to dissolve the lignin and free thefibers from one another or, in the case ofmechanical pulp and chemical-mechanical pulp,the abrasion of wood in a grinder or refiner tophysically tear the fibers apart.

Zlbid.3u,s, ~pmenl of Commerce, IWM U,S IIIAStria/ Outlook (Washington, DC; Intcmational Trade tiinw~iont 19~8)t PO 6-3

~~crlcm pWr ]n~l[utc, ~p, ~1[,, nolc ], p, 54, Nom: “S[C”r~fers[0“S~d~d lndustri~ Co&,” a classification of U.S. industries UWd by tkU.S. Depactmenl of Commerce.

6 • Technologies for Reducing Dioxin in the Manufacture of Bleached Wood PUlp

New pulping technologies are emerging, buttheir acceptance will depend on efficiency, pulpquality, cost effectiveness, or other factors thatmake them competitive with conventional tech-nology. To the extent that new pulping systemsmight reduce the amount of residual lignincarried over to the bleaching process, theamount of chemical bleaching required to attainthe desired brightness might be reduced andsubsequently the amount of chlorine used as ableaching agent trimmed.

Environmental Considerations

Effluents from bleached pulp mills contain avariety of substances, some of which are knownor suspected of being toxic, genotoxic, muta-genic, or teratogenic. Chlorinated organics,including TCDD and TCDF, that appear to beproduced in the chlorine bleaching and de-lignification processes, are of particular con-cern. The composition of bleaching effluent isextremely complex and varies widely from millto mill. The chlorinated components of thewaste stream consist of a variety of compounds,including simple phenols, high and low molecu-lar weight polymers, and neutral and acidicmaterials from the breakdown of the phenolicrings in lignin. An average North American pulpmill produces between 35 and 50 tons ofchlorinated substances daily. Greenpeace Inter-

national, the Environmental Defense Fund, andother environmental organizations advocate imme-diately minimizing and eventually eliminatingthe use of chlorine in order to avoid additionalreleases of these compounds into the environmentthrough effluent, sludge, or via paper products.

About 10 percent of the total solids in thewaste streams of bleaching plants contain chlo-rinated derivatives, but this varies among millsdepending on the degree of filtration. SomeEuropean countries, Sweden and the FederalRepublic of Germany in particular, have im-posed restrictions on the amount of total organi-cally bound chlorine (TOC1) that they will allowtheir pulp and paper industry to dischargesSweden has cut the amount of allowable TOC1discharge more than half, from the 5 to 6kilograms per metric ton (kg/t) of pulp normallyproduced with conventional chlorine bleaching,to 2 kg/t. Oxygen delignification and chlorinedioxide substitution have been the technology ofchoice to meet Sweden’s environmental re-quirements. The National Swedish Environ-mental Protection Board hopes to reduce TOC1to 0.1 kg/t by 2010 and to 1.5 kg/t by 1992. Withfurther reductions in allowable discharges, theSwedish industry may need to adopt closed-cycle processes to eliminate chlorinated dis-charges entirely.

SBetween 25CI ~d XXI ch~micals have been identified in pulp mill effluents. Many of them arc chlorinated organic compounds. SCe Lccna R. Suntio.Wan Ying Shiu, and Donald Mackay, “A Review of (he Nautre and Properties of Chemicals Present in Pulp Mill Effluents,” Chemo@ere, vol. 17, No.7, 1988, pp. 1249-1290. TCDD and TCDF make up a very small fraction of the total organochlonne emissions.

Chapter I--Introduction, Summary, and Findings ● 7

In 1988 the Board also considered the overallproblem of dioxin in the Swedish environmentand concluded that standards established forreducing TOC1 would suffice to reduce thedioxin levels from bleached pulp as well.Chlorinated organic effluents from pulp andpaper manufacture have created environmentalproblems in coastal waters and estuaries of theBaltic Sea, particularly in the enclosed Gulf ofBothnia. Unlike U.S. pulp mills that use second-ary biological sewage treatment technologies totreat waste effluents, mills in Sweden, some inFinland, and many in Canada do not usecomparable treatment methods before discharginginto the environment. Biological treatment canremove large amounts of chlorinated contami-nants (particularly those that adhere tightly tosediments such as TCDD and TCDF) from milleffluents when operated properly. Biologicaltreatment transfers the contaminants from thewater and concentrates it in the sludge. Thesludge must then be disposed of in a safe andacceptable manner. Biological treatment sys-tems require constant attention to ensure thatthey are working efficiently.

The Federal Republic of Germany will begintightening restrictions on the discharge of chlori-nated organics in 1989. Maximum alloweddischarges of organochlorines will be 1.0 kg/t ofbleached pulp under the new regulations. Untilrecent] y, West German regulations for pulp mill

discharges have not been overly strict. Feeslevied on discharged amounts of oxygen de-mand, solids, and measures of toxicity to fishhave allowed the German industry to “buy” theright to pollute.

Chlorinated organic residues from pulp bleach-ing have caused less concern in North Americawhere little research into adverse impacts onfisheries and aquatic biota has been conducted.There is ample experimental evidence, however,that pulp mill effluents, unless adequatelytreated, can be toxic to fish and some chlorinatedcompounds may ultimately find their waythrough the food web. Dioxin has been shown tobe extremely toxic to fish at very low concentra-tions under controlled laboratory experimentsand has been linked to periodic reductions inreproductive rates of some bird species. U.S.mills, unlike their European counterparts, haveinstalled extensive biological treatment systemsto reduce the biological and chemical oxygendemand (BOD and COD) of their waste efflu-ents before discharging them into streams. U.S.environmental requirements for the issuance ofa discharge permit for conventional pollutants,such as BOD, COD, and suspended solids, andtoxicity tests on mill effluents make biologicalwaste treatment a preferred technology for U.S.mills.

The detection of dioxin in some fish samplesreported in EPA’s National Dioxin Study has

8 ● Technologies for Reducing Dioxin in the Manufacture of Bleached Wood Pulp

raised concern about the possible effects onhuman health. Less than 20 percent of therandomly selected sites sampled by EPA in thestudy showed detectable quantities (more thanone part per trillion [ppt]) in whole fish samples.Other samples collected at over 300 regionalsites selected by EPA for high probability ofdioxin contamination showed that nearly one-third of the rivers, lakes, coastal waters, andestuaries contained fish with detectable amountsof dioxin that ranged up to 85 ppt—significantlyhigher than the 25 ppt determined to be unsafeby the Food and Drug Administration (con-tained in edible fillets, not in whole fish)-butit is not certain that pulp mill waste was thesource of all contamination. Only 4 of 57estuarine and coastal sites sampled in theNational Dioxin Study had detectable levels ofdioxin in fin fish or shellfish. Based on labora-tory tests, fish have been shown to accumulatedioxin in their bodies at rates approximately20,000 to 85,000 times the concentrations towhich they are exposed in water. Fishing hasbeen prohibited in several rivers because ofdioxin levels, and advisories have been issuedby Wisconsin, Maine, and Louisiana warning ofthe possible risks of eating contaminated fish.

A screening analysis of pulp waste at fivebleached kraft pulp mills, which was conductedjointly by EPA and the U.S. paper industry, andother research in Europe and Canada clearlylinks the formation of TCDD and TCDF to thechlorine bleaching process. TCDD was detectedin seven of nine bleached pulps sampled, somewith levels as high as 51 ppt. TCDF, which isless toxic than TCDD, was found in eight of ninepulps sampled, with levels ranging up to 330ppt.

Dioxin was detected in wastewater at four ofthe five mills, but concentrations varied greatlyamong the mills sampled. The highest levels ofTCDD and TCDF were associated with wastefrom the caustic extraction stage, which isdesigned to remove the dissolved materials afterthe chlorine treatment. EPA and the paper

industry are currently surveying 104 additionalmills to better determine the scope and extent ofdioxin and furan formation. Detailed analyses ofTCDD levels and bleaching processes at 25 pulpmill bleach lines will be conducted in the courseof the study.

The detection of TCDD and TCDF inbleached pulp samples raised concern aboutwhether residual dioxin was being carried for-ward into finished paper products. The NationalCouncil of the Paper Industry for Air and StreamImprovement, Inc. (NCASI), the industry’senvironmental research arm, commissioned anassessment of the potential risks to humanhealth from skin exposure to a variety of paperproducts, including disposable diapers, facialtissue, toilet tissue, sanitary napkins, coffeefilters, and paper towels. The assessment con-cluded that TCDD equivalents in all productstested presented a lifetime cancer risk of lessthan one in a million.

Environmentalists note that NCASI’s assess-ment only considered the risks for 2,3,7,8-TCDD and 2,3,7,8-TCDF without examining


the risks from the several hundred other chlori-nated byproducts that are also found in paperproducts, and that NCASI failed to consider thecumulative risk of using a wide range of paperproducts daily. Environmentalists also faultNCASI for not using appropriate testing proce-dures for evaluating the possible enhancedmobility of dioxin when in the presence oflipids, solvents, and fats, such as encounteredwhen using paper towels for cooking or whenusing creams and baby oils with disposablediapers.

OTA learned at its dioxin workshop that thereis considerable disagreement about the validityof the testing protocols for determining the rateof migration of dioxin from paper products andinto the human body. For instance, there arewidely differing opinions about how to estimatethe effect of urine leaching from disposablediapers into the skin of babies, and how tosimulate the environment of a tampon in determin-ing the movement of dioxin into a woman’sbody. Without standard protocols, disagreementsover the meaning of dioxin risk data leaves OTAwith no means to evaluate the industry’s find-ings and conclusions. Evaluation of these testingtechniques is beyond the scope of this as-sessment.

Agreement is also lacking on how to expresslevels of risk. Does one accept a one in a millionrisk of cancer as did the U.S. paper industry inits study of risks from dermal exposure todioxins, or should the acceptable risk be one ina thousand? There are no Federal regulatorystandards. Arbitrary levels of risk are used forconvenience, and experts differ over the mostappropriate 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 riskis zero,” or should the term “risk specific dose”be used since it does not imply that there is anydose above zero that is “safe”?

Couple the uncertainties over testing proto-cols with disagreement over levels of acceptablerisk, and add to it the lack of consensus about thepotency of dioxin to humans, and a confusedpicture is presented to those attempting to gaugethe danger of dioxin from pulp manufacture andpaper use.

OTA is aware of no published studies thathave attempted to define worker exposures todioxin in paper mills using environmental measure-ments or biological exposure measurements,such as serum or fat TCDD/TCDF concentrations.The National Institute of Occupational Safetyand Health (NIOSH), the Federal agencycharged with occupational and health research,is currently developing personal exposure moni-toring methods to address pulp and paperworkers’ exposure to dioxin.

EPA is currently coordinating an InteragencyTask Force on Dioxin in Paper aimed at gaugingthe cumulative risk of all of the media that couldcontribute to human exposure. Other coop-erating agencies include the Food and DrugAdministration and the Consumer Product Safe-ty Commission (CPSC). The interagency effortwas characterized by an EPA official at theNovember 1988 OTA dioxin workshop as aneffort to “determine whether there is a risk—nota ‘gold plated’ risk assessment,” and a “snapshotin time” as to the risk aimed at determiningwhether regulation is needed. The interagencystudy will build on both the EPA/Industry104-mill study and estimates of migration ratesof dioxin from paper products. It will include anassessment, based on product-use scenarios, oftechnologies that could reduce dioxin exposure.

OTA found that, as with many other Federalactivities, the analytical and regulatory authorityfor dealing with dioxin in paper is fragmentedamong several agencies, none of which haveclear responsibility for dealing with the problemin its entirety. While EPA has jurisdiction overwater pollution and dioxin in pulp (dealt with intwo separate offices), FDA has responsibility


for regulating dioxins in coffee filters andsanitary napkins, and CPSC has responsibilityfor dioxin in disposable diapers, paper towels,facial tissues, and toilet tissues.

The lack of published independent Federalresearch on the risks associated with dioxin inpaper products and the absence of governmentinformation on paper worker’s environmentalexposure to dioxin required OTA to rely almostwholly on industry-sponsored research. Thepaucity of government information on workers’risk and exposure to dioxin in paper productsmay change in the future as a result of theInteragency Task Force on Dioxin in Papermentioned above, but in the meantime we haveonly industry information available to evaluatethe 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 chemicaldigestion process to the bleaching stages in kraftmills. This residual lignin must be substantiallyreduced if pure cellulose products or brightpaper are to be produced. The less ligninretained in the pulp as it leaves the digester, theless bleaching needed. There are limits, how-ever, as to how much digestion wood fibers canendure without sacrificing pulp strength andpaper durability or significantly reducing pulpyield.

Bleaching is a continuation of the delignifica-tion process and is used to brighten and purifythe wood pulp. The purer the cellulose andbrighter the paper, the less lignin retained, andthe longer the paper will last without yellowingand becoming brittle. Chlorine gas has becomethe preferred bleaching agent because of itsrelatively 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 commerciallyavailable. Each has its advantages and disadvan-tages, and several are usually used in sequenceto achieve the final result. Between bleachingstages caustic chemicals are often used toremove the dissolved lignin and wash the fibersbefore they are subjected to additional bleach-ing. It is apparently during this extraction thatmuch of the dioxin created by chlorine bleach-ing finds its way into the waste stream.

It has been shown that by reducing the use ofchlorine gas in the bleaching process, theamount of chlorinated organic residues andcontaminates can be reduced. Oxygen delignifica-tion shows some promise for reducing theamount of lignin carried with the pulp to thebleach process, thus reducing the amount ofchlorine gas needed to bleach the pulp to thedesired brightness. Current commercial oxygendelignification technology can reduce the use ofchlorine gas by 40 to 50 percent. Further


reductions in the use of chlorine by more intenseoxygen delignification is limited by severelosses in pulp yield and strength properties.Pretreatment of the pulp with nitrogen dioxidebefore oxygen treatment shows promise forincreased delignification before bleaching.

The cost advantage that chlorine gas once hadover many other bleaching agents does not applyto oxygen. If the U.S. pulp and paper industry isrequired to reduce the amount of chlorinatedorganics and dioxin produced in bleaching pulp,oxygen may become a partial substitute forchlorine because of its low operating cost.Capital cost for oxygen treatment is very high,however, and installation of oxygen delignifica-tion in existing mills can reduce productioncapacity as much as 4 or 5 percent. Any eval-uation of the comparative costs of oxygentechnology with other delignification systemsshould consider both capital costs, life-cyclecosts based on operating costs and depreciation,and gains and losses in productivity and quality.Such analyses were not made by OTA in thisstudy.

Technologies for Reducing Dioxin

There are several ways to reduce the amountof pollution contributed by bleach plants. Theseinclude:

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further delignification of pulp before bleach-ing;improved washing of the unbleached pulp(brownstock);substitute nonchlorinated bleach agents;substitute chlorine dioxide for some or allchlorine gas;apply multiple additions of chlorine in splitcharges instead of using a single, massivecharge (low chlorine multiple);improve chemical mixing with the pulp;adjust the acidity of the unbleached stockbefore adding chlorine;

use “cleaner” oil-base defoamers that donot contain dioxin precursors;remove dioxin precursors prior to treat-ment of the pulp with chlorine; andimprove secondary waste treatment andwaste-disposal practices.

Pretreatment before bleaching can reducethe amount of chlorine or other bleachingchemical used. Technologies for prebleachdelignification include: 1) extended de-lignification, 2) oxygen delignification, 3)pretreatment with nitrogen dioxide beforeoxygen delingification (PRENOX allowsmore lingin to be removed without dam-aging fibers), 4) pretreatment with otherchemicals such as chlorine dioxide, and 5)extraction with sodium hydroxide supple-mented with oxygen and hydrogen perox-ide. Only extended delignification andoxygen delignification are currently usedcommercially.

Other technologies used to conserve waterand reduce energy consumption may also helpreduce the amount of chlorinated products in thewaste stream:

recycle process water from the chlorinationstage (although this may actually com-pound the dioxin problem),use countercurrent washing systems afterchlorination, andreduce water use by using higher ratios offiber pulp to water (higher consistency).

Elimination of chlorine in the bleach se-quence combined with internal recycling ofprocess water aimed at developing a “pollution-free” pulping system seems to offer a goodstrategy over the long term, but has not yet beenadopted for commercial use.


It is believed that if the use of chlorine gas isreduced or eliminated in the bleaching process,the amount of TCDD and TCDF formed will belowered or eliminated along with other chlori-nated organics. However, the relationship is notlinear, and other factors, such as mixing effi-ciency, have significant effects on the relation-ship. Oxygen is one of the most promisingbleaching chemicals for displacing some of thechlorine used in the bleaching cycle. In no casethus far has oxygen been able to completelyeliminate the need for chlorine.

Chlorine dioxide, a more efficient oxidantthan chlorine, can be substituted for substantialamounts of chlorine gas. The use of chlorinedioxide in conjunction with chlorine gas isincreasing at U.S. mills. It has been demon-strated that substitution of chlorine dioxide forsome optimum portion of chlorine gas cansignificant] y reduce the formation of TCDD andTCDF. However, the effects of chlorine dioxidesubstitution on the formation of other chlori-nated compounds are not known.

Recent research has also shown that theamount of dioxin can be reduced by lowering theacidity (raising the pH) of the pulp beforechlorination, by applying the chlorine charge inthree parts instead of one shot, and by usingchlorine dioxide after an initial charge ofchlorine gas. A very recent discovery by scien-tists at the Pulp and Paper Institute of Canadahas identified oil-based defoamers as one possi-ble source of the precursors that form dioxinwhen chlorinated. As a consequence, the U.S.industry and chemical suppliers are currentlyseeking cleaner oil-based defoamers.

Defoamers are added to the unbleached pulpin small amounts to improve washing. If adefoamer is made from used oil contaminatedwith unchlorinated dioxins (DBD) and furans(DBF), these precursors convert to their chlori-nated forms as TCDD and TCDF when exposedto chlorine gas. The United States regulates theuse of contaminated recycled oil, so furtherinvestigation is needed to determine whetherdefoamers are a problem for U.S. mills usingdomestically produced products. Recent in-vestigations by American scientists using test-ing protocols more sensitive than those used inCanada have raised doubts as to whether anyoil-based defoamers—whether made from ei-

Chapter 1—Introduction, Summary and Findings ● 13

ther virgin, hydro-treated oil, or used oil—arefree of contaminating precursors.

Water-based defoamers are also available,and tests show them to be free of DBD and DBFprecursors. Unfortunately, while water-baseddefoamers can be used at other washing stagesin the mill, they are not effective for washingbrownstock. The use of “cleaner” defoamers isanother option for reducing some of the dioxinproduced in the bleaching process, but moreresearch and development may be needed todevelop suitable products.

Oxygen delignification technology was discov-ered in 1952, and the first commercial unit wasinstalled in the 1960s. Since then, oxygentechnology has advanced steadily until it is nowconsidered a mature and proven process. In1988, world installed capacity using oxygendelignification is expected to exceed 10 millionmetric tons per year. Several manufacturers ofpulp and paper equipment market oxygen delig-nification systems. Over 50 oxygen units havebeen installed worldwide. About half the oxy-gen capacity is in Scandinavia and Europe,one-fifth is in Japan, and one-fifth is in NorthAmerica. About 92 percent of the oxygenbleaching capacity is installed in kraft mills, buta number of sulfite mills—mostly in the FederalRepublic of Germany—have also installed oxy-gen delignification units.

Oxygen can also be used in combination withextended digestion (modified cooking). By modify-ing the chemical addition and allowing it to cooklonger, the amount of residual lignin in the pulpcan be reduced prior to bleaching. Furtherpre-bleaching with oxygen can produce pulpwith even lower amounts of lignin. With pre-bleached pulps of low lignin content, it may betechnically possible to eliminate the use ofchlorine gas in the bleaching process if chlorinedioxide is used as a substitute.

No reliable data exist that directly link thereduction of TCDD and TCDF wholly tooxygen delignification, and in some cases oxy-

Modem mills are designed to match thecapacity of the chemical recovery plant with theplanned production capacity of the mill. Theaddition of an oxygen delignification stageincreases the volume of effluent that must behandled by the chemical recovery plant. In newmills recovery plants can be designed to handlethe additional load, but retrofitting an existingmill with an oxygen delignification system canresult in overloading the chemical recoverycapacity. If a larger size recovery furnace orevaporators are required, the additional capitalexpense may make oxygen delignification a lessattractive alternative for economic reasons. It isestimated that adding an oxygen stage to a millwhose chemical recovery plant is operating atfull capacity will reduce the productivity of thatmill 4 to 5 percent.

Given a range of equally effective technolo-gies to reduce the use of chlorine gas, capital and


operating costs may determine the most cost-effective strategy for reducing the amount ofchlorinated organic chemicals produced by pulpmills. Although operating costs may be lower ifoxygen delignification is used to replace someof the chlorine gas now used in the bleach cycle,the capital cost of oxygen systems is large. Bothcapital costs and operating costs must be consid-ered in a balanced assessment. Furthermore,cost factors will differ from mill-to-mill, mak-ing generalizations difficult. OTA did not at-tempt to assess these costs. On the other hand,by using “cleaner” defoamers, coupled withsubstitution of chlorine dioxide for chlorine, itmay be possible to achieve much lower levels ofTCDD and TCDF in pulp. Whether or not thiswould also lower the level of other chlorinatedcompounds is uncertain.

Further reductions in environmental releasesfrom pulp mill waste outfalls may result fromimproving the control of suspended solids in thesecondary waste treatment plant. TCDD andTCDF are relatively insoluble in water, but theyadhere tightly to fine colloidal material andsuspended solids. A well-designed, properlyoperated secondary treatment plant is capable ofremoving up to 90 percent of the TCDD and

TCDF released. Dioxins are retained intreatment plant sludge. The sludge is normallydisposed of in landfills, where limited studiesshow that it remains isolated and immobile.Some sludge is retained in sludge lagoons or isincinerated, but a portion is used for soilconditioners. The remaining 10 percent of thedioxin that remains in suspension can find itsway in to the water coarse and remain assediment in streambeds. The use of clarifiers,chemicals, and settling basins to improve theefficiency of waste treatment, coupled withchlorine dioxide substitution and/or perhapsother delignification technology might prove tobe the optimum solution for some existing pulpmills.

More questions remain than do answers as towhat risk dioxin from pulp and paper manufac-ture presents to humans and the environment:will additional regulations be needed to reducethe risk of human exposure from dioxin andother chlorinated compounds produced in pulpmills? and which technologies or mix of tech-nologies are best suited for reducing the produc-tion of dioxin in the pulp bleaching process? Thepulp and paper industry has a number oftechnical options available to meet the problem.Other technologies, such as extended delig-nification and the substitution of other oxidantsin the bleach process for chlorine gas, may be aseffective as oxygen delignification in reducingthe use of chlorine. All of these questionsrequire more detailed study before they can beanswered with acceptable certainty. Final deci-sions to meet regulatory requirements will haveto be made on a case-by-case, mill-by-mill basis.

Chapter 2

The Pulp and Paper MakingProcesses

CONTENTSPage

THE PULP AND PAPER MILL. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17Steps in the Pulp and Papermaking Process... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18Pulping Technologies... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

2-1.2-2.2-3.2-4.

FigureOverall View of Papermaking From Chemical Pulp by the Kraft Process.Stone Groundwood Pulp Mill Flow. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Refiner Groundwood Pulp MillSulfite Pulp Mill Process... . . .

Table2-1. Major Commercial Wood-Pulping

Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Table

Technologies . . . . . . . . . . . . . . . . . . . . . . . .

Page. . . . . . . . . 19

21. . . . . . . . . . . . . . . . . . 22. . . . . . . . . 25

Page. . . . . . . . . 18

Chapter 2

The Pulp and Paper Making Processes

The modem manufacture of paper evolved froman ancient art first developed in China, ca. 105 A.D.Although the modem product differs considerablyfrom its ancestral materials, papermaking retainsdistinct similarities to the processes developed byTs’ai Lun in the Imperial Chinese Court. ’ Inprinciple, paper is made by: 1) pulping, to separateand clean the fibers; 2) beating and refining thefibers; 3) diluting. to form a thin fiber slurry,suspended in solution; 4) forming a web of fibers ona thin screen; 5) pressing the web to increase thedensity of the material; 6) drying to remove theremaining moisture; and 7) finishing, to provide asuitable surface for the intended end use.

Pulp and paper are made from cellulosic fibers(i.e., fibers from trees) and other plant materials,although some synthetic materials may be used toimpart special qualities to the finished product. Mostpaper is made from wood fibers, but rags, flax,cotton linters, and bagasse (sugar cane residues) arealso used in some papers. Used paper is alsorecycled, and after purifying and sometimes de-inking, it is often blended with virgin fibers andreformed again into paper. Other products madefrom wood pulp (cellulose) include diapers, rayon,cellulose acetate, and cellulose esters, which areused for cloth, packaging films, and explosives.

Wood is composed of: 1) cellulose, 2) lignin, 3)hemicellulose, and 4) extractives (e.g., resins, fats,pectins, etc.). Cellulose, the fibers of primaryinterest in papermaking, comprises about 50 percentof wood by ovendry weight. Lignin, which cementsthe wood fibers together, is a complex organicchemical the structure and properties of which arenot fully understood, It is largely burned for thegeneration 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 forfuel (some is converted to chemical products) couldbecome a valuable feed stock for new chemicalproducts.

The pulping process is aimed at removing asmuch lignin as possible without sacrificing fiberstrength, thereby freeing the fibers and removing

impurities that cause discoloration and possiblefuture disintegration of the paper. Hemicellulose issimilar to cellulose in composition and function. Itplays an important role in fiber-to-fiber bonding inpapermaking. Several extractives (e.g., oleoresinsand waxes) are contained in wood but do notcontribute to its strength properties; these too areremoved during the pulping process.

The fiber from nearly any plant or tree can be usedfor paper. However, the strength and quality of fiber,and other factors that can complicate the pulpingprocess, varies among tree species. In general, thesoftwoods (e.g., pines, firs, and spruces) yield longand strong fibers that impart strength to paper andare used for boxes and packaging. Hardwoods, onthe other hand, generally have shorter fibers andtherefore produce a weaker paper, but one that issmoother, more opaque, and better suited for print-ing. Both softwoods and hardwoods are used forpapermaking and are sometimes mixed to provideboth strength and printability to the finished product.

THE PULP AND PAPER MILLAlthough there are several chemical and mechani-

cal pulping methods used for delignifying wood(table 2-l), separating fibers, and removing discol-oration, all integrated pulp and paper mills involvethe same general steps in the manufacture of pulpand paper. These steps include: 1 ) raw materialpreparation (e.g., debarking and chipping); 2) me-chanical and/or chemical separation of the woodfibers [i.e., grinding, refining, 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 andmanufacture.

A typical layout of a mill using the kraft chemicalpulping process is shown in figure 2-1. Mechanical,semichemical, and sulfite pulp mills differ in detail,particularly in wood preparation, fiber separation,and bleaching, but many of the downstream refining,bleaching, and papermaking processes are similar.In addition to the primary steps in pulp and papermanufacture, each mill has extensive facilities to

IA, H, Nlss~, p~p~~, WOO~ Ifs ~(ruC~~e U~~ p?ope?(ies, F.F, Wangaard (cd.) (University Park, PA: Pennsyivtia State University 198 1)! P 3~5.

-17-


Table 2-l—Major Commercial Wood-Pulping Technologies

Pulp grades use Wood type End-product useChemical pulps:sulfite pulp . . . . . . . . . . . . . . . . . . . . . . . . . . . Softwoods and hardwoods Fine and printing papersKraft sulfate pulp . . . . . . . . . . . . . . . . . . . . . . Softwoods and hardwoods Bleached-printing and writing papers, paperboard

Unbleached-heavy packaging papers, paperboard.Dissolving pulp . . . . . . . . . . . . . . . . . . . . . . . . Softwood S and hardwoods Viscose rayon, cellophane, acetate fibers, and filmSemichemical pulps:Cold-caustic process . . . . . . . . . . . . . . . . . . . Softwoods and hardwoods Newsprint and groundwood printing papersNeutral sulfite process . . . . . . . . . . . . . . . . . . Hardwoods Newsprint and groundwood printing papersMechanical pulpsStone groundwood , . . . . . . . . . . . . . . . . . . . Softwoods Corrugating mediumRefiner mechanical (RMP) . . . ., . . . . . . . . . Softwoods Newsprint and groundwood printing papersThermomechanical (TMP) . . . . . . . . . . . . . . .Softwooods Newsprint and groundwood printing papersSOURCE: Modifhd from George H. Soyd Ill and Chad E. Srown, Paper /~sfFy: ~tbok b Market Pu/p (New York, NY: KiddM, Peabody & (h, 19S1 ), p. 5.

produce and reclaim chemical agents used in thepulping process; collect, process, and bum ligninand waste wood to produce energy; and remove andtreat wastes from process water for release into theenvironment.

Steps in the Pulp and Papermaking Process

Raw Material Preparation

Wood received at a pulp mill may be in severaldifferent forms, depending on the pulping processand the origin of the raw material. It maybe receivedas bolts (short logs) of roundwood with the bark stillattached, as chips about the size of a half-dollar thatmay have been produced from sawmill or veneermill waste or pre-chipped from debarked roundwoodelsewhere, or as waste sawdust in the case of somepulping processes.

If roundwood is used, it is first debarked, usuallyby tumbling in large steel drums where wash watermay be applied. The debarked wood bolts are thenchipped in a chipper if the pulping process calls forchemical digestion or are fed into a grinder in thecase of some mechanical pulps. Chips are screenedfor size, cleaned, and temporarily stored for furtherprocessing.

Fiber Separation

The fiber separation stage is the point at which theseveral pulping technologies diverge. In kraft chemi-cal pulping, the chips are fed into a large pressurecooker (digester), into which is added the appropri-ate chemicals (white liquor). The chips are then

cooked (digested) with steam at specific tempera-tures long enough to separate the fibers and partiallydissolve the lignin and other extractives.

Some digesters operate continuously with a con-stant feed of chips (furnish) and liquor, others arecharged intermittently and treat a batch at a time.After digestion, the cooked pulp (brown stock) isdischarged into a pressure vessel (blow tank) wherethe steam and volatile materials are siphoned off.The cooking liquor, that by this time has turned darkbrown from the dissolved lignin (black liquor), isreturned to the chemical recovery cycle, In thechemical recovery plant, the lignin in the blackliquor is burned for the cogeneration of energy, andthe chemicals are recovered, purified, reconstituted,and returned to the digester as white liquor for reuse.

The brown stock containing the recovered fibers(having the consistency of cooked oatmeal) iswashed with water, screened to remove undigestedwood, and cleaned to remove other foreign matter.It is then ready for bleaching and further processing.

Fiber separation in mechanical pulping is lessdramatic. In the stone groundwood process, de-barked logs are forced against rotating stone grind-ing wheels that are constantly washed by a stream ofwater. The ground pulp is then screened to removecourse debris, thickened, and stored for the paper-making process.

Chips are used to produce refiner pulp andthermomechanical pulp. In both processes the chipsare ground by passing them through rapidly rotating

Chapter 2—The Pulp and Paper Making Processes ● 19

Figure 2-1-Overall View of Papermaking From Chemical Pulp by the Kraft ProcessContinuous digester

II

White liquorclarifier

Wash water

II

Stripped condensates A

SOURCE: Envirorwnent Ontario, Srop@ng Water Poflutioft Al Its Source (Toronto, OntarIO: Ministry of the Environment, 1988).


disk grinders. Thermomechanical pulp is refined(ground) under pressure after the chips are pretreatedwith steam (chemical thermomechanical pulp useschemicals and steam for pretreatment). After furtherrefining in a second stage, the pulp is screened,cleaned, and most of the process water is removed inpreparation for papermaking.

Bleaching or Brightening

Since the raw pulp (brown stock) still contains anappreciable amount of lignin and other discolora-tion, it must be bleached to produce light colored orwhite papers preferred for many products. Bleachingis normally done in several stages (multistagebleaching). Through chlorination and oxidation thefibers are further “delignified” by solubilizingadditional lignin from the cellulose.

A number of bleaching agents may be used andare applied in a stepwise fashion within a bleachingsequence. These include chlorine gas, chlorinedioxide, sodium hypochlorite, hydrogen peroxide,and oxygen. Between bleaching treatments, a strongalkali (usually sodium hydroxide) is used to extractthe dissolved lignin from the surface of the fibers.The bleaching agents and the sequence in which theyare used depend on a number of factors, such as therelative cost of the bleaching chemicals, type andcondition of the pulp, desired brightness of the paperto be produced, and sometimes in response toenvironmental guidelines and regulations.

Bleaching of mechanical pulp is much differentthan that for chemical pulp. Mechanical pulpingleaves the lignin and the cellulose intact, whereas thepurpose of chemical pulping is to chemically sepa-rate the lignin from the cellulose fibers and removeit from the pulp. A major advantage of mechanicalpulping is the high yields of pulp that can beachieved from a given volume of wood. Therefore,bleaching or brightening of mechanical pulps isdesigned to minimize the removal of the lignin thatwould reduce fiber yields.

Chemicals used for bleaching mechanical pulpsselectively destroy coloring impurities but leave thelignin and cellulosic materials intact, These includesodium bisulfite, sodium or zinc hydrosulfite (nolonger used in the United States), calcium or sodium

hypochlorite, hydrogen or sodium peroxide, and theSulfur Dioxide-Borol Process (a variation of thesodium hydrosulfite method). Originally, much ofthe mechanical pulp was not bleached, but thebleaching of groundwood has increased and im-proved technology now enables bleached ground-wood pulp to be used for printing papers, tissues, andtowelling.

Papermaking

The bleached or unbleached pulp may be furtherbeaten and refined to cut the fibers and roughen thesurface of the fibers (fibrillate) to improve formationand bonding of the fibers as they enter the papermachine. Before entering the paper machine, wateris added to the pulp slurry to make a thin mixturenormally containing less than 1 percent fiber. Thedilute slurry is then cleaned in cyclone cleaners andscreened in centrifugal screens before being fed intothe ‘‘wet end* of the paper-forming machine.

In the paper making process, the dilute stockpasses through a headbox that distributes the fiberslurry uniformly over the width of the paper sheet tobe formed. The “web” of fiber that will make thenew paper sheet is formed on a continuously movingbronze or polymer screen (Fourdrinier) or betweentwo such wire screens. Water drains from the slurrythrough the mesh of the screen, the wet paper web isconsolidated and the paper sheet gains some strengththrough fiber bonding.

The wet sheet of paper is continuously lifted fromthe screen (couched) and transferred to a woven feltbelt where additional water is squeezed from thepaper sheet by pressure rollers, The remaining wateris removed on steam-heated cylinders. When thepaper is dry it may be treated with stabilizingmaterials and surface finishes to improve durabilityor printability.

Pulping Technologies

Mechanical Pulping Processes

There are six basic mechanical pulping processes:1 ) stone groundwood, 2) refiner, 3) thermomechani-cal pulping, 4) chemical mechanical, 5) defibrated orexploded pulping, and 6) recycled paper.2 Mechani-cal pulping is generally used with softwoods be-

2~1s ~Uon ~ ~lplng IWhno]ogies IYCXTOWS heavily from a previous OTA assessment: Wood Use U.S. Co~etitiveness and Technolo~, Vol. //:Technical Report, OTA-M-224 (Springfkld, VA: National Technical Intormaiion Wvice, 1984), pp. 79-94.


cause of the added strength imparted by the longfiber length of softwood species. Some hardwoodsrequire chemical pretreatment (chemical mechancialpulping) to produce a suitable groundwood pulp.Fibers separated mechanically are substantially dam-aged in the process and therefore make weaker paperor paperboard. However, since both lignin andcellulose fibers remain intact, the yield of paper perunit volume of wood is still greater than thatproduced by chemical pulping. Pulp yields from allof the mechanical pulping processes typically arenear 90 to 95 percent recovery, which is a muchhigher yield per unit of wood than with the chemicalpulping methods because of the retention of lignin.However, paper made from mechanical pulp discol-ors and becomes brittle with age because of its lignincontent, which results in a shorter useful life thanpaper made from chemical pulp.

Mechanical pulps are used principally to manu-facture newsprint, printing papers, towelling, tissue,

and coated specialty papers that do not requirehigh-strength. Secondary uses include wallpaperand paperboard. Small amounts of chemical pulp areoften mixed with groundwood pulp for additionalstrength. Recycled pulp is used mainly for themanufacture of folding boxboard (gray board), tis-sue, corrugated board, and newsprint. Paper prod-ucts made from defibrated pulp include hardboards,construction boards, and roofing papers.

In the stone groundwood process, debarked shortlogs (roundwood) are fed whole against wet stonegrinders by hydraulic rams. Counter-revolving steeldisks are sometimes used in place of abrasive stonein the grinding process. The abrasion of the grindingwheel against the wood physically separates thewood fibers. The grinding process usually is auto-matic and continuous. The groundwood pulp is thenscreened, bleached or brightened, treated, and pre-pared for the paper machine (figure 2-2).

Figure 2-2-Stone Groundwood Pulp Mill Flow

W h ite Over f low

w a t e r1 I

P u l p B l e a c h i n g S ewe r

, I

SOURCE: Man M. Spdnger, Inolmrk?i Emwo rvmwtal Centi: RJ/p @ Psper hdmtry (New Yoik, NY: Jotm Wiley & Sons, 19SS), p. 147.

22 . Technologies for Reducing Dioxin in the Manufacture of Bleached Wood Pulp

Refiner mechanical pulping (RMP) uses chipsin lieu of roundwood and produces paper with higherstrength than conventional groundwood because ofless damage to the fibers in the pulping process. Thechips are passed through a refiner that has fixed androtating disks operating under a stream of water. Awider range of species, including hardwoods, can beprocessed by the refiner pulping process, Sawdustand other saw mill wastes can also be used (figure2-3).

Thermomechanical process (TMP) was devel-oped as a modification of the refiner mechanicalpulping process. In TMP, the wood chips aresteamed for several minutes under pressure andsubsequently refined in one or two stages. The ligninis softened by heating the wood chips with pressur-ized steam before they are refined (i.e., blended bypassing the fiber through rapidly rotating disks). Therefined wood pulp, although stiIl weaker thanchemical pulp, makes a stronger paper than ground-wood or refiner pulp with only a small sacrifice inyield but with large energy requirements. Somenewsprint is now produced wholly from ther-momechanical pulp, thus eliminating the need forthe addition of chemical pulp often needed forstrengthening paper made from mechanical pulp.

The neutral sulfite semichemical (NSSC) pulpingprocess is used at a number of U.S. mills to producecourser-grade products such as corrugated board,which has a yield of about 75 percent of the woodraw material. In NSSC pulping, wood chips aresoftened by briefly cooking them in a neutral sodiumor ammonium sulfite solution and then separatingthe fibers (defibrating) in a refiner (see also SulfitePulping below).

Recycling can effectively reduce the consumptionof both wood raw material and energy when used inconjunction with other mechanical pulping pro-cesses. It does so, however, with some sacrifice inpaper strength. Recycled pulp is manufactured fromwastepaper that is processed into paper stock. Asmall proportion of the paper stock (5 to 10 percent)is de-inked, usually with caustic soda-based chemi-cals. Most recycled paper, however, is pulpedwithout de-inking. Pulping is accomplished throughviolent agitation and shearing action performed athigh temperatures. The paper produced from recy-cled pulp is generally weaker than papers from

Figure 2-3-Refiner Groundwood Pulp Mill Process

1

SOURCE: AJIuI M. Sprhqor, l&S~EAWWWW c%@Vl: Al@wdP@NJnlAls&y(Nr# York, NY: John Wiley& Sarrs, 1986), p. 140.

virgin materials, because of the breakdown of theused fibers and loss of fiber bonding.


Three major developments in mechanical pulpingtechnologies show promise for improving pulpquality: 1) pressurized groundwood pulping, 2)chemical thermomechanical pulping, and 3) hard-wood chemical mechanical pulping. All of thesetechnologies have reached some stage of commer-cialization. Chemical thermomechanical pulping iscurrently used at several U.S. mills. Improvementsin mechanical pulping show promise for improvingthe quality (strength characteristics) of paper nowproduced by mechanical processes. The resultinghigher quality mechanical pulps may displace thekraft pulps that are currently mixed with mechanicalpulps to improve paper strength.

Pressurized Groundwood Pulping—h pressur-ized groundwood pulping, debarked logs are fed tothe grinding wheel through a heated, pressurizedchamber. The heat and pressure help separate thefiber, thus breaking fewer fibers in the grindingprocess and improving pulp quality. Paper producedfrom pressurized groundwood pulp is more tear-resistant than paper made from stone-ground pulp,but is slightly inferior to that of thermomechanicalpulp. Pressurized groundwood pulping may have thepotential for displacing some high-quality chemicalpulps in the manufacture of newsprint and otherprinting papers.

Chemical Thermomechanical Pulping— Chem-ical thermomechanical pulping involves treatingsoftwood chips with mild sulfite solutions to modifythe lignin and partially delignify the wood prior togrinding in a refiner. This “sulfonation” treatmentresults in paper with higher tear resistance thanthermomechanical, refiner, or stone-ground pulps.Pulp yields decrease slightly to between 85 and 90percent with chemical thermomechanical pulping,but these yields are still higher than chemicalpulping (40 to 56 percent).

Hardwood Chemical Mechanical Pulping— Me-chanical methods for producing pulp from hardwoodspecies involve pretreating hardwood chips withhydrogen peroxide or sodium hydroxide and proc-essing them like refiner mechanical pulps. Bothhardwoods and softwoods have been successfullypulped by this method, with fiber recoveries in the80 to 90 percent range. Pulp produced by hardwood

chemical mechanical pulping can be used to producenewsprint and printing papers.

Chemical Pulping

Chemical pulping involves treating wood chipswith chemicals to remove the lignin and hemicellu-lose, thus separating and cleaning the fibers. Delig-nification gives the fibers greater flexibility, result-ing in a substantially stronger paper (because ofgreater contact between the fibers in the finishedsheet) than can be manufactured from high-ligninfibers produced by mechanical pulping. Paperstrength and durability is gained at the expense offiber yield. Chemical processes may yield only halfthe fiber that can be recovered by the use ofmechanical pulping techniques.

Two major chemical pulping processes are cur-rently in commercial use: 1) kraft (sulfate) pulping,and 2) sulfite pulping. The kraft process dominatesthe pulp and paper industry, accounting for 76percent of the pulp produced for paper and paper-board in 1984.3 Paper produced from kraft pulpaccounts for most of the bleached boxboard andlinerboard used by the packaging industry (whichconsumes about 58 percent of the paper in the UnitedStates). Bleached softwood kraft pulps are oftenmixed with mechanical pulps to add strength tonewsprint and printing papers. Bleached hardwoodkraft pulps are added to bleached softwood pulp toimprove printability for specialty paper productslike magazine stock and coated papers. Both kraftpulp and sulfite pulp can be used for the productionof dissolving pulp, which is used for the productionof rayon and acetates.

Kraft Pulping—Kraft pulping involves treatingwood chips and sawdust with a sodium sulfide andsodium hydroxide solution (see figure 2-1). Thehighly alkaline chemical and wood mixture iscooked with steam under pressure (digested) forbetween 1 and 3 hours. Digestion may be either acontinuous process or treated in discontinuous‘‘batches.’ Most of the lignin and some of thehemicellulose is dissolved, leaving the remainingcellulose fibers separated.

The cooking liquor containing the dissolvedlignin and other extractives (black liquor) is routedto a chemical recovery plant where the lignin and


organic wastes are burned to produce energy neededin the pulping process. Valuable extractives (e.g.,turpentine, tall oil, and resin) are separated for saleas commodity chemicals. Process chemicals arerecovered with only a relatively small loss involume, and after replenishment with sodium salts,they are returned to the digester for reuse.

The brown pulp (brown stock) from the digesteris washed, screened, and passed through a battery ofcleaners. If the pulp is to be bleached, it is“thickened” by removing excess water and sentthrough a series of bleach operations. These can varywidely in the type of chemicals used and theirsequence. Bleached pulp is then ready for the papermaking process.

Both softwood and hardwoods can be pulped bythe kraft process. Fiber recovery is largely a functionof the wood species used, the time and temperatureof cooking, the degree of bleaching, and the paperstrength required. Generally, kraft pulp recoveriesfrom softwoods are approximately 47 percent forunbleached pulp and 44 percent for bleached.4

Hardwood recoveries range from 50 to 52 percent forunbleached kraft pulp to 50 percent for bleached.

Sulfitee Pulping—Lignin can be dissolved bysulfonation with an aqueous solution of sulfurdioxide and calcium, sodium, magnesium, or ammo-nium bisulfite cooked at high temperature andpressure in a digester (see figure 2-4). There are fourbasic sulfite pulping processes currently in commer-cial use: 1) acid sulfite, 2) bisulfite, 3) neutral sulfite,and 4) alkaline sulfite. The major differences be-tween the sulfite processes are the levels of acidityand alkalinity of the sulfite chemical solutions usedto break down the wood and remove the lignin.

Sulfite pulping processes are suitable only forspecies with low extractive contents (i.e., those lowin tannins, polyphenols, pigments, resins, fats, andthe like) because of the interference of thesesubstances with the sulfite pulping process. Al-though calcium is the cheapest sulfite base available,it forms insoluble compounds that cannot be re-claimed economically. Thus, calcium-based pulpingis seldom used. Because magnesium- and sodium-

based chemicals are recoverable, and ammonium-based chemicals are less expensive and can beburned without harmful environmental effects, theyare the most frequently used.

Sodium-based sulfite pulping can consist ofmultistage cooking, successive stages of whichdiffer in acidity. Because one stage optimizeschemical liquor penetration and the other the re-moval of lignin, more lignin may be removed withless fiber degradation, so that fiber yields are higher,fibers are stronger, and a wider range of woodspecies may be used. Sulfite pulping dissolves someof the hemicellulose as well as the lignin. Neutralsulfite pulping, using sodium and ammonium bases,recovers the largest proportion of fiber (75 to 90percent) of all the sulfite pulping methods.

Sulfite pulp is a light color and can sometimes beused without bleaching if high brightness is notrequired. Unbleached sulfite pulp is often blendedwith groundwood and other high-yield mechanicalpulps for strengthening newspaper stock. Sulfitepulp is easily bleached to very bright pulps forwriting and printing paper. It is also used for themanufacture of dissolving pulps (through the furtherremoval of hemicellulose) for the production ofviscose rayon, acetate fibers and films, plastic fillers,and cellophane.

Potential for New Pulping Technologies

The search for new pulping technologies andprocess improvements for established commercialtechnologies continues in the United States, Canada,Sweden, Finland, Japan, Germany, and elsewhere.In the United States, about $815 million is estimatedto have been spent on pulp and paper research anddevelopment in 1987.5 OTA could not determinewhat proportion of the R&D was directed atimproving pulping technologies. Nearly all R&D issponsored by the industry, with only $3 million(<0.4 percent) expended by the Federal Govern-ment.

Industry pulping R&D is largely focused onimproving established pulping and bleaching proc-esses rather than seeking new pulping technologies.Some of the research and development is driven by

4P.J. I-Iw~y, Cowri$on of Mills Ener~ Balance: Efects of Conventional Hydropyrolysis and ~ry Pyrolysis Recoveq system (Awlem WI:Institute of Paper Chemistry, 1978),

5Bat~]]e Memolj~ ~stitute, pro~le Leveb of R&D Expenditures in 1987: Forecau and Analysis (Columbus, OH: Battelle, 1986), P. 11.


the need for broadening the raw material base inresponse to concerns over forest resources. Restric-tions on water use and pollution control havecontributed to the impetus for seeking processimprovements.

Energy costs as reflected in both energy use by theindustry and their impact on the cost of chemicalshas led to process improvements in the past,although moderating energy prices have recentlyreduced these concerns. The emphasis on recyclingto reduce the massive problems of solid wastedisposal in metropolitan areas has also been anincentive to using more reclaimed material in papermanufacturing. Finally, the increasing cost of capitalto rebuild aging sectors of the pulp and paperindustry have fed the need for more R&D by theindustry.

There are several reasons why major advance-ments in pulp and paper technology appear to beglacial in comparison to some other more rapidlyadvancing technologies. First, the pulp and paperindustry is mature; the commercial technology,much of which was developed in the late 1700s and1800s, has undergone evolutionary change, andsatisfaction with the basic technology has led to littlereason to fix something that does not appear to bebroken. Concerns over future environmental prob-lems and competition from other materials couldchange this, and to some degree already has.

Second, R&D is fragmented by the emphasis onprocess improvement, therefore few scientists andengineers focus on new pulping processes. Inaddition, many researchers specialize in one pulpingprocess or another depending on the needs of aspecific firm; few are able to consider all technologi-cal options or innovations for improving pulp yieldor overall quality.

Third, R&D investment in incremental improve-ment in established processes is easier to sell tocorporate directors than risky, long-term, radicalchanges. Large existing investments in plant equip-ment stretch the amortization period of old equip-ment and slow the acceptance of new processes thatrequire substantial changes and alterations.

Finally, the absence of major government invest-ment in long-range, high-risk R&D to seek new,innovative pulping and bleaching technologies may

Figure 2-4—Sulflte Pulp Mill Process

Fresh water●

Wood ●

— - J - - - - :

SOURCE: Nan M. Sprhgar, /n&stMEnvhmantal Cmvfol: Pu/parWPqarltistry(Naw York, NY: John Wiby & Sons, 1986), p. 153.

limit the advancements that could be made throughcollective R&D efforts. Individual firms have littleincentive to undertake a major, long-term, high-investment R&D program to develop radically newtechnologies with uncertain payoff in the end,particularly in the current investment climate.

Organosolv Pulping (Ester Pulping)—Organo-solv pulping—sometimes called ester pulping—is atwo-stage process involving hydrolysis (decomposi-

.


tion of the wood by dilute acids or enzymes) and theremoval of lignin with an organic solvent, usually amixture of alcohol and water. The still experimentalprocess is suitable for both hardwoods and soft-woods. Sawdust as a byproduct from lumber manu-facture can also be pulped,6 Pulp recovery fromorganosolv pulping ranges between 50 and 60percent for hardwoods, and 40 and 45 percent forsoftwoods. Typical hardwood fiber recoveries com-pare favorably with those from kraft pulping.

Fibers produced by the organosolv process areweaker than those recovered by the kraft process.Thus, the papers produced from organosolv pulp aresuitable for uses where strength is not the mostimportant property (e.g., printing papers, fluff pulps,and dissolving pulp). Little waste is produced by theprocess, and low alcohols are recovered easily bydistillation, thus requiring relatively low capitalinvestment. 7 Commercial viability of this technol-ogy will require that markets be developed forbyproducts of the process.

A commercial demonstration plant using theAlcell process developed by Repap EnterprisesCorp. of Canada is currently under construction atNewcastle, New Brunswick. The 33-ton-per-daypilot plant will cost $65 million. The Canadian

government is underwriting half the cost of the plant.Alcell is an alcohol cellulose organosolv process.

Hydrotropic Pulping-Hydrotropic solutions areaqueous salt solutions that impart greater volubilityto slightly soluble substances (e.g., lignin) than doeswater at the same temperature. Sodium xylenesul-fonate, a hydrotropic salt, has been used experimen-tally to delignify wood.8 A hydrotropic pulpingprocess was patented by Ralph H. McKee in 1943.9

Laboratory pulping texts suggested that dissolutionof lignin with aqueous sodium xylenesulfonatesolutions of 30 to 40 percent had little or no effect onthe strength of the pulp and yielded a high alphacellulose content (important for dissolving pulp).

Pulping of poplar was conducted at temperaturesof about 150 °C for 11 to 12 hours. Tests yielded 52percent cellulose, compared to 47 percent fromcomparable sulfite pulp. Unlike sulfite or kraftpulping which uses contaminating inorganic chemi-cals, the lignin recovered through precipitation byhydrotropic pulping is relatively pure and is suitablefor conversion to other chemical products. Theprocess is not suited for resinous coniferous species,however, and comparatively little serious attentionhas been given to this process by the industry.

%edor N, Keinert, “Oranosolv Pulping With Aqueous Alcohol,” T@PIJ., August 1974, vol. 57, p, 99 et seq.TN. Sawyer, Stare OfN~ Pu@ng Processes: Problem and Perspectives (Madison, WI: U#S. %st hd~ts Ldxxatory, 1982), p. IQ scc ~sO

Raymond W. Young and Kenneth W, Baierl, “EsterPulpingof Wood: A Revolutionary Process,” Southerrt Pu@ & P~er, November 1985, pp. 15-17.sRalph H, MCKW, “u% of Hydrotropic solutions in Industry,” ]ndusrriaf and IMgimxring CIWRLW, VO1. 38, No. 4.1946 P. 382; ~ ~~ R~Ph

H. McKee, “Comparison of Wood Pulping Process,” Pulp and Paper Magazine of Canadu, vol. 55, No, 2, 1954, pp. 64-66,!IU.S. patent No. 2308564, Jm, 19, 1943.

Chapter 3


CONTENTSPage

CHLORINATED DERIVATIVES IN THE ENVIRONMENT-AN OVERVIEW... . . . 29DIOXIN . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31DIOXIN AND PULP AND PAPER MANUFACTURE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

National Dioxin Study . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32EPA/Paper Industry Joint Five-Mill Study . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

DIOXIN IN PULP AND PAPER PRODUCTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

BoxBox Page3-A. Detection Limits and Levels of Dioxin Contamination . . . . . . . . . . . . . . . . . . . . . . . . . . 32

TablesTable Page3-1. Major Chlorinated Derivatives Identified in Pulp and Paper Mill Effluents . . . . . . . . 303-2. Characteristics of the Bleached Kraft Pulp and Paper Mills Used in

the Five-Mill EPA/Industry Cooperative Dioxin Screening Study . . . . . . . . . . . . . . . . . 343-3. Concentration of TCDD and TCDF in Bleach Plant Wastewater . . . . . . . . . . . . . . . . . . 343-4. Mode of Environmental Release of TCDD and TCDF From Pulp

and Paper Mills . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 353-5. Safe-level Concentrations of TCDD in Paper Products . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

Chapter 3


Pulp manufacture, like most chemical processes,results in emissions, effluents, and solid residuesthat must be disposed of. This study focuses onchlorinated bleached pulp mill waste effluents andresidual chlorinated compounds in paper products(with emphasis on TCDD and TCDF), therefore airemissions and solid wastesl will not be consideredhere. Technologies for reducing the production ofchlorinated organics in the pulping process arediscussed in chapter 5.

CHLORINATED DERIVATIVESIN THE ENVIRONMENT—

AN OVERVIEWEffluents from bleached pulp mills contain a

variety of substances, some of which exhibit avariety of effects in biological tests, such as geno-toxicity, mutagenicity, or teratogenicity. These in-clude resin acids and fatty acids, chlorinated phe-nols, and other chlorinated organic substances. Thecomposition of bleaching effluent is extremelycomplex and varies from mill to mill depending onthe wood species being pulped, the pulping technol-ogy, bleaching reagents, and waste treatment sys-tems used.2 Comparatively little is known about theactual composition of mill waste effluents, althoughsubstantial scientific effort has been spent on re-search. A screening and verification survey of pulpand paper mill effluents conducted by the Environ-mental Protection Agency (EPA) tentatively identi-fied a number of chlorinated organic chemicals(table 3-l). The chlorinated components of the waste

stream consist mostly of simple phenols, phenolicand carbohydrate oligomers (high and low molecu-lar weight polymers), and neutral and acidic materi-als resulting from the breakdown of the phenolicrings in lignin.3

It has been estimated that no more than 10 percentof the total solids in the waste stream of a pulpbleaching plant contain chlorinated derivatives.However, their toxicity to aquatic biota has raisedconcerns among biologists.4 Chlorinated mill wasteshave only recently been focused on in the UnitedStates, although the toxicity of untreated, undilutedwaste to aquatic biota is well documented in thescientific literature. A technical committee of Envi-ronment Ontario, the provincial environmentalagency for Ontario, Canada, recently reviewed theavailable information on chlorinated organics anddioxin from mill waste. Based on its analysis, thecommittee recommended that Ontario adopt a long-term strategy aimed at completely eliminating theformation of organochlorines in kraft pulp mills.5

The committee also concluded, however, that ‘chlo-rinated dioxins do not represent an immediatedanger to human health and welfare,’ but it did notethat heavy fish-eaters consuming fish caught down-stream of some bleached kraft mills might exceedthe acceptable daily intake of TCDD.

Sweden and Finland, with pulp and paper millslocated adjacent to the Gulf of Bothnia and the BalticSea, have experienced environmental damage tomarine life from chlorinated organic substances

l$olld ~a~te disps~, p~lcul~ly tie dlspos~ ofcont~inat~ s~udgc from biologica] ~ea~ent pl~~, is ~ irnporf~l factor in the ultimate sO]utlO?l

of safely disposing of dioxin. U.S. EPA information on pulp mill sludge disposal provided to OTA by Karen Florini, Environmental Defense Fund, showsthat of the 104 bleached chemical pulp mills in the United States, 54 dispose of sludge in landfills, 20 use surface impoundments, 20 incinerate the sludge,6 convert it to compt 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 becausesome 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 haspublished several reports on related topics: Waste in Marine Environments, OTA-O-335, April 1987; From Pollution to Prevention: A Progress Reporton Waste Reduction, OTA-ITE-347, June 1987; Technologies and Management Strate@es for Hazardous Warte Control, OTA-M-197, March 1983;Serious Reduction of Hazardous Waste, OTA-lTE-3 18, September 1986.

2Bet~een 250 ad 3~ chemica]s have &n i&ntlfi~ in pu]p roil] effluen~. M~y of ~ese we chlorinated compounds. bena R. Suntio, WW) YingShui, and Donald Mackay, “A Review of the Nature and Propeflies of Chemicals Present in Pulp Mill Effluents, ” Chemosphere, vol. 17, No. 7, 1988,pp. 1249-1290,

J@l~n W. ~~e and Goran E. Annergren, ‘‘Chlorination, ” The Bleaching of P&p, Third Edition (Atlanta, GA: TAPPI, 1979), p. 69.41bid., p. 71.SEnvuoment ~t~o, Stopp[ng Water poll~ion at its Source ~ Kr@ Mill Efluents in Ontario, Repofl of [he ~chnic~ Advisory Committee, Pl,l]p

and Paper Sector, Municipal/Industrial Strategy for Abatement (Toromo, Ontario: Environment Omario, 1988), pp. 1-2.

-29-


Table 3-1-Major Chlorinated Derivatives Identified inPulp and Paper Mill Effluents

chlorobenzene1,1,1 Trichloroethanetrichlorophenol*2,4-dichlorophenoldichlorobromomethanechlorodibromomethanetrichloroethylenemonochlorodehydroabietic acid2,3,7,8-tetrachlorodibenzo-p-dioxin 1,2-dichloroethane *1,1,2,2-tetrachloroethane **chloroform ● *methylene chloride ● *trichlorofluoromethanetetrachloroethylene **3,4,5-trichloroguaiacol9,10-dichlorosteanic acid2,3,7,8-dibenzofurans*“ LI.sted as carcinogens in dws Four?h Annual Report on Caminogms, US. Department

of Health and Human Sedoes, Public Health service, Nabonal Toxuhgy Program,1965.

““ Listed as caroh+pms by the Natbnal Institute of Occupational Safety and Haalth,sea Office of Tectumlogy Assessment, Mantifying and %gtdating Caroinogana-Bac&ourrd Paper (Chelsea, Ml: Lawis Publishers, 1967), p. 64.

SOURCE: U.S. Envuonmental Protection Agency, Developrwnf Document for EffluentLinrifations Gutilinas and Starrdards br be Pulp, paper, and Paperboardarxl fhe Buil&ws’ P@ar and Board Mh--Pomt Sourcx Cakqpms, EPA440J1-82KW5 (Washington, DC: 1962), p. 46.

released into coastal waters.6 Sweden’s NationalEnvironmental Protection Board (Naturvardsverket)estimates that Scandinavian pulp and paper millscontribute between 300,000 and 400,000 tons ofchlorinated organic materials to the coastal waters ofSweden, Finland, and Norway annually.7 Evidenceof environmental harm in the estuaries of the BalticSea (including the accumulation of dioxin in theflesh of food fish), where water circulation andexchange are extremely slow, has led the SwedishGovernment to consider regulations to reduce theamount of chlorinated organic substances producedby bleached sulfate pulp mills by imposing regula-tions that require the use of oxygen bleaching andincreased use of chlorine dioxide bleach in place ofchlorine gas. Sweden is also considering steps to

promote the use of closed-cycle processes thatwould significantly reduce or even eliminate therelease of chlorinated wastes to the environment.

The Swedish experience, where biological wastetreatment is less prevalent, contrasts with that of theUnited States where nearly all bleached kraft pulpmills use secondary biological treatment to reducethe biological and chemical oxygen demands ofwastewater. In the course of biological treatment,many potentially toxic substances are removed andconcentrated in the treatment sludge.g Many Swed-ish mills, on the other hand, do not use biologicalwaste treatment, and discharge their effluent directlyinto the environment or use only primary wastetreatment.

The overall release of chlorinated organic com-pounds from pulp and paper mills has received lessattention in the United States up to now. Instead, themajor concern arose over TCDD and TCDF that areproduced along with other chlorination productsduring bleaching cycles and are considered to bepotentially harmful to human health.9 A similarpattern of concern over dioxin developed in Canada.A recent report on pollution from kraft mill effluentspublished by Environment Ontario warned againstfocusing too closely on dioxins as a result of mediapublicity, because it “may divert energies fromproductive avenues of pollution control into blindalleys of ill-conceived, routine, and expensive sur-veys of ‘dioxin’ concentrations."10

Products containing TCDD were at one time usedextensively as herbicides (agent orange, 2,4,5-T).They are also produced as byproducts from theincineration of municipal and industrial waste, thecombustion of wood in home furnaces, stoves, andfireplaces, metal smelters, and the incomplete com-bustion of dielectric fluids (PCBs) in electricaltransformers. The use of dioxin-containing materialsin industrial processes has since been significantly

~ommittee for the Gulf of Bothnia, Water Pollution Problems of Pulp and Paper Industries in Finland and Sweden, Report of the Special WorkingGroup, Naturvardsverkc\ Rapport 3348, in English (Solna, Sweden: Baltic Marine Environment Protection Commission, 1987), app. 3,

TNation~ Sw~sh Environment~ protWtion Board, Action Plan for Murine Po//ufwn (Solna, Sweden: Naturvardsverket, 1987), P 27.8A w=l].m~nt~~, Provr]y owral~ biOl@C~ Wrote ~eatment p]ant can remove 3010 so ~rcent of chlorin~ion 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 1988workshop indicates that it may be possible 10 remove up to Xl percent of the chlorinated organics with supplemental chemical treatment. Tlis work isstill experimental.

91‘Dloxins’ ~d ** fwms$ refer geneflc~ly t. ch]ofiat~ dibe~,o-p~a-diox ins (CDD) and chlorina[~ di~~fm~s (CDF), respectively, that haveone to eight chlorine substituents.

IOEnvironment htario, op. cit., nOIC 5, PP. 1-19.

Chapter 3—Environrmental Considerations ● 31

curtailed, but their inadvertent production throughchemical and industrial processes, and as combus-tion products continues.

DIOXIN 1l

Dioxin, as generally referred to, is 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). It is the mosttoxic of 75 chlorinated dioxins and over 135chlorinated furans (TCDFS). ’2 Dioxin is a byproductof, or a contaminant in, manufactured materials. Thechemical reactions and conditions under which theyare formed in the pulp mill are not completelyunderstood. TCDD was never produced as suchintentionally except in small experimental quantitiesfor research. In the United States, TCDD largelyearned its reputation as a “bad actor” in the AgentOrange controversies following the use of herbicidesas defoliants during the Vietnam War.

Agent Orange was a mixture of the herbicides2,4,5 -trichlorophenoxy acetic acid (2,4,5-T) and 2,4-dichlorophenoxy acetic acid (2,4-D). These samechemicals were extensively used in forestry andagriculture with little recognition of the health risksthat may be related to the dioxin that 2,4,5-Tcontained as an incidental ingredient (2,4,5-T is nolonger manufactured in the United States). Theexposure of military personnel to these chemicals inVietnam raised the consciousness of the publicabout the health risks of dioxins when returningveterans blamed a number of their health problemsand those of their families on past exposure to AgentOrange while serving in the military.

How serious a human health threat is the exposureto dioxins? On purely scientific bases, the questionof human risk has not yet been definitively an-swered. Epidemiological data are incomplete anddifficult to interpret. No study thus far has conclu-sively linked dioxins to the death of a human, or toa human disease other than chloracne, or conclu-

sively related exposure to dioxin to cancer (althoughthe carcinogenic potential is considered to be‘‘probable”), or to miscarriages .13 Abnormal behav-ior, genetic effects, immunological problems, enzy-matic disjunctions, and reproductive problems asso-ciated with dioxin exposure have been considered,but studies have not confirmed dioxins to be thecause.

With regard to certain-but not all—laboratorytest animals, dioxin has been shown to be extremelytoxic and lethal at low levels. Procedures forextrapolating from animal effects to humans arecontroversial. There is, however, sufficient scien-tific evidence to suggest that human exposure todioxin should be minimized within acceptable levelsof risk pending a better understanding of its healthimplications. The public perceives dioxins to bedangerous and a major health risk as a result ofpublicity surrounding the Agent Orange contro-versy, the Times Beach incident, the Love Canal,and problems related to the disposal of hazardouswastes. The regulatory agencies have opted for aconservative approach to regulating dioxins.

Research on rainbow trout, a species often used togauge the toxicity of chemicals, suggests that TCDDand TCDF can cause mortality, reduced growth, andabnormal behavior during the fishes’ early lifestages. TCDD was judged to be 10,000 times moretoxic than the pesticides endrin or toxaphene, whileTCDF was 1,000 times more toxic.14 Bioconcentra-tion factors (BCF) for TCDD were found to be muchhigher than originally estimated. TCDD accumu-lated to about 39,000 times the ambient concentra-tion of the water, and TCDF between 2,640 and4,449 times (but dioxin seems to concentrate in thegut and inedible parts of fish). It has also been shownthat similar preferential bioaccumulation or magni-fication of TCDD and TCDF occurs in aquaticbirds. 15 However, the effects of TCDD and TCDF on

I IT~~ dl~-~sion of tie human heal~ eff~~ of dioxin is not intended to & definitive OT analytical with regard tO the dangers of dioxin. Rather, itis an encapsulation of other recent surveys of existing knowledge about dioxin and its congeners.

]Q~sts on la~rtiv ~jm~s Su=est &at if t~ toxicity of 2,3,7,8 -TCDD is msign~ tie value of 1.0, the toxicity of 2,3,7,8 -TCDF is 0.1. Other dioxinsand furans also generally have toxicities that are estimated to range from one-tenth to one-thousandth that of 2,3,7,8 -TCDD.

13us. Envir~nm~nt~ ~otwtlon Agency, ~~t~o~~ f)j~xin st@, EpA/530-sw-&7-025° (Washington, DC: 1987), pp. l-~.Iqpaul M. Mehrlc et d.. “Toxicity and Bioconcentration of 2,3,7,8- Tetrachlorodibenzodioxin and 2,3,7,8 IM.rachlorodibenzofuran in Rainbow

Trout, ” Environmental Toxicology and Chemistry, vol. 6, 1988, pp. 47-62.15D.L, Stalling et d., “Pattetns of PCDD, PCDF, and PCB Contamination in Great Lake Fish and Birds and Their Characterization by Principal

Component Analysis, “ Chemosphere, vol. 14, No. W, 1985, PP. 627-643.


reproduction, survival, and behavior of bird popula-tions is uncertain.16

DIOXIN AND PULP AND PAPERMANUFACTURE

National Dioxin Study

The National Dioxin Study (NDS), a 2-year effortto explore the extent of dioxin contamination in theenvironment, detected the presence of TCDD andTCDF in fish and bottom sediment samples col-lected downstream of several U.S. pulp mills.17 Fishsamples for the NDS were selected in three ways:

● 90 sites were selected statistically,. 305 sites near urban and industrial areas were

nominated by EPA's Regional Offices or theOffice of Water Regulations and Standards(OWRS), and

. 57 estuarine or coastal sites were sampled.

Of the 90 sites sampled statistically for fishcontamination, 17 showed detectable levels ofTCDD up to 19 parts per trillion (ppt) in compositewhole fish samples (see box 3-A). Nearly one-thirdof the 305 regional samples showed detectablelevels of dioxin in whole fish samples. These samplesites included rivers, lakes, and some coastal andestuarine waters. TCDD levels in some samplesranged up to 85 ppt. Only 4 of the 57 estuarine andcoastal sites sampled had detectable levels of dioxinin fin fish or shellfish, and these ranged between 1.08and 3.5 ppt.

About 80 percent of the whole fish sampled fromsites in the Great Lakes were found to havedetectable levels of dioxin. A multitude of potentialdioxin sources are within the watershed of the GreatLakes and the turnover and flushing of the waterswithin the Lakes are extremely slow, Outside theGreat Lakes, detectable dioxin levels were mostfrequently found in the major river systems that flowthrough industrial and urban areas. Advisories havebeen issued by Wisconsin, Maine,, and Louisianawarning of possible risk of eating contaminated fish.

Box 3-A—Detection Limits and Levels ofDioxin Contamination

Determining the amount of dioxin in the naturalenvironment requires sophisticated analytical pro-cedures and careful statistical sampling and samplepreparation, High resolution gas chromatographyand mass spectrometry are used to quantify dioxinlevels. Gas chromatography separates the dioxinsfrom other compounds by selectively adsorbing thedioxins (based on their specific molecular weights)on an adsorbent such as activated charcoal, alu-mina, or silica gel. Mass spectrometry separates thedioxins specifically and quantitatively according totheir atomic weights. These technologies can meas-ure dioxins in the range of parts per quadrillion(Ppq) in water samples, but this level of detection isstill considered experimental for most biologicalsamples.

EPA selected a nominal detection limit of onepart per trillion (ppt) for fish and soil samplescollected in the course of the National DioxinStudy. This sensitivity pushed the limits of thestate-of-the-art in analytical technology. Compre-hending parts per quadrillion and parts per trillioncart boggle the mind. One ppt is equivalent to 1second in 32,000 years. One ppq is equivalent to 1second in 32,000,000 years. The potency of dioxinmakes measurements at this minute level of resolu-tion important.

The Food and Drug Administration (FDA) rec-ommends that consumption of fish be limited ifdioxin content exceeds 25 ppt, and consumption isbanned when levels reach 51 ppt. In general, EPAfound that dioxin levels of fish fillets-the edibleportions of the fish-had dioxin levels below thedetection limits even though whole fish samplesmay be judged to be contaminated.

At two-thirds of the sites where dioxin wasdetected, the maximum values encountered werebelow 5 ppt. At only four sites did dioxin levelsexceed 25 ppt (the level at which FDA recommendsthat fish consumption be limited). The high-levelsites were located in the Androscoggin River atLewiston, Maine (29 ppt), and the Rainy River at

16J,E@ El]iot et & ‘‘Levels of Polychiorinated Dibenzodioxins and Polychlorinated Dibenzofurans in Eggs of Great Blue Herons (Ardea herodias)in British Colurrtbia, 1983-1987: Possible Impacts on Reproductive Success, ” Progress Notes No, 176, Canadian Wildlife Service, April 1988, p, 7.

I’IIJ+SO ~viroment~ ~o~tlm Agency, Na(iona/ Dioxin Study, EPA/530-sw-87-025 (Washington, DC: 1987), pp. 111-32-33.

l~Ibid., pp. Ill-29.

Chapter 3—Environmental Considerations ● 33

International Falls, Minnesota (85 ppt), both ofwhich are located downstream of pulp and papermills.19 Additional investigations at these sitesshowed dioxin levels of up to 414 ppt in wastetreatment sludges from the mills.

As a follow-on to the NDS, EPA is investigatingother chemical pollutants that might accumulate infish. The National Bioaccumulation Study (NBS),which is currently underway, is focusing on a subsetof “priority” pollutants selected from among 400potential chemicals. These include non-conven-tional pesticides, semi-volatile organic chemicalsknown to accumulate in human fatty tissue, agricul-tural chemicals, industrial chemicals, and those inpulp mill effluents. Four hundred sites are beingsampled in targeted industrial, urban, and agricul-tural areas and below pulp mills. Approximately 95percent of the fish samples have been collected. Ofthe 75 samples that have been analyzed from fishcollected below pulp mills, 67 are reported to havedioxin above detectable levels. Samples from 10mill sites report TCDD and TCDF concentrations infish fillet tissue above the acceptable FDA limits of25 ppt.20

Recent data from the NBS based on fish sampledfrom 18 southern rivers receiving mill wastesshowed accumulations of dioxin in whole fishranging from about 1 ppt up to 164 ppt. Most wholefish samples had dioxin levels between 10 and 40ppt. Three whole fish samples had levels exceeding100 ppt. The edible part of the fish (fillets) containedmuch lower dioxin levels and none exceeded FDA’s25 ppt acceptable limits, although one fish showeda level of 24 ppt of dioxin.21

EPA/Paper Industry Joint Five-Mill Study

The results of the National Dioxin Study indicatedthat effluent from the manufacture of pulp canintroduce detectable levels of dioxin into the envi-ronment. This prompted the U.S. pulp and paper

industry, through the National Council of the PaperIndustry for Air and Stream Improvement, and EPAto undertake a joint investigation of five bleachedkraft pulp and paper mills in 1986.22 The cooperativescreening study focused on three mills known tohave dioxin in their waste sludge (all of the millssampled used the activated sludge waste treatmentprocess) plus two additional mills that were volun-teered by their firms to provide broader geographicalcoverage (table 3-2). The results of the cooperativefive-mill study indicated that the bleaching of kraftpulp with chlorine and chlorine derivatives isresponsible for the formation of 2,3,7,8 -TCDD and2,3,7,8 -TCDF as byproducts of the pulping process.

Dioxin in Bleached Pulps

Sensitive gas chromatographic procedures wereused to distinguish between the amounts of 2,3,7,8-TCDD and the related isomers of chlorinated furan(2,3,7,8-TCDF) in the bleached pulp and millwastes. TCDD was detected in seven of ninebleached pulps sampled, at levels up to 51 ppt. Themedian TCDD content was 4.9 ppt, and the meanwas 13 ppt.23

TCDF was found in eight of nine pulpsamples at levels ranging from below detectionlimits to 330 ppt. The median TCDF content was 50ppt, and the mean was 93 ppt.

Dioxin in Bleach Plant Wastewaters

Wastewater from each stage of the pulp bleachsequence was systematically sampled at each mill.TCDD was detected in wastewaters at three of thefive mills, and TCDF was detected at four of the fivemills sampled. The greatest discharge of both TCDDand TCDF was associated with the caustic extractionstage, which serves to flush away the lignin andother coloring agents that are mobilized during thebleaching stages. Lesser amounts of TCDD andTCDF were detected in the wastewaters of thehypochlorite bleaching stage and the chlorinationbleaching stages (see table 3-3).

lgIbid., pp. 111-30.Zostcven ~oner, us. Environment~ ~otmtion Agency, unpublished materials presented at OTA dioxin workshop. W=hjww ~. NOV. IA-ls~

1988,21Nati~n~ Bio~c~~~jon study dam pvjd~ w OTA by K~en flofinj, &vkonmen@ Defense Fund, Washington, ~, Feb. 6, 1989.2~, Amendola et ~., “The Occurrence and Fate of PCDDS and PCDFS in Five Bleached Kraft Pulp and Paper Mills,” presented at the Seventh

International Symposium on Chlorinated Dioxins and Related Compounds, Las Vegas, NV, October 1987.zsIbid., p. 8.


Table 3-2—Characteristics of the Bleached Kraft Pulp and Paper Mills Used inthe Five-Mill EPA/industry Cooperative Dioxin Screening Study

Daily

Dailyeffluent

productionFurnish (in percent) capacity Bleach sequences (million

Mill Hardwood softwood (tons) Hardwood softwood gallons)I . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85 15 500 CEOH & CEOHHP 23

CE0HHPII . . . . . . . . . . . . . . . . . . . . . . . . . . 20 775a CEHED CEHED 36Ill . . . . . . . . . . . . . . . . . . . . . . . . . . 100 NA 1,000 C/DEoD 36Iv . . . . . . . . . . . . . . . . . . . . . . . . . . NA 100 4 0 0b

— CEH— 18v . . . . . . . . . . . . . . . . . . . . . . . . . . 30 70 1 ,200’ CDEoH/D CDEOH/D 41NA = Not applicablea CM alafj wOdUq 300 tons par day of reftner mechanical gmu~ pulp.b H- a~b~ my tO pdIJ(X 8* hfW PIN *Y of ~u~.c Also can produce 130 tons of groundwwd daily.

SOURCE: G. Amendoia et al., “The @cwrence and Fate of PCDDS and PCDFS in FIVW Bbached Kraft Pulp and Paper Mills, -paper presented at the Seventh international Symposiumon Chbrlmted Dioxins and Related Compounds, Lea Vegas, NV, October 1987.

Table 3-3—Concentration of TCDD and TCDF in Bleach Plant Waetewater

TCDD (ppt) TCDF (ppt)Bleaching stage Range Median Mean Range Median MeanChlorination . . . . . . . . . . . . . . . . . . 0.01-0.24 0.04 0.07 0.06-3,8 0.24 0.65Caustic wash . . . . . . . . . . . . . . . . . 0.01-3.6 0,24 1,00 0.06-33.0 0,78 7.4Hypochlorite ., . . . . . . . . . . . . . . . 0.02- 1.9 0.20 0.40 0.09-9.2 0.59 2.3Chlorine dioxide . . . . . . . . . . . . . . 0.01-0.03 ND ND 0.01-0.13 ND NDND= Not detectabla

SOURCE: G. Amendola et al., “The &currerm and Fate of PCDDS and PCDFS in Five Bleached Kraff Pulp and Paper Mills,” paper presented at the Seventh Internatioml Symposiumon Chbrinated Dioxms and Related Compounds, Las Vegas, NV, October 1987.

Dioxin in Wastewaters and Sludges

Comparisons among the mills indicated that theTCDD and TCDF contents of pulp and wastewaterdiffered greatly from mill to mill. TCDD producedin the pulp bleaching process can be transported tothe environment as a residual in finished pulp, in thesludge recovered in the wastewater treatment proc-ess, or as treated effluent released into streams andpond (table 3-4). TCDD was found in wastewatertreatment sludges at each of the five mills sampled.24

Analyses of wastewater from the paper machinesshowed that some of the dioxin produced in thebleaching process was passed through to the papermaking process.

Continuing Efforts

Although the five-mill cooperative survey con-fined 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, wastesludge, and treated effluents. The results demon-strated the need for a comprehensive and systematicsurvey of the receiving waters and biota below thewaste outlets of U.S. pulp mills in order to betterunderstand the scope and intensity of the environ-mental loading of TCDD, The survey also indicatedthat more detailed information was needed aboutdioxin levels at various steps in the pulping processand in the bleaching sequence.

EPA has recently negotiated a cooperative agree-ment with the American Paper Institute (API) andthe National Council of the Pulp and Paper Industryfor Air and Stream Improvement (NCASI), bothassociated with the U.S. pulp and paper industry, tosurvey all 104 domestic pulp mills that manufacturechemical bleached pulp for production of dioxin and

24u,s. Envi~~ent~ ~o~c(jon Agency, U.S. EPAIP~er Industry Cooperative Dioxin Screew”ng Study, EPA440/1-88-025 (Washington, DC:1988), p, viii.

Chapter 3—Environmental Considerations ● 35

Table 3-4-Mode of Environmental Release of TCDD and TCDF From Pulp and Paper Mills (percent)

MillSource I II Ill Iv v

2,3,7,8-TCDDBleached pulp . . . . . . . . . . . . . . . . 19 66 — 30 57Waste sludge . . . . . . . . . . . . . . . . 16 16 100 70 22Treated effluent . . . . . . . . . . . . . . . 65 18 — — 21

2,3,7,8-TCDFBleached pulp . . . . . . . . . . . . . . . . 19 5 6 6 0 31 5 5Waste sludge . . . . . . . . . . . . . . . . 17 20 36 69 22Treated effluent . . . . . . . . . . . . . . . 64 24 4 — 23SOURCE: G. Amencbla et at., “The occurrence and Fate of PCDDS and PCDFS m Five Bleached Krafl Pulp and Paper Mills,” paper presented at the Seventh International Sympowm

on Chbrinated Dioxms and Related Compounds, Las Vegas, NV, October 1987.

furan isomers.25 In addition, an industry study will

undertake a detailed analysis of dioxin levels andbleaching processes at selected pulp mills.

The study will consider the full range of factorsthat might affect the production and dispersal ofdioxins, including annual effluent flow, wastewatertreatment, sludge disposal practices, bleach plantoperations, and an analysis of dioxin contents ofeffluents, pulps, and sludges. Detailed analyses ofdioxin levels in 25 bleach lines will be included. Thestudy began in the summer of 1988, and is expectedto be completed in the summer of 1989.

In a related effort, an EPA-led interagency grouphas undertaken a Multi-Media Risk Study that willutilize data collected in the EPA/Industry 104-millstudy. Under a court-approved consent agreement toconsider dioxin in paper, EPA is attempting toestimate the cumulative risk of dioxin from thissource in all media—pulp, sludge, and effluent.26

Data collected in the 104-mill study, and estimatesof migration rates of TCDD and TCDF from paperproducts adjusted by product-use scenarios, will beused to determine whether or not there is a humanrisk from dioxin in pulp and paper.27 The consentdecree also requires EPA to undertake an assessmentof environmental risks as well as human risks. TheFood and Drug Administration (FDA) and Con-

sumer Safety Product Commission (CSPC) haveinitiated product risk assessments that will becomepart of the interagency multi-media study.

DIOXIN IN PULP AND PAPERPRODUCTS

The finding of TCDD and TCDF in bleached pulpsamples raised questions as to whether residualTCDD might also find its way into finished paperproducts and present a potential health risk toconsumers through dermal contact (it did not con-sider other routes of exposure). The NCASI commis-sioned Envirologic Data, Inc. to assess the potentialrisks28 to human health from skin exposure to avariety of bleached pulp products, including dispos-able diapers, facial tissue, toilet tissue, sanitarynapkins, and paper towels.29

The results of Envirologic Data’s risk assessmentof concentrations of TCDD found in paper productswere related to a lifetime cancer risk of one in amillion in the general population—a regulatorystandard commonly used by the EPA and the FDAto 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).

25sW Federa/ Regi~fcr, “o]. 53, No. 27, Feb. If), 1988, p. s~sv; EPA office of Water Regulations and Standards, and office of Toxic Substances,

U.S. EPA/Paper Lndustry Cooperative Dioxin Study—Tier 1, Fact Sheet, Feb. 4, 1988.26Sce, Envi~o~~al Defe~e F~ & Nanoui Wildlife Federation v. Thomas, No. 85-0973 (D. D. C., consenl decree entered JUIY 27, 1988).27Dw~n Winter, us. EnViron~ent~ Protec[lm Agency, comrn~ication at he o’fA dioxin workshop, Washington, DC, NOV. 14-15, 1988.

28R1sk awewment is me ch~ac(eri~~ion of tie probability of plenlia]]y adverse hcatth eff~ls from human exposure [0 CtIVlrOtMTICIItd h~~ds. Therisk assessment process consists of four steps: 1) hazard identification, 2) dose-response assessment, 3) exposure assessment, and 4) risk chmactcrization.NCASI used EPA’s guidelines for Carcinogen Risk Assessment as a framework for the dioxin dermal exposure study.

2gNat10n~ Cowci] of tie paPr lndus~ for Air ~d s~e~ Improvement, ASScSS~nt of poteWia/ Health f?isks From Demal Exposure /0 Dioxinin Paper Products, ‘fkchnical Bulletin No. 534 (New York, NY: 1987), p. 107.


Table 3-5—Safe-level Concentrations of TCDD in Paper Products

Calculated TCDD equivalent (in ppt)Female Male Measured

Bleached pulps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5’Communication paper . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13b

Clerical worker . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9,000 9,100Manager .,.... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4,200 4,300

Personal care productsDisposable diapers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0c

Conventional . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 540,000 540,000Superabsorbent . . . . . . . . . . . . . . . . . . . . . . . . . . . 2,000,000 2,000,000

Facial tissueNormal use . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66,000,000 79,000,000Makeup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 230,000

Toilet tissue . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27,000,000 65,000,000Sanitary pads . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63,000,000Paper towels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7,900,000 9,500,000Composite personal care productsd . . . . . . . . . . . . . 160,000 510,000

Combined communication papers andpersonal care products . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4Clerical worker . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8,500 8,900Manager . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4,100 4,200

a~a~ur~ln7Nlp6~thlew16~d~xin~mle=~ lppft051P@,*tiamfrdianOf4,9fJP~

b~sutilnkfrdwmr.

cNo TCDDdatectad in dqxu4Medlaprs at datactionlimltsof 2.1 and2.6ppt.dExdudjWWWm~~t ctisposa~e ctiapers.

SOURCE: NatiornlCounctioftfm Paperlrrdus~forAirandStream lm~owmenL X~smenro/Po@@a/~/ti~sbFrom@rma/EX~mtoMXj~bpPpm@ts, Techn&alSulletrn No. S34(NewYork,NY: 1967~p107.

Calculated safe levels for bond paper, newspaper,and other paper used for communications rangedfrom 4,200 ppt of TCDD for female managers to9,100 ppt for male clerks. Safe levels for personalcare products were calculated to range from 230,000ppt of TCDD for female facial tissue used formakeup removal to 79,000,000 ppt for facial tissueby males. Safe levels for paper towels were calcu-lated at 7,900,000 ppt for females and 9,500,000 pptfor males. Toilet tissue safe limits for females werecalculated at 27,000,000 ppt for females and 65,000,000ppt for males. Actual concentrations of TCDD insamples of bond paper were detemined to be about13 ppt: in paper towels, 4 ppt: and no detectableTCDD was measured in disposable diapers.

NCASI published the results of an assessment ofpotential exposure to dioxin from coffee brewedusing bleached coffee filters in May 1988.30 Basedon an assumed consumption profile for an averageand heavy coffee drinker and assuming that 65 to 90

percent of the dioxin migrated from the filter,NCASI concluded that the calculated dioxin TEQsfor a risk ranging from zero to one in one-million tobe between 20 ppt TCDD TEQs for average coffeedrinkers to 11 ppt TCDD TEQs for heavy coffeedrinkers. The coffee filters tested had TEQ dioxincontents ranging from 2.2 to 6.6 ppt.

An assessment of dioxin in food packaging paperproducts has been initiated.31 NCASI commissionedENVIRON, Inc. to undertake the evaluation. Anumber of food contact products are scheduled to beundergo risk assessment, such as paper cups andplates, convenience food packaging, paper towels,and pizza boxes. NCASI has put the project on holdpending the development of acceptable test proce-dures to determine the absorption of dioxin into fattyfoods. The assessment will resume when test proto-cols are developed

Scientists at Health & Welfare Canada, a govern-ment agency, reported in August 1988 that they had

sON~ion~ ~W1l of tie Pavr [ndu~ for Air and Stream Improvement, Assessment of the Risks Associated With Potential Exposure to DioxinThrough the Consumption of Coffee Brewed Using Bleached Paper Coffee Filters, Technical Bulletin 546 (New York, NY: 1988), p. 34,

31Nation~ COUCII of ~e pawr lndu~t~ fm Air ad s~e~ ]rnp~vcrnent, First progress Report on the ASSeSSrne~ of Potential Health Risks FromUse of Bleached Board and Paper Food Packaging and Food Contact Products, Special Report 87-11 (New York, NY: 1987), p. 27.

Chapter 3--Environmental Considerations ● 37

detected 0.04 ppt of TCDD and 0.75 ppt of TCDF in packaged in non-bleached paper containers. NCASIwhole milk packaged in plasticized bleached paper is currently collaborating with Canadian scientists tocartons. 32 They r epoted no similar level in milk confirm these findings.

Chapter 4


CONTENTSPage

THE BLEACHING PROCESS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41Historical Development of Bleaching Technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41Extent of Bleaching in the Industry.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42Bleaching M ethods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42Bleaching Systems. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46

BoxesBox P a g e4-A. What Is Pulp “Brightness’’? How Is It Measured?. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 424-B. Shorthand for Describing Bleaching Sequences . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 444-C. Assessing Lignin Content and Pulp Bleach ability . . . . . . . . . . . . . . . . . . . . . . . . . 46

FiguresFigure Page4-1. Bleaching Sequences Used in U.S. and Canadian Pulp Mills. . . . . . . . . . . . . . . . . . . . . . 444-2. Sequence for the Production of Fully Bleached Chemical Pulp. . . . . . . . . . . . . . . . . . . . 474-3. Pulp Brightness at Stages of the Bleaching Sequence CEHDED. . . . . . . . . . . . . . . . . . 484-4. Schematic Diagram of the Chlorination Process .,.,... . . . . . . . . . . . . . . . . . . . . . . . . . . 50

TablesTable Page4-1. Domestic Bleached Pulp Capacity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 434-2. Bleaching Chemicals: Form, Function, Advantages, Disadvantages . . . . . . . . . . . . . . . 444-3. Examples of Prebleaching Sequences for Pulp Delignification. . . . . . . . . . . . . . . . . . . . 484-4. Common Sequences Used To Bleach Kraft Pulp to Various Degrees of Brightness. 49

Chapter 4


THE BLEACHING PROCESSBleaching is the treatment of cellulosic fiber with

chemicals to increase brightness (see box 4-A).Brightness may be achieved by either lignin removal(delignification) or lignin decolonization. Ligninremains a major constituent of pulp even afterdigestion by chemical pulping. For example, kraftpulp may contain up to 6 percent lignin based on itsdry weight.1 Unbleached groundwood spruce pulpmay contain 27 percent lignin.

If chemical pulping removes the lignin from woodfibers, why then does some lignin remain after thepulping process? The strength of paper is largely dueto the chemical bonds (hydrogen bonds) formedbetween cellulose fibers. Although longer and moresevere pulping might remove more of the lignin, thusreducing the amount of bleaching needed, thecellulose molecules might be degraded and theirbonding power diminished. Should this happen, thestrength of the pulp would be reduced. The removalof lignin by bleaching is regarded as a continuationof the pulping process, albeit somewhat gentler andless destructive, but bleaching too can degradecellulose if done improperly.

Lignin imparts a color to the raw pulp (hence itsname “brown stock”) and unless removed, willcontinue to darken with age (note the yellowing,darkening, and enbrittlement of newspaper exposedto sunlight). Bleaching by removing the lignin giveshigher brightness to the paper than is possible byleaving the lignin in the pulp and brightening bydecolonization, and also leads to a more durable andstable paper.

In addition to the removal and decolonization oflignin, bleaching serves to clean the pulp of dirt andforeign matter that escaped the digestion process.Bleaching also removes hemicellulose and extrac-tives (hemicellulose is nearly completely removedfor the production of dissolved pulps). Bleachingpulp adds significantly to its value as market pulpbecause the demand for bleached paper is increasing.

Historical Development of BleachingTechnology

Bleaching of fibers for decolonization has beenpracticed since early times. Sunlight was one of theearliest bleaching agents. Japanese paper makerswere known to bleach fibers by soaking them inwater from high mountain streams that containedozone (the first use of oxygen for bleaching). In1774, Karl Wilhelm Scheele discovered chlorineand its bleaching action on vegetable fibers. Severalyears later in 1799, Charles Tennant invented‘ ‘bleaching powder’ (calcium hypochlorite),thereby converting chlorine to an easily transport-able form. For the next 130 years it remained theonly available bleaching agent. The first time a U.S.paper mill used chlorine for bleaching was in 1804.

Rapid developments in bleaching technologyoccurred between 1900 and 1930. Multistage bleach-ing using calcium hypochlorite followed by analkaline extraction stage, then a repeat of thehypochlorite bleach stage was first adopted by theindustry. Later, technologies that allowed the use ofgaseous chlorine began to displace hypochlorite inthe first bleach stage. The use of chlorine reducedbleaching costs. New equipment developments fur-ther improved bleaching efficiency.

Improvements in the manufacture of chlorinedioxide and dioxide bleaching technology weredeveloped in the 1940s. By the 1950s, thesedevelopments led to the adoption of the five-stagebleaching sequence that is still used extensively inthe industry: (1) chlorine+(2) alkaline extrac-

sequence allowed very bright pulp to be producedwith minor losses in fiber strength.

Oxygen bleaching was discovered in 1952 byV.M. Nikitin and G.L. Akim in the Soviet Union. Inthe late 1960s, oxygen bleaching was commercial-ized, followed by the installation of the first dis-placement bleach plant in the 1970s. This resulted inmore rapid bleaching by displacing chemicalsthrough a pulp mat rather than mixing the chemicals

1 Douglas W, Reeve, ‘‘The Principles of Bleaching, 19/37 Bleach Plant Operations Seminar, TAPPI Notes (Atlama, GA: TAPP1 Press, 1987), p, 3.

-41–


Box 4-A—What Is Pulp “Brightness’y? How Is It Measured?“Brightness’ is the reflecting properties of a sheet of pulp. It is a physical and measurable phenomenon. Some

mistakenly equate “whiteness” with brightness, but whiteness is a physiological phenomenon, measuredsubjectively by the impression and perception of the human eye. For instance, if blue dye is added to a lightly yellowtinted paper, the sheet will look whiter but the sheet will measure less reflected light (less brightness).

Since reflectance is affected by the nature and the angle of incident light, the surface properties of the pulpsheet, and other factors, the measurement of brightness has been standardized: Brightness is the reflectance of bluelight with a spectral peak at 457 millimicrons from an opaque sample of pulp sheets compared to a specified standardsurface.

There are two recognized methods for measuring pulp brightness in North America: 1) the TAPPI method(Technical Association of the Pulp and Paper industry), Standard T-452; and 2) the CPPA method (Canadian Pulpand Paper Association).

TAPPI MethodReported in units of %GE Brightness. Illuminating light is aimed at 45 degrees to the sample and the reflected

light is measured perpendicular to the sample (90 degrees). Reflectance is compared to magnesium oxide powder(98 to 99 percent absolute reflectance). Calibrated opal glass standards are used for routine measurements.

CPPA MethodReported in units of ISO Brightness. The sample is illuminated with diffused light using a highly reflecting

integrating sphere. Reflected light measurement is taken at 90 degrees to the sample. Reflectance is compared toabsolute reflectance from an imaginary perfectly deflecting, perfectly diffusing surface. Calibrated opal glassstandards are used on a routine basis.

A third brightness standard is used throughout the rest of the world: The Zeiss Elrepho standard. It is measuredin units termed Elrepho Brightness. GE Brightness is measured with a reflectance meter manufactured by theGeneral Electric Corp., while Elrepho Brightness is measured by an instrument manufactured by Zeiss, the Germanoptical company. Since the two meters have different light geometries, there is no simple relationship between thetwo 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 States as well.2 Overall, the tendency has been to1970s development has taken place in the use ofoxygen enrichment in alkaline extraction stages, tofurther delignify pulp after extended cooking (modi-fications of the cooking process to improve deligni-fication), and in short bleaching sequences whereoxygen is used to supplement chlorine.

Extent of Bleaching in the Industry

Nearly 55 percent of the chemical pulp currentlyproduced in the United States is bleached (table 4-l).By far the greatest proportion of bleached chemicalpulp is produced by the kraft process (about 88percent of the pulp bleached in 1987 was kraft pulp).Very little mechanical pulp has been bleached in thepast, however, this is currently changing. Mechani-cal pulp bleaching is growing at more than twice therate of chemical pulp bleaching worldwide and it islikely that this trend will continue in the United

bleach more pulp as the demand for products usingbleached paper increases (nearly 39 percent of thedomestic paper and paperboard currently producedis from bleached pulp).

Bleaching Methods

Bleaching Agents

Pulp cooking can safely dissolve up to about 90percent of the lignin without degrading the cellulosefiber. Additional delignification is done by bleach-ing. Bleaching of high-yield chemical pulps isachieved by decolonizing with either an oxidizingagent (combines oxygen) or a reducing agent(combines hydrogen). Chlorine gas, sodium hypo-chlorite, chlorine dioxide, oxygen gas, and hydrogenperoxide are oxidants. Sodium hydrosulfite is areductant. Alkali is used to remove the solubilized

zIbid., p. 2.

Chapter 4—Pulp Bleaching Technology ● 43

Table 4-l-Domestic Bleached Pulp Capacity(thousand metric tons)

may be offset by the cost of the chemical or theequipment needed to handle it.

Grade 1971 1987’Bleached sulfite ... , . . . . . . . . . . . . . . . . 1,774 1,256Bleached kraft. . . . . . . . . . . . . . . . . . . . . . 13,364 21,259Dissolving (bleached) ., . . . . . . . . . . . . . 1,604 1,455

Total chemical pulp produced . . . . . . . 32,779 43,800Percent of chemical pulp bleached . . 51 .lO/O 54.70/0

a EstimateSOURCE: Oouglas W. Rowe, “The Prmctples of Sfeaching,” 1987 EUauch Pknt

C@rations Seminar, TAPP/ Notes (Alfanta, GA. TAPPI Press, 1987), p. 2,

lignin from the cellulose. Each has its advantages,disadvantages, and limitations (table 4-2).

Since the 1930s, chlorine gas has been thepredominant chemical used for the delignification ofpulp. Chlorine dioxide can brighten pulp withoutdamaging the cellulose. Oxygen is comparativelyinexpensive and is now coming into its own both fordelignification (immediately after digestion andbefore the bleach cycle) and as a supplement in thefirst extraction (alkali) stage of the bleach sequence.Hydrogen peroxide is expensive, so it is used muchless than other bleaching agents. The effectivenessof a bleaching agent, although a major factor indetermining its use in a pulp bleaching sequence,

A critical determinant in choosing a bleachingchemical is the ‘‘selectivity” of the agent. Selectiv-ity refers to the capacity of the chemical to attacklignin while doing minimal damage to the cellulosefibers. Unbleached pulp (brown stock) contains highlevels of lignin, therefore less selective chemicals(e.g., oxygen and chlorine) can be used in the initialstages of the bleach cycle. With further delignifica-tion and lower residual lignin content of the pulp,more chemical is available to react with the celluloseand pulp strength may suffer.

Chlorine dioxide and hydrogen peroxide arehighly selective, thus they react rapidly with ligninbut affect cellulose very little. The highly selectivechemicals are generally used in later bleach stageswhen the lignin content is low and the cellulose issusceptible to degradation. However, both chemi-cals are expensive and are therefore used sparingly.Sodium hydrosulfite, a reducing agent, and hydro-gen peroxide are used for bleaching lignin-richmechanical pulp.

Table 4-2-Bleaching Chemicals: Form, Function, Advantages, Disadvantages

Chemicals Function Advantages DisadvantagesOxidantsChlorine . . . . . . . . . . . Oxidize and chlorinate Iignin

Hypochlorite . . . . . . . Oxidize, brighten and solubilizeIignin

Chlorine dioxide . . . . 1 ) Oxidize, brighten and solubilizeIignin

2) In small amounts with chlorinegas to protect againstdegradation of pulp

Oxygen . . . . . . . . . . . Oxidize and solubilize Iignin

Hydrogen peroxide . . Oxidize and brighten Iignin inchemical and high-yield pulps

ReductantHydrosulfite . . . . . . . . Reduce and decolonize Iignin in

high-yield pulpsAlkali

Sodium hydroxide . . . Hydrolize chlorolignin and solubilizeligin

Effective, economical delignification

Easy to make and use

Achieves high brightness withoutpulp degradation

Low chemical cost

Easy to use. Low capital cost

Easy to use. Low capital cost

Effective and economical

Can cause loss of pulp strength ifused improperly

Can cause loss of pulp strength ifused improperly

Must be made at the mill site

Used in large amounts. Requiresexpensive equipment. Can causeloss of pulp strength

Expensive

Decomposes readily. Limitedbrightness gain

Darkens pulp

SOURCE: Oougias W, Reeve, ‘The Pnnc@as of Blaachmg,” 1W7 Bleach Wanf Operatkwts Semmar, TWPI Notes (Atfanta, GA: TAPPI Press), p. 6.


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 inwhich they are used make up the “bleachingsequence. ” Bleaching sequences generally containtwo phases within each sequence: 1) a delignifica-tion segment, whose function is to remove thelignin; and 2) a brightening segment, whose princi-ple function is to increase the brightness of the pulp.Examples of delignifaction stages or segmentsinclude oxidation by chlorine (C) followed by

Box 4-B-Shorthand for DescribingBleaching Sequences

The pulp and paper industry has developed aseries of shorthand descriptors for the multistagebleaching sequences. The following abbreviationsare used to designate the bleaching agents:

c ChlorinationE Extraction with sodium hydroxideH Hypochlorite (sodium or calcium)D Chlorine dioxideP Hydrogen peroxideo OxygenN Nitrogen dioxidez Ozone

Bleaching sequences are designated by listingeach treatment serially. For example, “CEDED”represents a commonly used five-stage bleachingsequence consisting of a first-stage chlorine treat-ment, followed by a second-stage alkali extractionstage, followed by a third-stage chlorine dioxidetreatment, followed by a fourth-stage alkali extrac-tion treatment, and a final fifth-stage chlorinedioxide treatment. Washing is conducted betweeneach chemical application.

Two bleaching agents may be used in a singlestage. For instance, chlorine gas and chlorinedioxide are sometimes combined in an early bleach-ing stage. If chlorine gas is the predominant agentin the mixture, the treatment would be designated as“cD.” On the other hand, if the mixture containsmore chlorine dioxide than chlorine gas, the treat-ment would be designated as Dc. Other commonlyencountered oxidative extraction treatments in-clude EO (or E/P), EP, E/H etc.

extraction of the dissolved lignin with sodiumhydroxide (E). Brightening segments use sodiumhypochlorite (H) and/or chlorine dioxide (D). Oxy-gen can be used for delignification and for reinforc-ing extraction of the dissolved lignin in the alkalistage.

Several of the more commonly used bleachsequences in U.S. and Canadian mills are: 1)CEDED, 2) CEDH, 3) CEHDED, 4) CEH, and 5)CED (figure 4-l). Although these are the mostprominent bleaching sequences currently in use, anincreasing number of mills are now using oxygen incombination with alkali for extraction (Eo), andchlorine dioxide (CD, Dc) in the chlorination stages.In addition, there area number of unique bleachingsequences used by some mills (e.g., CEHDH,CEHEDP, CEDPD, CEDEHD and CEHCHDED).

Factors Affecting the Bleaching Process

Process engineers and paper chemists have a widerange of chemicals, processes, equipment, andoperating conditions to choose from in optimizing ableaching sequence. Cost of chemicals, capital costof equipment, energy requirements, and other oper-ating costs figure heavily in bleach plant decisions.While cost control is an important factor, thephysical and chemical composition of the wood rawmaterial (furnish) and the desired characteristics(brightness and strength) of the finished paper areoften more important in selecting bleaching tech-nologies. Bleaching sequences also depend on thepulping process used for initial delignification, some

Figure 4-l-Bleaching Sequences Used inU.S. and Canadian Pulp Mills

CEDED

19%

BOURCE: Darn tmm David R. Forbos, “IUpgmdlng Exlsti~ BlsactI Plants,” 1987- -f Qwsfkwts Semhw, T@P/Notvs (AduIta, GA: TAPPI Press,1967), p. 116.

Chapter 4-Pulp Bleaching Technology ● 45

of which leave higher residual lignin levels remain-ing in the brown stock than do others.

The efficiency of the bleach cycle (related to itscost effectiveness), also depends on controlling theoperating environment within each bleaching stage.Bleaching is achieved through chemical reactions.Operating conditions are related to temperature,time, chemical concentrations, and degree of acidityor alkalinity (pH).3 These factors must be kept inbalance to achieve the desired degree of bleaching,while at the same time minimizing damage to thecellulose fiber. In addition, the “consistency”(amount of fiber being bleached in relation to thevolume of liquid) of the fiber slurry being bleachedaffects chemical penetration and therefore must alsobe controlled. Computerization and improved sen-sors now allow nearly real-time control over theoperating environment in all stages of the pulpingand bleaching processes.4

Lignin Content— Lignin is a large, complex,organic molecule that still holds mysteries for woodchemists. Complete quantitative analysis of ligninfrom pulp samples would be expensive if performedwith precision. This is not necessary from thestandpoint of controlling the digestion and bleachingprocess, however, as simpler methods have beenfound. Index systems for ranking the lignin contentof wood pulp have been devised by the industry (box4-c).

The lignin content of unbleached pulp, expressedby its kappa number, determines the amount ofbleaching required in the bleach sequence to obtainthe brightness desired in the finished pulp. The keyis to find the optimum point between cooking andbleaching (i.e., the best kappa number for un-

bleached pulp). For bleachable grades, kappa num-bers of unbleached kraft softwood pulp may rangebetween 20 and 40 and hardwood between 15 and 25as it leaves the digesters in some mills (kappanumber 35 represents approximately 5 percentlignin).

Using modem computer-controlled cooking, ap-propriate chip pretreatment, and chip equalizingsystems, it is now possible for well-run kraft mills toproduce unbleached softwood pulp consistentlywith kappa numbers between 28 and 32.5 Un-bleached hardwood pulps can be produced withkappa numbers between 20 and 25.6 Pulp can bedelignified to extremely low kappa numbers (2 to 4)by using chlorination followed by a alkali/oxygen(Eo) extraction stage.7

Lignin and Brightness— The lignin content,kappa number, and brightness of chemical pulps aresomewhat interrelated. Since pulp brightness is themajor objective of bleaching, measurements ofbrightness expressed as either GE Brightness or asISO Brightness (see box 4-A) are often used to trackprogress 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 reflected light by the residual lignin.8 Kraftpulp can be bleached to a very high brightness of 90percent GE by decreasing its lgnin content to nearzero without affecting the strength of the pulp. It ispossible to bleach kraft pulp to a brightness of 91 to92 percent GE for special papers.9 Unbleached acidsulfite pulp with GE brightness of about 60 to 66percent can also be bleached to 90 percent byremoving nearly all of the lgnin. Groundwood pulpmay have brightness values of about 62 percent GE.

3Ru&a P. Sin@, ‘‘Principles of Pulp Bleaching,” The Bleaching of Pulp-Third Edition (Atlanta, GA: TAPPI Press, 1979), p. 17.‘tThomm J. Boyle and Carr Smith, *’Bleaeh Plant Instrumentation and Computer Control, ” The Bleaching of PuiP-Third Edition (Atlanta, GA:

TAPPI Press, 1979), p. 487 et seq.

51ngemU Croon, Alf de RUVO, ~-d GunnwTarnvik, ‘‘Bleaching of Kraft Pulps: Oxygen Tkehniques Today and in the Future, ’ Svensk PapperstidnmgNo 4, reprinted by Sunds Defibrator in English (Stoekholrn, Sweden: Sunds DeXbrator, 1985), p. 2.

bKristina Idner, “Oxygen Bleaching of Kraft Pulp-High Consistency vs. Medium Consistency,” 1987 International Oxygen Delinl~icationCo~erence, TAPPINotes (Atlanta, GA: TAPPI Press, 1987), p. 197.

?N. Lic@~t and B, van Licrop, “Extraction, Part I: Oxidative Extraction, ‘‘ 1987 Bleach Plant Seminar, TAPPI Notes (At.tanta, GA: TAPPI Ress),p, 46,

6Rwve, op, Clt,, f-note ‘! p, 4’

9w, HOw~d R~rt ~d Gene Be S(mjla, “~l~ne DjO~jde B]e~hing,” The Bfeaching of Pulpj Third Edition (Attanta, GA: TAPPI preSS, 1979),p, 142.


Box 4-C—Assessing Lignin Content andPulp Bleachability

The Technical Association of the Pulp and PaperIndustry (TAPPI) has devised two standardizedprocedures for determining and reporting the lignincontent of pulp: 1) Permanganate (K) number(TAPPI Test Method T214), and 2) Kappa number(TAPPI Test Method T236). These indices are usedby the industry to control cooking within thedigester during pulping and for determining thebleachability of the pulp.

Both methods are chlorine demand tests and arebased on the amount of permanganate needed tooxidize the contained lignin, The Permanganate, orK number, is used for determining the bleachabilityof chemical pulps having lignin contents below 6percent (based on weight of oven-dry pulp). Thekappa 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 70percent.

Standard procedures have been established forboth bleaching indices. Both are based on theamount of potassium permanganate that can reactwith dry pulp samples. Most modem pulp millsnow use automated, continuous oxidation-reduction measurements or optical devices such asbrightness meters for on-line measurements togauge the progress of delignification and the needfor additional bleaching. However, permanganatetests are still used in mill laboratories for verifica-tion of the instrument reading.

Bleaching Systems

Bleaching sequences apply various bleachingagents in different orders and combinations. Be-tween each bleaching stage the pulp is generally (butnot always) flushed with an alkali extraction solu-tion to remove the dissolved lignin before it is sentto the next bleaching stage (figure 4-2). The first stepof a bleaching sequence is designed to remove thebulk of the residual lignin (delignification) andinvolves little or no improvement in the brightnessof the pulp (figure 4-3). This step, along with thefollowing extraction stage, is called “prebleach-

ing. ” The purpose of prebleaching is to remove asmuch lignin from the pulp as possible to minimizethe volume of more expensive bleaching chemicals(e.g., chlorine dioxide, hypochlorite, and hydrogenperoxide) needed in subsequent bleaching stages.

Chlorine gas and sodium hydroxide (CE) havebeen the preferred chemicals for the prebleach stageof the bleaching process. More recently, mixtures ofchlorine and chlorine dioxide have been used inplace of (or in addition to) pure chlorine treatment(table 4-3).10 Other prebleaching processes areslowly displacing CE as the first stages of the bleachsequence at some mills. Prebleach oxygen delignifi-cation and extended cooking may shorten thebleaching sequence by reducing the amount of lignincarried forward to the bleaching process. 11

Impetus for considering alternatives to the con-ventional CE stage are based partially on reducingthe cost of bleach plant operations and partially onconcerns about environmental impacts from dis-charged bleaching effluents. These concerns resultfrom detection of chlorinated organic material con-taining chloroform and dioxins in bleached pulp millsludge. Most of the chlorinated organics containedin bleach plant effluent originate from the firstchlorine, alkaline extraction, and hypochloritestages.

Brightening stages that follow prebleaching re-move less lignin but bring out the brilliance of thepulp through bleaching action. The brighter the pulpdesired, 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 finalbleaching. Choices are largely determined by therelative costs and efficiencies of the bleach optionsand the required brightness of the pulp beingproduced. The entire bleaching sequence is then acombination of the prebleaching stages and thebrightening stages.

Prebleach Delignification

Chlorination-Chlorine selectively reacts withlignin and under normal bleaching conditions doeslittle harm to cellulose fibers. Because of its relative

l%u&a p, Wngh and Edward S. Atkinson, “The Alkaline Extraction,” The Bleaching of Pulp, Third Editbn (Atlanta, GA: TAPP1 Press, 1979), p.89.

lljoh~ GulliChXn, ‘The Past and Future of Pulp Bleaching,” 1987 Bleach Plant Operations Seminar, TMPI Notes (Atlanta, GA: TAPPI Press,1987), p, 19.


Figure 4-2-Sequence for the Production of Fully Bleached Chemical

Raw water, Raw water Raw water Raw waterc o n d e n s a t e o r o r o r o r

white water w h i t e w a t e r w h i t e w a t e r

Pulp

Raw watero r

white water

Unsreeened

B I a c k B l a c k I i q u o r C h I o r i n a t i o n E x t r a c t i o nI i q u o r lost i n e f f I u e n t e f f I u e n t

t o r e c o v e r y s c r e e n i n g o r

c h I o r i n a t i o n

SOURCE: Caribn W. Denos snd (30ran E. Annergren, “Chlorination,” Tbe 8kach”ng of Pdp, Thd Edition (Attants, GA: TAPPI Press, 1978), p. 51.

cheapness in comparison with other bleach chemi-cals it became widely used for delignification afterthe pulping process. Chlorination in the prebleachcycle begins with washed brown stock pulp slurry atlow consistency (3 to 5 percent weight of pulp towater) being pumped into a chlorination mixer.Chlorine gas, which is often dispersed in water, isadded to the pulp slurry in the mixer and isvigorously mixed (figure 4-4).

The reaction between chlorine and lignin beginsimmediately in the mixer, and the reaction iscompleted in a chlorination tower designed to givethe proper retention time. If chlorination is con-ducted at low temperatures (5 to 45 “C) retentiontime may range between 15 and 60 minutes. Highertemperatures reduce the time necessary to completethe chemical reaction. The chlorinated pulp iswashed before being sent to the alkali extractionstage.

Chlorination is sometimes repeated after extrac-tion if additional delignification is needed, butbecause of possible cellulose damage a chlorine

dioxide stage is often used. Throughout the processbrightness, kappa number, and other indicators ofpulp quality are monitored.12

There is a trend toward modification of the firstchlorination stage by including other bleachingagents (e.g., chlorine dioxide, with the chlorinecharge). Inclusion of these chemicals can reducecellulose degradation, improve pulp strength, andreduce environmental releases. Chlorine dioxide issometimes used sequentially preceding the chlorinetreatment and has been shown to be more effectivethan when the two chemicals are mixed. ’s Chlorinedioxide can be used to completely replace chlorinein the first delignification stage, but its gains in pulpquality do not offset the additional expense, and pulpbrightness equivalent to t-hat produced by chlorinecan not be achieved. 14 Pretreatment with sodiumhypochlorite prior to chlorination has improved thedelignification of resinous woods.

Alkaline Extraction-Hot alkaline extraction isthe second stage in the pulp bleaching process andcompletes the prebleaching delignification process.

Wouglss W. Reeve, ‘‘Pulp Chlorination,’ 1987 Bleach Plant Operations Seminar, TAPP1 Notes (Atlanta, GA: TAPP1 Press, 1987), p. 37.IJcullon W, Dence and Goran E. Annergrcn, “Chlorination,” The Bleaching of Pufp, Third Edirwn (Atlanta, GA: TAPP1 press, 1979), p. 65.ldDougl~ C. ~ke, ‘chlorine Dioxide in the ~lorination stage, 1987 Bleach Piant Operation Sem”rwr, TAPPINotes (Atlanta, GA: TAPPI Press,

1987), p. 55.


Figure 4-3-Pulp Brightness at Stages of theBleaching Sequence CEHDED

Bleaching stage

SOURCE: N. Lkrgott and 6. Van Lierop, “Oxidative Waaching A Rev!ew Part 1:Oel@nificalion,’< P@& Paper Carreb, vof. 87, No. 8, IWS, p. 58.

Moderate temperature or cold temperature alkalineextraction is also used after later bleach stages in thebrightening process of multistage bleaching se-quences. Cold alkaline extraction is particularlyimportant in the production of dissolving pulps forthe manufacture of rayon and acetate. Alkalineextraction removes the soluble colored componentsand lignin released in the preceding delignificationstage (chlorination or oxygen), therefore reducingthe amount of bleaching chemicals needed insubsequent stages and improving the durability ofthe pulp.

Sodium hydroxide has been shown to be the mostefficient alkali for decreasing the kappa number ofpulp. Efficient extraction is nearly as important asprebleach delignification in the first bleaching stage.For instance, after chlorination and washing, butbefore alkaline extraction, about 30 to 50 percent ofthe chlorinated lignin in removed. After alkalineextraction, 80 to 90 percent of the lignin is re-moved. 15 A second hot alkaline extraction is some-

Table 4-3-Examples of Prebleaching Sequencesfor Pulp Delignification

CE . . . . . Chlorine - Alkali extractionCDE . . . (Chlorine+ Chlorine dioxide) - Alkali extractionCE O . . . Chlorine - (Alkali + Oxygen extraction)OCDE . . Oxygen - (Chlorine + Chlorine dioxide) - Alkali

extractionDCE . . . Chlorine dioxide - Chlorine - Alkali extractionSOURCE: Dougies W. Reeve, “The principles of Bieaohi~,- f987 SkMch Plant

Qemfiorts Seminar, TAPP/Notes(AUanta, GA: TAPPI Press, 1987), p. 10.

times used later in the bleach sequence afterbleaching with chlorine dioxide or sodium hypo-chorite (e.g., CEHED, CEHDED, or CEDED se-quences) to improve pulp brightness stability andconserve bleach chemicals. ’b

The pulp is subjected to the alkaline extractiontreatment for 60 to 90 minutes at most mills in thefirst post-chlorination extraction stage, althoughsome operate on a shorter schedule. The secondalkaline extraction usually lasts from 30 to 60minutes.

The first alkaline extraction stage contributes thelargest potential pollutant load released from thepulp bleach plant. It may be possible to reduce thepollutant loss from the extraction stage considerablyby substituting oxidative extraction, particularlysodium hypochlorite, for the first alkaline extractionstage (e.g., CHED, bleaching sequences) .17 Sodiumhypochlorite added to the sodium hydroxide extrac-tion solution (EH) may reduce the color (a roughindicator of pollution load) in waste water by aboutone-half.18 Even larger reductions in waste watercolor (about three-quarters) have resulted whenhydrogen peroxide is added at the alkaline extractionstage.

Oxidative Extraction-Oxygen gas added tosodium hydroxide in the extraction stage (Eo)decreases the kappa number, conserves chemicals insubsequent bleaching stages (in some cases it canreduce the number of bleaching stages), and reducespulp strength loss and coloration in the wasteWater.19 Studies have shown that the addition of

15F3. VUI Lierop et id., ‘‘Caustic Extraction, Part I: Reaetion %riables,’ 1987 Bleach Plaru Operatwns Seminar, TWPl Notes (Atlanta, GA: TAPPIPIXXS, 1987), p. 44.

lbSln@ ~d Atkinson, op. cit., footno(e 10, p. 91.

171bid., p. 99lgLie~rgo~ and van Lierop, op. cit., fOOttlote 7, p. 46.

lgIbid., p, 47,

Chapter 4—Pulp Bleaching Technology . 49

other oxidizers, such as hydrogen peroxide orsodium hypochorite, to an EO extraction stage mightallow for a shortened, three-stage bleaching se-quence capable of bleaching pulp to high brightness(88 to 90 percent ISO). The EO extraction stage hasgained 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 Ifcoupled with a following chlorine dioxide bleachstage, the COD sequence can produce fully bleachedpulp with major savings in chemicals. A three-stagesequence using oxygen in the second stage follow-ing a chlorine dioxide-chlorine delignification stage(( DC)OD) has been used by the Chesapeake Corp. atWest Point, Virginia since 1972. The Chesapeakemill was the first commercial application of oxygenin the extraction stage.

Brightening Stages

Chlorine Dioxide—Chlorine dioxide is very se-lective in attacking lignin without significantlydegrading cellulose, while producing high bright-ness pulp. In the dioxide bleach stage, chlorinedioxide is generated as a gas at the mill and dissolvedin cold water. The aqueous chlorine dioxide solutionis mixed with the prebleached pulp, heated to about70 “C, and is normally held in a reaction vessel forapproximately 3 hours.21

Because of chlorine dioxide’s high cost, it is mostcommonly used at or near the end of bleachingsequences (e.g., CEHD, CEHED, CEDED, andCEHDED). Sequences using chlorine dioxide inonly one bleach stage generally produce lowerbrightness pulps.

22 For instance, the CEHD se-quence on softwood kraft pulp would probably belimited to 85 percent G.E. brightness. In order toachieve the highest brightness (90+ percent G.E.),two chlorine dioxide stages are generally required(e.g., CEDED and CEHDED sequences). Chlorinedioxide can also be used in conjunction with a

Table 4-4-Common Sequences Used To BleachKraft Pulp to Various Degrees of Brightness

Range of GE%brightness Sequence

CEH70-80 CEHH

CHEH

CEHEH80-85 CCHEHH

CEDCEHD

CHED85-92 CEHDD

C CH E DH

CEDEDCEDHEDC D E O D E DO CDE H D

SOURCE. Adapted from AJlan M. springer, krdustnal Erwironrnarrkd Corrtrol: Pub andPapar /ndJstry (Naw York, NY: JohrI Wiley IS Sons, 19S6), p. 161.

hydrogen peroxide bleach stage (CEHDP or CEDPD)to produce 90+ percent G.E. brightness pulp.23

Peroxide-Hydrogen peroxide is a very effectivecellulose-preserving bleach agent and is well suitedfor improving the brightness of highly lignifiedpulps, such as mechanical groundwood or chemi-mechanical pulps, without significantly reducing itsyield. Hydrogen peroxide is an extremely versatiledelignifying chemical and has been proposed for useas a chip pretreatment before kraft pulping and as adelignifier in the prebleach stage prior to, or as asubstitute for, the C, CD, or Dc prebleachingstages. 24 It is also used in association with sodiumhydroxide in alkaline extraction (EP) Because of itshigh cost, hydrogen peroxide is used most often inthe later stages of pulp bleaching.

Peroxide is used in the intermediate stages of thebleaching sequence as a replacement for hypochlo-rite or chlorine dioxide. It is frequently used as thelast stage in the bleach sequence where it can add afew points of brightness to the pulp and improve itsbrightness stability. Peroxide alone is a relatively

zOR~&a P. Sin@ and Bjom C. Diilner, “Oxygen Bleaching, ” The Bleaching of Pulp, Third Editwn (Atlanta, GA: TAPPI Press, 1979), p. 181.21Rapson @ Stt-lmila, op. cit., footnote 8, p. 114.zz~ug]m W. Reeve, “Chlorine Dioxide Bleaching,” 1987 Bleach Plant Operatiom Seminar, 7iVPlNotes (Allama, GA: TAPPI Press, 1987), p.

67.23RWWn ~ StIUmlla, op. cit.! p. 1a”

ZAJ.R. ~es]ey and R.R, Kindron, ‘Hydrogen Peroxide Bleaching,’ 1987 Bleach Plant Operations Seminar, TAPPINotes (Atlanta, GA: TAPPI Press,1987), p, 75.

50 ● Technologies for Reducing Dioxin in the A-manufacture of Bleached Wood Pulp

Figure 4-4-Schematic Diagram of the Chlorination Process

SOURCE, Carlton W. Denos and Goran E. Annergren, “Chbrinatmn,” l% Bkwchmg of Pu/p, Third Edition (Atlanta, GA: TAPPI Press,1979), p. 52,

ineffective means for bleaching krafl pulp.25 How-ever, when used in sequences with chlorine-basedbleaching agents, peroxide is an efficient delignifierand brightner. Peroxide is also used for intermediatebleaching stages of the kraft bleaching sequence oras a final treatment to increase and stabilize thebrightness of chemical pulps.26

The peroxide bleach liquor is usually in the rangeof 1 to 3 percent hydrogen peroxide. An appropriatevolume of peroxide liquor, sodium hydroxide, andother chemicals to stabilize the peroxide are mixedwith pulp and heated with steam to the reactiontemperature (35 to 70 ‘C). The peroxide-pulpmixture is allowed to react under controlled tem-perature for an optimum time (1 to 5 hours). Whenthe reaction is complete, the pulp is washed and sentto the next bleaching stage or washed and neutral-ized with sulfur dioxide if it is the final bleach stage.

Peroxide, coupled with oxygen and/or ozone,shows some promise in research laboratory evalu-

ations for formulating chlorine-free bleach sequencesto reduce release of chlorinated organics in the wastestream. A three-stage sequence OZP (oxygen, ozone,peroxide) has yielded brightness values of 85percent GE in eucalyptus kraft pulp.27 However,the pulp suffered a substantial loss in tear strength,The ZP bleaching sequence produced southern pineoxygen pulps of 80 percent GE brightness with goodbrightness stability. In contrast to peroxide andoxygen bleaching, ozone bleaching has not beendeveloped to the point of commercialization.

Hypchlorite—The use of hypochlorites for bleach-ing wood pulp began in the early 1880s. Althoughthe development of chlorine bleaching technology inthe 1900s led to a decrease in the use of sodium andcalcium hypochlorite, still about 40 percent of thekraft pulp mills in the United States and Canada useat least one hypochlorite stage in their bleachsequence. Hypochlorites have been used effectivelyon sulfite pulps where an alkaline extraction stage isinterposed with two hypochlorite stages (HEH), 28

ZSD.H, ~~ws ~d R,p. Singh, “Peroxide Bleaching, ” The Bleaching of Pulp, Third Edition (Atlanta, GA: TAPPI Press, 1987), p, 237.~lbid., p. 212.271bid,, p, 243,zs~ E, ~~n md H, dev, Partridge, “Bleaching With Hypoehlorites,” The Bleaching of Pub, Third Editions (Altanta, GA: TAPPI PIESS, 1979),

p. 101.


Kraft pulps, being more difficult to bleach thansulfite pulps, require that a chlorine and alkalineextraction stage be added in the prebleach segmentof the bleach sequence. Until chlorine dioxide andperoxides became available in the 1940s, kraft pulpsof 85 percent GE brightness were the brightest thatcould be produced with hypochlorite bleaching andstill maintain acceptable pulp strength, but thesepulps had poor brightness stability.

Hypochlorite is nonspecific, that is, it attackscellulose as well as lignin, therefore it requirescareful control if a reduction in pulp strength is to beavoided. 29 Bleaching sequences such as CEHD,CEHED, and CEHHD are used widely for producingpulps of 86 to 88 percent GE brightness, CEHDEDis used for pulps of 88 to 90+ percent GE brightness,and CEHDP and CEHEDP for pulps of 90 percentGE brightness using peroxides. Hypochlorite is alsoused in small amounts for oxidative extraction (seeabove). Some mills use hypochlorite as a replace-ment for the first alkaline extraction stage to reducethe color in bleach plant effluent.30

Retention times and chemical concentrations varyfor hypochlorite bleaching depending on whichstage it is being used in the bleaching sequence.Retention times range from a low of 30 minutes atsome mills to 3.5 hours for those using hypochloritein the brightening stage. Reaction temperatures forhypochlorite stages are generally kept low (85 to 110‘F) to minimize cellulose degradation.

A “simplified bleaching” process for hypochlo-rite has recently been developed.31 Simplified bleach-ing uses a short (l O-minute) bleach cycle at highertemperatures than normally used (180 ‘F). Thehypochlorite treated pulp is sent without washing toa chlorine dioxide stage. This ostensibly producespulps of higher brightness at lower cost.

Studies have shown that one of the largestcontributors to the environmental release of chloro-

forms from a bleach plant is the effluent from thehypochlorite stage.

32 It is hypothesized that chloro-form is produced under specific conditions existingin the hypochlorite stage rather than simply as theresult of chlorine-based chemicals. The specificconditions and reactions contributing to the produc-tion of chloroform are not well known, however, andmore research is needed to establish causation.These early findings of the linkage between hypo-chlorite reactions and chloroform production has ledsome to propose that the release of chloroformcompounds from pulp mills could be reduced byeliminating the large-scale use of hypochlorite in thebleach sequence.

Ozone-Ozone is one of the most powerfulbleaching and oxidizing agents. It is a special formof oxygen produced by the discharge of an electricalcurrent in oxygen gas. While oxygen atoms nor-mally occur in pairs, the electrical discharge makesthree atoms associate with one another, thus givingextraordinary oxidative properties to ozone. Itsdecomposition to oxygen after bleaching producesneither a residue, nor undesirable inorganic by-products. Ozone, in a bleaching sequence withhydrogen peroxide, can produce high-brightnesspulps. Ozone is not used commercially for pulpbleaching. Some pilot plant studies have beenconducted, but additional development work wouldbe needed to permit its widespread commercial use.

Ozone, in conjunction with preliminary oxygendelignification, holds promise for reducing theamount of chlorine and hypochlorite used in theprebleach and brightening stages of the bleachingsequence. Ozone bleaching is particularly wellsuited to bleaching sulfite pulps because of their low

H i g h - b r i g h t n e s s ,residual lignin content.33 high-quality, hardwood kraft pulps can be produced byusing ozone in the first stage of the bleaching

Z9E.13. Althouse, J.H. Bostwick, and D.K. Jain, “Using Hydrogen Peroxide and Oxygen to Replace Sodium Hypochlorite in Chemical PulpBleaching, ’ T#P/ J , vol. 70, No. 6, June, 1987, p. 113.

s~uwn ~d partridge, Op. clt.~ P. 102.

31R.G. Hise imd H.L. Hintz, “HypoehIorite Bleaching, ” 1987 Bleach Plant Operations Semiw, TAPPI Notes (Atlanta, GA: TAPPI Press, 19871,p> 65.

szIbid.SSR, Patt et al,, “Laboratory and Pilot Plant Bleaching of Mrious Pulps With Ozone, ‘‘1984 OqgenDelignfication Sy~osiwn, TAPPINotes (Atlanta,

GA: TAPP1 Press, 1984), p. 33.


sequence (e.g., ZEP, ZEZP, and ZED).34 Because itis a very powerful and nonselective chemical, its useis usually limited to the early bleaching stages.

Kraft softwood pulps must be delignified, prefer-ably with oxygen or through extended cooking toreduce the need for chlorination in the prebleachsegment, before bleaching with ozone. Low kappanumber kraft softwood pulps bleached with theOZEP sequence produced pulp comparable in bright-ness and strength to those produced from theCEHED five-stage sequence.

35Brighter kraft soft-

wood pulp can be produced with oxygen-ozone-peroxide and/or chlorine dioxide bleaching se-quences (e.g., OZEP, OZEPP, OZEPD, OZED, andOZEPD). For the highest brightness, chlorine diox-ide was needed in the final bleaching stage,

Ozone is not currently used commercially by theindustry. Experimental results and pilot plant opera-tions indicate that ozone might have future promiseas an alternative bleaching agent. Further discussionof oxygen-based, nonchlorine bleaching technology,including ozone bleaching, is found in chapter 5.

~qsleven s.K, Ow and Rudrii P. Sin@, “Advances in Ozone Bleaching, Part II: Blcaehing of Softwood Kraft Pulps With Oxygen & OzxmeCombination,” Oxygen Delignfl’cution .~~posium, TAPP1 Notes (Atlanta, GA: TAPPI Press, 1984), p. 43,

J51bid., p. 49.

Chapter 5

Technologies for ReducingChlorinated Organics in

Pulp Manufacture

CONTENTSPage

OXYGEN DELIGNIFICATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56Oxygen Delignification Technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59Effects of Oxygen Delignification on Pollutant Loads . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64Extended Delignification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64Pretreatment of Oxygen Pulps With Nitrogen Dioxide (PRENOX) .., . . . . . . . . . . . . . . . 66Soda-Anthraquinone/Oxygen Pulping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67

DISPLACEMENT BLEACHING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67CLOSED-CYCLE BLEACHED PULP MILL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68WASTE-OUTLET TREATMENT TECHNOLOGIES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69

Primary Treatment of Suspended Solids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71Secondary Biological Treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71Oxidation Ponds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71Activated Sludge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72New or Developing Treatment Technologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72

FiguresFigure Page5-1. Chlorinated Waste From Bleaching of Hardwood Pulp . . . . . . . . . . . . . . . . . . . . . . . . . . 575-2. World Production Capacity of Oxygen-Delignified Pulp . . . . . . . . . . . . . . . . . . . . . . . . . 585-3. High Consistency Oxygen Delignification Stage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 625-4. Oxygen Delignification System Installed in a Bleached Kraft Pulp Mill . . . . . . . . . . . 635-5. Phenolic Compounds Formed With and Without Oxygen Bleaching . . . . . . . . . . . . . . 655-6. Continuous Digester for Extended Delignification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 675-7. Multistage Displacement Bleach System Single Tower Bleach Plant Showing

an EDED Wash Sequence. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .695-8. Closed-Cycle Bleached Kraft Pulp Mill . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70

TablesTable Page5-1. Oxygen Delignification Systems Installed Worldwide . . . . . . . . . . . . . . . . . . . . . . . . . . . 605-2. Effluent Characteristics of Oxygen-Treated Bleached Pulp and Pulp Bleached

by the Common Chlorinated Bleach Processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 655-3. Effluent Characteristics for Softwood Kraft Pulps With and Without

Oxygen Delignification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 655-4, Emissions of PCDD and PCDF (expressed as TEF) in Receiving Waters From

Swedish Pulp Mills With and Without Oxygen Delignification . . . . . . . . . . . . . . . . . . . 66

Chapter 5

Technologies for Reducing ChlorinatedOrganics in Pulp Manufacture

The bleaching plant is the major source ofwaterborne environmental pollutants produced bypulp and paper mills. As much as 40 percent of thebiological oxygen demand (BOD), 25 percent of thesuspended solids (SS), 70 percent of the color, andnearly all of chlorinated organic materials (TOC1)produced by pulp mills originate in the bleachingprocess. The amount and nature of bleach plantpollutants vary considerably among the bleachingsequences used, wood species, and pulping proc-esses.

Water is used in large quantities in the bleachingprocess (averaging 20,000 to 30,000 gallons per tonof pulp produced, although modem mills use muchless than this). In contrast to pulping chemicals, thebleaching chemicals are generally not recovered butare discharged as wastes after treatment.

Chlorination, often the first bleaching stage afterpulping in many bleaching sequences, requires largevolumes of water to dilute the unbleached pulp to alow consistency for subsequent bleaching (about 3percent fiber by weight compared to water), and istherefore a major source of chlorine-contaminatedwater. Chlorination sometimes uses as much wateras all of the subsequent bleaching stages combined.To reduce the amount of fresh water consumed, thechlorination filtrate is often recycled in the bleach-ing stage, or paper mill white water is used fordilution.

General hypotheses have been advanced to ex-plain the possible occurrence of dioxins in pulp,paper, and mill wastes. If true, these hypotheses maysuggest ways to reduce the amount of dioxinsproduced in pulp and paper manufacture. First, sincelignin or wood extracts probably contain somedioxin precursors, the reduction of the amount oflignin exposed to chlorine in the bleach plant mightreduce the volume available for dioxin formation. It

is now believed that lignin may not be a major sourceof 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 eveneliminated.

Third, a recent discovery by the Pulp and PaperInstitute of Canada that oil-based defoamers madewith contaminated used oil may be the source ofnonchlorinated precursors of dioxin and furan thatbecome tetrachloro-p-dibenzodioxin (TCDD) andtetrachlorodibenzofuran (TCDF) with chlorination.1By 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 theformation of TCDD and TCDF can be reduced bymodifying conventional chlorine bleach sequences.For instance, by applying chlorine gas in smaller,successive split charges rather than a single largecharge, research has shown reductions in TCDD.2

By carefully controlling the acidity (pH) of theunbleached pulp within an optimum range, TCDDhas also been reduced. Applying chlorine bleachbefore the addition of chlorine dioxide (CD) tends toproduce less TCDD than if chlorine and chlorinedioxide are mixed (CD) or if chlorine dioxide isapplied separately before the addition of chlorine.TCDD and TCDF formation is also sensitive to theratio of chlorine to lignin. The Swedish Pulp andPaper Research Institute has found that if chlorineadditions are kept below 15 percent (chlorine tolignin), TCDD and TCDF can be kept to low levels.3

These observations need verification.

Fifth, improvements in secondary biological wastetreatment can further reduce the amount of fine,colloidal suspended solids on which TCDD and

IR. H. Voss et al., “Some New Insights Into the Origins of Dioxins Formed During Chemical Pulp Bleaching, ’ Canadian Pdp and Paper AssociationEnviromnenr Conference Proceedings, Vancouver, B. C., Oct. 25-26, 1988, p. 31.

7-Ron~d B ~~dge ~d Wl]llm fi~ke, Ame~c~ Paper [nsti(ute, material presented at the OTA dioxin workshop, Wfihing(on, DC! NOV. 14-15,1988.

JKnut P. Kringstad et d., ‘‘Bleaching and the Environment,’ 1988 Pulp Bleaching Conference, Orlando, FL, June 5-9, 1988 (Atlanta, GA: TAPPI,1988).

-55-


TCDF are transported.4 Supplemental treatmentwith chemical coagulant, precipitant, absorbents,and perhaps destruction by ultraviolet light andchemicals could increase the efficiency of theexisting waste treatment facilities. Further evalu-ation is needed on these options,

Should it prove necessary to reduce the amount ofdioxins produced from pulp and paper manufacturefor health or environmental reasons, one approach toconsider is prebleaching technologies that wouldreduce the amount of lignin passing to the bleachplant. Another approach would utilize brighteningtechnologies that reduce or eliminate chlorine gasfrom the bleaching sequence. The several optionsavailable to reduce the formation of TCDD, TCDF,and other chlorinated organics are not mutuallyexclusive and can be linked at stages throughout thepulping, bleaching, and waste disposal processes toachieve low levels of discharge.

Because of the pollution potential of chlorinatedcompounds and the volume of wastewater producedby chlorine bleaching, the chlorination stage is thefocal point for efforts to control pollution resultingfrom the manufacture of pulp. There are severalpossible approaches to reducing the amount ofdioxin and other chlorinated organics formed duringchlorine bleaching:

●

●

●

Delignify pulp to a further degree beforebleaching-extended delignification-oxygen delignification—pretreatment with nitrogen (more lignin can

be removed without fiber damage)

Substitute other bleaching chemicals for chlo-rine—chlorine dioxide for part or all of the chlorine—hydrogen peroxide—alkaline extraction supplemented with oxy-

gen and peroxide-ozone (precommercial)

Modify chlorination procedures (more researchneeded)-optimize the acidity (pH) of the pulp

●

●

—use smaller multiple charges of chlorineinstead of one

—apply chlorine first, then bleach with chlo-rine dioxide

Remove known sources of precursors (e.g.,contaminated defoamers or other additives,prewash pulp, etc.)

Improve waste treatment systems (more re-search needed)-chemical coagulant—sorption or precipitation enhancers-destruction with ultraviolet light or chemi-

cals—anaerobic treatment

Elimination of chlorine in the bleach sequencecombined with internal recycling of process wateraimed at developing an overall “pollution free”pulping system probably offers the best theoreticalstrategy for reducing the pollution from bleachplants over the long term, but because of practicallimitations, it may not be commercially viable forsometimes

OXYGEN DELIGNIFICATIONAlthough the introduction of any chlorinated

bleaching chemical (e.g., chlorine gas, sodium, orcalcium hypochlorite), can generate some chlori-nated organic compounds, chlorine gas used in theprebleaching stages of the bleaching sequenceproduces the largest amount (figure 5-l). Chlori-nated organics produced by pulp mills contain smallamounts of TCDD and TCDF (see ch. 2).6 Theirpresence in pulp mill wastes is ascribed to chlorina-tion in the bleach sequence although the precisechemical reactions and mechanisms that producedioxins are not known. Other factors might alsocontribute to dioxin formation.

Ever since the connection was made betweendioxin and the use of chlorine, the emphasis of thoseadvocating process changes to reduce the formationof TCDD, TCDF, and other chlorinated organics hasfocused on oxygen delignification technology. TheSwedish example has served as a demonstration of

4u.s. ~viromcnt~ protection Agency, USEPA Bench Scale Wastewater Treatability Study PUIP and Paper Mill Discharges of 2378 -TCDD ati2378-TCDF: Proposed Interim Control Measures Interim NPDES Permit Strategy (WestMe, OH: EPA Region 5, 1988), p. 17.

SA]l~ M, springer, Itiustrial Environmental Control: Pulp and Paper Industry (New York, NY: Jok Wiley & Son% 1986), P. 161.SU.S. fiviroment~ pro[~tion Agency, National Dioxin Study, EPA050-SW-87-025, August 1987, p. 111-33.

Chapter 5—Technologies for Reducing Chlorinated Organics in Pulp Manufacture ● 57

Figure 5-l—Chlorinated Waste From Bleaching ofHardwood Pulp

Lbs/tons70

60

60

40

30

20

10

0

SOURCE. Man M. Scmnoeri /ndusfna/ Emmmrrer?tal CorIfro/; Pulp and Paper hdustry(New York, N; John Wiley& Sons, 1986), p. 166.

one method to reduce the amount of chlorine neededin the bleaching process. Sweden has encouraged theinternational move toward adopting oxygen deligni-fication to solve the chlorine problem. It should notbe overlooked that first, most of the bleached kraftmills in Sweden have already installed oxygendelignification at a considerable capital expense, andsecond, oxygen delignification is a Swedish technol-ogy largely manufactured in Sweden.

The effectiveness of oxygen delignification toreduce the amount of chlorine needed is welldocumented. It should be noted, however, that othertechnologies and process modifications are alsoavailable that can reduce the amount of dioxins

produced, but none will reduce the amount ofchlorine-based bleach needed to the degree thatoxygen can. Conversion to oxygen delignification isnot always the best solution. However, the substitu-tion of oxygen bleaching for chlorination in prebleach-ing and brightening sequences is considered bysome pulp and paper experts to be a technologicaltrend that likely defines the future state-of-the-art inlow-chlorine bleach plant design.

Effluent from oxygen bleaches, such as oxygengas, ozone, or peroxides, can be recycled internallyto destroy harmful byproducts that might be formed.Transition from conventional chlorine bleaching tooxygen delignification and bleaching has been fasterin Scandinavian countries-particularly Sweden—than it has in the United States and Canada.7 InSweden, oxygen bleaching has been used in place ofbiological waste treatment that is commonly used inNorth America. Although oxygen delignificationwas developed in the Soviet Union, the process wascommercialized in Sweden in the late 1960s and inthe Union of South Africa in the early 1970s.

Early interest in oxygen delignification stemmedprimarily from its ability to reduce pulp millpollution. Substantial reductions in BOD, color, andchlorinated organics in the effluent can be realized,as well as savings in bleaching agents. Oxygenprebleaching may not significantly alter the kinds ofchemicals formed (although this has not beendetermined with certainty), but, when properlyconducted, it will probably generate smaller quanti-ties of all these compounds.8 Moreover, bleachingoperations using oxygen do not normally call for ahypochlorite stage, as a result, little chloroform isreleased (other non-oxygen bleaching sequencesalso do not use hypochlorite stages, e.g., CEDED).

Chlorinated organics—not specifically dioxins—are major pollutants in the Baltic Sea. Nowhere inthe United States have chlorinated organics pre-sented the problems that have been experienced inthe Baltic region. BOD has been the major environ-mental concern in the United States (U.S. standardsfor BOD were stricter than Sweden’s); attempts to

7F1ftwn Swdlsh ~u]p ~llls ~mufac~ure bleached sulfate pulp, Nine ml]]s currently usc oxygen in the first prcbleachlng Stage of the bleachingsequence and two others have plans to install oxygen stages in the bleach sequence. Committee for the Gulf of Bothnia, Water Pollution Problems ofPulp and Paper Industries in Finland and Sweden, Nahmvardsvcrket Rappofi 3384, May 1987, p. 55.

gKnut Kringstad and Krister Lindstrom, “Spent Liquors From Pulp Bleaching, “ Environmental Science and Technology, vol. 18, No. 8, 1984,p. 246A,

58 ● Technologies for Reducing Dioxin in the Mnufacture of Bleached Wood Pulp

address this problem have relied on secondarybiological waste treatment and internal changes inpulp mill processes.

With the exception of Sweden and Germany(Germany has no kraft mills), where environmentalrequirements have forced acceptance, economicshave been a significant motivating factor in adoptionof oxygen delignification technology. For instance,in Japan where oxygen delignification systems haverecently gained acceptance, oxygen is cheap becauseit is produced as a byproduct of nitrogen recovery,which is needed in the manufacture of printedelectronic circuits. At North American mills whereoxygen delignification has been installed, wasteimprovements have been accompanied by economicbenefits from shorter bleaching sequences andsmaller and cheaper effluent treatment systems.

Oxygen delignification can cause degradation ofthe pulp and reduce paper strength. In 1963, thisproblem was reduced with the discovery that theaddition of magnesium chemicals (e.g., magnesiumcarbonate or sulfate) can reduce or prevent degrada-tion of cellulose fibers.9 Oxygen delignification andbleaching produces pulp that compares favorablywith conventional bleached pulps in most ways.Strength properties of oxygen pulps are slightly lessthan conventional pulps, but may be acceptable forsome products if delignification by oxygen does notexceed about 40 to 50 percent of the pretreatmentlignin level.10 Experts differ as to the viscosity andstrength of oxygen pulp compared to conventionalpulp. Brightness stability for oxygen pulps is equalto or better than that of conventional pulps.

Since the construction of the first pulp mill to useoxygen delignification in 1970 at Enstra, SouthAfrica, there has been a steady increase in the annualworld production of oxygen pulps (figure 5-2). In1988, world installed capacity was expected toexceed 10 million metric tons per year. About halfthe oxygen capacity is in Scandinavia and Europe,one-fifth is in North America, and one-fifth is inJapan. About 92 percent of the installed capacity ofoxygen delignification systems is in kraft mills, and60 percent is in bleached softwoods (table 5-l). Theuse of oxygen delignification is expected to expand

Figure 5-2—World Production Capacity ofoxygen-Delignified Pulp

30 Capacity--thousand metric tons per day1

Y e a r s

SOURCE: Larry Tench and Stuart I+arpar, “Oxygen-Sleaching Practlcas and Seneflte:An Overwew,”’ TAPPI hurnal, Nowmber 1987, p. 57.

worldwide as environmental standards are tightenedand will likely accelerate even more if savings in thecost of bleach plant operations favor oxygen pulps.

Capital cost of oxygen delignification systems ishigh. Cost estimates based on prior conversionsfrom a conventional chlorine bleaching process tooxygen delignification range between $20 millionand $30 million for an existing pulp mill with acapacity of 750 to 1,000 tons per day, depending onthe need to modify supporting equipment, such asrecovery boilers, evaporators, etc. If the mill re-quired expansion of supporting equipment, such asevaporators and brownstock washers, the cost wouldescalate to $40 million to $50 million. If, in addition,the mill did not have sufficient reserve recoveryboiler capacity, the additional liquor treatmentrequired for oxygen delignification would raise thecost to $80 million. However, if the mill did notrequire recovery modifications, costs may go as lowas $8 million to $10 million.

The American Paper Institute estimates that if the98 bleached chemical pulp mills in the United Statesthat have not yet installed oxygen delignificationequipment (5 have already done so for a combined

91. Crwn ad D.H. ~&ews, 1‘Adv~~es in oxygen Bleaching: 1, Demonstration of Its Fea..ibility ~d Scope, ” TAPPI AwnaL vOI. $$, No. z, Il.1893 et seq.

IT. Tcnch ad St- ‘~r’ “Oxygen-Bleaching Practices and Benefits: An Ovemiew, ” TAPPI Journal, November 1987, p. 55.


total of 6,100 tons per day of oxygen pulp capacity).The total capital outlay to the U.S. industry wouldthen be about $3 billion or $40,000 per daily ton ofcapacity to fit out the U.S. industry with oxygensystems (if capital costs were annualized andchanges in operating costs were considered, thecosts of oxygen delignification would be lower).

For greenfield construction of a new bleachedchemical pulp mill using oxygen delignification, thecapital cost of an oxygen system is more attractive.The capital cost of installing an oxygen delignifica-tion system in a mill of 1,000 tons per day capacitycould be between $25 million to $35 million morethan for conventional chlorine bleaching. The highercost of oxygen delignification compared to conven-tional chlorine bleaching is part due to the cost of theoxygen generating plant ($1 2 million to $14 mil-lion), and part due to the need for larger recoveryboilers. 11

Oxygen Delignification Technology

The objective of chemical pulping is to reduce theamount of lignin carried forward with the brown-stock pulp to the brightening stages of the bleachplant. The less lignin that prebleached pulp contains,the less bleaching that is required, Conventionalkraft pulping, for example, produces pulp with akappa number between 32 and 35. By subjectingconventional pulp to oxygen delignification, thekappa number may be reduced to 16 or 17.12 Thisallows the use of a short bleaching sequence, sincethe amount of lignin to be bleached is reduced up to50 percent. An even higher proportion of lignin canbe removed from sulfite pulp. Oxygen delignifica-tion is considered to be an extension of the cookingprocess. With an oxygen bleached pulpit is possibleto reduce the amount of chlorine gas bleach. Withadditional research and development, it may bepossible to eliminate it altogether by using ozone,hydrogen peroxide, and/or chlorine dioxide.

Effluent from the oxygen delignification stage isdisposed of by cycling it through the pulp millchemical recovery cycle. If chlorine is used as a

bleaching chemical, the effluents cannot be disposedof in the recovery plant because of the corrosivenessof chlorides and the difficulty in purging chloridesfrom a closed recovery cycle. White liquor, the samereagent that is used in the kraft pulping process, canalso be used in the oxygen delignification processand then recycled through the recovery plant.Installation of an oxygen delignification unit mayrequire that the capacity of the chemical and energyrecovery plant be increased to accommodate theadditional dissolved organic and inorganic chemi-cals. 13

Several commercial oxygen delignification sys-tems have been developed. They differ more indetail than in operating principles. These units are oftwo general types: 1) high consistency, and 2)medium consistency. Consistency refers to theamount of wood fiber in relation to the volume ofsolution in a reactor vessel. The higher the ratio offiber to water, the higher the consistency. Highconsistency oxygen delignification systems use pulpslurries containing 20 to 32 percent fibers. Mediumconsistency is 10 to 15 percent fibers. At highconsistencies, the pulp is more of a ‘‘fluff” than afluid. The key to effective oxygen delignificationsystems is the ability to disperse oxygen finelyenough to last for the necessary reaction time. Thiscannot be done efficiently with pulp consistencies ofless than 6 percent because of energy considerations.

Differences among the oxygen delignificationsystems are mainly in the design of the reactionvessel and associated pumping and gas handlingequipment. In general, the capital equipment cost islower for a medium consistency system than for ahigh consistency system.14 On the other hand, theconsumption of oxygen and alkali is somewhathigher for medium consistency oxygen delignifica-tion. Medium consistency systems require a longerretention time than high consistency reactors toachieve the same degree of delignification, thereforecapacity for medium consistency units must belarger to maintain the same rate of production.Medium consistency oxygen stages are often used to

I IRon~d J. S]tin, vice p~slden~, American Paper Institute, personal correspondence, August 1988. Cost data supplid by the induw~ have not beenverified with other sources.

IZ~m ~d An&ews, op. cit., nOte 91 P, 1896,

13 Spfiger, op. cit., no~ 5, P. 172.

ldK~yr, inc., ~Vgen De/ignific~Wn, bull. No. KGDI8OI-WN1O87 (Glens F~h Ny: 1987)? P. 11.


..,. . . . . .. . . . . . . . .. . . . . . . . .. . . . . . . . .. . . . . . . .,. . . . . . . . . . .

. . . a: . . . .

. .! .. .. .. .. .. .. .. .

, .

. .. .. .. .. .. ..

. .. ... ... .

. . . . . . . .,. . . . . . . . .. . . . . . . . .. . . . . . . . .. . . . . . . ,.. . . . . . . . .. . . . . . . . .. . . . . . . . .

Chapter S---Technologies for Reducing Chlorinated Organics in Pulp Manufacture ● 61

n .

. .. .. .. .. .. .. .. .. .. .. .

. . .. .. . .. . .. .. . .. . .. . .. . .. . .. . .. . . . .

62 •. Technologies for Reducing Dioxin in the Manufacture of Bleached Wood Pulp

achieve 35 to 40 percent delignification. Bothmedium and high consistency bleaching systemsrequire efficient washing of the oxygen bleachedpulp to prevent carry over of the dissolved organicsinto the bleaching system.

Medium consistency oxygen delignificationcauses less degradation of the wood fibers than highconsistency systems, but because of the slower rateof delignification it is more difficult to delignify tovery low kappa numbers using medium consistency.Two-stage medium consistency oxygen systemshave 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.

. Almost no carbon monoxide is produced.

. Little danger of explosion from gas accumula-tions.16

High Consistency Systems

High consistency oxygen delignification systemscan produce high brightness pulps up to 90+ GE withsupplemental bleaching, In the high consistencyprocesses well-washed brownstock pulp, that isdewatered after discharge from the digester to 28 to32 percent consistency, is treated with alkali (oxi-dized white liquor) and magnesium salts (a protectoror inhibiter) in a mixer, it is then fluffed and fed intoan oxygen reactor (figure 5-3). The pulp is heatedwith pressurized steam (90 to 120 ‘C) and oxygengas is injected into the bottom of the reactor. Theatmosphere in the reactor is maintained at about 80percent oxygen. Gases produced by oxidation of thelignin are purged to avoid combustion. The oxygen-ated pulp is washed after discharge from the reactorbefore being sent to the bleach plant.

Medium Consistency Systems

The steps for preparing the brownstock foroxygen treatment that are used in the mediumconsistency process are similar to those used for

Figure 5-3—High Consistency OxygenDelignification Stage

Dewatered stockDewateredstock

SOURCE: Adapted from Kanneth E. Smith, “Oxygan Bleaching Systam Oparating Wallat Unmn Camp’s Franklin Mill,” Pup & Rsper, Octobar 1982.

high consistency oxygen delignification. Pulp con-sistency is adjusted to between 10 and 15 percent.Magnesium salts may or may not be added to thepulp depending on the selected procedure. Oxygengas is dispersed throughout the pulp mixture, andsteam is added to bring the mixture to a temperatureof about 100 “C before it is injected at the base of theoxygen reactor (figure 5-4). The mixing of the

I%id., p. 12,lbMich~l D. Meredith and Joseph M. Bentvelzen, ‘‘oCO&The Bleaeh Sequence of the ‘80s,’ 1984 Oxygen Delignification $mposiurn (Atlanta,

GA: TAPPI i%SS, 1%4), p. 112.


Figure 5-4-Oxygen Delignification System Installed in a Bleached Kraft Pulp Mill

continuous digester Excess

Gas toFoul incineration I

unbleachedwhite

oxygen and steam with the pulp prior to transport to immediately, and about half the oxygen is consumed

the reactor is the major difference in materials in the first 2 minutes. The oxygenated pulp is

handling between the two systems. Reaction begins normally retained in the reactor for 45 to 60 minutes.


Effects of Oxygen Delignificationon Pollutant Loads

The processes and conditions under which diox-ins are formed during pulping, bleaching, andbrightening are not well known. Dioxins are usuallyformed in small quantities, and the conditionsleading to their formation are poorly understood.Heat, light, and catalytic action have been shown tostimulate the conversion of chlorinated precursors todioxins. 17 Chlorine makes up about 44 percent of aTCDD molecule. Lignin is a complex polymercontaining many resin acid, fatty acid, and phenolicderivatives that could serve as precursors for theformation of dioxins in the presence of chlorineunder appropriate physiochemical conditions. Othernon-lignin components (e.g., extractives, nonchlori-nated dioxins, and catachols), may also be precur-sors of TCDD or TCDF.

Impco Division of Ingersoll-Rand, an equipmentmanufacturer that produces oxygen delignificationsystems, claims that oxygen treatment of pre-bleached pulp before a short chlorination brighten-ing sequence significantly reduces the pollution loadin kraft mill effluents compared to the most commonfive-stage bleach sequence CEDED (table 5-2).Similar results are reported from prebleach oxygendelignification using a chlorine-chlorine dioxidefirst stage bleach sequence (CDEoD) compared witha long bleach sequence (CDEoDED) without oxygenpretreatment (table 5-3). Furthermore, the totalamount of chlorinated phenolic compounds formedin the bleaching process decrease considerably as aresult of oxygen delignification (figure 5-5). Thephenolic compounds released from conventionaland oxygen-delignified pulps are lower in quantitybut probably do not differ much in compositioncompared to those produced from conventionalpulp, 18 To the extent that phenolic compounds maybe linked with the production of TCDD, this may

indicate that oxygen delignification could reduce theamounts of phenolic precursors present in the pulp.

Limited data are available that directly relate thereduction of TCDD and TCDF to oxygen delignifi-cation. There is substantial evidence that oxygendelignification can reduce the amount of wastechlorinated organics produced, as well as reduceBOD and COD (chemical oxygen demand), and cansignificantly reduce effluent color. Few analyseshave been made of dioxin in effluents and pulps fromoxygen-treated pulps. The Swedish Pulp and PaperResearch Institute recently published some prelimi-nary information on the effects of oxygen delignifi-cation on dioxins and furans.

Sweden’s experience with oxygen delignificationand appropriate bleaching sequences show that therelease of polychlorinated dibenzo-p-dioxins (PCDD)and polychlorinated dibenzofurans (PCDF)19 inkraft pulp mill effluents can be reduced to relativelylow levels (table 5-4). Similar reductions in PCDDand PCDF have been recorded in prebleachedoxygen pulps. Conventionally bleached softwoodpulps contained levels of PCDD and PCDF ofbetween 9 and 29 ppt; oxygen pulps containedbetween 0.2 and 5 ppt (expressed as TEQ-toxicequivalents) .20 A similar sample of bleached pulpfrom five U.S. kraft mills without oxygen delignifi-cation averaged 13 ppt TCDD and 93 ppt TCDF(TEQ about 22 ppt, within the range of Swedishmills reported above).

21 One U.S. mill with asoftwood oxygen delignification line and a shortbleaching sequence including chlorine dioxide (CD)in the first bleaching stage reports no detectablelevels of TCDD in either bleached pulp or effluent.22

Extended Delignification

Alkaline digestion (cooking) and oxygen deligni-fication are both aimed at reducing the amount oflignin in the wood pulp before it is brightened in thebleach plant. Therefore, oxygen is considered an

ITU.S. EnvirOnmen~ fiotw~ion Agency, Dioxim, EPA-@o/2-80-197 (Cincinnati, OH: EPA Industrial Research Laboratory, 19~).

1X-J. Germgard et al., ‘‘Oxygen Bleaching and IIS Impact on the Environmcnl,’ 1984 Oxygen Delignification Symposium (Atlama, GA: TAPP1 Press,1984), p. 101.

]g~DD ~d ~DF imlude TCDD ad TCDF as ~el] m other rela~ isomers.20fingS~ e[ al., op. cit., note J, PP. 2-3”

Zlu,se Environment~~ot~[ion AgeMy, U.S, EpA/p~ef/ti~f~cOOPeTutive DioxinScreeningStudy, EPA-44M-88025 (Wmhingon, ~: 1988)1p. 76.

22u.s. Environrnentd Protection Agency, op. cit,, note 4 p. 1.


Table 5-2—Effluent Characteristics of Oxygen-Treated Bleached Pulp and PulpBleached by the Common Chlorinated Bleach Processesa (pounds per ton)

Bleach sequence BOD5 COD Color C L -

CEDED . . . . . . . . . . . . . . . . . . . . . 10-39 80-90 180-250 60-70OCED . . . . . . . . . . . . . . . . . . . . . . 10-15 40-50 30-50 30-40

Percent Difference . . . . . . . . . . 45-55 45-50 80-85 35-55a ~W~ ~ 50 ~r~flt mllgnifi~tion,

SOURCE: lrrgarsoll-Rand, Oxygen Deligtifcafion Techrrohgy. An U@afe (Nashua, NH: Ingersoll-Rand, 1986), p. 4.

Table 5-3-Effluent Characteristics for Softwood Kraft PulpsWith and Without Oxygen Delignificationa

(kg/metric ton)

Bleach Sequence BOD5 COD Color TOCL

CDEODED . . . . . . . . . . . . . . . . . . . 15-21 65-75 200-300 5-8OCDEOD . . . . . . . . . . . . . . . . . . . 8-11 30-40 80-120 3-4

Reduction (Percent) . . . . . . . . . 40-50 45-55 60-75 35-50%xygan stage reducbon m kappa number 4550 percent.

SOURCE: lary Tench and Stuart Harper, ~xygen Bleaching Practms and Benefits An Overvtew,” TAPP/ Jouma/, November 1967, p. 56.

Figure 5-5—Phenolic Compounds Formed Withand Without Oxygen Bleaching

Chlor inated phenol ic compounds - -g / t

17 22 33

Kappa number

SOURCE. U Germard et al., “Oxygen Bleachng and Ita Impact on tha Enwronmen[,”I(XJ4 Oqgen &#igrwfrcabon Syrnposrum (Atlanta, GA. TAPPI Press, 1984),p 101.

extension of the cooking process. Retaining the pulpin the digester for a longer period and exposing it toa modified time-temperature-alkaline cycle canreduce the amount of lignin retained in the brown-

stock. There is a practical limit to the amount ofcooking that can be done without dissolving thedesired components of the wood fiber and reducingthe pulp yield. A balance, therefore, between cook-ing and other delignification processes must be used.

A combination of extended cooking—sometimescalled ‘‘modified” cooking—and oxygen delignifi-cation has been used to achieve kappa numbersbetween 10 and 12 for unbleached pulp.23 Othersbelieve that kappa numbers as low as 7 can beachieved without loss of pulp strength.24 Withprebleached pulps of such low lignin content, it maybe technically possible to eliminate the use ofchlorine in the bleaching process altogether.

The standard kraft cooking procedure generallyyields softwood pulp with kappa numbers between30 and 35. Oxygen delignification can reduce thestandard prebleached pulp to 16 to 20, althoughcommercial practice is typically limited to 20 kappabecause of strength considerations. Extended cook-ing plus oxygen treatment can produce pulps withkappa numbers between 7 and 12. By starting thekraft cook at a relatively low concentration of alkali

z3Stig Andtbacka, ‘‘ imw Kappa Puiping Followed by Oxygen Deli unification, Australian Pulp and Paper Industry Technical A.~sociation Journal,March 1986, p. 129.

NKmyr, ]nc, Kmyr con~nmu cooking PIUS &f~dlfied conflnuo~ cooking Plus Medium Consistency Oxygen (Glens Fails, NY: date unkno~).

66 ● Technologies for Reducing Dioxin in the Manufacture ofl Bleached Wood Pulp

Table 5-4-Emissions of PCDD and PCDF (expressed as TEF)a in Receiving WatersFrom Swedish Pulp Mills With and Without Oxygen Delignification

Production TEFPulp Bleaching process (tons/yr) (g/yr)

Without oxygen delignification1 softwood ( C9 5+ D5) (E()) H D 300,000 2-5.8. . . . . . . . . . .

Hardwood (C95+D5) (EO) H D E D2 . . . . . . . . . . . softwood C (E-O) H D (EP) D 125,000 0.7

Hardwood (C10+D90) E D E DWith oxygen delignification3 softwood O (C85+D15) E D E D 160,000 0.4. . . . . . . . . . .

Hardwood O (C70+D15) E D E D 160,0004 . . . . . . . . . . . Softwood O(C85+D15) (EO) D D 75,000 0.7

Hardwood O (C70+D15) (EO) D D5 . . . . . . . . . . . softwood O (C85+D 15) (EO) D E D 235,000 0.1

Hardwood O (C70+Dl 5) (EO) D E D 85,000a TEF IS the ‘ToxIcIty Emwmn Factor- according to G. Eadon as discussed in J.S Ballini and D.G. Barnes, Toxiidogy and /rrtjustria/

Hadth, VOI. 1, No. 4, 1985, F) 235.

SOURCE: Knut P. Krhgstad et al., addendum 10 “BJeachmg and the Environment,” 1988 lntcwwionai Pulp 8Jeaching Conferenceq

Orlando, FL, June 5-9, 1988 (Atlanta, GA: TAPPI, 1988), pp. 2.3.

and lengthening the retention period, low-ligninpulps are produced with comparable strength topulps containing more lignin that are produced bystandard cooking methods.

Extended delignification can be either a batchprocess or a continuous process. Continuous modi-fied cooking digesters have been developed byKamyr, Inc. and others, which split the white liquor(alkali) charge and introduce it into different pointsin the impregnation and cooking stages and at thebase of the reactor (figure 5-6).

As its name implies, extended delignificationrequires a longer cooking time than standard pulp-ing, While standard kraft pulp is cooked for 1 to 3hours, extended delignification, including impreg-nation (steeping) and cooking, may require over 4hours. 25 Initial concentration of alkali in the cookingliquor is about half of that used in the standard cook.By the end of the cook, however, about 2 percentmore alkali is used in the extended process than inthe standard. Pilot tests have shown that pulp lignin

concentration at the end of extended delignificationmay be 40 to 60 percent of that contained in standardpulp.

Pretreatment of Oxygen Pulps WithNitrogen Dioxide (PRENOX)

Laboratory experiments and pilot plant studieshave demonstrated that treatment of pulp with acombination of nitrogen dioxide and oxygen prior tooxygen delignification can also reduce the lignin inprebleached pulps.

26 Nitrogen dioxide has beenshown to promote more selective delignification inthe oxygen delignification stage, thus resulting inless damage to the fiber.27 The upper limit ofdelignification for most softwood kraft pulps isconsidered to be about 50 to 55 percent for oxygentreatment. Delignification to these limits, however,causes fiber degradation. To avoid this, oxygendelignitication is generally reduced to about 40 to 45percent. By using PRENOX before the oxygenstage, delignification rates of about 75 percent(kappa number 8-10) may be possible to achieve.28

25 Andtbacka, op. cit., note 23, p. 130.zbRolf Brannland C[ al., “Oxidation of Pulp With NO~Oz Prior to Oxygen Delignification-A Novel Process With Potentially tss Pollution,’ S-93E;

04/87 2000 MarknadsRadct, Reprinted by Sunds Defibrator, May 1986.ZTD. Lachena] and C, DE Choudens, “High Efficiency Oxygen and Peroxide fkhgnlficatlon, ’ Cellulose Chem. Technol., vol. 20, p. 557,ZgBryan L, Sorensen, ‘‘New Bleach Plants: A Review of Present State of the Art, f 987 Bleach P/ant Operations Seminar (Atlanta, GA: TA.PP1 Press,

1987), p. 183,


Figure 5-6-Continuous Digester for Extended DelignificationChips in

To recovery

diffuser

SOURCE: Stg Andtticka, “Low Kappa Pulping Followad by Oxygan Deligrufwation,” Ausfrahan Pulp and PaPar Industry Twhmcal Association Journal, March 1986, p. f30.

Soda-Anthraquinone/Oxygen PulpingAlthough the kraft pulping process efficiently

produces pulps with unmatched quality and may beused with any wood species, there are continuingefforts to develop a pulping system that eliminatessulfur chemicals from the pulping process becauseof the odor and environmental concerns. The mostexpensive capital investment in a kraft pulp mill isthe chemical recovery plant. A soda-anthraquinoneplant requires a similar recovery plant to that of aconventional system, however, the odor controlequipment used in conventional mills is not needed.

The soda process was a forerunner of the kraftprocess. 29 Its major drawbacks are low pulp yieldsand inferior pulp quality that result from longcooking times, high temperatures, and the strongsolution of sodium hydroxide needed to producebleachable 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 pulpyields. Anthraquinones have also been used with

kraft pulping, but a larger amount of the expensivechemical must be used to be effective and residualamounts of AQ can interfere with the chemicalrecovery plant. Furthermore, AQ is regulated by theFood and Drug Administration, and only smallresidual amounts are permitted in finished products.

Laboratory experiments with a two-stage soda/AQ-oxygen-sodium hydroxide delignification of hard-wood has produced pulp of kappa numbers between10 to 12 with about 5 percent higher pulp yields thancomparable kraft pulp. Lignin content of softwoodspulped (kappa number 20) by the two-stage processdid not match those of the hardwood pulp, how-ever.30 Researchers found that to avoid fiber degra-dation, the soda cooking must be stopped at a highkappa number and the remainder of delignificationdone with oxygen.

DISPLACEMENT BLEACHINGThere is no difference between the chemistry of

displacement bleaching and that used in conven-tional bleaching, only in the efficiency of the mass

2gHutch Holton, * ISoftW~d Pulping: A Major New pr~esst “ Pulp& Paper Canada, vol. 78, No, 10, Oclober 1977, p. 19.30y.C, Tsal, H.m. ch~g, ~d J,S Gra~,], $ ‘optimi~ati~n of Soda-AQ/Oxygen Pu]ping Of southern pine, ‘‘ 1984 Oxygen Delign.ij?cation Symposium

(Atlanta, GA: TAPPI Press, 1984), p. 31.


transfer. 31 When bleaching chemicals are displacedthrough a pulp mat, the fibers are continuallyexposed to highly concentrated bleaching chemi-cals, therefore bleaching is very rapid.

Displacement (sometimes referred to as “dy-namic”) bleaching is generally conducted in amultistage displacement tower (figure 5-7). Filtratewithdrawn at each stage is supplemented withmakeup chemicals and reused. Unbleached pulpmoves sequentially upward, while white liquormoves downward through the bleaching column.Displacement bleaching uses a minimum of water,thus reducing the amount of waste effluent andenabling the recycling of bleaching chemicals. Thesystem is suited for use in a “closed-mill” configu-ration (see above).

Any water soluble bleaching chemical, includingchlorine gas, can be used in displacement bleaching.Softwood and hardwood pulps of 86 to 88 GEbrightness have been produced by displacementsystems. 32 Displacement bleaching technology hasproduced mixed results. Although it requires morechemicals, and causes corrosion and scaling prob-lems in some instances, displacement bleaching isstill considered promising.

CLOSED-CYCLE BLEACHEDPULP MILL

During the past 20 years the concept of aclosed-cycle pulp mill that could significantly re-duce the amount of waste effluents has receivedsome attention. The closed-cycle concept involves:

●

●

●

reducing the amount of water consumed byusing filtrate as wash water,cycling the spent bleaching chemicals to thechemical recovery plant, andrecovering the salts introduced into the recov-ery system for either re-use in manufacturingthe bleaching chemicals or for disposal (figure5-8).

By closing the water cycle of a bleached kraftmill, the demand for heat is reduced and consider-able energy savings can result. Excess heat producedin the process is disposed of as low-grade waste heatinto cooling waters. A closed-cycle mill bleach plantmight use only 3,900 gallons per air-dry ton of pulpproduced, while an average North American Millwould use 20,000 gallons.33 A mixture of chlorinedioxide and chlorine gas is substituted for purechlorine in the first bleaching stage to avoidreducing the pulp strength during the high-temperature-chlorination (60 °C).34

Pulp manufacture and bleaching, being chemicalprocesses, obey the general principles of chemicalcombination and mass balance, In a mill in whichwater and all chemicals are recycled, all of thechemicals must be kept in proper balance or either anexcess or shortage of bleaching chemicals willoccur. An excess of certain chemicals, particularlysodium chloride (common salt), can contribute tocorrosion of the recovery boiler. Some of therecovered salt can be reused to produce chlorinedioxide, but the excess must be disposed of.Corrosion-resistant materials and careful chemicalcontrol have largely overcome these problems.

The Great Lakes Paper Co. at Thunderbay,Ontario, installed the first closed-cycle mill in 1977,but the process has not been successful, and the millis not now operating in the closed-cycle mode.Severe operating problems have been encounteredwhich have resulted in abandoning the process. Ifthese problems could be overcome by furtherdevelopment, the closed-cycle concept might reducethe cost of biological waste treatment, reduce energycosts, increase fiber yield, decrease water usage, andreduce chemical costs.

It is possible that some variation of the closed-cycle bleached kraft mill concept could be devel-oped to incorporate oxygen delignification, ex-tended delignification, or displacement bleaching,thus avoiding some of the problems encounteredwith conventional bleaching sequences.

31 JAm Gu\lich~n, “Displacement Bleaching, ” The Bleaching of PuJp-Third Edition (AtlanQ GA: TAPPI press, 1979), p. 276.gzsp~wr, op. cit., note Z P. 170,

33w. Howard Rapson, “The Closed-Cycle Bleached K.raft Pulp Mill,” The Bleaching of Pu&Third Edirion (Atlanta, GA: TAPPI Press, 1979),p. 415.

34Spfi&r, 0p. cit., note 5, P. ’29.

Chapter 5-Technologies for Reducing Chlorinated Organics in Pulp Munufucture ● 69

Fgaure 5-7-Multistage Displacement Bleach System Single Tower Bleach PlantShowing an EDED Wash Sequence

Clo

SOURCE: John Gullichsen, “Dlsplacament Blaachmg,” 7ba Bleaching of Pu/~Thrd Edtion (Atlanta, GA: TAPPI Press, 1979)4 p 288.

WASTE-OUTLET TREATMENTTECHNOLOGIES

Several physical and chemical treatments havebeen developed to cleanse pulp and paper effluents.Waste treatment processes include resin separationand ion exchange; use of chemicals such as alumi-num oxide, metallic ions, lime, amines, and ozone;adsorption by wood; biological treatments such asenzymes and bacteria; filtration systems such as

activated charcoal; membrane separation; and ul-trafiltration, irradiation, and reverse osmosis. It istechnically feasible, but extremely costly, to cleanpulp-mill wastes to the purity of drinking water. Injudging the economics of waste treatment systems,the cost of the exotic waste treatment technologiesis generally compared with the cost of secondarybiological treatment, which is the U.S. industrystandard for removing most foreign materials otherthan color.


Figure 5-8—Closed-Cycle Bleached Kraft Pulp Mill

W a t e r

C o n d e n s i n g

Water Condensate I S t r i p p i n gW h i t e l i q u o r

e v a p o r a t o r a t e rL i q u o r

p r e p a r a t i o n

F u r n a c e

+

B l a c k I i q u o r

W a t e r e v a p o r a t o r

W a t e r

F r e s hw a t e r

D c E D E D /W a s h i n g 4 o x y g e n

Unbleached Pulp

SOURCE: W. Howard Fhpson, “The Cboed-Cyclo Bbahed Xrafl Pulp Mall,” The BJsactnng of Fu/p-Third Et#tion (Atlanta, GA: TAPPI Prwso, 1979), p. 414.

The effectiveness of any waste treatment systemis related to the internal processes used for delignify -ing and brightening the pulp. The load on the wastetreatment plant can be reduced by using some of thetechnologies discussed above, e.g., oxygen deligni-fication, soda-AQ pulping, or displacement bleach-ing. Use of bleaching sequences including oxygen,chlorine dioxide, hypochlorite, peroxide, or ozonecan also change the nature of the treatable wastes.Waste treatment and pulping and bleaching tech-nologies, therefore, may be considered as an integralsystem that can be balanced to achieve the desiredperformance standards.

Theoretically, the optimum way to make wastetreatment a less costly venture is to integrate it intothe manufacturing process. Benefits can then begained from conservation of raw material, fibers,

additives, energy, and water. Cost analyses ofinternal controls v. external controls generally showthat internal modifications may be less capitalintensive.35 On the other hand, external pollutioncontrol does not interfere with established manufac-turing processes and provides flexibility for a rangeof operating conditions and exigencies. Realisti-cally, successful control probably requires a combi-nation of both.

Color and solids are the first targets for cleanup ofthe pulp waste stream as it leaves the mill outlets.Untreated pulp-mill effluents are generally of a deepmahogany color. Most of that color is derived fromthe bleaching process, and most of that is from thecaustic extraction process after the first bleachingstage. Most of the color is attributable to lignin in thewaste effluent. Lignin is resistant to biodegradation,

35H. Gehm, State-o f.~e.Art Revj~, of Pulp and Paper Waste Management, EPA ‘kchnology Series EPA-R2-73- 184 (Washington, DC: U.S.Government Printing Office, 1973), p. 32.

Chapter 5-Technologies for Reducing Chlorinated Organics in Pulp Manufacture ● 71

and secondary biological treatment plants removeonly 30 percent or less of the color component.36

Primary Treatment of Suspended Solids

Suspended solids in the waste stream consist ofbark, fiber, fillers, clay, and coloring agents. Theseare removed by sedimentation, flotation, and screen-ing. Grit chambers, screens, and chemicals are usedas sediment clarifiers to remove grit. Removal ofsuspended solids and grit currently constitute pri-mary (first stage) treatment of pulp and paper millwaste.

Secondary Biological Treatment

Removal of suspended solids in the first stage isgenerally followed by biological waste treatmentthat is principally aimed at reducing the BOD oftreated water. Secondary biological treatment haslittle effect on effluent color, but it may significantlyreduce levels of toxic pollutants .37 As its nameimplies, biological treatment relies on the assimila-tion and conversion of potential pollutants in thewaste effluent by bacteria, fungi, algae, and otherliving organisms. Since biological treatment relieson the physiological processes of living plants andanimals to reduce the pollution load, the secondstage of waste treatment is similar to farming-theplants and animals must be kept healthy, productive,and reproducing. In order to promote biologicalactivity, adequate air and nutrients must be providedto the biota.

Biological treatment can remove 80 to 95 percentof the BOD. Research has shown that chlorinatedbleach plant derivatives are more difficult to degradeby biological processes than nonchlorinated wastes.Several biological treatment systems are currentlyused to treat pulp mill waste (e.g, oxidation ponds,activated sludge, and aerated basins). Emergingbiological treatment technologies include: rotatingbiological surfaces, fixed-film activated sludge,aerated activated carbon, and deep tank aeration.

Bench-scale wastewater treatability studies con-ducted by EPA indicated that the addition of alum or

lime can remove more than 95 percent of TCDD andTCDF in bleach plant wastewater by improving therecovery of suspended sediments to which theyadhere. Use of chemical treatments would probablyrequire additional clarification and sludge dewater-ing facilities at most mills. The application of anon-ionic polymer to an oxidation pond reducedTCDD and TCDF to less than detectable levels.38

Oxidation Ponds

Oxidation ponds or basins depend primarily onsurface exchange with the atmosphere for aeration,although some oxygen may be supplied photosyn-thetically by aquatic plants. Large surface areas aregenerally needed to provide sufficient air to main-tain biological activity, therefore oxidation pondstend to be large and shallow. Since the rate ofbiological activity increases with temperature, oxi-dation ponds work best in warmer southern climates.Oxidation ponds are relatively inexpensive, requirelittle mechanical equipment, produce little secon-dary waste products that must be disposed of, and inemergencies can serve as temporary impoundmentsshould an accidental discharge of harmful chemicalsoccur in the mill. Racetrack-shaped oxidationditches are sometimes used to eliminate the primaryclarification stage. Oxidation ditches require me-chanical aeration because of the smaller watersurface and perform more like an extended aerationactivated sludge process than like a conventionaloxidation pond system.

Oxidation basins are frequently equipped withaerators to increase the rate of biological activity(aerated stabilization basins). Nitrogen and phos-phorous fertilizers are sometimes added if the wastestream is nutrient deficient. These supplementaltreatments can reduce the retention time in theoxidation pond to 8 to 10 days in order to reduceBOD to a low level. The addition of mechanicalaerators increases the cost of oxidation ponds, butthey are generally cheaper than activated sludgesystems.

36spr~ger, op. cit. , note j! PO 182

37c,c, w~den ad J,c, Mu~l]er, ]nve~tigafion of the Effe~l of ~OD Reduction syste~$ on ro~”~’1~, CPAR Rept, FJo, 150-] (Ottiwa, otlt: CanadianForest Service, 1973).

3SU.S. Environmental Protection Agency, op. cit., note 4, P. 9.


Activated Sludge

Activated sludge treatment systems are often usedwhere space limits the use of oxidation ponds. Anadaptation from sanitary sewage treatment, acti-vated sludge is a high-rate biological process thatcan reduce waste treatment retention time to 3 to 8hours. The biological mass that is produced in theaeration stage of the treatment process is separatedin a secondary clarifier and the active biologicalcomponents are returned to the process, thus furtheraccelerating biological activity.

Activated sludge systems are flexible and can beadapted to treat a wide range of wastes. However,these systems cannot withstand shocks from emer-gency mill releases nearly as well as oxidationponds, and the biological balance of the process ismore sensitive to chemical and biological perturba-tions. The capital and operating cost of activatedsludge systems are generally twice as great asaerated ponds.

The pulp and paper industry is the largest user ofpure oxygen-activated sludge (UNOX) technology,with 16 plants in operation in 1986.39 Pure oxygensystems can remove 87 to 97 percent of BOD. Theprocess has several advantages over conventionalactivated sludge systems such as, smaller aerationtanks required better tolerance to “shock loading,”and better sludge settling. A buildup of carbondioxide at some UNOX plants has raised concernabout the potential toxicity to fish. Retention time inpure oxygen systems is about 3 to 4 hours.

Two-stage activated sludge systems, such as theZurn Attisholz process, using two oxygen levelssequentially with high recycling rates can reduceBOD 95 percent and the process is very stable. Costof the two-stage process may be slightly less thanwith conventional activated sludge.40 There is someevidence that the two-stage oxygen activation proc-ess is more efficient in reducing toxicity of kraftwastewater. 41

New or Developing Treatment Technologies

A number of of coagulant that remove color havebeen tested. Although several are effective, e.g.,alum, ferric sulfate, sulfuric acid, activated carbon,etc., they are expensive. Lime is the least costlyprecipitation agent, and can be reclaimed in a kraftmill by oxidizing in the lime kiln. Pretreatment ofwaste effluents with enzymes before precipitationwith lime can increase efficiency, as can theadditions of magnesium sulfate.

Fungi have also been used to remove color fromwastewater. White rot fungus (Phanerochaete chryso-sporium burds) can metabolize the lignin responsi-ble for most of the color of wastewaters. It can alsoeliminate toxic chlorinated and halogenated com-pounds from the waste stream, including dioxins.42

Although the process (MYCOR) is economical, thefungi 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 frompulping and bleaching wastes, but specialized syn-thetic resins seem promising. Resins are sensitive tooverloading from suspended solids and contami-nants that reduce their effectiveness, thus they havebeen used primarily on both small waste streams andisolated output from the chlorination and firstextractive stages of the bleaching sequence. Ionexchange resins have thus far found only limited usein the pulp and paper industry. Activated carbon,because of its large adsorption surface area, is aneffective scrubbing agent. The charcoal process hasbeen used commercially, but its high cost andregeneration requirements make it less attractive.The efficiency of membrane processes is consideredgood, but their costs remain high.

Rotating Biological Surface

A new developing technology, rotating biologicalsurfaces (RBS), involves rotating polyethylene discsalternately through the wastewater and into the air asthey rotate on a shaft. The process is analogous to the

39springer, op. cit., note 5> P. 2W0

@Ibid,, p. 242.dlwmte Water ~hnoloW Cen(er, An Assessme~ of Kr@ B[eaChery Efluent Toxicity Red~tLOn Using Actlvafe(f sludge, EPS 4-W-77-3 (ottawa,

Ont: Environment Canada, 1977).42u,s, &vlromenla] ~otwtlon Age~y, Natio~/ f)loxln study, Ep~530-sw-87-@5 (Washington, DC: 1987), p. Vi-2.

Chapter 5—-Technologies for Reducing Chlorinated Organics in Pulp Manufacture . 73

simple “trickle filter’ where wastewater is perco-lated through a porous medium exposed to air in theinterstices. RBS usually operates in three stages, andremoval of up to 90 percent of the BOD has beenachieved with this technology. Efficiency of theprocess is proportional to the surface area of thediscs, rotation speed, and submergence depth of thediscs. Application of RBS is probably limited tosmall- or medium-size pulp mills because of costs inscaling the equipment to large volumes. In smalleroperations the cost of RBS is about the same as theactivated sludge process. Although pilot-scale test-ing of RBS has shown promise for removing BODand toxicity, mill-scale studies have encounteredoperating difficulty .43

Innovations in Activated Sludge Technology

High-Rate, Fixed-Film Activated Sludge—Solid particles (e.g., clays, sand, or calcium carbon-ate) added to activated sludge systems can improvesludge settling and prolong solids retention. Cal-cium carbonate has been particularly effective be-cause of its buffering capacity with regard to acidcomponents in the waste effluent. The particlesdevelop a fixed-film growth, allowing a high level ofbiological growth to develop on the expansiveparticle surfaces. Preliminary studies indicate thatnearly all of the solids can be retained in the systemexcept a small amount that is lost with the treatedwater, Most of the solids are lost through therespiratory processes as carbon dioxide and water.

The oxitron process is a variation of the fixed-filmactivated sludge system. Effluent, free of suspendedsolids, is fed to a fluidized bed of sand through whichpure oxygen is injected. Biological growth developsaround the sand particles, finally causing them torise in the fluidized bed where they are selectivelyremoved. The sand is later cleaned of the growth andreturned to the fluidized bed. Tests of the processshow it to be efficient and compact, but it is unableto tolerate any solids in the wastes to be treated. Theoxitron system is best suited for large waste volumesof 2 to 3 million gallons daily. There are nocommercial installations in service, but new tech-nologies may utilize elements of the process.

Aerated Activated Carbon Filter—Experimentshave shown that due to its immense surface areaactivated charcoal is an efficient filter medium.Coarse granular charcoal placed in a filter bed hasbeen shown to be effective in removing contami-nants from waste effluent. To keep the systemaerobic, air is pumped through the filter bed. Thesystem is tolerant of excessive suspended solids, butthe 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 thepulp and paper industry.

Captor System—An English innovation, the Cap-tor System is similar to the conventional diffused-aeration activated sludge process, but it encapsulatesthe biological growth in small polyurethanesponges. With the biomass contained in the sponges,the treated effluent is discharged through a screensized to retain the impregnated sponges. Sponges arecleaned for reuse by squeezing through a wringer.Developers claim that a secondary clarifier is notneeded, therefore the plant size may be reduced byabout 20 percent from that of a conventionalactivated sludge plant. The Captor System has beenused for municipal wastes, but it is not certain thatthis treatment would be sufficient to meet U.S. waterstandards. It may prove useful in upgrading existingoverloaded biological treatment systems.

Deep Tank Aeration-Deep tank aeration in-creases the amount of time that air is in contact withthe waste effluent. As its name implies, the processuses a deep tank or deep shaft as an aerationchamber. This eliminates the need for a secondaryclarifier but still requires a floating clarifier. Deepaeration occupies less space than conventionalactivated sludge systems and is capable of handlingemergency ‘‘shock’ loads from the paper mill.

Anaerobic Treatment

Anaerobic waste treatment is neither new, norinnovative. However, with reduced water usage bymany of the modem pulp mills, there has been ageneral increase in the potency of the pollution load,and anaerobic treatment is being reconsidered as ameans to improve waste treatment. Anaerobic sys-tems are well suited for treating high-strength waste,

43u.$ Environment~ ~otec[ion Agency, Devefop~nt Document for Efluenr Limitatwns Guidehnes and Sta&rds for the pulP, p~ert atiPaperboard and the Btulders” Paper and Board Mills: Point Source Categories, EPA 440/1-82/025 (Washington DC: 1982), p. 342.


but with modifications such as, attached-film andexpanded bed reactors, it may also be possible totreat low-strength waste anaerobically as well.

Anaerobic (absence of oxygen) decomposition isa microbial process, which is primarily dependent onbacteria. Unlike aerobic processes (e.g., activatedsludge and oxidation ponds to which air is supplied)anaerobic processes depend on bacterial action thatobtains oxygen from sulfate and nitrate ions in thewaste stream. When applied in the pulp and paper

industry, anaerobic treatment is generally used as awaste pretreatment before release to the aerobicstages of the treatment process. Anaerobic systemsare sensitive to imbalances in the ambient waste, butthe process is durable and can be applied to a widerange of waste effluents. High temperatures areneeded to reduce treatment time, long startupperiods are required, odor emissions can be signifi-cant, and there is difficulty in achieving low BODlevels with anaerobic decomposition.