Technologies for Reducing Dioxin in the

Manufacture of Bleached Wood Pulp

May 1989

NTIS order #PB89-223291
Recommended Citation:

U.S. Congress, Office of Technology Assessment, Technologies for Reducing Dioxin in the

Manufacture of Bleached Wood Pulp, OTA-BP-O-54 (Washington, DC: U.S. Government

Printing Office, May 1989),

Library of Congress Catalog Card Number 89-600719

For sale by the Superintendent of Documents

U.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 many

products commonly found in the home and workplace. Recently, dioxins have been detected

in wastes resulting from the manufacture of wood pulp. Paper products made from wood pulp

are used in tremendous volumes for food packaging, hygienic products, printing paper, writing

paper, paperboard for shipping containers, and numerous other household items. Between 650

and 700 pounds of paper products are used annually by each American, with domestic and

foreign consumption continuing to 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 the

formation 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 the

problem is not yet known. Studies currently under way by the Environmental Protection

Agency, the Food and Drug Administration, the Consumer Products Safety Commission, the

National Institute for Occupational Safety and Health, and the U.S. paper industry will shed

light on this when completed later in 1989.

Alternative technologies using oxygen as a pretreatment to chlorine bleaching and

improved delignification that removes more of the potential reactants from the wood can

reduce the amount of dioxin in bleached pulp. Substituting other bleaching chemicals for

chlorine also shows promise for reducing the amount of dioxin produced in the bleaching

process should regulation be required. This study provides an assessment of these

technologies; it does not address the policy issues related to regulating dioxin in paper

products and controlling environmental release.

u Director

iii
Workshop Participants: Technologies for Reducing Dioxin

in the Manufacture of Bleached Wood Pulp

Michael Babich

U.S. Consumer Product Safety Commission

Phillip Cook

U.S. Environmental Protection Agency

Michael Crandall

National Institute of Occupational Safety and Health

Ronald Estridge

James River Corp.

David Firestone

U.S. Food and Drug Administration

Karen Florini

Environmental Defense Fund

William Gillespie

National Council of the Paper Industry for Air and

Stream Improvement

Michael Gough

Resources for the Future

Russell Keenan

Envirologic Data

Gregory Kramer

U.S. Food and Drug Administration

William Kraske

Boise Cascade Corp.

Renata Kroesa

Greenpeace Foundation

Steve Kroner

U.S. Environmental Protection Agency

Russell Kross

Mead Corp.

Thomas O’Farrell

U.S. Environmental Protection Agency

Greg Schweer

U.S. Environmental Protection Agency

David Stallings

U.S. Fish and Wildlife Service

Mel Stratmeyer

U.S. Food and Drug Administration

Clare Sullivan

United Paperworkers International Union

Dwain Winters

U.S. Environmental Protection Agency

NOTE: OTA is grateful for the valuable assistance and thoughtful critiques provided by the workshop participants. The views

expressed in this OTA background paper, however, are the sole responsibility of the Office of Technology Assessment.

iv
OTA Project Staff: Technologies for Reducing Dioxin

in the Manufacture of Bleached Wood Pulp

John Andelin, Assistant Director, OTA

Science, 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
Contents

Chapter

1. Introduction, Summary, and Findings . . . . . . . . . . . . . . . . . . . Page

2. The Pulp and Paper Making Processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

. . . . . . . . . . . . . . . . . . 3. Environmental Considerations ... .. .. ...... .. .. ... .. . ... ......: . . 172 9

 . . . . . . ..

4. Pulp Bleaching Technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . ....... 41

5. Technologies for Reducing Chlorinated Organics in Pulp Manufacture . . .. . .. . .. ........ 55

vi
Chapter 1

Introduction, Summary,

and Findings
CONTENTS

Page

PURPOSE AND SCOPE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

THE U.S. PULP AND PAPER INDUSTRY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

SUMMARY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

The Pulp and Paper Making Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Environmental Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

3

4

5

5

6

Pulp Bleaching Technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

Technologies 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 as

chlorinated dibenzo-para-dioxins and chlorinated

dibenzofurans. The most toxic member of this

large family of compounds is 2,3,7,8 -tetrachlorop-dibenzodioxin (2,3,7,8 -TCDD or TCDD); the

closely related compound 2,3,7,8 -tetrachlorodibenzofuran (2,3,7,8 -TCDF or TCDF) is believed to be about one-tenth as potent. Both of

these chemicals can be formed during the

manufacture of bleached wood pulp if chlorine

is used as a bleaching agent. The amount of

TCDD and TCDF formed in the bleaching

process is measured in parts per trillion (ppt).

The relative toxicity of these chemicals is

measured in ppt and expressed as “Toxic

Equivalents” (TEQs) using TCDD as the standard. TCDD is considered to be highly toxic

based on laboratory animal experiments and has

been linked to malignancies, birth defects, and

physical deterioration in animals, Evidence of

human health effects is less certain and remains

a contentious issue among scientists. The U.S.

Environmental Protection Agency (EPA) classifies it as a “probable human carcinogen.”

The release of EPA’s National Dioxin Study in

1987 focused attention on evidence that showed

a pattern of dioxin concentrations in stream

bottom sediment and fish below pulp mill waste

outfalls. Subsequent studies by the paper industries and government agencies in North America

and Europe have confirmed that detectable

amounts of TCDD and TCDF are produced and

released into the environment at many bleached

kraft pulp mills. Measurements of dioxin levels

in the three sources of pollution (pulp, effluent,

and sludge) are being conducted at all U.S.

chemical pulp mills using chlorine bleach under

the joint sponsorship of EPA and the American

paper industry. The results of these studies and

others underway in Canada and Europe will

- 3 -

contribute much more to the information base

now available, but final results are months away.

Bleached wood pulp is used in paper products

such as writing and printing papers, rayon,

tissues, towels, disposable diapers, food packaging, and a myriad of other products. Paper is

used so widely in modern societies that every

person comes in contact with bleached paper

products almost daily. The knowledge that

TCDD and TCDF are produced and released

during the manufacture of chlorine-bleached

kraft pulp has prompted action by the Federal

agencies charged with protecting health and the

environment and has stimulated research by a

paper industry faced with increasing public

concern and, as a consequence, possible regulatory action.

Environmental and consumer groups have

organized an international action program to

inform the public, prompt consumer action, and

stimulate political and regulatory action in the

pulp-producing countries of North America and

Europe. The United Paperworkers International

Union, the labor union that represents many of

the paper mill employees in North America, and

the National Institute for Occupational Health

(NIOSH) are monitoring the potential health

impacts of exposure of workers to dioxin in pulp

and paper mills.

This study reviews what is known about the

production and fate of TCDD and TCDF and

other associated chlorinated compounds during

the course of pulp bleaching and brightening.

The study is specifically aimed at assessing the

state of pulping and bleaching technologies that

show promise for reducing the amount of those

chemicals resulting from the manufacture of

bleached kraft pulp, such as extended delignification, oxygen delignification, the use of bleaching chemicals that might substitute for some or

all of the chlorine commonly used, modifications of the timing, amount, and operating
4 • Technologies for Reducing Dioxin in the Manufacture of Bleached Wood Pulp

conditions of the chlorine bleach sequence, and

a review of pulping and/or bleaching technologies that are not yet commercially available, but

might be in the future.

Because this is a technical background report,

neither an assessment of policy options related

to the need for regulatory action, nor the form

such regulations might take, are included. Furthermore, this assessment assumes—based on past

trends and industry projections—that a strong

demand for high-brightness bleached pulp and

paper products will continue in the future.

Should the public boycott or avoid the use of

bleached paper products in the future because of

a perceived health risk, the paper industry would

have to adjust to the changing markets within its

own constraints.

Markets for unbleached hygienic paper products, disposable diapers, coffee filters, and milk

cartons, for example, have developed in Sweden

where “environment-graded” or “environmentpositive” lines of unbleached paper products are

retailed along with bleached grades. Recently,

the Canadian pulp and paper industry has agreed

to reduce the amount of dioxin in paper stock

used for milk containers after Health and

Welfare Canada, a government agency, discovered trace amounts of dioxin leaching into milk

from plastic-coated paper cartons. The pulp and

paper industry claims that paper products requiring high strength and long-term durability

generally must be made with a highly bleached

pulp, and industry analysts believe that this will

likely continue to be a major proportion of the

pulp produced.

Market pulp is a commodity traded widely in

world markets. The possibility exists that some

countries in the future may impose limits on the

amount of residual chlorine compounds or

dioxin in pulp and paper products that can be

imported. If similar collective action were to be

taken by a large number of trading partners, such

as the European Economic Community or a

coalition of Pacific Rim countries, a significant

portion of North America’s export market could

be affected. If such trade restrictions were

broadly adopted, it could, for practical purposes,

impose de facto international performance standards on those firms wanting to compete in

international markets.

OTA convened a workshop in November

1988 to discuss the latest information regarding

three aspects of dioxin and bleached wood pulp

manufacture:

1.

2.

3.

What is known about the distribution and

effects of TCDD and TCDF resulting from

environmental releases by pulp mills?

What is known about the amount of

residual TCDD and TCDF in paper products, and what is the level of risk to

consumers and mill workers?

What is the current understanding of the

precursors that react with chlorine to form

TCDD and TCDF, and what is known

about the means to reduce their formation?

As a result of the workshop, we were impressed

by how fast knowledge about TCDD and TCDF

in bleached wood pulp is accumulating. This

report is on] y a “snapshot” of the technology and

knowledge about the formation of dioxin and

possible means to reduce it as of December

1988.

THE U.S. PULP AND PAPER

INDUSTRY

The United States leads the world in per

capita paper consumption. In 1986 U.S. consumers used over 660 pounds of paper products

per person.

1 This new record of paper consumption continues the steady increase in the domestic use of paper that saw per capita consumption

rise about 16 percent between 1976 and 1986.

Paper, which is indirectly correlated with genl~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 consumed at the rate of about 21,000 tons per billion

dollars of the real gross national product (GNP)

generated. For a variety of reasons, including the

reduction of solid waste, efforts to recycle scrap

paper are expanding, particularly in urban areas.

Nearly 73 million tons of paper, board, and

construction board were produced in the United

States in 1986.2 Although 4.8 million tons of

paper products were exported by U.S. producers, almost 12 million tons were imported for

domestic consumption. Most of the imports

originated in Canada and were newsprint. More

than 70 percent of U.S. exports and 80 percent

of imports in 1987 were bleached or semibleached pulps.3

Pulp products valued at $3.9 billion were

shipped from U.S. pulpmills in 1987. The paper

and allied products industry (SIC 26) employed

over 674,000 persons in 1986, with about

one-third of those directly involved in the

production of paper and pulp (SIC 261, 262,

266).4 In general, pulp mills have tended to

concentrate in the South and Northwest near

major sources of wood, while allied industries

and conversion facilities are more broadly

distributed near primary markets.

Future prospects for the pulp and paper

industry are bright. In the Pacific Rim countries

where industrial expansion is expected to increase dramatically, forest-deficient countries

like Japan, Korea, and Taiwan are considered

large potential markets. The United States and

Canada, with their vast forest resources, their

established industries, and technological capacity, are well positioned to take advantage of new

export markets. However, international competition for new markets is sharpening, with overseas suppliers in Brazil, Portugal, Spain, Morocco, and South Africa expanding their capacity.

SUMMARY

The Pulp and Paper Making Process

The manufacture of pulp and paper involves

the separation and purification of cellulosic

wood fibers from which paper is formed. About

half of the wood raw material is cellulosic fiber,

and half is lignin, hemicellulose, and other

extractive compounds that cement and strengthen the fibers. The pulping process involves

either the use of chemicals, heat, and pressure in

a digester to dissolve the lignin and free the

fibers from one another or, in the case of

mechanical pulp and chemical-mechanical pulp,

the abrasion of wood in a grinder or refiner to

physically 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 tk

U.S. Depactmenl of Commerce.
6 • Technologies for Reducing Dioxin in the Manufacture of Bleached Wood PUlp

New pulping technologies are emerging, but

their acceptance will depend on efficiency, pulp

quality, cost effectiveness, or other factors that

make them competitive with conventional technology. To the extent that new pulping systems

might reduce the amount of residual lignin

carried over to the bleaching process, the

amount of chemical bleaching required to attain

the desired brightness might be reduced and

subsequently the amount of chlorine used as a

bleaching agent trimmed.

Environmental Considerations

Effluents from bleached pulp mills contain a

variety of substances, some of which are known

or suspected of being toxic, genotoxic, mutagenic, or teratogenic. Chlorinated organics,

including TCDD and TCDF, that appear to be

produced in the chlorine bleaching and delignification processes, are of particular concern. The composition of bleaching effluent is

extremely complex and varies widely from mill

to mill. The chlorinated components of the

waste stream consist of a variety of compounds,

including simple phenols, high and low molecular weight polymers, and neutral and acidic

materials from the breakdown of the phenolic

rings in lignin. An average North American pulp

mill produces between 35 and 50 tons of

chlorinated substances daily. Greenpeace International, the Environmental Defense Fund, and

other environmental organizations advocate immediately minimizing and eventually eliminating

the use of chlorine in order to avoid additional

releases of these compounds into the environment

through effluent, sludge, or via paper products.

About 10 percent of the total solids in the

waste streams of bleaching plants contain chlorinated derivatives, but this varies among mills

depending on the degree of filtration. Some

European countries, Sweden and the Federal

Republic of Germany in particular, have imposed restrictions on the amount of total organically bound chlorine (TOC1) that they will allow

their pulp and paper industry to discharges

Sweden has cut the amount of allowable TOC1

discharge more than half, from the 5 to 6

kilograms per metric ton (kg/t) of pulp normally

produced with conventional chlorine bleaching,

to 2 kg/t. Oxygen delignification and chlorine

dioxide substitution have been the technology of

choice to meet Sweden’s environmental requirements. The National Swedish Environmental Protection Board hopes to reduce TOC1

to 0.1 kg/t by 2010 and to 1.5 kg/t by 1992. With

further reductions in allowable discharges, the

Swedish industry may need to adopt closedcycle processes to eliminate chlorinated discharges 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 overall

problem of dioxin in the Swedish environment

and concluded that standards established for

reducing TOC1 would suffice to reduce the

dioxin levels from bleached pulp as well.

Chlorinated organic effluents from pulp and

paper manufacture have created environmental

problems in coastal waters and estuaries of the

Baltic Sea, particularly in the enclosed Gulf of

Bothnia. Unlike U.S. pulp mills that use secondary biological sewage treatment technologies to

treat waste effluents, mills in Sweden, some in

Finland, and many in Canada do not use

comparable treatment methods before discharging

into the environment. Biological treatment can

remove large amounts of chlorinated contaminants (particularly those that adhere tightly to

sediments such as TCDD and TCDF) from mill

effluents when operated properly. Biological

treatment transfers the contaminants from the

water and concentrates it in the sludge. The

sludge must then be disposed of in a safe and

acceptable manner. Biological treatment systems require constant attention to ensure that

they are working efficiently.

The Federal Republic of Germany will begin

tightening restrictions on the discharge of chlorinated organics in 1989. Maximum allowed

discharges of organochlorines will be 1.0 kg/t of

bleached pulp under the new regulations. Until

recent] y, West German regulations for pulp mill

discharges have not been overly strict. Fees

levied on discharged amounts of oxygen demand, solids, and measures of toxicity to fish

have allowed the German industry to “buy” the

right to pollute.

Chlorinated organic residues from pulp bleaching have caused less concern in North America

where little research into adverse impacts on

fisheries and aquatic biota has been conducted.

There is ample experimental evidence, however,

that pulp mill effluents, unless adequately

treated, can be toxic to fish and some chlorinated

compounds may ultimately find their way

through the food web. Dioxin has been shown to

be extremely toxic to fish at very low concentrations under controlled laboratory experiments

and has been linked to periodic reductions in

reproductive rates of some bird species. U.S.

mills, unlike their European counterparts, have

installed extensive biological treatment systems

to reduce the biological and chemical oxygen

demand (BOD and COD) of their waste effluents before discharging them into streams. U.S.

environmental requirements for the issuance of

a discharge permit for conventional pollutants,

such as BOD, COD, and suspended solids, and

toxicity tests on mill effluents make biological

waste treatment a preferred technology for U.S.

mills.

The detection of dioxin in some fish samples

reported 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 on

human health. Less than 20 percent of the

randomly selected sites sampled by EPA in the

study showed detectable quantities (more than

one part per trillion [ppt]) in whole fish samples.

Other samples collected at over 300 regional

sites selected by EPA for high probability of

dioxin contamination showed that nearly onethird of the rivers, lakes, coastal waters, and

estuaries contained fish with detectable amounts

of dioxin that ranged up to 85 ppt—significantly

higher than the 25 ppt determined to be unsafe

by the Food and Drug Administration (contained in edible fillets, not in whole fish)-but

it is not certain that pulp mill waste was the

source of all contamination. Only 4 of 57

estuarine and coastal sites sampled in the

National Dioxin Study had detectable levels of

dioxin in fin fish or shellfish. Based on laboratory tests, fish have been shown to accumulate

dioxin in their bodies at rates approximately

20,000 to 85,000 times the concentrations to

which they are exposed in water. Fishing has

been prohibited in several rivers because of

dioxin levels, and advisories have been issued

by Wisconsin, Maine, and Louisiana warning of

the possible risks of eating contaminated fish.

A screening analysis of pulp waste at five

bleached kraft pulp mills, which was conducted

jointly by EPA and the U.S. paper industry, and

other research in Europe and Canada clearly

links the formation of TCDD and TCDF to the

chlorine bleaching process. TCDD was detected

in seven of nine bleached pulps sampled, some

with levels as high as 51 ppt. TCDF, which is

less toxic than TCDD, was found in eight of nine

pulps sampled, with levels ranging up to 330

ppt.

Dioxin was detected in wastewater at four of

the five mills, but concentrations varied greatly

among the mills sampled. The highest levels of

TCDD and TCDF were associated with waste

from the caustic extraction stage, which is

designed to remove the dissolved materials after

the chlorine treatment. EPA and the paper

industry are currently surveying 104 additional

mills to better determine the scope and extent of

dioxin and furan formation. Detailed analyses of

TCDD levels and bleaching processes at 25 pulp

mill bleach lines will be conducted in the course

of the study.

The detection of TCDD and TCDF in

bleached pulp samples raised concern about

whether residual dioxin was being carried forward into finished paper products. The National

Council of the Paper Industry for Air and Stream

Improvement, Inc. (NCASI), the industry’s

environmental research arm, commissioned an

assessment of the potential risks to human

health from skin exposure to a variety of paper

products, including disposable diapers, facial

tissue, toilet tissue, sanitary napkins, coffee

filters, and paper towels. The assessment concluded that TCDD equivalents in all products

tested presented a lifetime cancer risk of less

than one in a million.

Environmentalists note that NCASI’s assessment only considered the risks for 2,3,7,8-

TCDD and 2,3,7,8-TCDF without examining
Chapter 1—Introduction, Summary, and Findings ● 9

the risks from the several hundred other chlorinated byproducts that are also found in paper

products, and that NCASI failed to consider the

cumulative risk of using a wide range of paper

products daily. Environmentalists also fault

NCASI for not using appropriate testing procedures for evaluating the possible enhanced

mobility of dioxin when in the presence of

lipids, solvents, and fats, such as encountered

when using paper towels for cooking or when

using creams and baby oils with disposable

diapers.

OTA learned at its dioxin workshop that there

is considerable disagreement about the validity

of the testing protocols for determining the rate

of migration of dioxin from paper products and

into the human body. For instance, there are

widely differing opinions about how to estimate

the effect of urine leaching from disposable

diapers into the skin of babies, and how to

simulate the environment of a tampon in determining the movement of dioxin into a woman’s

body. Without standard protocols, disagreements

over the meaning of dioxin risk data leaves OTA

with no means to evaluate the industry’s findings and conclusions. Evaluation of these testing

techniques is beyond the scope of this assessment.

Agreement is also lacking on how to express

levels of risk. Does one accept a one in a million

risk of cancer as did the U.S. paper industry in

its study of risks from dermal exposure to

dioxins, or should the acceptable risk be one in

a thousand? There are no Federal regulatory

standards. Arbitrary levels of risk are used for

convenience, and experts differ over the most

appropriate levels to use.

Semantics also contribute to the confusion:

Should dioxin exposure levels be expressed as

“virtually safe dose,” which implies that there

“is a level of exposure at which the cancer risk

is zero,” or should the term “risk specific dose”

be used since it does not imply that there is any

dose above zero that is “safe”?

Couple the uncertainties over testing protocols with disagreement over levels of acceptable

risk, and add to it the lack of consensus about the

potency of dioxin to humans, and a confused

picture is presented to those attempting to gauge

the danger of dioxin from pulp manufacture and

paper use.

OTA is aware of no published studies that

have attempted to define worker exposures to

dioxin in paper mills using environmental measurements or biological exposure measurements,

such as serum or fat TCDD/TCDF concentrations.

The National Institute of Occupational Safety

and Health (NIOSH), the Federal agency

charged with occupational and health research,

is currently developing personal exposure monitoring methods to address pulp and paper

workers’ exposure to dioxin.

EPA is currently coordinating an Interagency

Task Force on Dioxin in Paper aimed at gauging

the cumulative risk of all of the media that could

contribute to human exposure. Other cooperating agencies include the Food and Drug

Administration and the Consumer Product Safety Commission (CPSC). The interagency effort

was characterized by an EPA official at the

November 1988 OTA dioxin workshop as an

effort to “determine whether there is a risk—not

a ‘gold plated’ risk assessment,” and a “snapshot

in time” as to the risk aimed at determining

whether regulation is needed. The interagency

study will build on both the EPA/Industry

104-mill study and estimates of migration rates

of dioxin from paper products. It will include an

assessment, based on product-use scenarios, of

technologies that could reduce dioxin exposure.

OTA found that, as with many other Federal

activities, the analytical and regulatory authority

for dealing with dioxin in paper is fragmented

among several agencies, none of which have

clear responsibility for dealing with the problem

in its entirety. While EPA has jurisdiction over

water pollution and dioxin in pulp (dealt with in

two separate offices), FDA has responsibility
10 ● Technologies for Reducing Dioxin in the Manufacture of Bleached Wood Pulp

for regulating dioxins in coffee filters and

sanitary napkins, and CPSC has responsibility

for dioxin in disposable diapers, paper towels,

facial tissues, and toilet tissues.

The lack of published independent Federal

research on the risks associated with dioxin in

paper products and the absence of government

information on paper worker’s environmental

exposure to dioxin required OTA to rely almost

wholly on industry-sponsored research. The

paucity of government information on workers’

risk and exposure to dioxin in paper products

may change in the future as a result of the

Interagency Task Force on Dioxin in Paper

mentioned above, but in the meantime we have

only industry information available to evaluate

the potential risks from dioxin in paper.

Pulp Bleaching Technology

About 10 percent of the lignin found originally in wood is carried over from the chemical

digestion process to the bleaching stages in kraft

mills. This residual lignin must be substantially

reduced if pure cellulose products or bright

paper are to be produced. The less lignin

retained in the pulp as it leaves the digester, the

less bleaching needed. There are limits, however, as to how much digestion wood fibers can

endure without sacrificing pulp strength and

paper durability or significantly reducing pulp

yield.

Bleaching is a continuation of the delignification process and is used to brighten and purify

the wood pulp. The purer the cellulose and

brighter the paper, the less lignin retained, and

the longer the paper will last without yellowing

and becoming brittle. Chlorine gas has become

the preferred bleaching agent because of its

relatively low cost and high effectiveness.

However, a number of other bleaching chemicals are also used, including hypochlorite,

chlorine dioxide, oxygen, and hydrogen peroxide. Ozone has also been tried under experimental conditions, but is not yet commercially

available. Each has its advantages and disadvantages, and several are usually used in sequence

to achieve the final result. Between bleaching

stages caustic chemicals are often used to

remove the dissolved lignin and wash the fibers

before they are subjected to additional bleaching. It is apparently during this extraction that

much of the dioxin created by chlorine bleaching finds its way into the waste stream.

It has been shown that by reducing the use of

chlorine gas in the bleaching process, the

amount of chlorinated organic residues and

contaminates can be reduced. Oxygen delignification shows some promise for reducing the

amount of lignin carried with the pulp to the

bleach process, thus reducing the amount of

chlorine gas needed to bleach the pulp to the

desired brightness. Current commercial oxygen

delignification technology can reduce the use of

chlorine gas by 40 to 50 percent. Further
Chapter 1—Introduction, Summary, and Findings ● 11

reductions in the use of chlorine by more intense

oxygen delignification is limited by severe

losses in pulp yield and strength properties.

Pretreatment of the pulp with nitrogen dioxide

before oxygen treatment shows promise for

increased delignification before bleaching.

The cost advantage that chlorine gas once had

over many other bleaching agents does not apply

to oxygen. If the U.S. pulp and paper industry is

required to reduce the amount of chlorinated

organics and dioxin produced in bleaching pulp,

oxygen may become a partial substitute for

chlorine because of its low operating cost.

Capital cost for oxygen treatment is very high,

however, and installation of oxygen delignification in existing mills can reduce production

capacity as much as 4 or 5 percent. Any evaluation of the comparative costs of oxygen

technology with other delignification systems

should consider both capital costs, life-cycle

costs based on operating costs and depreciation,

and gains and losses in productivity and quality.

Such analyses were not made by OTA in this

study.

Technologies for Reducing Dioxin

There are several ways to reduce the amount

of pollution contributed by bleach plants. These

include:

●

●

●

●

●

●

●

further delignification of pulp before bleaching;

improved washing of the unbleached pulp

(brownstock);

substitute nonchlorinated bleach agents;

substitute chlorine dioxide for some or all

chlorine gas;

apply multiple additions of chlorine in split

charges instead of using a single, massive

charge (low chlorine multiple);

improve chemical mixing with the pulp;

adjust the acidity of the unbleached stock

before adding chlorine;

use “cleaner” oil-base defoamers that do

not contain dioxin precursors;

remove dioxin precursors prior to treatment of the pulp with chlorine; and

improve secondary waste treatment and

waste-disposal practices.

Pretreatment before bleaching can reduce

the amount of chlorine or other bleaching

chemical used. Technologies for prebleach

delignification include: 1) extended delignification, 2) oxygen delignification, 3)

pretreatment with nitrogen dioxide before

oxygen delingification (PRENOX allows

more lingin to be removed without damaging fibers), 4) pretreatment with other

chemicals such as chlorine dioxide, and 5)

extraction with sodium hydroxide supplemented with oxygen and hydrogen peroxide. Only extended delignification and

oxygen delignification are currently used

commercially.

Other technologies used to conserve water

and reduce energy consumption may also help

reduce the amount of chlorinated products in the

waste stream:

recycle process water from the chlorination

stage (although this may actually compound the dioxin problem),

use countercurrent washing systems after

chlorination, and

reduce water use by using higher ratios of

fiber pulp to water (higher consistency).

Elimination of chlorine in the bleach sequence combined with internal recycling of

process water aimed at developing a “pollutionfree” pulping system seems to offer a good

strategy over the long term, but has not yet been

adopted for commercial use.
12 ● Technologies for Reducing Dioxin in the Manufacture of Bleached Wood Pulp

It is believed that if the use of chlorine gas is

reduced or eliminated in the bleaching process,

the amount of TCDD and TCDF formed will be

lowered or eliminated along with other chlorinated organics. However, the relationship is not

linear, and other factors, such as mixing efficiency, have significant effects on the relationship. Oxygen is one of the most promising

bleaching chemicals for displacing some of the

chlorine used in the bleaching cycle. In no case

thus far has oxygen been able to completely

eliminate the need for chlorine.

Chlorine dioxide, a more efficient oxidant

than chlorine, can be substituted for substantial

amounts of chlorine gas. The use of chlorine

dioxide in conjunction with chlorine gas is

increasing at U.S. mills. It has been demonstrated that substitution of chlorine dioxide for

some optimum portion of chlorine gas can

significant] y reduce the formation of TCDD and

TCDF. However, the effects of chlorine dioxide

substitution on the formation of other chlorinated compounds are not known.

Recent research has also shown that the

amount of dioxin can be reduced by lowering the

acidity (raising the pH) of the pulp before

chlorination, by applying the chlorine charge in

three parts instead of one shot, and by using

chlorine dioxide after an initial charge of

chlorine gas. A very recent discovery by scientists at the Pulp and Paper Institute of Canada

has identified oil-based defoamers as one possible source of the precursors that form dioxin

when chlorinated. As a consequence, the U.S.

industry and chemical suppliers are currently

seeking cleaner oil-based defoamers.

Defoamers are added to the unbleached pulp

in small amounts to improve washing. If a

defoamer is made from used oil contaminated

with unchlorinated dioxins (DBD) and furans

(DBF), these precursors convert to their chlorinated forms as TCDD and TCDF when exposed

to chlorine gas. The United States regulates the

use of contaminated recycled oil, so further

investigation is needed to determine whether

defoamers are a problem for U.S. mills using

domestically produced products. Recent investigations by American scientists using testing protocols more sensitive than those used in

Canada have raised doubts as to whether any

oil-based defoamers—whether made from ei-
Chapter 1—Introduction, Summary and Findings ● 13

ther virgin, hydro-treated oil, or used oil—are

free of contaminating precursors.

Water-based defoamers are also available,

and tests show them to be free of DBD and DBF

precursors. Unfortunately, while water-based

defoamers can be used at other washing stages

in the mill, they are not effective for washing

brownstock. The use of “cleaner” defoamers is

another option for reducing some of the dioxin

produced in the bleaching process, but more

research and development may be needed to

develop suitable products.

Oxygen delignification technology was discovered in 1952, and the first commercial unit was

installed in the 1960s. Since then, oxygen

technology has advanced steadily until it is now

considered a mature and proven process. In

1988, world installed capacity using oxygen

delignification is expected to exceed 10 million

metric tons per year. Several manufacturers of

pulp and paper equipment market oxygen delignification systems. Over 50 oxygen units have

been installed worldwide. About half the oxygen capacity is in Scandinavia and Europe,

one-fifth is in Japan, and one-fifth is in North

America. About 92 percent of the oxygen

bleaching capacity is installed in kraft mills, but

a number of sulfite mills—mostly in the Federal

Republic of Germany—have also installed oxygen delignification units.

Oxygen can also be used in combination with

extended digestion (modified cooking). By modifying the chemical addition and allowing it to cook

longer, the amount of residual lignin in the pulp

can be reduced prior to bleaching. Further

pre-bleaching with oxygen can produce pulp

with even lower amounts of lignin. With prebleached pulps of low lignin content, it may be

technically possible to eliminate the use of

chlorine gas in the bleaching process if chlorine

dioxide is used as a substitute.

No reliable data exist that directly link the

reduction of TCDD and TCDF wholly to

oxygen delignification, and in some cases oxyModem mills are designed to match the

capacity of the chemical recovery plant with the

planned production capacity of the mill. The

addition of an oxygen delignification stage

increases the volume of effluent that must be

handled by the chemical recovery plant. In new

mills recovery plants can be designed to handle

the additional load, but retrofitting an existing

mill with an oxygen delignification system can

result in overloading the chemical recovery

capacity. If a larger size recovery furnace or

evaporators are required, the additional capital

expense may make oxygen delignification a less

attractive alternative for economic reasons. It is

estimated that adding an oxygen stage to a mill

whose chemical recovery plant is operating at

full capacity will reduce the productivity of that

mill 4 to 5 percent.

Given a range of equally effective technologies to reduce the use of chlorine gas, capital and
14 ● Technologies for Reducing Dioxin in the Manufacture of Bleached Wood Pulp

operating costs may determine the most costeffective strategy for reducing the amount of

chlorinated organic chemicals produced by pulp

mills. Although operating costs may be lower if

oxygen delignification is used to replace some

of the chlorine gas now used in the bleach cycle,

the capital cost of oxygen systems is large. Both

capital costs and operating costs must be considered in a balanced assessment. Furthermore,

cost factors will differ from mill-to-mill, making generalizations difficult. OTA did not attempt to assess these costs. On the other hand,

by using “cleaner” defoamers, coupled with

substitution of chlorine dioxide for chlorine, it

may be possible to achieve much lower levels of

TCDD and TCDF in pulp. Whether or not this

would also lower the level of other chlorinated

compounds is uncertain.

Further reductions in environmental releases

from pulp mill waste outfalls may result from

improving the control of suspended solids in the

secondary waste treatment plant. TCDD and

TCDF are relatively insoluble in water, but they

adhere tightly to fine colloidal material and

suspended solids. A well-designed, properly

operated secondary treatment plant is capable of

removing up to 90 percent of the TCDD and

TCDF released. Dioxins are retained in

treatment plant sludge. The sludge is normally

disposed of in landfills, where limited studies

show that it remains isolated and immobile.

Some sludge is retained in sludge lagoons or is

incinerated, but a portion is used for soil

conditioners. The remaining 10 percent of the

dioxin that remains in suspension can find its

way in to the water coarse and remain as

sediment in streambeds. The use of clarifiers,

chemicals, and settling basins to improve the

efficiency of waste treatment, coupled with

chlorine dioxide substitution and/or perhaps

other delignification technology might prove to

be the optimum solution for some existing pulp

mills.

More questions remain than do answers as to

what risk dioxin from pulp and paper manufacture presents to humans and the environment:

will additional regulations be needed to reduce

the risk of human exposure from dioxin and

other chlorinated compounds produced in pulp

mills? and which technologies or mix of technologies are best suited for reducing the production of dioxin in the pulp bleaching process? The

pulp and paper industry has a number of

technical options available to meet the problem.

Other technologies, such as extended delignification and the substitution of other oxidants

in the bleach process for chlorine gas, may be as

effective as oxygen delignification in reducing

the use of chlorine. All of these questions

require more detailed study before they can be

answered with acceptable certainty. Final decisions to meet regulatory requirements will have

to be made on a case-by-case, mill-by-mill basis.
Chapter 2

The Pulp and Paper Making

Processes
CONTENTS

Page

THE PULP AND PAPER MILL. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

Steps in the Pulp and Papermaking Process... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

Pulping Technologies... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

2-1.

2-2.

2-3.

2-4.

Figure

Overall View of Papermaking From Chemical Pulp by the Kraft Process.

Stone Groundwood Pulp Mill Flow. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Refiner Groundwood Pulp Mill

Sulfite Pulp Mill Process... . . .

Table

2-1. 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 from

an ancient art first developed in China, ca. 105 A.D.

Although the modem product differs considerably

from its ancestral materials, papermaking retains

distinct similarities to the processes developed by

Ts’ai Lun in the Imperial Chinese Court. ’ In

principle, paper is made by: 1) pulping, to separate

and clean the fibers; 2) beating and refining the

fibers; 3) diluting. to form a thin fiber slurry,

suspended in solution; 4) forming a web of fibers on

a thin screen; 5) pressing the web to increase the

density of the material; 6) drying to remove the

remaining moisture; and 7) finishing, to provide a

suitable 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 to

impart special qualities to the finished product. Most

paper is made from wood fibers, but rags, flax,

cotton linters, and bagasse (sugar cane residues) are

also used in some papers. Used paper is also

recycled, and after purifying and sometimes deinking, it is often blended with virgin fibers and

reformed again into paper. Other products made

from wood pulp (cellulose) include diapers, rayon,

cellulose acetate, and cellulose esters, which are

used for cloth, packaging 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 primary

interest in papermaking, comprises about 50 percent

of wood by ovendry weight. Lignin, which cements

the wood fibers together, is a complex organic

chemical the structure and properties of which are

not fully understood, It is largely burned for the

generation of energy used in pulp and paper mills.

As the chemistry of lignin becomes better understood, what is now mostly a waste product used for

fuel (some is converted to chemical products) could

become a valuable feed stock for new chemical

products.

The pulping process is aimed at removing as

much lignin as possible without sacrificing fiber

strength, thereby freeing the fibers and removing

impurities that cause discoloration and possible

future disintegration of the paper. Hemicellulose is

similar to cellulose in composition and function. It

plays an important role in fiber-to-fiber bonding in

papermaking. Several extractives (e.g., oleoresins

and waxes) are contained in wood but do not

contribute to its strength properties; these too are

removed during the pulping process.

The fiber from nearly any plant or tree can be used

for paper. However, the strength and quality of fiber,

and other factors that can complicate the pulping

process, varies among tree species. In general, the

softwoods (e.g., pines, firs, and spruces) yield long

and strong fibers that impart strength to paper and

are used for boxes and packaging. Hardwoods, on

the other hand, generally have shorter fibers and

therefore produce a weaker paper, but one that is

smoother, more opaque, and better suited for printing. Both softwoods and hardwoods are used for

papermaking and are sometimes mixed to provide

both strength and printability to the finished product.

THE PULP AND PAPER MILL

Although there are several chemical and mechanical pulping methods used for delignifying wood

(table 2-l), separating fibers, and removing discoloration, all integrated pulp and paper mills involve

the same general steps in the manufacture of pulp

and paper. These steps include: 1 ) raw material

preparation (e.g., debarking and chipping); 2) mechanical and/or chemical separation of the wood

fibers [i.e., grinding, refining, or digestion (cooking)] to dissolve the lignin and extractives; 3)

removal of coloring agents (primarily residual lignin) by bleaching; and 4) paper formation and

manufacture.

A typical layout of a mill using the kraft chemical

pulping 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 paper

manufacture, 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-
18 ● Technologies for Reducing Dioxin in the Manufacture of Bleached Wood Pulp

Table 2-l—Major Commercial Wood-Pulping Technologies

Pulp grades use Wood type End-product use

Chemical pulps:

sulfite pulp . . . . . . . . . . . . . . . . . . . . . . . . . . . Softwoods and hardwoods Fine and printing papers

Kraft sulfate pulp . . . . . . . . . . . . . . . . . . . . . . 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 film

Semichemical pulps:

Cold-caustic process . . . . . . . . . . . . . . . . . . . Softwoods and hardwoods Newsprint and groundwood printing papers

Neutral sulfite process . . . . . . . . . . . . . . . . . . Hardwoods Newsprint and groundwood printing papers

Mechanical pulps

Stone groundwood , . . . . . . . . . . . . . . . . . . . Softwoods Corrugating medium

Refiner mechanical (RMP) . . . ., . . . . . . . . . Softwoods Newsprint and groundwood printing papers

Thermomechanical (TMP) . . . . . . . . . . . . . . .Softwooods Newsprint and groundwood printing papers

SOURCE: 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 the

pulping process; collect, process, and bum lignin

and waste wood to produce energy; and remove and

treat wastes from process water for release into the

environment.

Steps in the Pulp and Papermaking Process

Raw Material Preparation

Wood received at a pulp mill may be in several

different forms, depending on the pulping process

and the origin of the raw material. It maybe received

as bolts (short logs) of roundwood with the bark still

attached, as chips about the size of a half-dollar that

may have been produced from sawmill or veneer

mill waste or pre-chipped from debarked roundwood

elsewhere, or as waste sawdust in the case of some

pulping processes.

If roundwood is used, it is first debarked, usually

by tumbling in large steel drums where wash water

may be applied. The debarked wood bolts are then

chipped in a chipper if the pulping process calls for

chemical digestion or are fed into a grinder in the

case of some mechanical pulps. Chips are screened

for size, cleaned, and temporarily stored for further

processing.

Fiber Separation

The fiber separation stage is the point at which the

several pulping technologies diverge. In kraft chemical pulping, the chips are fed into a large pressure

cooker (digester), into which is added the appropriate chemicals (white liquor). The chips are then

cooked (digested) with steam at specific temperatures long enough to separate the fibers and partially

dissolve the lignin and other extractives.

Some digesters operate continuously with a constant feed of chips (furnish) and liquor, others are

charged intermittently and treat a batch at a time.

After digestion, the cooked pulp (brown stock) is

discharged into a pressure vessel (blow tank) where

the steam and volatile materials are siphoned off.

The cooking liquor, that by this time has turned dark

brown from the dissolved lignin (black liquor), is

returned to the chemical recovery cycle, In the

chemical recovery plant, the lignin in the black

liquor is burned for the cogeneration of energy, and

the chemicals are recovered, 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) is

washed with water, screened to remove undigested

wood, and cleaned to remove other foreign matter.

It is then ready for bleaching and further processing.

Fiber separation in mechanical pulping is less

dramatic. In the stone groundwood process, debarked logs are forced against rotating stone grinding wheels that are constantly washed by a stream of

water. The ground pulp is then screened to remove

course debris, thickened, and stored for the papermaking process.

Chips are used to produce refiner pulp and

thermomechanical pulp. In both processes the chips

are ground by passing them through rapidly rotating
Chapter 2—The Pulp and Paper Making Processes ● 19

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

Continuous digester

II

White liquor

clarifier

Wash water

I

I

Stripped condensates A

SOURCE: Envirorwnent Ontario, Srop@ng Water Poflutioft Al Its Source (Toronto, OntarIO: Ministry of the Environment, 1988).
20 ● Technologies for Reducing Dioxin in the Manufacture of Bleached Wood Pulp

disk grinders. Thermomechanical pulp is refined

(ground) under pressure after the chips are pretreated

with steam (chemical thermomechanical pulp uses

chemicals and steam for pretreatment). After further

refining in a second stage, the pulp is screened,

cleaned, and most of the process water is removed in

preparation for papermaking.

Bleaching or Brightening

Since the raw pulp (brown stock) still contains an

appreciable amount of lignin and other discoloration, it must be bleached to produce light colored or

white papers preferred for many products. Bleaching

is normally done in several stages (multistage

bleaching). Through chlorination and oxidation the

fibers are further “delignified” by solubilizing

additional lignin from the cellulose.

A number of bleaching agents may be used and

are applied in a stepwise fashion within a bleaching

sequence. These include chlorine gas, chlorine

dioxide, sodium hypochlorite, hydrogen peroxide,

and oxygen. Between bleaching treatments, a strong

alkali (usually sodium hydroxide) is used to extract

the dissolved lignin from the surface of the fibers.

The bleaching agents and the sequence in which they

are used depend on a number of factors, such as the

relative cost of the bleaching chemicals, type and

condition of the pulp, desired brightness of the paper

to be produced, and sometimes in response to

environmental guidelines and regulations.

Bleaching of mechanical pulp is much different

than that for chemical pulp. Mechanical pulping

leaves the lignin and the cellulose intact, whereas the

purpose of chemical pulping is to chemically separate the lignin from the cellulose fibers and remove

it from the pulp. A major advantage of mechanical

pulping is the high yields of pulp that can be

achieved from a given volume of wood. Therefore,

bleaching or brightening of mechanical pulps is

designed to minimize the removal of the lignin that

would reduce fiber yields.

Chemicals used for bleaching mechanical pulps

selectively destroy coloring impurities but leave the

lignin and cellulosic materials intact, These include

sodium bisulfite, sodium or zinc hydrosulfite (no

longer used in the United States), calcium or sodium

hypochlorite, hydrogen or sodium peroxide, and the

Sulfur Dioxide-Borol Process (a variation of the

sodium hydrosulfite method). Originally, much of

the mechanical pulp was not bleached, but the

bleaching of groundwood has increased and improved technology now enables bleached groundwood pulp to be used for printing papers, tissues, and

towelling.

Papermaking

The bleached or unbleached pulp may be further

beaten and refined to cut the fibers and roughen the

surface of the fibers (fibrillate) to improve formation

and bonding of the fibers as they enter the paper

machine. Before entering the paper machine, water

is added to the pulp slurry to make a thin mixture

normally containing less than 1 percent fiber. The

dilute slurry is then cleaned in cyclone cleaners and

screened in centrifugal screens before being fed into

the ‘‘wet end* of the paper-forming machine.

In the paper making process, the dilute stock

passes through a headbox that distributes the fiber

slurry uniformly over the width of the paper sheet to

be formed. The “web” of fiber that will make the

new paper sheet is formed on a continuously moving

bronze or polymer screen (Fourdrinier) or between

two such wire screens. Water drains from the slurry

through the mesh of the screen, the wet paper web is

consolidated and the paper sheet gains some strength

through fiber bonding.

The wet sheet of paper is continuously lifted from

the screen (couched) and transferred to a woven felt

belt where additional water is squeezed from the

paper sheet by pressure rollers, The remaining water

is removed on steam-heated cylinders. When the

paper is dry it may be treated with stabilizing

materials and surface finishes to improve durability

or printability.

Pulping Technologies

Mechanical Pulping Processes

There are six basic mechanical pulping processes:

1 ) stone groundwood, 2) refiner, 3) thermomechanical pulping, 4) chemical mechanical, 5) defibrated or

exploded pulping, and 6) recycled paper.2 Mechanical pulping is generally used with softwoods be2~1s ~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.
Chapter 2—The Pulp and Paper Making Processes ● 21

cause of the added strength imparted by the long

fiber length of softwood species. Some hardwoods

require chemical pretreatment (chemical mechancial

pulping) to produce a suitable groundwood pulp.

Fibers separated mechanically are substantially damaged in the process and therefore make weaker paper

or paperboard. However, since both lignin and

cellulose fibers remain intact, the yield of paper per

unit volume of wood is still greater than that

produced by chemical pulping. Pulp yields from all

of the mechanical pulping processes typically are

near 90 to 95 percent recovery, which is a much

higher yield per unit of wood than with the chemical

pulping methods because of the retention of lignin.

However, paper made from mechanical pulp discolors and becomes brittle with age because of its lignin

content, which results in a shorter useful life than

paper made from chemical pulp.

Mechanical pulps are used principally to manufacture newsprint, printing papers, towelling, tissue,

and coated specialty papers that do not require

high-strength. Secondary uses include wallpaper

and paperboard. Small amounts of chemical pulp are

often mixed with groundwood pulp for additional

strength. Recycled pulp is used mainly for the

manufacture of folding boxboard (gray board), tissue, corrugated board, and newsprint. Paper products made from defibrated pulp include hardboards,

construction boards, and roofing papers.

In the stone groundwood process, debarked short

logs (roundwood) are fed whole against wet stone

grinders by hydraulic rams. Counter-revolving steel

disks are sometimes used in place of abrasive stone

in the grinding process. The abrasion of the grinding

wheel against the wood physically separates the

wood fibers. The grinding process usually is automatic and continuous. The groundwood pulp is then

screened, bleached or brightened, treated, and prepared for the paper machine (figure 2-2).

Figure 2-2-Stone Groundwood Pulp Mill Flow

W h ite Over f low

wate r

1 I

Pul p Bleachin 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 chips

in lieu of roundwood and produces paper with higher

strength than conventional groundwood because of

less damage to the fibers in the pulping process. The

chips are passed through a refiner that has fixed and

rotating disks operating under a stream of water. A

wider range of species, including hardwoods, can be

processed by the refiner pulping process, Sawdust

and other saw mill wastes can also be used (figure

2-3).

Thermomechanical process (TMP) was developed as a modification of the refiner mechanical

pulping process. In TMP, the wood chips are

steamed for several minutes under pressure and

subsequently refined in one or two stages. The lignin

is softened by heating the wood chips with pressurized steam before they are refined (i.e., blended by

passing the fiber through rapidly rotating disks). The

refined wood pulp, although stiIl weaker than

chemical pulp, makes a stronger paper than groundwood or refiner pulp with only a small sacrifice in

yield but with large energy requirements. Some

newsprint is now produced wholly from thermomechanical pulp, thus eliminating the need for

the addition of chemical pulp often needed for

strengthening paper made from mechanical pulp.

The neutral sulfite semichemical (NSSC) pulping

process is used at a number of U.S. mills to produce

courser-grade products such as corrugated board,

which has a yield of about 75 percent of the wood

raw material. In NSSC pulping, wood chips are

softened by briefly cooking them in a neutral sodium

or ammonium sulfite solution and then separating

the fibers (defibrating) in a refiner (see also Sulfite

Pulping below).

Recycling can effectively reduce the consumption

of both wood raw material and energy when used in

conjunction with other mechanical pulping processes. It does so, however, with some sacrifice in

paper strength. Recycled pulp is manufactured from

wastepaper that is processed into paper stock. A

small proportion of the paper stock (5 to 10 percent)

is de-inked, usually with caustic soda-based chemicals. Most recycled paper, however, is pulped

without de-inking. Pulping is accomplished through

violent agitation and shearing action performed at

high temperatures. The paper produced from recycled 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 the

used fibers and loss of fiber bonding.
Chapter 2—The Pulp and Paper Making Processes ● 23

Three major developments in mechanical pulping

technologies show promise for improving pulp

quality: 1) pressurized groundwood pulping, 2)

chemical thermomechanical pulping, and 3) hardwood chemical mechanical pulping. All of these

technologies have reached some stage of commercialization. Chemical thermomechanical pulping is

currently used at several U.S. mills. Improvements

in mechanical pulping show promise for improving

the quality (strength characteristics) of paper now

produced by mechanical processes. The resulting

higher quality mechanical pulps may displace the

kraft pulps that are currently mixed with mechanical

pulps to improve paper strength.

Pressurized Groundwood Pulping—h pressurized groundwood pulping, debarked logs are fed to

the grinding wheel through a heated, pressurized

chamber. The heat and pressure help separate the

fiber, thus breaking fewer fibers in the grinding

process and improving pulp quality. Paper produced

from pressurized groundwood pulp is more tearresistant than paper made from stone-ground pulp,

but is slightly inferior to that of thermomechanical

pulp. Pressurized groundwood pulping may have the

potential for displacing some high-quality chemical

pulps in the manufacture of newsprint and other

printing papers.

Chemical Thermomechanical Pulping— Chemical thermomechanical pulping involves treating

softwood chips with mild sulfite solutions to modify

the lignin and partially delignify the wood prior to

grinding in a refiner. This “sulfonation” treatment

results in paper with higher tear resistance than

thermomechanical, refiner, or stone-ground pulps.

Pulp yields decrease slightly to between 85 and 90

percent with chemical thermomechanical pulping,

but these yields are still higher than chemical

pulping (40 to 56 percent).

Hardwood Chemical Mechanical Pulping— Mechanical methods for producing pulp from hardwood

species involve pretreating hardwood chips with

hydrogen peroxide or sodium hydroxide and processing them like refiner mechanical pulps. Both

hardwoods and softwoods have been successfully

pulped by this method, with fiber recoveries in the

80 to 90 percent range. Pulp produced by hardwood

chemical mechanical pulping can be used to produce

newsprint and printing papers.

Chemical Pulping

Chemical pulping involves treating wood chips

with chemicals to remove the lignin and hemicellulose, thus separating and cleaning the fibers. Delignification gives the fibers greater flexibility, resulting in a substantially stronger paper (because of

greater contact between the fibers in the finished

sheet) than can be manufactured from high-lignin

fibers produced by mechanical pulping. Paper

strength and durability is gained at the expense of

fiber yield. Chemical processes may yield only half

the fiber that can be recovered by the use of

mechanical pulping techniques.

Two major chemical pulping processes are currently in commercial use: 1) kraft (sulfate) pulping,

and 2) sulfite pulping. The kraft process dominates

the pulp and paper industry, accounting for 76

percent of the pulp produced for paper and paperboard in 1984.3 Paper produced from kraft pulp

accounts for most of the bleached boxboard and

linerboard used by the packaging industry (which

consumes about 58 percent of the paper in the United

States). Bleached softwood kraft pulps are often

mixed with mechanical pulps to add strength to

newsprint and printing papers. Bleached hardwood

kraft pulps are added to bleached softwood pulp to

improve printability for specialty paper products

like magazine stock and coated papers. Both kraft

pulp and sulfite pulp can be used for the production

of dissolving pulp, which is used for the production

of rayon and acetates.

Kraft Pulping—Kraft pulping involves treating

wood chips and sawdust with a sodium sulfide and

sodium hydroxide solution (see figure 2-1). The

highly alkaline chemical and wood mixture is

cooked with steam under pressure (digested) for

between 1 and 3 hours. Digestion may be either a

continuous process or treated in discontinuous

‘‘batches.’ Most of the lignin and some of the

hemicellulose is dissolved, leaving the remaining

cellulose fibers separated.

The cooking liquor containing the dissolved

lignin and other extractives (black liquor) is routed

to a chemical recovery plant where the lignin and
24 ● Technologies for Reducing Dioxin in the Manufacture of Bleached Wood Pulp

organic wastes are burned to produce energy needed

in the pulping process. Valuable extractives (e.g.,

turpentine, tall oil, and resin) are separated for sale

as commodity chemicals. Process chemicals are

recovered with only a relatively small loss in

volume, and after replenishment with sodium salts,

they are returned to the digester for reuse.

The brown pulp (brown stock) from the digester

is washed, screened, and passed through a battery of

cleaners. If the pulp is to be bleached, it is

“thickened” by removing excess water and sent

through a series of bleach operations. These can vary

widely in the type of chemicals used and their

sequence. Bleached pulp is then ready for the paper

making process.

Both softwood and hardwoods can be pulped by

the kraft process. Fiber recovery is largely a function

of the wood species used, the time and temperature

of cooking, the degree of bleaching, and the paper

strength required. Generally, kraft pulp recoveries

from softwoods are approximately 47 percent for

unbleached pulp and 44 percent for bleached.4

Hardwood recoveries range from 50 to 52 percent for

unbleached kraft pulp to 50 percent for bleached.

Sulfitee Pulping—Lignin can be dissolved by

sulfonation with an aqueous solution of sulfur

dioxide and calcium, sodium, magnesium, or ammonium bisulfite cooked at high temperature and

pressure in a digester (see figure 2-4). There are four

basic sulfite pulping processes currently in commercial use: 1) acid sulfite, 2) bisulfite, 3) neutral sulfite,

and 4) alkaline sulfite. The major differences between the sulfite processes are the levels of acidity

and alkalinity of the sulfite chemical solutions used

to break down the wood and remove the lignin.

Sulfite pulping processes are suitable only for

species with low extractive contents (i.e., those low

in tannins, polyphenols, pigments, resins, fats, and

the like) because of the interference of these

substances with the sulfite pulping process. Although calcium is the cheapest sulfite base available,

it forms insoluble compounds that cannot be reclaimed economically. Thus, calcium-based pulping

is seldom used. Because magnesium- and sodiumbased chemicals are recoverable, and ammoniumbased chemicals are less expensive and can be

burned without harmful environmental effects, they

are the most frequently used.

Sodium-based sulfite pulping can consist of

multistage cooking, successive stages of which

differ in acidity. Because one stage optimizes

chemical liquor penetration and the other the removal of lignin, more lignin may be removed with

less fiber degradation, so that fiber yields are higher,

fibers are stronger, and a wider range of wood

species may be used. Sulfite pulping dissolves some

of the hemicellulose as well as the lignin. Neutral

sulfite pulping, using sodium and ammonium bases,

recovers the largest proportion of fiber (75 to 90

percent) of all the sulfite pulping methods.

Sulfite pulp is a light color and can sometimes be

used without bleaching if high brightness is not

required. Unbleached sulfite pulp is often blended

with groundwood and other high-yield mechanical

pulps for strengthening newspaper stock. Sulfite

pulp is easily bleached to very bright pulps for

writing and printing paper. It is also used for the

manufacture of dissolving pulps (through the further

removal of hemicellulose) for the production of

viscose rayon, acetate fibers and films, plastic fillers,

and cellophane.

Potential for New Pulping Technologies

The search for new pulping technologies and

process improvements for established commercial

technologies continues in the United States, Canada,

Sweden, Finland, Japan, Germany, and elsewhere.

In the United States, about $815 million is estimated

to have been spent on pulp and paper research and

development in 1987.5 OTA could not determine

what proportion of the R&D was directed at

improving pulping technologies. Nearly all R&D is

sponsored by the industry, with only $3 million

(<0.4 percent) expended by the Federal Government.

Industry pulping R&D is largely focused on

improving established pulping and bleaching processes 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.
Chapter 2—The Pulp and Paper Making Processes ● 25

the need for broadening the raw material base in

response to concerns over forest resources. Restrictions on water use and pollution control have

contributed to the impetus for seeking process

improvements.

Energy costs as reflected in both energy use by the

industry and their impact on the cost of chemicals

has led to process improvements in the past,

although moderating energy prices have recently

reduced these concerns. The emphasis on recycling

to reduce the massive problems of solid waste

disposal in metropolitan areas has also been an

incentive to using more reclaimed material in paper

manufacturing. Finally, the increasing cost of capital

to rebuild aging sectors of the pulp and paper

industry have fed the need for more R&D by the

industry.

There are several reasons why major advancements in pulp and paper technology appear to be

glacial in comparison to some other more rapidly

advancing technologies. First, the pulp and paper

industry is mature; the commercial technology,

much of which was developed in the late 1700s and

1800s, has undergone evolutionary change, and

satisfaction with the basic technology has led to little

reason to fix something that does not appear to be

broken. Concerns over future environmental problems and competition from other materials could

change this, and to some degree already has.

Second, R&D is fragmented by the emphasis on

process improvement, therefore few scientists and

engineers focus on new pulping processes. In

addition, many researchers specialize in one pulping

process or another depending on the needs of a

specific firm; few are able to consider all technological options or innovations for improving pulp yield

or overall quality.

Third, R&D investment in incremental improvement in established processes is easier to sell to

corporate directors than risky, long-term, radical

changes. Large existing investments in plant equipment stretch the amortization period of old equipment and slow the acceptance of new processes that

require substantial changes and alterations.

Finally, the absence of major government investment 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 through

collective R&D efforts. Individual firms have little

incentive to undertake a major, long-term, highinvestment R&D program to develop radically new

technologies with uncertain payoff in the end,

particularly in the current investment climate.

Organosolv Pulping (Ester Pulping)—Organosolv pulping—sometimes called ester pulping—is a

two-stage process involving hydrolysis (decomposi-
26 ● Technologies for Reducing Dioxin in the Manufacture of Bleached Wood Pulp

tion of the wood by dilute acids or enzymes) and the

removal of lignin with an organic solvent, usually a

mixture of alcohol and water. The still experimental

process is suitable for both hardwoods and softwoods. Sawdust as a byproduct from lumber manufacture can also be pulped,6 Pulp recovery from

organosolv pulping ranges between 50 and 60

percent for hardwoods, and 40 and 45 percent for

softwoods. Typical hardwood fiber recoveries compare favorably with those from kraft pulping.

Fibers produced by the organosolv process are

weaker than those recovered by the kraft process.

Thus, the papers produced from organosolv pulp are

suitable for uses where strength is not the most

important property (e.g., printing papers, fluff pulps,

and dissolving pulp). Little waste is produced by the

process, and low alcohols are recovered easily by

distillation, thus requiring relatively low capital

investment. 7 Commercial viability of this technology will require that markets be developed for

byproducts of the process.

A commercial demonstration plant using the

Alcell process developed by Repap Enterprises

Corp. of Canada is currently under construction at

Newcastle, New Brunswick. The 33-ton-per-day

pilot plant will cost $65 million. The Canadian

government is underwriting half the cost of the plant.

Alcell is an alcohol cellulose organosolv process.

Hydrotropic Pulping-Hydrotropic solutions are

aqueous salt solutions that impart greater volubility

to slightly soluble substances (e.g., lignin) than does

water at the same temperature. Sodium xylenesulfonate, a hydrotropic salt, has been used experimentally to delignify wood.8 A hydrotropic pulping

process was patented by Ralph H. McKee in 1943.9

Laboratory pulping texts suggested that dissolution

of lignin with aqueous sodium xylenesulfonate

solutions of 30 to 40 percent had little or no effect on

the strength of the pulp and yielded a high alpha

cellulose content (important for dissolving pulp).

Pulping of poplar was conducted at temperatures

of about 150 °C for 11 to 12 hours. Tests yielded 52

percent cellulose, compared to 47 percent from

comparable sulfite pulp. Unlike sulfite or kraft

pulping which uses contaminating inorganic chemicals, the lignin recovered through precipitation by

hydrotropic pulping is relatively pure and is suitable

for conversion to other chemical products. The

process is not suited for resinous coniferous species,

however, and comparatively little serious attention

has been given to this process by the industry.

%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

Environmental Considerations
CONTENTS

Page

CHLORINATED DERIVATIVES IN THE ENVIRONMENT-AN OVERVIEW... . . . 29

DIOXIN . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

DIOXIN AND PULP AND PAPER MANUFACTURE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

National Dioxin Study . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

EPA/Paper Industry Joint Five-Mill Study . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

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

Box

Box Page

3-A. Detection Limits and Levels of Dioxin Contamination . . . . . . . . . . . . . . . . . . . . . . . . . . 32

Tables

Table Page

3-1. Major Chlorinated Derivatives Identified in Pulp and Paper Mill Effluents . . . . . . . . 30

3-2. Characteristics of the Bleached Kraft Pulp and Paper Mills Used in

the Five-Mill EPA/Industry Cooperative Dioxin Screening Study . . . . . . . . . . . . . . . . . 34

3-3. Concentration of TCDD and TCDF in Bleach Plant Wastewater . . . . . . . . . . . . . . . . . . 34

3-4. Mode of Environmental Release of TCDD and TCDF From Pulp

and Paper Mills . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

3-5. Safe-level Concentrations of TCDD in Paper Products . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
Chapter 3

Environmental Considerations

Pulp manufacture, like most chemical processes,

results in emissions, effluents, and solid residues

that must be disposed of. This study focuses on

chlorinated bleached pulp mill waste effluents and

residual chlorinated compounds in paper products

(with emphasis on TCDD and TCDF), therefore air

emissions and solid wastesl will not be considered

here. Technologies for reducing the production of

chlorinated organics in the pulping process are

discussed in chapter 5.

CHLORINATED DERIVATIVES

IN THE ENVIRONMENT—

AN OVERVIEW

Effluents from bleached pulp mills contain a

variety of substances, some of which exhibit a

variety of effects in biological tests, such as genotoxicity, mutagenicity, or teratogenicity. These include resin acids and fatty acids, chlorinated phenols, and other chlorinated organic substances. The

composition of bleaching effluent is extremely

complex and varies from mill to mill depending on

the wood species being pulped, the pulping technology, bleaching reagents, and waste treatment systems used.2 Comparatively little is known about the

actual composition of mill waste effluents, although

substantial scientific effort has been spent on research. A screening and verification survey of pulp

and paper mill effluents conducted by the Environmental Protection Agency (EPA) tentatively identified a number of chlorinated organic chemicals

(table 3-l). The chlorinated components of the waste

stream consist mostly of simple phenols, phenolic

and carbohydrate oligomers (high and low molecular weight polymers), and neutral and acidic materials resulting from the breakdown of the phenolic

rings in lignin.3

It has been estimated that no more than 10 percent

of the total solids in the waste stream of a pulp

bleaching plant contain chlorinated derivatives.

However, their toxicity to aquatic biota has raised

concerns among biologists.4 Chlorinated mill wastes

have only recently been focused on in the United

States, although the toxicity of untreated, undiluted

waste to aquatic biota is well documented in the

scientific literature. A technical committee of Environment Ontario, the provincial environmental

agency for Ontario, Canada, recently reviewed the

available information on chlorinated organics and

dioxin from mill waste. Based on its analysis, the

committee recommended that Ontario adopt a longterm strategy aimed at completely eliminating the

formation of organochlorines in kraft pulp mills.5

The committee also concluded, however, that ‘chlorinated dioxins do not represent an immediate

danger to human health and welfare,’ but it did note

that heavy fish-eaters consuming fish caught downstream of some bleached kraft mills might exceed

the acceptable daily intake of TCDD.

Sweden and Finland, with pulp and paper mills

located adjacent to the Gulf of Bothnia and the Baltic

Sea, have experienced environmental damage to

marine 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, shows

that 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 because

some mills use more than one method of disposal). The subject of solid waste disposal has many aspects that range far beyond the focus of this study,

including disposal of contaminated municipal sewage sludge, disposal of toxic and hazardous materials, and disposal of incinerator residues. OTA has

published several reports on related topics: Waste in Marine Environments, OTA-O-335, April 1987; From Pollution to Prevention: A Progress Report

on 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) Ying

Shui, 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-
30 ● Technologies for Reducing Dioxin in the Manufacture of Bleached Wood Pulp

Table 3-1-Major Chlorinated Derivatives Identified in

Pulp and Paper Mill Effluents

chlorobenzene

1,1,1 Trichloroethane

trichlorophenol*

2,4-dichlorophenol

dichlorobromomethane

chlorodibromomethane

trichloroethylene

monochlorodehydroabietic acid

2,3,7,8-tetrachlorodibenzo-p-dioxin 

1,2-dichloroethane *

1,1,2,2-tetrachloroethane **

chloroform ● *

methylene chloride ● *

trichlorofluoromethane

tetrachloroethylene **

3,4,5-trichloroguaiacol

9,10-dichlorosteanic acid

2,3,7,8-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 CaroinoganaBac&ourrd Paper (Chelsea, Ml: Lawis Publishers, 1967), p. 64.

SOURCE: U.S. Envuonmental Protection Agency, Developrwnf Document for Effluent

Linrifations Gutilinas and Starrdards br be Pulp, paper, and Paperboard

arxl fhe Buil&ws’ P@ar and Board Mh--Pomt Sourcx Cakqpms, EPA

440J1-82KW5 (Washington, DC: 1962), p. 46.

released into coastal waters.6 Sweden’s National

Environmental Protection Board (Naturvardsverket)

estimates that Scandinavian pulp and paper mills

contribute between 300,000 and 400,000 tons of

chlorinated organic materials to the coastal waters of

Sweden, Finland, and Norway annually.7 Evidence

of environmental harm in the estuaries of the Baltic

Sea (including the accumulation of dioxin in the

flesh of food fish), where water circulation and

exchange are extremely slow, has led the Swedish

Government to consider regulations to reduce the

amount of chlorinated organic substances produced

by bleached sulfate pulp mills by imposing regulations that require the use of oxygen bleaching and

increased use of chlorine dioxide bleach in place of

chlorine gas. Sweden is also considering steps to

promote the use of closed-cycle processes that

would significantly reduce or even eliminate the

release of chlorinated wastes to the environment.

The Swedish experience, where biological waste

treatment is less prevalent, contrasts with that of the

United States where nearly all bleached kraft pulp

mills use secondary biological treatment to reduce

the biological and chemical oxygen demands of

wastewater. In the course of biological treatment,

many potentially toxic substances are removed and

concentrated in the treatment sludge.g Many Swedish mills, on the other hand, do not use biological

waste treatment, and discharge their effluent directly

into the environment or use only primary waste

treatment.

The overall release of chlorinated organic compounds from pulp and paper mills has received less

attention in the United States up to now. Instead, the

major concern arose over TCDD and TCDF that are

produced along with other chlorination products

during bleaching cycles and are considered to be

potentially harmful to human health.9 A similar

pattern of concern over dioxin developed in Canada.

A recent report on pollution from kraft mill effluents

published by Environment Ontario warned against

focusing too closely on dioxins as a result of media

publicity, because it “may divert energies from

productive avenues of pollution control into blind

alleys of ill-conceived, routine, and expensive surveys of ‘dioxin’ concentrations."10

Products containing TCDD were at one time used

extensively as herbicides (agent orange, 2,4,5-T).

They are also produced as byproducts from the

incineration of municipal and industrial waste, the

combustion of wood in home furnaces, stoves, and

fireplaces, metal smelters, and the incomplete combustion of dielectric fluids (PCBs) in electrical

transformers. The use of dioxin-containing materials

in 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 Working

Group, 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 1988

workshop indicates that it may be possible 10 remove up to Xl percent of the chlorinated organics with supplemental chemical treatment. Tlis work is

still 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 have

one to eight chlorine substituents.

IOEnvironment htario, op. cit., nOIC 5, PP. 1-19.
Chapter 3—Environrmental Considerations ● 31

curtailed, but their inadvertent production through

chemical and industrial processes, and as combustion products continues.

DIOXIN1l

Dioxin, as generally referred to, is 2,3,7,8-

tetrachlorodibenzo-p-dioxin (TCDD). It is the most

toxic of 75 chlorinated dioxins and over 135

chlorinated furans (TCDFS). ’2 Dioxin is a byproduct

of, or a contaminant in, manufactured materials. The

chemical reactions and conditions under which they

are formed in the pulp mill are not completely

understood. TCDD was never produced as such

intentionally except in small experimental quantities

for research. In the United States, TCDD largely

earned its reputation as a “bad actor” in the Agent

Orange controversies following the use of herbicides

as defoliants during the Vietnam War.

Agent Orange was a mixture of the herbicides

2,4,5 -trichlorophenoxy acetic acid (2,4,5-T) and 2,4-

dichlorophenoxy acetic acid (2,4-D). These same

chemicals were extensively used in forestry and

agriculture with little recognition of the health risks

that may be related to the dioxin that 2,4,5-T

contained as an incidental ingredient (2,4,5-T is no

longer manufactured in the United States). The

exposure of military personnel to these chemicals in

Vietnam raised the consciousness of the public

about the health risks of dioxins when returning

veterans blamed a number of their health problems

and those of their families on past exposure to Agent

Orange while serving in the military.

How serious a human health threat is the exposure

to dioxins? On purely scientific bases, the question

of human risk has not yet been definitively answered. Epidemiological data are incomplete and

difficult to interpret. No study thus far has conclusively linked dioxins to the death of a human, or to

a human disease other than chloracne, or conclusively related exposure to dioxin to cancer (although

the carcinogenic potential is considered to be

‘‘probable”), or to miscarriages .13 Abnormal behavior, genetic effects, immunological problems, enzymatic disjunctions, and reproductive problems associated with dioxin exposure have been considered,

but studies have not confirmed dioxins to be the

cause.

With regard to certain-but not all—laboratory

test animals, dioxin has been shown to be extremely

toxic and lethal at low levels. Procedures for

extrapolating from animal effects to humans are

controversial. There is, however, sufficient scientific evidence to suggest that human exposure to

dioxin should be minimized within acceptable levels

of risk pending a better understanding of its health

implications. The public perceives dioxins to be

dangerous and a major health risk as a result of

publicity surrounding the Agent Orange controversy, the Times Beach incident, the Love Canal,

and problems related to the disposal of hazardous

wastes. The regulatory agencies have opted for a

conservative approach to regulating dioxins.

Research on rainbow trout, a species often used to

gauge the toxicity of chemicals, suggests that TCDD

and TCDF can cause mortality, reduced growth, and

abnormal behavior during the fishes’ early life

stages. TCDD was judged to be 10,000 times more

toxic than the pesticides endrin or toxaphene, while

TCDF was 1,000 times more toxic.14 Bioconcentration factors (BCF) for TCDD were found to be much

higher than originally estimated. TCDD accumulated to about 39,000 times the ambient concentration of the water, and TCDF between 2,640 and

4,449 times (but dioxin seems to concentrate in the

gut and inedible parts of fish). It has also been shown

that similar preferential bioaccumulation or magnification of TCDD and TCDF occurs in aquatic

birds. 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, it

is an encapsulation of other recent surveys of existing knowledge about dioxin and its congeners.

]Q~sts on la~rtiv ~jm~sSu=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 dioxins

and 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~xinst@,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.
32 ● Technologies for Reducing Dioxin in the Manufacture of Bleached Wood Pulp

reproduction, survival, and behavior of bird populations is uncertain.16

DIOXIN AND PULP AND PAPER

MANUFACTURE

National Dioxin Study

The National Dioxin Study (NDS), a 2-year effort

to explore the extent of dioxin contamination in the

environment, detected the presence of TCDD and

TCDF in fish and bottom sediment samples collected downstream of several U.S. pulp mills.17 Fish

samples 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 the

Office of Water Regulations and Standards

(OWRS), and

. 57 estuarine or coastal sites were sampled.

Of the 90 sites sampled statistically for fish

contamination, 17 showed detectable levels of

TCDD up to 19 parts per trillion (ppt) in composite

whole fish samples (see box 3-A). Nearly one-third

of the 305 regional samples showed detectable

levels of dioxin in whole fish samples. These sample

sites included rivers, lakes, and some coastal and

estuarine waters. TCDD levels in some samples

ranged up to 85 ppt. Only 4 of the 57 estuarine and

coastal sites sampled had detectable levels of dioxin

in fin fish or shellfish, and these ranged between 1.08

and 3.5 ppt.

About 80 percent of the whole fish sampled from

sites in the Great Lakes were found to have

detectable levels of dioxin. A multitude of potential

dioxin sources are within the watershed of the Great

Lakes and the turnover and flushing of the waters

within the Lakes are extremely slow, Outside the

Great Lakes, detectable dioxin levels were most

frequently found in the major river systems that flow

through industrial and urban areas. Advisories have

been issued by Wisconsin, Maine,, and Louisiana

warning of possible risk of eating contaminated fish.

Box 3-A—Detection Limits and Levels of

Dioxin Contamination

Determining the amount of dioxin in the natural

environment requires sophisticated analytical procedures and careful statistical sampling and sample

preparation, High resolution gas chromatography

and mass spectrometry are used to quantify dioxin

levels. Gas chromatography separates the dioxins

from other compounds by selectively adsorbing the

dioxins (based on their specific molecular weights)

on an adsorbent such as activated charcoal, alumina, or silica gel. Mass spectrometry separates the

dioxins specifically and quantitatively according to

their atomic weights. These technologies can measure dioxins in the range of parts per quadrillion

(Ppq) in water samples, but this level of detection is

still considered experimental for most biological

samples.

EPA selected a nominal detection limit of one

part per trillion (ppt) for fish and soil samples

collected in the course of the National Dioxin

Study. This sensitivity pushed the limits of the

state-of-the-art in analytical technology. Comprehending parts per quadrillion and parts per trillion

cart boggle the mind. One ppt is equivalent to 1

second in 32,000 years. One ppq is equivalent to 1

second in 32,000,000 years. The potency of dioxin

makes measurements at this minute level of resolution important.

The Food and Drug Administration (FDA) recommends that consumption of fish be limited if

dioxin content exceeds 25 ppt, and consumption is

banned when levels reach 51 ppt. In general, EPA

found that dioxin levels of fish fillets-the edible

portions of the fish-had dioxin levels below the

detection limits even though whole fish samples

may be judged to be contaminated.

At two-thirds of the sites where dioxin was

detected, the maximum values encountered were

below 5 ppt. At only four sites did dioxin levels

exceed 25 ppt (the level at which FDA recommends

that fish consumption be limited). The high-level

sites were located in the Androscoggin River at

Lewiston, 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 of

which are located downstream of pulp and paper

mills.19 Additional investigations at these sites

showed dioxin levels of up to 414 ppt in waste

treatment sludges from the mills.

As a follow-on to the NDS, EPA is investigating

other chemical pollutants that might accumulate in

fish. The National Bioaccumulation Study (NBS),

which is currently underway, is focusing on a subset

of “priority” pollutants selected from among 400

potential chemicals. These include non-conventional pesticides, semi-volatile organic chemicals

known to accumulate in human fatty tissue, agricultural chemicals, industrial chemicals, and those in

pulp mill effluents. Four hundred sites are being

sampled in targeted industrial, urban, and agricultural areas and below pulp mills. Approximately 95

percent of the fish samples have been collected. Of

the 75 samples that have been analyzed from fish

collected below pulp mills, 67 are reported to have

dioxin above detectable levels. Samples from 10

mill sites report TCDD and TCDF concentrations in

fish fillet tissue above the acceptable FDA limits of

25 ppt.20

Recent data from the NBS based on fish sampled

from 18 southern rivers receiving mill wastes

showed accumulations of dioxin in whole fish

ranging from about 1 ppt up to 164 ppt. Most whole

fish samples had dioxin levels between 10 and 40

ppt. Three whole fish samples had levels exceeding

100 ppt. The edible part of the fish (fillets) contained

much lower dioxin levels and none exceeded FDA’s

25 ppt acceptable limits, although one fish showed

a level of 24 ppt of dioxin.21

EPA/Paper Industry Joint Five-Mill Study

The results of the National Dioxin Study indicated

that effluent from the manufacture of pulp can

introduce detectable levels of dioxin into the environment. This prompted the U.S. pulp and paper

industry, through the National Council of the Paper

Industry for Air and Stream Improvement, and EPA

to undertake a joint investigation of five bleached

kraft pulp and paper mills in 1986.22 The cooperative

screening study focused on three mills known to

have dioxin in their waste sludge (all of the mills

sampled used the activated sludge waste treatment

process) plus two additional mills that were volunteered by their firms to provide broader geographical

coverage (table 3-2). The results of the cooperative

five-mill study indicated that the bleaching of kraft

pulp with chlorine and chlorine derivatives is

responsible for the formation of 2,3,7,8 -TCDD and

2,3,7,8 -TCDF as byproducts of the pulping process.

Dioxin in Bleached Pulps

Sensitive gas chromatographic procedures were

used 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 mill

wastes. TCDD was detected in seven of nine

bleached pulps sampled, at levels up to 51 ppt. The

median TCDD content was 4.9 ppt, and the mean

was 13 ppt.23 TCDF was found in eight of nine pulp

samples at levels ranging from below detection

limits to 330 ppt. The median TCDF content was 50

ppt, and the mean was 93 ppt.

Dioxin in Bleach Plant Wastewaters

Wastewater from each stage of the pulp bleach

sequence was systematically sampled at each mill.

TCDD was detected in wastewaters at three of the

five mills, and TCDF was detected at four of the five

mills sampled. The greatest discharge of both TCDD

and TCDF was associated with the caustic extraction

stage, which serves to flush away the lignin and

other coloring agents that are mobilized during the

bleaching stages. Lesser amounts of TCDD and

TCDF were detected in the wastewaters of the

hypochlorite bleaching stage and the chlorination

bleaching stages (see table 3-3).

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

Table 3-2—Characteristics of the Bleached Kraft Pulp and Paper Mills Used in

the Five-Mill EPA/industry Cooperative Dioxin Screening Study

Daily

Daily

effluent

production

Furnish (in percent) capacity Bleach sequences (million

Mill Hardwood softwood (tons) Hardwood softwood gallons)

I . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85 15 500 CEOH & CEOHHP 23

CE0HHP

II . . . . . . . . . . . . . . . . . . . . . . . . . . 20 775a CEHED CEHED 36

Ill . . . . . . . . . . . . . . . . . . . . . . . . . . 100 NA 1,000 C/DEoD 36

Iv . . . . . . . . . . . . . . . . . . . . . . . . . . NA 100 4 0 0

b — CEH—

18

v . . . . . . . . . . . . . . . . . . . . . . . . . . 30 70 1 ,200’ CDEoH/D CDEOH/D 41

NA = Not applicable

a 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 Symposium

on 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 Mean

Chlorination . . . . . . . . . . . . . . . . . . 0.01-0.24 0.04 0.07 0.06-3,8 0.24 0.65

Caustic wash . . . . . . . . . . . . . . . . . 0.01-3.6 0,24 1,00 0.06-33.0 0,78 7.4

Hypochlorite ., . . . . . . . . . . . . . . . 0.02- 1.9 0.20 0.40 0.09-9.2 0.59 2.3

Chlorine dioxide . . . . . . . . . . . . . . 0.01-0.03 ND ND 0.01-0.13 ND ND

ND= 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 Symposium

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

Dioxin in Wastewaters and Sludges

Comparisons among the mills indicated that the

TCDD and TCDF contents of pulp and wastewater

differed greatly from mill to mill. TCDD produced

in the pulp bleaching process can be transported to

the environment as a residual in finished pulp, in the

sludge recovered in the wastewater treatment process, or as treated effluent released into streams and

pond (table 3-4). TCDD was found in wastewater

treatment sludges at each of the five mills sampled.24

Analyses of wastewater from the paper machines

showed that some of the dioxin produced in the

bleaching process was passed through to the paper

making process.

Continuing Efforts

Although the five-mill cooperative survey confined that chlorinated bleaches can produce dioxins in the manufacture of bleached wood pulp, the

study revealed great variability in dioxin concentrations among the mills and within the pulps, waste

sludge, and treated effluents. The results demonstrated the need for a comprehensive and systematic

survey of the receiving waters and biota below the

waste outlets of U.S. pulp mills in order to better

understand the scope and intensity of the environmental loading of TCDD, The survey also indicated

that more detailed information was needed about

dioxin levels at various steps in the pulping process

and in the bleaching sequence.

EPA has recently negotiated a cooperative agreement with the American Paper Institute (API) and

the National Council of the Pulp and Paper Industry

for Air and Stream Improvement (NCASI), both

associated with the U.S. pulp and paper industry, to

survey all 104 domestic pulp mills that manufacture

chemical bleached pulp for production of dioxin and

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)

Mill

Source I II Ill Iv v

2,3,7,8-TCDD

Bleached pulp . . . . . . . . . . . . . . . . 19 66 — 30 57

Waste sludge . . . . . . . . . . . . . . . . 16 16 100 70 22

Treated effluent . . . . . . . . . . . . . . . 65 18 — — 21

2,3,7,8-TCDF

Bleached pulp . . . . . . . . . . . . . . . . 19 5 6 6 0 31 5 5

Waste sludge . . . . . . . . . . . . . . . . 17 20 36 69 22

Treated effluent . . . . . . . . . . . . . . . 64 24 4 — 23

SOURCE: 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 and

bleaching processes at selected pulp mills.

The study will consider the full range of factors

that might affect the production and dispersal of

dioxins, including annual effluent flow, wastewater

treatment, sludge disposal practices, bleach plant

operations, and an analysis of dioxin contents of

effluents, pulps, and sludges. Detailed analyses of

dioxin levels in 25 bleach lines will be included. The

study began in the summer of 1988, and is expected

to be completed in the summer of 1989.

In a related effort, an EPA-led interagency group

has undertaken a Multi-Media Risk Study that will

utilize data collected in the EPA/Industry 104-mill

study. Under a court-approved consent agreement to

consider dioxin in paper, EPA is attempting to

estimate the cumulative risk of dioxin from this

source in all media—pulp, sludge, and effluent.26

Data collected in the 104-mill study, and estimates

of migration rates of TCDD and TCDF from paper

products adjusted by product-use scenarios, will be

used to determine whether or not there is a human

risk from dioxin in pulp and paper.27 The consent

decree also requires EPA to undertake an assessment

of environmental risks as well as human risks. The

Food and Drug Administration (FDA) and Consumer Safety Product Commission (CSPC) have

initiated product risk assessments that will become

part of the interagency multi-media study.

DIOXIN IN PULP AND PAPER

PRODUCTS

The finding of TCDD and TCDF in bleached pulp

samples raised questions as to whether residual

TCDD might also find its way into finished paper

products and present a potential health risk to

consumers through dermal contact (it did not consider other routes of exposure). The NCASI commissioned Envirologic Data, Inc. to assess the potential

risks28 to human health from skin exposure to a

variety of bleached pulp products, including disposable diapers, facial tissue, toilet tissue, sanitary

napkins, and paper towels.29

The results of Envirologic Data’s risk assessment

of concentrations of TCDD found in paper products

were related to a lifetime cancer risk of one in a

million in the general population—a regulatory

standard commonly used by the EPA and the FDA

to gauge risk. Based on this measure of risk, a

“virtually safe concentration’ of TCDD equivalents 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 adversehcatth eff~ls from human exposure [0 CtIVlrOtMTICIItd h~~ds. The

risk 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/ Healthf?isks From Demal Exposure /0 Dioxin

in Paper Products, ‘fkchnical Bulletin No. 534 (New York, NY: 1987), p. 107.
36 ● Technologies for Reducing Dioxin in the Manufacture of Bleached Wood Pulp

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 9,100

Manager .,.... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4,200 4,300

Personal care products

Disposable diapers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0

c

Conventional . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 540,000 540,000

Superabsorbent . . . . . . . . . . . . . . . . . . . . . . . . . . . 2,000,000 2,000,000

Facial tissue

Normal use . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66,000,000 79,000,000

Makeup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 230,000

Toilet tissue . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27,000,000 65,000,000

Sanitary pads . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63,000,000

Paper towels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7,900,000 9,500,000

Composite personal care productsd . . . . . . . . . . . . . 160,000 510,000

Combined communication papers and

personal care products . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

Clerical worker . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8,500 8,900

Manager . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4,100 4,200

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&al

Sulletrn No. S34(NewYork,NY: 1967~p107.

Calculated safe levels for bond paper, newspaper,

and other paper used for communications ranged

from 4,200 ppt of TCDD for female managers to

9,100 ppt for male clerks. Safe levels for personal

care products were calculated to range from 230,000

ppt of TCDD for female facial tissue used for

makeup removal to 79,000,000 ppt for facial tissue

by males. Safe levels for paper towels were calculated at 7,900,000 ppt for females and 9,500,000 ppt

for males. Toilet tissue safe limits for females were

calculated at 27,000,000 ppt for females and 65,000,000

ppt for males. Actual concentrations of TCDD in

samples of bond paper were detemined to be about

13 ppt: in paper towels, 4 ppt: and no detectable

TCDD was measured in disposable diapers.

NCASI published the results of an assessment of

potential exposure to dioxin from coffee brewed

using bleached coffee filters in May 1988.30 Based

on an assumed consumption profile for an average

and heavy coffee drinker and assuming that 65 to 90

percent of the dioxin migrated from the filter,

NCASI concluded that the calculated dioxin TEQs

for a risk ranging from zero to one in one-million to

be between 20 ppt TCDD TEQs for average coffee

drinkers to 11 ppt TCDD TEQs for heavy coffee

drinkers. The coffee filters tested had TEQ dioxin

contents ranging from 2.2 to 6.6 ppt.

An assessment of dioxin in food packaging paper

products has been initiated.31 NCASI commissioned

ENVIRON, Inc. to undertake the evaluation. A

number of food contact products are scheduled to be

undergo risk assessment, such as paper cups and

plates, convenience food packaging, paper towels,

and pizza boxes. NCASI has put the project on hold

pending the development of acceptable test procedures to determine the absorption of dioxin into fatty

foods. The assessment will resume when test protocols are developed

Scientists at Health & Welfare Canada, a government 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 Dioxin

Through 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 From

Use 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. NCASI

whole milk packaged in plasticized bleached paper is currently collaborating with Canadian scientists to

cartons. 32 They r epoted no similar level in milk confirm these findings.
Chapter 4

Pulp Bleaching Technology
CONTENTS

Page

THE BLEACHING PROCESS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

Historical Development of Bleaching Technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

Extent of Bleaching in the Industry.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

Bleaching M ethods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

Bleaching Systems. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46

Boxes

Box Pag e

4-A. What Is Pulp “Brightness’’? How Is It Measured?. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

4-B. Shorthand for Describing Bleaching Sequences . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44

4-C. Assessing Lignin Content and Pulp Bleach ability . . . . . . . . . . . . . . . . . . . . . . . . . 46

Figures

Figure Page

4-1. Bleaching Sequences Used in U.S. and Canadian Pulp Mills. . . . . . . . . . . . . . . . . . . . . . 44

4-2. Sequence for the Production of Fully Bleached Chemical Pulp. . . . . . . . . . . . . . . . . . . . 47

4-3. Pulp Brightness at Stages of the Bleaching Sequence CEHDED. . . . . . . . . . . . . . . . . . 48

4-4. Schematic Diagram of the Chlorination Process.,.,... . . . . . . . . . . . . . . . . . . . . . . . . . . 50

Tables

Table Page

4-1. Domestic Bleached Pulp Capacity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

4-2. Bleaching Chemicals: Form, Function, Advantages, Disadvantages . . . . . . . . . . . . . . . 44

4-3. Examples of Prebleaching Sequences for Pulp Delignification. . . . . . . . . . . . . . . . . . . . 48

4-4. Common Sequences Used To Bleach Kraft Pulp to Various Degrees of Brightness. 49
Chapter 4

Pulp Bleaching Technology

THE BLEACHING PROCESS

Bleaching 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. Lignin

remains a major constituent of pulp even after

digestion by chemical pulping. For example, kraft

pulp may contain up to 6 percent lignin based on its

dry weight.1 Unbleached groundwood spruce pulp

may contain 27 percent lignin.

If chemical pulping removes the lignin from wood

fibers, why then does some lignin remain after the

pulping process? The strength of paper is largely due

to the chemical bonds (hydrogen bonds) formed

between cellulose fibers. Although longer and more

severe pulping might remove more of the lignin, thus

reducing the amount of bleaching needed, the

cellulose molecules might be degraded and their

bonding power diminished. Should this happen, the

strength of the pulp would be reduced. The removal

of lignin by bleaching is regarded as a continuation

of the pulping process, albeit somewhat gentler and

less destructive, but bleaching too can degrade

cellulose if done improperly.

Lignin imparts a color to the raw pulp (hence its

name “brown stock”) and unless removed, will

continue to darken with age (note the yellowing,

darkening, and enbrittlement of newspaper exposed

to sunlight). Bleaching by removing the lignin gives

higher brightness to the paper than is possible by

leaving the lignin in the pulp and brightening by

decolonization, and also leads to a more durable and

stable paper.

In addition to the removal and decolonization of

lignin, bleaching serves to clean the pulp of dirt and

foreign matter that escaped the digestion process.

Bleaching also removes hemicellulose and extractives (hemicellulose is nearly completely removed

for the production of dissolved pulps). Bleaching

pulp adds significantly to its value as market pulp

because the demand for bleached paper is increasing.

Historical Development of Bleaching

Technology

Bleaching of fibers for decolonization has been

practiced since early times. Sunlight was one of the

earliest bleaching agents. Japanese paper makers

were known to bleach fibers by soaking them in

water from high mountain streams that contained

ozone (the first use of oxygen for bleaching). In

1774, Karl Wilhelm Scheele discovered chlorine

and its bleaching action on vegetable fibers. Several

years later in 1799, Charles Tennant invented

‘ ‘bleaching powder’ (calcium hypochlorite),

thereby converting chlorine to an easily transportable form. For the next 130 years it remained the

only available bleaching agent. The first time a U.S.

paper mill used chlorine for bleaching was in 1804.

Rapid developments in bleaching technology

occurred between 1900 and 1930. Multistage bleaching using calcium hypochlorite followed by an

alkaline extraction stage, then a repeat of the

hypochlorite bleach stage was first adopted by the

industry. Later, technologies that allowed the use of

gaseous chlorine began to displace hypochlorite in

the first bleach stage. The use of chlorine reduced

bleaching costs. New equipment developments further improved bleaching efficiency.

Improvements in the manufacture of chlorine

dioxide and dioxide bleaching technology were

developed in the 1940s. By the 1950s, these

developments led to the adoption of the five-stage

bleaching sequence that is still used extensively in

the industry: (1) chlorine+(2) alkaline extracsequence allowed very bright pulp to be produced

with minor losses in fiber strength.

Oxygen bleaching was discovered in 1952 by

V.M. Nikitin and G.L. Akim in the Soviet Union. In

the late 1960s, oxygen bleaching was commercialized, followed by the installation of the first displacement bleach plant in the 1970s. This resulted in

more rapid bleaching by displacing chemicals

through 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–
42 ● Technologies for Reducing Dioxin in the Manufacture of Bleached Wood Pulp

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, measured

subjectively by the impression and perception of the human eye. For instance, if blue dye is added to a lightly yellow

tinted 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 pulp

sheet, and other factors, the measurement of brightness has been standardized: Brightness is the reflectance of blue

light with a spectral peak at 457 millimicrons from an opaque sample of pulp sheets compared to a specified standard

surface.

There are two recognized methods for measuring pulp brightness in North America: 1) the TAPPI method

(Technical Association of the Pulp and Paper industry), Standard T-452; and 2) the CPPA method (Canadian Pulp

and Paper Association).

TAPPI Method

Reported 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 Method

Reported 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 to

absolute reflectance from an imaginary perfectly deflecting, perfectly diffusing surface. Calibrated opal glass

standards are used on a routine basis.

A third brightness standard is used throughout the rest of the world: The Zeiss Elrepho standard. It is measured

in units termed Elrepho Brightness. GE Brightness is measured with a reflectance meter manufactured by the

General Electric Corp., while Elrepho Brightness is measured by an instrument manufactured by Zeiss, the German

optical company. Since the two meters have different light geometries, there is no simple relationship between the

two measurements. In general, Elrepho Brightness is, on average, 0.5 to 1.0 percent higher than GE Brightness.

with the pulp in the conventional way. Since the late States as well.2 Overall, the tendency has been to

1970s development has taken place in the use of

oxygen enrichment in alkaline extraction stages, to

further delignify pulp after extended cooking (modifications of the cooking process to improve delignification), and in short bleaching sequences where

oxygen is used to supplement chlorine.

Extent of Bleaching in the Industry

Nearly 55 percent of the chemical pulp currently

produced in the United States is bleached (table 4-l).

By far the greatest proportion of bleached chemical

pulp is produced by the kraft process (about 88

percent of the pulp bleached in 1987 was kraft pulp).

Very little mechanical pulp has been bleached in the

past, however, this is currently changing. Mechanical pulp bleaching is growing at more than twice the

rate of chemical pulp bleaching worldwide and it is

likely that this trend will continue in the United

bleach more pulp as the demand for products using

bleached paper increases (nearly 39 percent of the

domestic paper and paperboard currently produced

is from bleached pulp).

Bleaching Methods

Bleaching Agents

Pulp cooking can safely dissolve up to about 90

percent of the lignin without degrading the cellulose

fiber. Additional delignification is done by bleaching. Bleaching of high-yield chemical pulps is

achieved by decolonizing with either an oxidizing

agent (combines oxygen) or a reducing agent

(combines hydrogen). Chlorine gas, sodium hypochlorite, chlorine dioxide, oxygen gas, and hydrogen

peroxide are oxidants. Sodium hydrosulfite is a

reductant. 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 the

equipment needed to handle it.

Grade 1971 1987’

Bleached sulfite ... , . . . . . . . . . . . . . . . . 1,774 1,256

Bleached kraft. . . . . . . . . . . . . . . . . . . . . . 13,364 21,259

Dissolving (bleached) ., . . . . . . . . . . . . . 1,604 1,455

Total chemical pulp produced . . . . . . . 32,779 43,800

Percent of chemical pulp bleached . . 51 .lO

/O 54.70/0

a Estimate

SOURCE: 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 the

predominant chemical used for the delignification of

pulp. Chlorine dioxide can brighten pulp without

damaging the cellulose. Oxygen is comparatively

inexpensive and is now coming into its own both for

delignification (immediately after digestion and

before the bleach cycle) and as a supplement in the

first extraction (alkali) stage of the bleach sequence.

Hydrogen peroxide is expensive, so it is used much

less than other bleaching agents. The effectiveness

of a bleaching agent, although a major factor in

determining its use in a pulp bleaching sequence,

A critical determinant in choosing a bleaching

chemical is the ‘‘selectivity” of the agent. Selectivity refers to the capacity of the chemical to attack

lignin while doing minimal damage to the cellulose

fibers. Unbleached pulp (brown stock) contains high

levels of lignin, therefore less selective chemicals

(e.g., oxygen and chlorine) can be used in the initial

stages of the bleach cycle. With further delignification and lower residual lignin content of the pulp,

more chemical is available to react with the cellulose

and pulp strength may suffer.

Chlorine dioxide and hydrogen peroxide are

highly selective, thus they react rapidly with lignin

but affect cellulose very little. The highly selective

chemicals are generally used in later bleach stages

when the lignin content is low and the cellulose is

susceptible to degradation. However, both chemicals are expensive and are therefore used sparingly.

Sodium hydrosulfite, a reducing agent, and hydrogen peroxide are used for bleaching lignin-rich

mechanical pulp.

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

Chemicals Function Advantages Disadvantages

Oxidants

Chlorine . . . . . . . . . . . Oxidize and chlorinate Iignin

Hypochlorite . . . . . . . Oxidize, brighten and solubilize

Iignin

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

Iignin

2) In small amounts with chlorine

gas to protect against

degradation of pulp

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

Hydrogen peroxide . . Oxidize and brighten Iignin in

chemical and high-yield pulps

Reductant

Hydrosulfite . . . . . . . . Reduce and decolonize Iignin in

high-yield pulps

Alkali

Sodium hydroxide . . . Hydrolize chlorolignin and solubilize

ligin

Effective, economical delignification

Easy to make and use

Achieves high brightness without

pulp 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 if

used improperly

Can cause loss of pulp strength if

used improperly

Must be made at the mill site

Used in large amounts. Requires

expensive equipment. Can cause

loss of pulp strength

Expensive

Decomposes readily. Limited

brightness 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.
44 ● Technologies for Reducing Dioxin in the Manufacture of Bleached Wood Pulp

Bleaching Sequences

A combination of bleaching and extracting treatments is generally used for bleaching chemical pulps

(box 4-B). The bleaching chemicals and the order in

which they are used make up the “bleaching

sequence. ” Bleaching sequences generally contain

two phases within each sequence: 1) a delignification segment, whose function is to remove the

lignin; and 2) a brightening segment, whose principle function is to increase the brightness of the pulp.

Examples of delignifaction stages or segments

include oxidation by chlorine (C) followed by

Box 4-B-Shorthand for Describing

Bleaching Sequences

The pulp and paper industry has developed a

series of shorthand descriptors for the multistage

bleaching sequences. The following abbreviations

are used to designate the bleaching agents:

c Chlorination

E Extraction with sodium hydroxide

H Hypochlorite (sodium or calcium)

D Chlorine dioxide

P Hydrogen peroxide

o Oxygen

N Nitrogen dioxide

z Ozone

Bleaching sequences are designated by listing

each treatment serially. For example, “CEDED”

represents a commonly used five-stage bleaching

sequence consisting of a first-stage chlorine treatment, followed by a second-stage alkali extraction

stage, followed by a third-stage chlorine dioxide

treatment, followed by a fourth-stage alkali extraction treatment, and a final fifth-stage chlorine

dioxide treatment. Washing is conducted between

each chemical application.

Two bleaching agents may be used in a single

stage. For instance, chlorine gas and chlorine

dioxide are sometimes combined in an early bleaching stage. If chlorine gas is the predominant agent

in the mixture, the treatment would be designated as

“cD.” On the other hand, if the mixture contains

more chlorine dioxide than chlorine gas, the treatment would be designated as Dc. Other commonly

encountered oxidative extraction treatments include EO (or E/P), EP

, E/H etc.

extraction of the dissolved lignin with sodium

hydroxide (E). Brightening segments use sodium

hypochlorite (H) and/or chlorine dioxide (D). Oxygen can be used for delignification and for reinforcing extraction of the dissolved lignin in the alkali

stage.

Several of the more commonly used bleach

sequences in U.S. and Canadian mills are: 1)

CEDED, 2) CEDH, 3) CEHDED, 4) CEH, and 5)

CED (figure 4-l). Although these are the most

prominent bleaching sequences currently in use, an

increasing number of mills are now using oxygen in

combination with alkali for extraction (Eo), and

chlorine dioxide (CD, Dc) in the chlorination stages.

In addition, there area number of unique bleaching

sequences used by some mills (e.g., CEHDH,

CEHEDP, CEDPD, CEDEHD and CEHCHDED).

Factors Affecting the Bleaching Process

Process engineers and paper chemists have a wide

range of chemicals, processes, equipment, and

operating conditions to choose from in optimizing a

bleaching sequence. Cost of chemicals, capital cost

of equipment, energy requirements, and other operating costs figure heavily in bleach plant decisions.

While cost control is an important factor, the

physical and chemical composition of the wood raw

material (furnish) and the desired characteristics

(brightness and strength) of the finished paper are

often more important in selecting bleaching technologies. Bleaching sequences also depend on the

pulping process used for initial delignification, some

Figure 4-l-Bleaching Sequences Used in

U.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 remaining in the brown stock than do others.

The efficiency of the bleach cycle (related to its

cost effectiveness), also depends on controlling the

operating environment within each bleaching stage.

Bleaching is achieved through chemical reactions.

Operating conditions are related to temperature,

time, chemical concentrations, and degree of acidity

or alkalinity (pH).3 These factors must be kept in

balance to achieve the desired degree of bleaching,

while at the same time minimizing damage to the

cellulose fiber. In addition, the “consistency”

(amount of fiber being bleached in relation to the

volume of liquid) of the fiber slurry being bleached

affects chemical penetration and therefore must also

be controlled. Computerization and improved sensors now allow nearly real-time control over the

operating environment in all stages of the pulping

and bleaching processes.4

Lignin Content— Lignin is a large, complex,

organic molecule that still holds mysteries for wood

chemists. Complete quantitative analysis of lignin

from pulp samples would be expensive if performed

with precision. This is not necessary from the

standpoint of controlling the digestion and bleaching

process, however, as simpler methods have been

found. Index systems for ranking the lignin content

of wood pulp have been devised by the industry (box

4-c).

The lignin content of unbleached pulp, expressed

by its kappa number, determines the amount of

bleaching required in the bleach sequence to obtain

the brightness desired in the finished pulp. The key

is to find the optimum point between cooking and

bleaching (i.e., the best kappa number for unbleached pulp). For bleachable grades, kappa numbers of unbleached kraft softwood pulp may range

between 20 and 40 and hardwood between 15 and 25

as it leaves the digesters in some mills (kappa

number 35 represents approximately 5 percent

lignin).

Using modem computer-controlled cooking, appropriate chip pretreatment, and chip equalizing

systems, it is now possible for well-run kraft mills to

produce unbleached softwood pulp consistently

with kappa numbers between 28 and 32.5 Unbleached hardwood pulps can be produced with

kappa numbers between 20 and 25.6 Pulp can be

delignified 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 are

somewhat interrelated. Since pulp brightness is the

major objective of bleaching, measurements of

brightness expressed as either GE Brightness or as

ISO Brightness (see box 4-A) are often used to track

progress through the bleaching sequence.

Unbleached kraft pulp has a very low GE Brightness (10 to 20 percent) because of the high absorption of reflected light by the residual lignin.8 Kraft

pulp can be bleached to a very high brightness of 90

percent GE by decreasing its lgnin content to near

zero without affecting the strength of the pulp. It is

possible to bleach kraft pulp to a brightness of 91 to

92 percent GE for special papers.9 Unbleached acid

sulfite pulp with GE brightness of about 60 to 66

percent can also be bleached to 90 percent by

removing nearly all of the lgnin. Groundwood pulp

may 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 Papperstidnmg

No 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~ication

Co~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.
46 ● Technologies for Reducing Dioxin in the Manufacture of Bleached Wood Pulp

Box 4-C—Assessing Lignin Content and

Pulp Bleachability

The Technical Association of the Pulp and Paper

Industry (TAPPI) has devised two standardized

procedures for determining and reporting the lignin

content of pulp: 1) Permanganate (K) number

(TAPPI Test Method T214), and 2) Kappa number

(TAPPI Test Method T236). These indices are used

by the industry to control cooking within the

digester during pulping and for determining the

bleachability of the pulp.

Both methods are chlorine demand tests and are

based on the amount of permanganate needed to

oxidize the contained lignin, The Permanganate, or

K number, is used for determining the bleachability

of chemical pulps having lignin contents below 6

percent (based on weight of oven-dry pulp). The

kappa number is applicable to all grades of chemical and semi-chemical wood pulps, including higher lignin content pulps with yields as high as 70

percent.

Standard procedures have been established for

both bleaching indices. Both are based on the

amount of potassium permanganate that can react

with dry pulp samples. Most modem pulp mills

now use automated, continuous oxidationreduction measurements or optical devices such as

brightness meters for on-line measurements to

gauge the progress of delignification and the need

for additional bleaching. However, permanganate

tests are still used in mill laboratories for verification of the instrument reading.

Bleaching Systems

Bleaching sequences apply various bleaching

agents in different orders and combinations. Between each bleaching stage the pulp is generally (but

not always) flushed with an alkali extraction solution to remove the dissolved lignin before it is sent

to the next bleaching stage (figure 4-2). The first step

of a bleaching sequence is designed to remove the

bulk of the residual lignin (delignification) and

involves little or no improvement in the brightness

of the pulp (figure 4-3). This step, along with the

following extraction stage, is called “prebleaching. ” The purpose of prebleaching is to remove as

much lignin from the pulp as possible to minimize

the volume of more expensive bleaching chemicals

(e.g., chlorine dioxide, hypochlorite, and hydrogen

peroxide) needed in subsequent bleaching stages.

Chlorine gas and sodium hydroxide (CE) have

been the preferred chemicals for the prebleach stage

of the bleaching process. More recently, mixtures of

chlorine and chlorine dioxide have been used in

place of (or in addition to) pure chlorine treatment

(table 4-3).10 Other prebleaching processes are

slowly displacing CE as the first stages of the bleach

sequence at some mills. Prebleach oxygen delignification and extended cooking may shorten the

bleaching sequence by reducing the amount of lignin

carried forward to the bleaching process. 11

Impetus for considering alternatives to the conventional CE stage are based partially on reducing

the cost of bleach plant operations and partially on

concerns about environmental impacts from discharged bleaching effluents. These concerns result

from detection of chlorinated organic material containing chloroform and dioxins in bleached pulp mill

sludge. Most of the chlorinated organics contained

in bleach plant effluent originate from the first

chlorine, alkaline extraction, and hypochlorite

stages.

Brightening stages that follow prebleaching remove less lignin but bring out the brilliance of the

pulp through bleaching action. The brighter the pulp

desired, the more bleaching stages that must be used

(table 4-2). Mill operators have a number of mix-andmatch brightening processes to chose from for final

bleaching. Choices are largely determined by the

relative costs and efficiencies of the bleach options

and the required brightness of the pulp being

produced. The entire bleaching sequence is then a

combination of the prebleaching stages and the

brightening stages.

Prebleach Delignification

Chlorination-Chlorine selectively reacts with

lignin and under normal bleaching conditions does

little harm to cellulose 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.
Chapter 4—Pulp Bleaching Technology ● 47

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

Raw water, Raw water Raw water Raw water

condensate o r o r o r o r

white water white wate r white wate r

Pulp

Raw water

o r

white water

Unsreeened

BIac k

Black Iiquo r

ChIorinatio n Extractio n

Iiquo r

lost i n

effIuen t effIuen t

to recover y

screening o r

chIorinatio 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 chemicals it became widely used for delignification after

the pulping process. Chlorination in the prebleach

cycle begins with washed brown stock pulp slurry at

low consistency (3 to 5 percent weight of pulp to

water) being pumped into a chlorination mixer.

Chlorine gas, which is often dispersed in water, is

added to the pulp slurry in the mixer and is

vigorously mixed (figure 4-4).

The reaction between chlorine and lignin begins

immediately in the mixer, and the reaction is

completed in a chlorination tower designed to give

the proper retention time. If chlorination is conducted at low temperatures (5 to 45 “C) retention

time may range between 15 and 60 minutes. Higher

temperatures reduce the time necessary to complete

the chemical reaction. The chlorinated pulp is

washed before being sent to the alkali extraction

stage.

Chlorination is sometimes repeated after extraction if additional delignification is needed, but

because of possible cellulose damage a chlorine

dioxide stage is often used. Throughout the process

brightness, kappa number, and other indicators of

pulp quality are monitored.12

There is a trend toward modification of the first

chlorination stage by including other bleaching

agents (e.g., chlorine dioxide, with the chlorine

charge). Inclusion of these chemicals can reduce

cellulose degradation, improve pulp strength, and

reduce environmental releases. Chlorine dioxide is

sometimes used sequentially preceding the chlorine

treatment and has been shown to be more effective

than when the two chemicals are mixed. ’s Chlorine

dioxide can be used to completely replace chlorine

in the first delignification stage, but its gains in pulp

quality do not offset the additional expense, and pulp

brightness equivalent to t-hat produced by chlorine

can not be achieved. 14 Pretreatment with sodium

hypochlorite prior to chlorination has improved the

delignification of resinous woods.

Alkaline Extraction-Hot alkaline extraction is

the second stage in the pulp bleaching process and

completes 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.
48 ● Technologies for Reducing Dioxin in the Manufacture of Bleached Wood Pulp

Figure 4-3-Pulp Brightness at Stages of the

Bleaching 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 alkaline

extraction is also used after later bleach stages in the

brightening process of multistage bleaching sequences. Cold alkaline extraction is particularly

important in the production of dissolving pulps for

the manufacture of rayon and acetate. Alkaline

extraction removes the soluble colored components

and lignin released in the preceding delignification

stage (chlorination or oxygen), therefore reducing

the amount of bleaching chemicals needed in

subsequent stages and improving the durability of

the pulp.

Sodium hydroxide has been shown to be the most

efficient alkali for decreasing the kappa number of

pulp. Efficient extraction is nearly as important as

prebleach delignification in the first bleaching stage.

For instance, after chlorination and washing, but

before alkaline extraction, about 30 to 50 percent of

the chlorinated lignin in removed. After alkaline

extraction, 80 to 90 percent of the lignin is removed. 15 A second hot alkaline extraction is someTable 4-3-Examples of Prebleaching Sequences

for Pulp Delignification

CE . . . . . Chlorine - Alkali extraction

CDE . . . (Chlorine+ Chlorine dioxide) - Alkali extraction

CE O . . . Chlorine - (Alkali + Oxygen extraction)

OCDE . . Oxygen - (Chlorine + Chlorine dioxide) - Alkali

extraction

DCE . . . Chlorine dioxide - Chlorine - Alkali extraction

SOURCE: 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 after

bleaching with chlorine dioxide or sodium hypochorite (e.g., CEHED, CEHDED, or CEDED sequences) to improve pulp brightness stability and

conserve bleach chemicals. ’b

The pulp is subjected to the alkaline extraction

treatment for 60 to 90 minutes at most mills in the

first post-chlorination extraction stage, although

some operate on a shorter schedule. The second

alkaline extraction usually lasts from 30 to 60

minutes.

The first alkaline extraction stage contributes the

largest potential pollutant load released from the

pulp bleach plant. It may be possible to reduce the

pollutant loss from the extraction stage considerably

by substituting oxidative extraction, particularly

sodium hypochlorite, for the first alkaline extraction

stage (e.g., CHED, bleaching sequences) .17 Sodium

hypochlorite added to the sodium hydroxide extraction solution (EH

) may reduce the color (a rough

indicator of pollution load) in waste water by about

one-half.18 Even larger reductions in waste water

color (about three-quarters) have resulted when

hydrogen peroxide is added at the alkaline extraction

stage.

Oxidative Extraction-Oxygen gas added to

sodium hydroxide in the extraction stage (Eo)

decreases the kappa number, conserves chemicals in

subsequent bleaching stages (in some cases it can

reduce the number of bleaching stages), and reduces

pulp strength loss and coloration in the waste

Water.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: TAPPI

PIXXS, 1987), p. 44.

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

171bid., p. 99

lgLie~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 or

sodium hypochorite, to an EO extraction stage might

allow for a shortened, three-stage bleaching sequence capable of bleaching pulp to high brightness

(88 to 90 percent ISO). The EO extraction stage has

gained rapid acceptance since only modest additional investment in new equipment is needed.

Oxygen has been substituted for alkaline extraction immediately after the chlorination stage.20 If

coupled with a following chlorine dioxide bleach

stage, the COD sequence can produce fully bleached

pulp with major savings in chemicals. A three-stage

sequence using oxygen in the second stage following a chlorine dioxide-chlorine delignification stage

(( DC)OD) has been used by the Chesapeake Corp. at

West Point, Virginia since 1972. The Chesapeake

mill was the first commercial application of oxygen

in the extraction stage.

Brightening Stages

Chlorine Dioxide—Chlorine dioxide is very selective in attacking lignin without significantly

degrading cellulose, while producing high brightness pulp. In the dioxide bleach stage, chlorine

dioxide is generated as a gas at the mill and dissolved

in cold water. The aqueous chlorine dioxide solution

is mixed with the prebleached pulp, heated to about

70 “C, and is normally held in a reaction vessel for

approximately 3 hours.21

Because of chlorine dioxide’s high cost, it is most

commonly used at or near the end of bleaching

sequences (e.g., CEHD, CEHED, CEDED, and

CEHDED). Sequences using chlorine dioxide in

only one bleach stage generally produce lower

brightness pulps.

22 For instance, the CEHD sequence on softwood kraft pulp would probably be

limited to 85 percent G.E. brightness. In order to

achieve the highest brightness (90+ percent G.E.),

two chlorine dioxide stages are generally required

(e.g., CEDED and CEHDED sequences). Chlorine

dioxide can also be used in conjunction with a

Table 4-4-Common Sequences Used To Bleach

Kraft Pulp to Various Degrees of Brightness

Range of GE%

brightness Sequence

CEH

70-80 CEHH

CHEH

CEHEH

80-85 CCHEHH

CED

CEHD

CHED

85-92 CEHDD

C CH E DH

CEDED

CEDHED

C D E OD E D

O CDE H D

SOURCE. Adapted from AJlan M. springer, krdustnal Erwironrnarrkd Corrtrol: Pub and

Papar /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 effective

cellulose-preserving bleach agent and is well suited

for improving the brightness of highly lignified

pulps, such as mechanical groundwood or chemimechanical pulps, without significantly reducing its

yield. Hydrogen peroxide is an extremely versatile

delignifying chemical and has been proposed for use

as a chip pretreatment before kraft pulping and as a

delignifier in the prebleach stage prior to, or as a

substitute for, the C, CD, or Dc prebleaching

stages. 24 It is also used in association with sodium

hydroxide in alkaline extraction (EP) Because of its

high cost, hydrogen peroxide is used most often in

the later stages of pulp bleaching.

Peroxide is used in the intermediate stages of the

bleaching sequence as a replacement for hypochlorite or chlorine dioxide. It is frequently used as the

last stage in the bleach sequence where it can add a

few points of brightness to the pulp and improve its

brightness stability. Peroxide alone is a relatively

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 However, when used in sequences with chlorine-based

bleaching agents, peroxide is an efficient delignifier

and brightner. Peroxide is also used for intermediate

bleaching stages of the kraft bleaching sequence or

as a final treatment to increase and stabilize the

brightness of chemical pulps.26

The peroxide bleach liquor is usually in the range

of 1 to 3 percent hydrogen peroxide. An appropriate

volume of peroxide liquor, sodium hydroxide, and

other chemicals to stabilize the peroxide are mixed

with pulp and heated with steam to the reaction

temperature (35 to 70 ‘C). The peroxide-pulp

mixture is allowed to react under controlled temperature for an optimum time (1 to 5 hours). When

the reaction is complete, the pulp is washed and sent

to the next bleaching stage or washed and neutralized with sulfur dioxide if it is the final bleach stage.

Peroxide, coupled with oxygen and/or ozone,

shows some promise in research laboratory evaluations for formulating chlorine-free bleach sequences

to reduce release of chlorinated organics in the waste

stream. A three-stage sequence OZP (oxygen, ozone,

peroxide) has yielded brightness values of 85

percent GE in eucalyptus kraft pulp.27 However,

the pulp suffered a substantial loss in tear strength,

The ZP bleaching sequence produced southern pine

oxygen pulps of 80 percent GE brightness with good

brightness stability. In contrast to peroxide and

oxygen bleaching, ozone bleaching has not been

developed to the point of commercialization.

Hypchlorite—The use of hypochlorites for bleaching wood pulp began in the early 1880s. Although

the development of chlorine bleaching technology in

the 1900s led to a decrease in the use of sodium and

calcium hypochlorite, still about 40 percent of the

kraft pulp mills in the United States and Canada use

at least one hypochlorite stage in their bleach

sequence. Hypochlorites have been used effectively

on sulfite pulps where an alkaline extraction stage is

interposed 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.
Chapter 4—Pulp Bleaching Technology ● 51

Kraft pulps, being more difficult to bleach than

sulfite pulps, require that a chlorine and alkaline

extraction stage be added in the prebleach segment

of the bleach sequence. Until chlorine dioxide and

peroxides became available in the 1940s, kraft pulps

of 85 percent GE brightness were the brightest that

could be produced with hypochlorite bleaching and

still maintain acceptable pulp strength, but these

pulps had poor brightness stability.

Hypochlorite is nonspecific, that is, it attacks

cellulose as well as lignin, therefore it requires

careful control if a reduction in pulp strength is to be

avoided. 29 Bleaching sequences such as CEHD,

CEHED, and CEHHD are used widely for producing

pulps of 86 to 88 percent GE brightness, CEHDED

is used for pulps of 88 to 90+ percent GE brightness,

and CEHDP and CEHEDP for pulps of 90 percent

GE brightness using peroxides. Hypochlorite is also

used in small amounts for oxidative extraction (see

above). Some mills use hypochlorite as a replacement for the first alkaline extraction stage to reduce

the color in bleach plant effluent.30

Retention times and chemical concentrations vary

for hypochlorite bleaching depending on which

stage it is being used in the bleaching sequence.

Retention times range from a low of 30 minutes at

some mills to 3.5 hours for those using hypochlorite

in the brightening stage. Reaction temperatures for

hypochlorite stages are generally kept low (85 to 110

‘F) to minimize cellulose degradation.

A “simplified bleaching” process for hypochlorite has recently been developed.31 Simplified bleaching uses a short (l O-minute) bleach cycle at higher

temperatures than normally used (180 ‘F). The

hypochlorite treated pulp is sent without washing to

a chlorine dioxide stage. This ostensibly produces

pulps of higher brightness at lower cost.

Studies have shown that one of the largest

contributors to the environmental release of chloroforms from a bleach plant is the effluent from the

hypochlorite stage.

32 It is hypothesized that chloroform is produced under specific conditions existing

in the hypochlorite stage rather than simply as the

result of chlorine-based chemicals. The specific

conditions and reactions contributing to the production of chloroform are not well known, however, and

more research is needed to establish causation.

These early findings of the linkage between hypochlorite reactions and chloroform production has led

some to propose that the release of chloroform

compounds from pulp mills could be reduced by

eliminating the large-scale use of hypochlorite in the

bleach sequence.

Ozone-Ozone is one of the most powerful

bleaching and oxidizing agents. It is a special form

of oxygen produced by the discharge of an electrical

current in oxygen gas. While oxygen atoms normally occur in pairs, the electrical discharge makes

three atoms associate with one another, thus giving

extraordinary oxidative properties to ozone. Its

decomposition to oxygen after bleaching produces

neither a residue, nor undesirable inorganic byproducts. Ozone, in a bleaching sequence with

hydrogen peroxide, can produce high-brightness

pulps. Ozone is not used commercially for pulp

bleaching. Some pilot plant studies have been

conducted, but additional development work would

be needed to permit its widespread commercial use.

Ozone, in conjunction with preliminary oxygen

delignification, holds promise for reducing the

amount of chlorine and hypochlorite used in the

prebleach and brightening stages of the bleaching

sequence. Ozone bleaching is particularly well

suited to bleaching sulfite pulps because of their low

 High-brightness ,

residual lignin content.33 highquality, hardwood kraft pulps can be produced by

using 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 Pulp

Bleaching, ’ 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.
52 ● Technologies for Reducing Dioxin in the Manufacture of Bleached Wood Pulp

sequence (e.g., ZEP, ZEZP, and ZED).34 Because it

is a very powerful and nonselective chemical, its use

is usually limited to the early bleaching stages.

Kraft softwood pulps must be delignified, preferably with oxygen or through extended cooking to

reduce the need for chlorination in the prebleach

segment, before bleaching with ozone. Low kappa

number kraft softwood pulps bleached with the

OZEP sequence produced pulp comparable in brightness and strength to those produced from the

CEHED five-stage sequence.

35Brighter kraft softwood pulp can be produced with oxygen-ozoneperoxide and/or chlorine dioxide bleaching sequences (e.g., OZEP, OZEPP, OZEPD, OZED, and

OZEPD). For the highest brightness, chlorine dioxide was needed in the final bleaching stage,

Ozone is not currently used commercially by the

industry. Experimental results and pilot plant operations indicate that ozone might have future promise

as an alternative bleaching agent. Further discussion

of oxygen-based, nonchlorine bleaching technology,

including ozone bleaching, is found in chapter 5.

~qsleven s.K, Ow and Rudrii P. Sin@, “Advances in Ozone Bleaching, Part II: Blcaehing of Softwood Kraft Pulps With Oxygen & Ozxme

Combination,” Oxygen Delignfl’cution .~~posium, TAPP1 Notes (Atlanta, GA: TAPPI Press, 1984), p. 43,

J51bid., p. 49.
Chapter 5

Technologies for Reducing

Chlorinated Organics in

Pulp Manufacture
CONTENTS

Page

OXYGEN DELIGNIFICATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56

Oxygen Delignification Technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59

Effects of Oxygen Delignification on Pollutant Loads . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64

Extended Delignification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64

Pretreatment of Oxygen Pulps With Nitrogen Dioxide (PRENOX) .., . . . . . . . . . . . . . . . 66

Soda-Anthraquinone/Oxygen Pulping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67

DISPLACEMENT BLEACHING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67

CLOSED-CYCLE BLEACHED PULP MILL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68

WASTE-OUTLET TREATMENT TECHNOLOGIES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69

Primary Treatment of Suspended Solids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71

Secondary Biological Treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71

Oxidation Ponds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71

Activated Sludge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72

New or Developing Treatment Technologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72

Figures

Figure Page

5-1. Chlorinated Waste From Bleaching of Hardwood Pulp . . . . . . . . . . . . . . . . . . . . . . . . . . 57

5-2. World Production Capacity of Oxygen-Delignified Pulp . . . . . . . . . . . . . . . . . . . . . . . . . 58

5-3. High Consistency Oxygen Delignification Stage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62

5-4. Oxygen Delignification System Installed in a Bleached Kraft Pulp Mill . . . . . . . . . . . 63

5-5. Phenolic Compounds Formed With and Without Oxygen Bleaching . . . . . . . . . . . . . . 65

5-6. Continuous Digester for Extended Delignification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67

5-7. Multistage Displacement Bleach System Single Tower Bleach Plant Showing

an EDED Wash Sequence. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .69

5-8. Closed-Cycle Bleached Kraft Pulp Mill . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70

Tables

Table Page

5-1. Oxygen Delignification Systems Installed Worldwide . . . . . . . . . . . . . . . . . . . . . . . . . . . 60

5-2. Effluent Characteristics of Oxygen-Treated Bleached Pulp and Pulp Bleached

by the Common Chlorinated Bleach Processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65

5-3. Effluent Characteristics for Softwood Kraft Pulps With and Without

Oxygen Delignification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65

5-4, 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 Chlorinated

Organics in Pulp Manufacture

The bleaching plant is the major source of

waterborne environmental pollutants produced by

pulp and paper mills. As much as 40 percent of the

biological oxygen demand (BOD), 25 percent of the

suspended solids (SS), 70 percent of the color, and

nearly all of chlorinated organic materials (TOC1)

produced by pulp mills originate in the bleaching

process. The amount and nature of bleach plant

pollutants vary considerably among the bleaching

sequences used, wood species, and pulping processes.

Water is used in large quantities in the bleaching

process (averaging 20,000 to 30,000 gallons per ton

of pulp produced, although modem mills use much

less than this). In contrast to pulping chemicals, the

bleaching chemicals are generally not recovered but

are discharged as wastes after treatment.

Chlorination, often the first bleaching stage after

pulping in many bleaching sequences, requires large

volumes of water to dilute the unbleached pulp to a

low consistency for subsequent bleaching (about 3

percent fiber by weight compared to water), and is

therefore a major source of chlorine-contaminated

water. Chlorination sometimes uses as much water

as all of the subsequent bleaching stages combined.

To reduce the amount of fresh water consumed, the

chlorination filtrate is often recycled in the bleaching stage, or paper mill white water is used for

dilution.

General hypotheses have been advanced to explain the possible occurrence of dioxins in pulp,

paper, and mill wastes. If true, these hypotheses may

suggest ways to reduce the amount of dioxins

produced in pulp and paper manufacture. First, since

lignin or wood extracts probably contain some

dioxin precursors, the reduction of the amount of

lignin exposed to chlorine in the bleach plant might

reduce the volume available for dioxin formation. It

is now believed that lignin may not be a major source

of precursors as originally thought.

Second, by reducing the amount of chlorine used,

or eliminating the use of chlorine bleach altogether,

the formation of dioxins might be reduced or even

eliminated.

Third, a recent discovery by the Pulp and Paper

Institute of Canada that oil-based defoamers made

with contaminated used oil may be the source of

nonchlorinated precursors of dioxin and furan that

become tetrachloro-p-dibenzodioxin (TCDD) and

tetrachlorodibenzofuran (TCDF) with chlorination.1

By using “cleaner” oil- or water-based defoamers

(although water-based defoamers may not be effective in washing brownstock), this source of precursors can be eliminated.

Fourth, preliminary findings indicate that the

formation of TCDD and TCDF can be reduced by

modifying conventional chlorine bleach sequences.

For instance, by applying chlorine gas in smaller,

successive split charges rather than a single large

charge, research has shown reductions in TCDD.2

By carefully controlling the acidity (pH) of the

unbleached pulp within an optimum range, TCDD

has also been reduced. Applying chlorine bleach

before the addition of chlorine dioxide (CD) tends to

produce less TCDD than if chlorine and chlorine

dioxide are mixed (CD) or if chlorine dioxide is

applied separately before the addition of chlorine.

TCDD and TCDF formation is also sensitive to the

ratio of chlorine to lignin. The Swedish Pulp and

Paper Research Institute has found that if chlorine

additions are kept below 15 percent (chlorine to

lignin), TCDD and TCDF can be kept to low levels.3

These observations need verification.

Fifth, improvements in secondary biological waste

treatment 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 Association

Enviromnenr Conference Proceedings, Vancouver, B. C., Oct. 25-26, 1988, p. 31.

7-Ron~d B~~dge~dWl]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-
56 ● Technologies for Reducing Dioxin in the Manufacture of Bleached Wood Pulp

TCDF are transported.4 Supplemental treatment

with chemical coagulant, precipitant, absorbents,

and perhaps destruction by ultraviolet light and

chemicals could increase the efficiency of the

existing waste treatment facilities. Further evaluation is needed on these options,

Should it prove necessary to reduce the amount of

dioxins produced from pulp and paper manufacture

for health or environmental reasons, one approach to

consider is prebleaching technologies that would

reduce the amount of lignin passing to the bleach

plant. Another approach would utilize brightening

technologies that reduce or eliminate chlorine gas

from the bleaching sequence. The several options

available to reduce the formation of TCDD, TCDF,

and other chlorinated organics are not mutually

exclusive and can be linked at stages throughout the

pulping, bleaching, and waste disposal processes to

achieve low levels of discharge.

Because of the pollution potential of chlorinated

compounds and the volume of wastewater produced

by chlorine bleaching, the chlorination stage is the

focal point for efforts to control pollution resulting

from the manufacture of pulp. There are several

possible approaches to reducing the amount of

dioxin and other chlorinated organics formed during

chlorine bleaching:

●

●

●

Delignify pulp to a further degree before

bleaching

-extended delignification

-oxygen delignification

—pretreatment with nitrogen (more lignin can

be removed without fiber damage)

Substitute other bleaching chemicals for chlorine

—chlorine dioxide for part or all of the chlorine

—hydrogen peroxide

—alkaline extraction supplemented with oxygen and peroxide

-ozone (precommercial)

Modify chlorination procedures (more research

needed)

-optimize the acidity (pH) of the pulp

●

●

—use smaller multiple charges of chlorine

instead of one

—apply chlorine first, then bleach with chlorine dioxide

Remove known sources of precursors (e.g.,

contaminated defoamers or other additives,

prewash pulp, etc.)

Improve waste treatment systems (more research needed)

-chemical coagulant

—sorption or precipitation enhancers

-destruction with ultraviolet light or chemicals

—anaerobic treatment

Elimination of chlorine in the bleach sequence

combined with internal recycling of process water

aimed at developing an overall “pollution free”

pulping system probably offers the best theoretical

strategy for reducing the pollution from bleach

plants over the long term, but because of practical

limitations, it may not be commercially viable for

sometimes

OXYGEN DELIGNIFICATION

Although the introduction of any chlorinated

bleaching chemical (e.g., chlorine gas, sodium, or

calcium hypochlorite), can generate some chlorinated organic compounds, chlorine gas used in the

prebleaching stages of the bleaching sequence

produces the largest amount (figure 5-l). Chlorinated organics produced by pulp mills contain small

amounts of TCDD and TCDF (see ch. 2).6 Their

presence in pulp mill wastes is ascribed to chlorination in the bleach sequence although the precise

chemical reactions and mechanisms that produce

dioxins are not known. Other factors might also

contribute to dioxin formation.

Ever since the connection was made between

dioxin and the use of chlorine, the emphasis of those

advocating process changes to reduce the formation

of TCDD, TCDF, and other chlorinated organics has

focused on oxygen delignification technology. The

Swedish 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 ati

2378-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 of

Hardwood Pulp

Lbs/tons

70

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 needed

in the bleaching process. Sweden has encouraged the

international move toward adopting oxygen delignification to solve the chlorine problem. It should not

be overlooked that first, most of the bleached kraft

mills in Sweden have already installed oxygen

delignification at a considerable capital expense, and

second, oxygen delignification is a Swedish technology largely manufactured in Sweden.

The effectiveness of oxygen delignification to

reduce the amount of chlorine needed is well

documented. It should be noted, however, that other

technologies and process modifications are also

available that can reduce the amount of dioxins

produced, but none will reduce the amount of

chlorine-based bleach needed to the degree that

oxygen can. Conversion to oxygen delignification is

not always the best solution. However, the substitution of oxygen bleaching for chlorination in prebleaching and brightening sequences is considered by

some pulp and paper experts to be a technological

trend that likely defines the future state-of-the-art in

low-chlorine bleach plant design.

Effluent from oxygen bleaches, such as oxygen

gas, ozone, or peroxides, can be recycled internally

to destroy harmful byproducts that might be formed.

Transition from conventional chlorine bleaching to

oxygen delignification and bleaching has been faster

in Scandinavian countries-particularly Sweden—

than it has in the United States and Canada.7 In

Sweden, oxygen bleaching has been used in place of

biological waste treatment that is commonly used in

North America. Although oxygen delignification

was developed in the Soviet Union, the process was

commercialized in Sweden in the late 1960s and in

the Union of South Africa in the early 1970s.

Early interest in oxygen delignification stemmed

primarily from its ability to reduce pulp mill

pollution. Substantial reductions in BOD, color, and

chlorinated organics in the effluent can be realized,

as well as savings in bleaching agents. Oxygen

prebleaching may not significantly alter the kinds of

chemicals formed (although this has not been

determined with certainty), but, when properly

conducted, it will probably generate smaller quantities of all these compounds.8 Moreover, bleaching

operations using oxygen do not normally call for a

hypochlorite stage, as a result, little chloroform is

released (other non-oxygen bleaching sequences

also do not use hypochlorite stages, e.g., CEDED).

Chlorinated organics—not specifically dioxins—

are major pollutants in the Baltic Sea. Nowhere in

the United States have chlorinated organics presented the problems that have been experienced in

the Baltic region. BOD has been the major environmental concern in the United States (U.S. standards

for BOD were stricter than Sweden’s); attempts to

7F1ftwn Swdlsh ~u]p ~llls ~mufac~ure bleached sulfate pulp, Nine ml]]s currently usc oxygenin the first prcbleachlng Stage of the bleaching

sequence and two others have plans to install oxygen stages in the bleach sequence. Committee for the Gulf of Bothnia, Water Pollution Problems of

Pulp 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 secondary

biological waste treatment and internal changes in

pulp mill processes.

With the exception of Sweden and Germany

(Germany has no kraft mills), where environmental

requirements have forced acceptance, economics

have been a significant motivating factor in adoption

of oxygen delignification technology. For instance,

in Japan where oxygen delignification systems have

recently gained acceptance, oxygen is cheap because

it is produced as a byproduct of nitrogen recovery,

which is needed in the manufacture of printed

electronic circuits. At North American mills where

oxygen delignification has been installed, waste

improvements have been accompanied by economic

benefits from shorter bleaching sequences and

smaller and cheaper effluent treatment systems.

Oxygen delignification can cause degradation of

the pulp and reduce paper strength. In 1963, this

problem was reduced with the discovery that the

addition of magnesium chemicals (e.g., magnesium

carbonate or sulfate) can reduce or prevent degradation of cellulose fibers.9 Oxygen delignification and

bleaching produces pulp that compares favorably

with conventional bleached pulps in most ways.

Strength properties of oxygen pulps are slightly less

than conventional pulps, but may be acceptable for

some products if delignification by oxygen does not

exceed about 40 to 50 percent of the pretreatment

lignin level.10 Experts differ as to the viscosity and

strength of oxygen pulp compared to conventional

pulp. Brightness stability for oxygen pulps is equal

to or better than that of conventional pulps.

Since the construction of the first pulp mill to use

oxygen delignification in 1970 at Enstra, South

Africa, there has been a steady increase in the annual

world production of oxygen pulps (figure 5-2). In

1988, world installed capacity was expected to

exceed 10 million metric tons per year. About half

the oxygen capacity is in Scandinavia and Europe,

one-fifth is in North America, and one-fifth is in

Japan. About 92 percent of the installed capacity of

oxygen delignification systems is in kraft mills, and

60 percent is in bleached softwoods (table 5-l). The

use of oxygen delignification is expected to expand

Figure 5-2—World Production Capacity of

oxygen-Delignified Pulp

30 Capacity--thousand metric tons per day

1

Year 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 tightened

and will likely accelerate even more if savings in the

cost of bleach plant operations favor oxygen pulps.

Capital cost of oxygen delignification systems is

high. Cost estimates based on prior conversions

from a conventional chlorine bleaching process to

oxygen delignification range between $20 million

and $30 million for an existing pulp mill with a

capacity of 750 to 1,000 tons per day, depending on

the need to modify supporting equipment, such as

recovery boilers, evaporators, etc. If the mill required expansion of supporting equipment, such as

evaporators and brownstock washers, the cost would

escalate to $40 million to $50 million. If, in addition,

the mill did not have sufficient reserve recovery

boiler capacity, the additional liquor treatment

required for oxygen delignification would raise the

cost to $80 million. However, if the mill did not

require recovery modifications, costs may go as low

as $8 million to $10 million.

The American Paper Institute estimates that if the

98 bleached chemical pulp mills in the United States

that have not yet installed oxygen delignification

equipment (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.
Chapter 5—Technologies for Reducing Chlorinated Organics in Pulp Manufacture ● 59

total of 6,100 tons per day of oxygen pulp capacity).

The total capital outlay to the U.S. industry would

then be about $3 billion or $40,000 per daily ton of

capacity to fit out the U.S. industry with oxygen

systems (if capital costs were annualized and

changes in operating costs were considered, the

costs of oxygen delignification would be lower).

For greenfield construction of a new bleached

chemical pulp mill using oxygen delignification, the

capital cost of an oxygen system is more attractive.

The capital cost of installing an oxygen delignification system in a mill of 1,000 tons per day capacity

could be between $25 million to $35 million more

than for conventional chlorine bleaching. The higher

cost of oxygen delignification compared to conventional chlorine bleaching is part due to the cost of the

oxygen generating plant ($1 2 million to $14 million), and part due to the need for larger recovery

boilers. 11

Oxygen Delignification Technology

The objective of chemical pulping is to reduce the

amount of lignin carried forward with the brownstock pulp to the brightening stages of the bleach

plant. The less lignin that prebleached pulp contains,

the less bleaching that is required, Conventional

kraft pulping, for example, produces pulp with a

kappa number between 32 and 35. By subjecting

conventional pulp to oxygen delignification, the

kappa number may be reduced to 16 or 17.12 This

allows the use of a short bleaching sequence, since

the amount of lignin to be bleached is reduced up to

50 percent. An even higher proportion of lignin can

be removed from sulfite pulp. Oxygen delignification is considered to be an extension of the cooking

process. With an oxygen bleached pulpit is possible

to reduce the amount of chlorine gas bleach. With

additional research and development, it may be

possible to eliminate it altogether by using ozone,

hydrogen peroxide, and/or chlorine dioxide.

Effluent from the oxygen delignification stage is

disposed of by cycling it through the pulp mill

chemical recovery cycle. If chlorine is used as a

bleaching chemical, the effluents cannot be disposed

of in the recovery plant because of the corrosiveness

of chlorides and the difficulty in purging chlorides

from a closed recovery cycle. White liquor, the same

reagent that is used in the kraft pulping process, can

also be used in the oxygen delignification process

and then recycled through the recovery plant.

Installation of an oxygen delignification unit may

require that the capacity of the chemical and energy

recovery plant be increased to accommodate the

additional dissolved organic and inorganic chemicals. 13

Several commercial oxygen delignification systems have been developed. They differ more in

detail than in operating principles. These units are of

two general types: 1) high consistency, and 2)

medium consistency. Consistency refers to the

amount of wood fiber in relation to the volume of

solution in a reactor vessel. The higher the ratio of

fiber to water, the higher the consistency. High

consistency oxygen delignification systems use pulp

slurries containing 20 to 32 percent fibers. Medium

consistency is 10 to 15 percent fibers. At high

consistencies, the pulp is more of a ‘‘fluff” than a

fluid. The key to effective oxygen delignification

systems is the ability to disperse oxygen finely

enough to last for the necessary reaction time. This

cannot be done efficiently with pulp consistencies of

less than 6 percent because of energy considerations.

Differences among the oxygen delignification

systems are mainly in the design of the reaction

vessel and associated pumping and gas handling

equipment. In general, the capital equipment cost is

lower for a medium consistency system than for a

high consistency system.14 On the other hand, the

consumption of oxygen and alkali is somewhat

higher for medium consistency oxygen delignification. Medium consistency systems require a longer

retention time than high consistency reactors to

achieve the same degree of delignification, therefore

capacity for medium consistency units must be

larger to maintain the same rate of production.

Medium consistency oxygen stages are often used to

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

verified 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.
60 ● Technologies for Reducing Dioxin in the Manufacture of Bleached Wood Pulp

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

n .

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

achieve 35 to 40 percent delignification. Both

medium and high consistency bleaching systems

require efficient washing of the oxygen bleached

pulp to prevent carry over of the dissolved organics

into the bleaching system.

Medium consistency oxygen delignification

causes less degradation of the wood fibers than high

consistency systems, but because of the slower rate

of delignification it is more difficult to delignify to

very low kappa numbers using medium consistency.

Two-stage medium consistency oxygen systems

have been proposed to overcome this problem.15

Medium consistency may have several other advantages 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 accumulations.16

High Consistency Systems

High consistency oxygen delignification systems

can produce high brightness pulps up to 90+ GE with

supplemental bleaching, In the high consistency

processes well-washed brownstock pulp, that is

dewatered after discharge from the digester to 28 to

32 percent consistency, is treated with alkali (oxidized white liquor) and magnesium salts (a protector

or inhibiter) in a mixer, it is then fluffed and fed into

an oxygen reactor (figure 5-3). The pulp is heated

with pressurized steam (90 to 120 ‘C) and oxygen

gas is injected into the bottom of the reactor. The

atmosphere in the reactor is maintained at about 80

percent oxygen. Gases produced by oxidation of the

lignin are purged to avoid combustion. The oxygenated pulp is washed after discharge from the reactor

before being sent to the bleach plant.

Medium Consistency Systems

The steps for preparing the brownstock for

oxygen treatment that are used in the medium

consistency process are similar to those used for

Figure 5-3—High Consistency Oxygen

Delignification Stage

Dewatered stock Dewatered

stock

SOURCE: Adapted from Kanneth E. Smith, “Oxygan Bleaching Systam Oparating Wall

at Unmn Camp’s Franklin Mill,” Pup & Rsper, Octobar 1982.

high consistency oxygen delignification. Pulp consistency is adjusted to between 10 and 15 percent.

Magnesium salts may or may not be added to the

pulp depending on the selected procedure. Oxygen

gas is dispersed throughout the pulp mixture, and

steam is added to bring the mixture to a temperature

of about 100 “C before it is injected at the base of the

oxygen 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.
Chapter 5—Technologies for Reducing Chlorinated Organics in Pulp Manufacture ● 63

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

continuous digester Excess

Gas to

Foul incineration I

unbleached

white

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

Effects of Oxygen Delignification

on Pollutant Loads

The processes and conditions under which dioxins are formed during pulping, bleaching, and

brightening are not well known. Dioxins are usually

formed in small quantities, and the conditions

leading to their formation are poorly understood.

Heat, light, and catalytic action have been shown to

stimulate the conversion of chlorinated precursors to

dioxins. 17 Chlorine makes up about 44 percent of a

TCDD molecule. Lignin is a complex polymer

containing many resin acid, fatty acid, and phenolic

derivatives that could serve as precursors for the

formation of dioxins in the presence of chlorine

under appropriate physiochemical conditions. Other

non-lignin components (e.g., extractives, nonchlorinated dioxins, and catachols), may also be precursors of TCDD or TCDF.

Impco Division of Ingersoll-Rand, an equipment

manufacturer that produces oxygen delignification

systems, claims that oxygen treatment of prebleached pulp before a short chlorination brightening sequence significantly reduces the pollution load

in kraft mill effluents compared to the most common

five-stage bleach sequence CEDED (table 5-2).

Similar results are reported from prebleach oxygen

delignification using a chlorine-chlorine dioxide

first stage bleach sequence (CDEoD) compared with

a long bleach sequence (CDEoDED) without oxygen

pretreatment (table 5-3). Furthermore, the total

amount of chlorinated phenolic compounds formed

in the bleaching process decrease considerably as a

result of oxygen delignification (figure 5-5). The

phenolic compounds released from conventional

and oxygen-delignified pulps are lower in quantity

but probably do not differ much in composition

compared to those produced from conventional

pulp, 18 To the extent that phenolic compounds may

be linked with the production of TCDD, this may

indicate that oxygen delignification could reduce the

amounts of phenolic precursors present in the pulp.

Limited data are available that directly relate the

reduction of TCDD and TCDF to oxygen delignification. There is substantial evidence that oxygen

delignification can reduce the amount of waste

chlorinated organics produced, as well as reduce

BOD and COD (chemical oxygen demand), and can

significantly reduce effluent color. Few analyses

have been made of dioxin in effluents and pulps from

oxygen-treated pulps. The Swedish Pulp and Paper

Research Institute recently published some preliminary information on the effects of oxygen delignification on dioxins and furans.

Sweden’s experience with oxygen delignification

and appropriate bleaching sequences show that the

release of polychlorinated dibenzo-p-dioxins (PCDD)

and polychlorinated dibenzofurans (PCDF)19 in

kraft pulp mill effluents can be reduced to relatively

low levels (table 5-4). Similar reductions in PCDD

and PCDF have been recorded in prebleached

oxygen pulps. Conventionally bleached softwood

pulps contained levels of PCDD and PCDF of

between 9 and 29 ppt; oxygen pulps contained

between 0.2 and 5 ppt (expressed as TEQ-toxic

equivalents) .20 A similar sample of bleached pulp

from five U.S. kraft mills without oxygen delignification averaged 13 ppt TCDD and 93 ppt TCDF

(TEQ about 22 ppt, within the range of Swedish

mills reported above).

21 One U.S. mill with a

softwood oxygen delignification line and a short

bleaching sequence including chlorine dioxide (CD)

in the first bleaching stage reports no detectable

levels of TCDD in either bleached pulp or effluent.22

Extended Delignification

Alkaline digestion (cooking) and oxygen delignification are both aimed at reducing the amount of

lignin in the wood pulp before it is brightened in the

bleach plant. Therefore, oxygen is considered an

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)1

p. 76.

22u.s. Environrnentd Protection Agency, op. cit,, note 4 p. 1.
Chapter 5—Technologies for Reducing Chlorinated Organics in Pulp Manufacture ● 65

Table 5-2—Effluent Characteristics of Oxygen-Treated Bleached Pulp and Pulp

Bleached by the Common Chlorinated Bleach Processesa (pounds per ton)

Bleach sequence BOD5 COD Color C L -

CEDED . . . . . . . . . . . . . . . . . . . . . 10-39 80-90 180-250 60-70

OCED . . . . . . . . . . . . . . . . . . . . . . 10-15 40-50 30-50 30-40

Percent Difference . . . . . . . . . . 45-55 45-50 80-85 35-55

a ~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 Pulps

With and Without Oxygen Delignificationa (kg/metric ton)

Bleach Sequence BOD5 COD Color TOCL

CDEODED . . . . . . . . . . . . . . . . . . . 15-21 65-75 200-300 5-8

OCDEOD . . . . . . . . . . . . . . . . . . . 8-11 30-40 80-120 3-4

Reduction (Percent) . . . . . . . . . 40-50 45-55 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 With

and Without Oxygen Bleaching

Chlorinated phenolic 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 pulp

in the digester for a longer period and exposing it to

a modified time-temperature-alkaline cycle can

reduce the amount of lignin retained in the brownstock. There is a practical limit to the amount of

cooking that can be done without dissolving the

desired components of the wood fiber and reducing

the pulp yield. A balance, therefore, between cooking and other delignification processes must be used.

A combination of extended cooking—sometimes

called ‘‘modified” cooking—and oxygen delignification has been used to achieve kappa numbers

between 10 and 12 for unbleached pulp.23 Others

believe that kappa numbers as low as 7 can be

achieved without loss of pulp strength.24 With

prebleached pulps of such low lignin content, it may

be technically possible to eliminate the use of

chlorine in the bleaching process altogether.

The standard kraft cooking procedure generally

yields softwood pulp with kappa numbers between

30 and 35. Oxygen delignification can reduce the

standard prebleached pulp to 16 to 20, although

commercial practice is typically limited to 20 kappa

because of strength considerations. Extended cooking plus oxygen treatment can produce pulps with

kappa numbers between 7 and 12. By starting the

kraft cook at a relatively low concentration of alkali

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 Waters

From Swedish Pulp Mills With and Without Oxygen Delignification

Production TEF

Pulp Bleaching process (tons/yr) (g/yr)

Without oxygen delignification

1 softwood ( C9 5+ D5) (E()) H D 300,000 2-5.8 . . . . . . . . . . .

Hardwood (C95+D5) (EO) H D E D

2 . . . . . . . . . . . softwood C (E-O) H D (EP) D 125,000 0.7

Hardwood (C10+D90) E D E D

With oxygen delignification

3

softwood O (C85+D15) E D E D 160,000 0.4 . . . . . . . . . . .

Hardwood O (C70+D15) E D E D 160,000

4 . . . . . . . . . . . Softwood O(C85+D15) (EO) D D 75,000 0.7

Hardwood O (C70+D15) (EO) D D

5 . . . . . . . . . . . softwood O (C85+D 15) (EO) D E D 235,000 0.1

Hardwood O (C70+Dl 5) (EO) D E D 85,000

a 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-lignin

pulps are produced with comparable strength to

pulps containing more lignin that are produced by

standard cooking methods.

Extended delignification can be either a batch

process or a continuous process. Continuous modified cooking digesters have been developed by

Kamyr, Inc. and others, which split the white liquor

(alkali) charge and introduce it into different points

in the impregnation and cooking stages and at the

base of the reactor (figure 5-6).

As its name implies, extended delignification

requires a longer cooking time than standard pulping, While standard kraft pulp is cooked for 1 to 3

hours, extended delignification, including impregnation (steeping) and cooking, may require over 4

hours. 25 Initial concentration of alkali in the cooking

liquor is about half of that used in the standard cook.

By the end of the cook, however, about 2 percent

more alkali is used in the extended process than in

the standard. Pilot tests have shown that pulp lignin

concentration at the end of extended delignification

may be 40 to 60 percent of that contained in standard

pulp.

Pretreatment of Oxygen Pulps With

Nitrogen Dioxide (PRENOX)

Laboratory experiments and pilot plant studies

have demonstrated that treatment of pulp with a

combination of nitrogen dioxide and oxygen prior to

oxygen delignification can also reduce the lignin in

prebleached pulps.

26 Nitrogen dioxide has been

shown to promote more selective delignification in

the oxygen delignification stage, thus resulting in

less damage to the fiber.27 The upper limit of

delignification for most softwood kraft pulps is

considered to be about 50 to 55 percent for oxygen

treatment. Delignification to these limits, however,

causes fiber degradation. To avoid this, oxygen

delignitication is generally reduced to about 40 to 45

percent. By using PRENOX before the oxygen

stage, 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,
Chapter 5—Technologies for Reducing Chlorinated Organics in Pulp Manufacture ● 67

Figure 5-6-Continuous Digester for Extended Delignification

Chips 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 Pulping

Although the kraft pulping process efficiently

produces pulps with unmatched quality and may be

used with any wood species, there are continuing

efforts to develop a pulping system that eliminates

sulfur chemicals from the pulping process because

of the odor and environmental concerns. The most

expensive capital investment in a kraft pulp mill is

the chemical recovery plant. A soda-anthraquinone

plant requires a similar recovery plant to that of a

conventional system, however, the odor control

equipment used in conventional mills is not needed.

The soda process was a forerunner of the kraft

process.

29 Its major drawbacks are low pulp yields

and inferior pulp quality that result from long

cooking times, high temperatures, and the strong

solution of sodium hydroxide needed to produce

bleachable grade pulps.

The addition of small amounts of anthraquinone

(AQ) to the pulping liquor are effective in accelerating the soda pulping process and improving pulp

yields. Anthraquinones have also been used with

kraft pulping, but a larger amount of the expensive

chemical must be used to be effective and residual

amounts of AQ can interfere with the chemical

recovery plant. Furthermore, AQ is regulated by the

Food and Drug Administration, and only small

residual amounts are permitted in finished products.

Laboratory experiments with a two-stage soda/AQoxygen-sodium hydroxide delignification of hardwood has produced pulp of kappa numbers between

10 to 12 with about 5 percent higher pulp yields than

comparable kraft pulp. Lignin content of softwoods

pulped (kappa number 20) by the two-stage process

did not match those of the hardwood pulp, however.30 Researchers found that to avoid fiber degradation, the soda cooking must be stopped at a high

kappa number and the remainder of delignification

done with oxygen.

DISPLACEMENT BLEACHING

There is no difference between the chemistry of

displacement bleaching and that used in conventional 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.
68 ● Technologies for Reducing Dioxin in the Manufacture of Bleached Wood Pulp

transfer. 31 When bleaching chemicals are displaced

through a pulp mat, the fibers are continually

exposed to highly concentrated bleaching chemicals, therefore bleaching is very rapid.

Displacement (sometimes referred to as “dynamic”) bleaching is generally conducted in a

multistage displacement tower (figure 5-7). Filtrate

withdrawn at each stage is supplemented with

makeup chemicals and reused. Unbleached pulp

moves sequentially upward, while white liquor

moves downward through the bleaching column.

Displacement bleaching uses a minimum of water,

thus reducing the amount of waste effluent and

enabling the recycling of bleaching chemicals. The

system is suited for use in a “closed-mill” configuration (see above).

Any water soluble bleaching chemical, including

chlorine gas, can be used in displacement bleaching.

Softwood and hardwood pulps of 86 to 88 GE

brightness have been produced by displacement

systems. 32 Displacement bleaching technology has

produced mixed results. Although it requires more

chemicals, and causes corrosion and scaling problems in some instances, displacement bleaching is

still considered promising.

CLOSED-CYCLE BLEACHED

PULP MILL

During the past 20 years the concept of a

closed-cycle pulp mill that could significantly reduce the amount of waste effluents has received

some attention. The closed-cycle concept involves:

●

●

●

reducing the amount of water consumed by

using filtrate as wash water,

cycling the spent bleaching chemicals to the

chemical recovery plant, and

recovering the salts introduced into the recovery system for either re-use in manufacturing

the bleaching chemicals or for disposal (figure

5-8).

By closing the water cycle of a bleached kraft

mill, the demand for heat is reduced and considerable energy savings can result. Excess heat produced

in the process is disposed of as low-grade waste heat

into cooling waters. A closed-cycle mill bleach plant

might use only 3,900 gallons per air-dry ton of pulp

produced, while an average North American Mill

would use 20,000 gallons.33 A mixture of chlorine

dioxide and chlorine gas is substituted for pure

chlorine in the first bleaching stage to avoid

reducing the pulp strength during the hightemperature-chlorination (60 °C).34

Pulp manufacture and bleaching, being chemical

processes, obey the general principles of chemical

combination and mass balance, In a mill in which

water and all chemicals are recycled, all of the

chemicals must be kept in proper balance or either an

excess or shortage of bleaching chemicals will

occur. An excess of certain chemicals, particularly

sodium chloride (common salt), can contribute to

corrosion of the recovery boiler. Some of the

recovered salt can be reused to produce chlorine

dioxide, but the excess must be disposed of.

Corrosion-resistant materials and careful chemical

control have largely overcome these problems.

The Great Lakes Paper Co. at Thunderbay,

Ontario, installed the first closed-cycle mill in 1977,

but the process has not been successful, and the mill

is not now operating in the closed-cycle mode.

Severe operating problems have been encountered

which have resulted in abandoning the process. If

these problems could be overcome by further

development, the closed-cycle concept might reduce

the cost of biological waste treatment, reduce energy

costs, increase fiber yield, decrease water usage, and

reduce chemical costs.

It is possible that some variation of the closedcycle bleached kraft mill concept could be developed to incorporate oxygen delignification, extended delignification, or displacement bleaching,

thus avoiding some of the problems encountered

with 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 Plant

Showing 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 TREATMENT

TECHNOLOGIES

Several physical and chemical treatments have

been developed to cleanse pulp and paper effluents.

Waste treatment processes include resin separation

and ion exchange; use of chemicals such as aluminum oxide, metallic ions, lime, amines, and ozone;

adsorption by wood; biological treatments such as

enzymes and bacteria; filtration systems such as

activated charcoal; membrane separation; and ultrafiltration, irradiation, and reverse osmosis. It is

technically feasible, but extremely costly, to clean

pulp-mill wastes to the purity of drinking water. In

judging the economics of waste treatment systems,

the cost of the exotic waste treatment technologies

is generally compared with the cost of secondary

biological treatment, which is the U.S. industry

standard for removing most foreign materials other

than color.
70 ● Technologies for Reducing Dioxin in the Manufacture of Bleached Wood Pulp

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

Wate r

Condensin g

Water Condensate I

Strippin g

White liquo r

evaporato r ate r

Liquo r

preparatio n

Furnac e

+

Black Iiquo r

Wate r

evaporato r

Wate r

Fres h

wate r

DcEDED /

Washin g 4

oxyge 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 system

is related to the internal processes used for delignify -

ing and brightening the pulp. The load on the waste

treatment plant can be reduced by using some of the

technologies discussed above, e.g., oxygen delignification, soda-AQ pulping, or displacement bleaching. Use of bleaching sequences including oxygen,

chlorine dioxide, hypochlorite, peroxide, or ozone

can also change the nature of the treatable wastes.

Waste treatment and pulping and bleaching technologies, therefore, may be considered as an integral

system that can be balanced to achieve the desired

performance standards.

Theoretically, the optimum way to make waste

treatment a less costly venture is to integrate it into

the manufacturing process. Benefits can then be

gained from conservation of raw material, fibers,

additives, energy, and water. Cost analyses of

internal controls v. external controls generally show

that internal modifications may be less capital

intensive.35 On the other hand, external pollution

control does not interfere with established manufacturing processes and provides flexibility for a range

of operating conditions and exigencies. Realistically, successful control probably requires a combination of both.

Color and solids are the first targets for cleanup of

the pulp waste stream as it leaves the mill outlets.

Untreated pulp-mill effluents are generally of a deep

mahogany color. Most of that color is derived from

the bleaching process, and most of that is from the

caustic extraction process after the first bleaching

stage. Most of the color is attributable to lignin in the

waste 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 remove

only 30 percent or less of the color component.36

Primary Treatment of Suspended Solids

Suspended solids in the waste stream consist of

bark, fiber, fillers, clay, and coloring agents. These

are removed by sedimentation, flotation, and screening. Grit chambers, screens, and chemicals are used

as sediment clarifiers to remove grit. Removal of

suspended solids and grit currently constitute primary (first stage) treatment of pulp and paper mill

waste.

Secondary Biological Treatment

Removal of suspended solids in the first stage is

generally followed by biological waste treatment

that is principally aimed at reducing the BOD of

treated water. Secondary biological treatment has

little effect on effluent color, but it may significantly

reduce levels of toxic pollutants .37 As its name

implies, biological treatment relies on the assimilation and conversion of potential pollutants in the

waste effluent by bacteria, fungi, algae, and other

living organisms. Since biological treatment relies

on the physiological processes of living plants and

animals to reduce the pollution load, the second

stage of waste treatment is similar to farming-the

plants and animals must be kept healthy, productive,

and reproducing. In order to promote biological

activity, adequate air and nutrients must be provided

to the biota.

Biological treatment can remove 80 to 95 percent

of the BOD. Research has shown that chlorinated

bleach plant derivatives are more difficult to degrade

by biological processes than nonchlorinated wastes.

Several biological treatment systems are currently

used to treat pulp mill waste (e.g, oxidation ponds,

activated sludge, and aerated basins). Emerging

biological treatment technologies include: rotating

biological surfaces, fixed-film activated sludge,

aerated activated carbon, and deep tank aeration.

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

lime can remove more than 95 percent of TCDD and

TCDF in bleach plant wastewater by improving the

recovery of suspended sediments to which they

adhere. Use of chemical treatments would probably

require additional clarification and sludge dewatering facilities at most mills. The application of a

non-ionic polymer to an oxidation pond reduced

TCDD and TCDF to less than detectable levels.38

Oxidation Ponds

Oxidation ponds or basins depend primarily on

surface exchange with the atmosphere for aeration,

although some oxygen may be supplied photosynthetically by aquatic plants. Large surface areas are

generally needed to provide sufficient air to maintain biological activity, therefore oxidation ponds

tend to be large and shallow. Since the rate of

biological activity increases with temperature, oxidation ponds work best in warmer southern climates.

Oxidation ponds are relatively inexpensive, require

little mechanical equipment, produce little secondary waste products that must be disposed of, and in

emergencies can serve as temporary impoundments

should an accidental discharge of harmful chemicals

occur in the mill. Racetrack-shaped oxidation

ditches are sometimes used to eliminate the primary

clarification stage. Oxidation ditches require mechanical aeration because of the smaller water

surface and perform more like an extended aeration

activated sludge process than like a conventional

oxidation pond system.

Oxidation basins are frequently equipped with

aerators to increase the rate of biological activity

(aerated stabilization basins). Nitrogen and phosphorous fertilizers are sometimes added if the waste

stream is nutrient deficient. These supplemental

treatments can reduce the retention time in the

oxidation pond to 8 to 10 days in order to reduce

BOD to a low level. The addition of mechanical

aerators increases the cost of oxidation ponds, but

they are generally cheaper than activated sludge

systems.

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: Canadian

Forest Service, 1973).

3SU.S. Environmental Protection Agency, op. cit., note 4, P. 9.
72 ● Technologies for Reducing Dioxin in the Manufacture of Bleached Wood Pulp

Activated Sludge

Activated sludge treatment systems are often used

where space limits the use of oxidation ponds. An

adaptation from sanitary sewage treatment, activated sludge is a high-rate biological process that

can reduce waste treatment retention time to 3 to 8

hours. The biological mass that is produced in the

aeration stage of the treatment process is separated

in a secondary clarifier and the active biological

components are returned to the process, thus further

accelerating biological activity.

Activated sludge systems are flexible and can be

adapted to treat a wide range of wastes. However,

these systems cannot withstand shocks from emergency mill releases nearly as well as oxidation

ponds, and the biological balance of the process is

more sensitive to chemical and biological perturbations. The capital and operating cost of activated

sludge systems are generally twice as great as

aerated ponds.

The pulp and paper industry is the largest user of

pure oxygen-activated sludge (UNOX) technology,

with 16 plants in operation in 1986.39 Pure oxygen

systems can remove 87 to 97 percent of BOD. The

process has several advantages over conventional

activated sludge systems such as, smaller aeration

tanks required better tolerance to “shock loading,”

and better sludge settling. A buildup of carbon

dioxide at some UNOX plants has raised concern

about the potential toxicity to fish. Retention time in

pure oxygen systems is about 3 to 4 hours.

Two-stage activated sludge systems, such as the

Zurn Attisholz process, using two oxygen levels

sequentially with high recycling rates can reduce

BOD 95 percent and the process is very stable. Cost

of the two-stage process may be slightly less than

with conventional activated sludge.40 There is some

evidence that the two-stage oxygen activation process is more efficient in reducing toxicity of kraft

wastewater. 41

New or Developing Treatment Technologies

A number of of coagulant that remove color have

been tested. Although several are effective, e.g.,

alum, ferric sulfate, sulfuric acid, activated carbon,

etc., they are expensive. Lime is the least costly

precipitation agent, and can be reclaimed in a kraft

mill by oxidizing in the lime kiln. Pretreatment of

waste effluents with enzymes before precipitation

with lime can increase efficiency, as can the

additions of magnesium sulfate.

Fungi have also been used to remove color from

wastewater. White rot fungus (Phanerochaete chrysosporium burds) can metabolize the lignin responsible for most of the color of wastewaters. It can also

eliminate toxic chlorinated and halogenated compounds from the waste stream, including dioxins.42

Although the process (MYCOR) is economical, the

fungi culture cannot sustain itself and may collapse,

thus it must constantly be recultured.

Conventional ion exchange resins have not proven technically successful in removing color from

pulping and bleaching wastes, but specialized synthetic resins seem promising. Resins are sensitive to

overloading from suspended solids and contaminants that reduce their effectiveness, thus they have

been used primarily on both small waste streams and

isolated output from the chlorination and first

extractive stages of the bleaching sequence. Ion

exchange resins have thus far found only limited use

in the pulp and paper industry. Activated carbon,

because of its large adsorption surface area, is an

effective scrubbing agent. The charcoal process has

been used commercially, but its high cost and

regeneration requirements make it less attractive.

The efficiency of membrane processes is considered

good, but their costs remain high.

Rotating Biological Surface

A new developing technology, rotating biological

surfaces (RBS), involves rotating polyethylene discs

alternately through the wastewater and into the air as

they rotate on a shaft. The process is analogous to the

39springer, op. cit., note 5> P. 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 percolated through a porous medium exposed to air in the

interstices. RBS usually operates in three stages, and

removal of up to 90 percent of the BOD has been

achieved with this technology. Efficiency of the

process is proportional to the surface area of the

discs, rotation speed, and submergence depth of the

discs. Application of RBS is probably limited to

small- or medium-size pulp mills because of costs in

scaling the equipment to large volumes. In smaller

operations the cost of RBS is about the same as the

activated sludge process. Although pilot-scale testing of RBS has shown promise for removing BOD

and toxicity, mill-scale studies have encountered

operating difficulty .43

Innovations in Activated Sludge Technology

High-Rate, Fixed-Film Activated Sludge—

Solid particles (e.g., clays, sand, or calcium carbonate) added to activated sludge systems can improve

sludge settling and prolong solids retention. Calcium carbonate has been particularly effective because of its buffering capacity with regard to acid

components in the waste effluent. The particles

develop a fixed-film growth, allowing a high level of

biological growth to develop on the expansive

particle surfaces. Preliminary studies indicate that

nearly all of the solids can be retained in the system

except a small amount that is lost with the treated

water, Most of the solids are lost through the

respiratory processes as carbon dioxide and water.

The oxitron process is a variation of the fixed-film

activated sludge system. Effluent, free of suspended

solids, is fed to a fluidized bed of sand through which

pure oxygen is injected. Biological growth develops

around the sand particles, finally causing them to

rise in the fluidized bed where they are selectively

removed. The sand is later cleaned of the growth and

returned to the fluidized bed. Tests of the process

show it to be efficient and compact, but it is unable

to tolerate any solids in the wastes to be treated. The

oxitron system is best suited for large waste volumes

of 2 to 3 million gallons daily. There are no

commercial installations in service, but new technologies may utilize elements of the process.

Aerated Activated Carbon Filter—Experiments

have shown that due to its immense surface area

activated charcoal is an efficient filter medium.

Coarse granular charcoal placed in a filter bed has

been shown to be effective in removing contaminants from waste effluent. To keep the system

aerobic, air is pumped through the filter bed. The

system is tolerant of excessive suspended solids, but

the capital and operating costs are likely to be high.

While the system has been used on sanitary wastes,

there has been limited operating experience in the

pulp and paper industry.

Captor System—An English innovation, the Captor System is similar to the conventional diffusedaeration activated sludge process, but it encapsulates

the biological growth in small polyurethane

sponges. With the biomass contained in the sponges,

the treated effluent is discharged through a screen

sized to retain the impregnated sponges. Sponges are

cleaned for reuse by squeezing through a wringer.

Developers claim that a secondary clarifier is not

needed, therefore the plant size may be reduced by

about 20 percent from that of a conventional

activated sludge plant. The Captor System has been

used for municipal wastes, but it is not certain that

this treatment would be sufficient to meet U.S. water

standards. It may prove useful in upgrading existing

overloaded biological treatment systems.

Deep Tank Aeration-Deep tank aeration increases the amount of time that air is in contact with

the waste effluent. As its name implies, the process

uses a deep tank or deep shaft as an aeration

chamber. This eliminates the need for a secondary

clarifier but still requires a floating clarifier. Deep

aeration occupies less space than conventional

activated sludge systems and is capable of handling

emergency ‘‘shock’ loads from the paper mill.

Anaerobic Treatment

Anaerobic waste treatment is neither new, nor

innovative. However, with reduced water usage by

many of the modem pulp mills, there has been a

general increase in the potency of the pollution load,

and anaerobic treatment is being reconsidered as a

means to improve waste treatment. Anaerobic systems 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 ati

Paperboard and the Btulders” Paper and Board Mills: Point Source Categories, EPA 440/1-82/025 (Washington DC: 1982), p. 342.
74 ● Technologies for Reducing Dioxin in the Manufacture of Bleached Wood Pulp

but with modifications such as, attached-film and

expanded bed reactors, it may also be possible to

treat low-strength waste anaerobically as well.

Anaerobic (absence of oxygen) decomposition is

a microbial process, which is primarily dependent on

bacteria. Unlike aerobic processes (e.g., activated

sludge and oxidation ponds to which air is supplied)

anaerobic processes depend on bacterial action that

obtains oxygen from sulfate and nitrate ions in the

waste stream. When applied in the pulp and paper

industry, anaerobic treatment is generally used as a

waste pretreatment before release to the aerobic

stages of the treatment process. Anaerobic systems

are sensitive to imbalances in the ambient waste, but

the process is durable and can be applied to a wide

range of waste effluents. High temperatures are

needed to reduce treatment time, long startup

periods are required, odor emissions can be significant, and there is difficulty in achieving low BOD

levels with anaerobic decomposition.