Waste Treatment in the Process Industries - Chapter 10 ppt

The major air emissions are fine and coarse particulates from recovery furnaces andburners, sulfur oxides SOx from sulfite mills, reduced sulfur gases and associated odorproblems from Kr

Cleveland State University, Cleveland, Ohio, U.S.A.

Pulp and paper mills are a major source of industrial pollution worldwide The pulping andbleaching steps generate most of the liquid, solid, and gaseous wastes (Table 1) [1] Pulping is aprocess in which the raw material is treated mechanically or chemically to remove lignin in order

to facilitate cellulose and hemicellulose fiber separation and to improve the papermakingproperties of fibers Bleaching is a multistage process to whiten and brighten the pulp throughremoval of residual lignin Pulping and bleaching operations are energy intensive and typicallyconsume huge volumes of fresh water and large quantities of chemicals such as sodiumhydroxide, sodium carbonate, sodium sulfide, bisulfites, elemental chlorine or chlorine dioxide,calcium oxide, hydrochloric acid, and so on A partial list of the various types of compoundsfound in spent liquors generated from pulping and bleaching steps is shown inTable 2[2 – 4].The effluents generated by the mills are associated with the following major problems: Dark brown coloration of the receiving water bodies result in reduced penetration oflight, thereby affecting benthic growth and habitat The color responsible for causingaesthetic problems is attributable to lignin and its degradation products

High content of organic matter, which contributes to the biological oxygen demand(BOD) and depletion of dissolved oxygen in the receiving ecosystems

Presence of persistent, bio-accumulative, and toxic pollutants

Contribution to adsorbable organic halide (AOX) load in the receiving ecosystems Measurable long-distance transport (.100 km) of organic halides (such as chloro-guaiacols), thereby contaminating remote parts of seas and lakes [5]

Cross-media pollutant transfer through volatilization of compounds and absorption ofchlorinated organics to wastewater particulates and sludge

Significant solid wastes from pulp and paper mills include bark, reject fibers, wastewatertreatment plant sludge, scrubber sludge, lime mud, green liquor dregs, and boiler and furnaceash The bulk of the solid wastes is generated during wastewater treatment Sludge disposal is aserious environmental problem due to the partitioning of chlorinated organics from effluents to

453

solids The major air emissions are fine and coarse particulates from recovery furnaces andburners, sulfur oxides (SOx) from sulfite mills, reduced sulfur gases and associated odorproblems from Kraft pulping and chemical recovery operations, volatile organic compounds(VOC) from wood chip digestion, spent liquor evaporation and bleaching, nitrogen oxides(NOx), and SOx from combustion processes Volatile organics include carbon disulfide,methanol, methyl ethyl ketone, phenols, terpenes, acetone, alcohols, chloroform, chloro-methane, and trichloroethane [1].

The extent of pollution and toxicity depends upon the raw material used, pulping method,and pulp bleaching process adapted by the pulp and paper mills For example, the pollution loadfrom hardwood is lower than softwood On the other hand, the spent liquor generated frompulping of nonwood fiber has a high silica content Volumes of wastewater discharged may varyfrom near zero to 400 m3per ton of pulp depending on the raw material used, manufacturingprocess, and size of the mill [6] Thus, the variability of effluent characteristics and volume fromone mill to another emphasizes the requirement for a variety of pollution prevention andtreatment technologies, tailored for a specific industry

Pollution generating step

Pollution

Wood debarking and

chipping, chip washing

Chemical (Kraft) pulping,

black liquor evaporation, and

chemical recovery steps

sulfide, methyl mercaptan,dimethyl sulfide, dimethyldisulfide), VOC

Wood chip digestion, spent pulping liquor

evaporator condensates

reduced sulfur compounds,resin acids

Pulp screening, thickening, and cleaning

operations

BOD, colorSmelt dissolution, clarification to generate

green liquor

Recausticizing of green liquor, clarification

to generate white liquor

organics, resin acids

chemical sludge

nitrogen oxides, SO2

SS, suspended solids; VOC, volatile organics; BOD, biochemical oxygen demand.

Source: Ref 1.

Table 2 Low-Molecular-Weight Organic Compounds Found in the Spent Liquors from Pulping and Bleaching Processes

Class of compoundsAcidic

Wood extractives

Lignin/carbohydrate

Category: Fatty acid

Monohydroxy benzoicacid

Dihydroxy benzoicacid

Guaiacolic acidSyringic acid

Category: PhenolicMonochlorophenolsDichlorophenolsTrichlorophenolsTetrachlorophenolPentachlorophenolCategory: GuaiacolicDichloroguaiacolsTrichloroguaiacolsTetrachloroguaiacolCategory: CatecholicDichlorocatecholsTrichlorocatecholsCategory: SyringicTrichlorosyringolChlorosyringaldehyde

HemicellulosesMethanolChlorinated acetonesChloroformDichloromethaneTrichloroetheneChloropropenalChlorofuranone1,1-dichloro-methylsulfoneAldehydesKetonesChlorinated sulfurReduced sulfurcompounds

Category: Dioxins2,3,7,8-tetrachloro-dibenzodioxin (2,3,7,8-TCDD)2,3,7,8-tetrachloro-dibenzofuran(2,3,7,8-TCDF)

Wood derivativesMonoterpenesSesquiterpenesDiterpenes: PimarolAbienol

JuvabionesJuvabiolJuvabioneLignin derivativesEugenol

IsoeugenolStilbeneTannins (monomeric, condensedand hydrolysable)

The focus of this chapter is to trace the origin and nature of the major pollution (especiallywater) problems within the pulp and paper industries and to present an overview of the pollutionmitigation strategies and technologies that are currently in practice or being developed(emerging technologies).

BY PULP AND PAPER INDUSTRIES

The pulp and paper industries use three types of raw materials, namely, hard wood, soft wood,and nonwood fiber sources (straw, bagasse, bamboo, kenaf, and so on) Hard woods (oaks,maples, and birches) are derived from deciduous trees Soft woods (spruces, firs, hemlocks,pines, cedar) are obtained from evergreen coniferous trees

10.2.1 Composition of Wood and Nonwood Fibers

Soft and hard woods contain cellulose (40 – 45%), hemicellulose (20 – 30%), lignin (20 – 30%),and extractives (2 – 5%) [7] Cellulose is a linear polymer composed ofb-D-glucose units linked

by 1 – 4 glucosidic bonds Hemicelluloses are branched and varying types of this polymer arefound in soft and hard woods and nonwood species In soft woods, galactoglucomannans(15 – 20% by weight), arabinoglucurono-xylan, (5 – 10% by weight), and arabinogalactan (2 – 3%

by weight) are the common hemicelluloses, while in hard woods, glucuronoxylan (20 – 30% byweight) and glucomannan (1 – 5% by weight) are found [2,3] Lignin is a complex heterogeneousphenylpropanoid biopolymer containing a diverse array of stable carbon – carbon bonds witharyl/alkyl ether linkages and may be cross-linked to hemicelluloses [8] Lignins are amorphous,stereo irregular, water-insoluble, nonhydrolyzable, and highly resistant to degradation by mostorganisms and must be so in order to impart resistance to plants against many physical andenvironmental stresses This recalcitrant biopolymer is formed in plant cell walls by the enzyme-catalyzed coupling of p-hydroxycinnamyl alcohols, namely, p-coumaryl, coniferyl, and sinapylalcohols that make up significant proportion of the biomass in terrestrial higher plants Inhardwoods, lignin is composed of coniferyl and sinapyl alcohols and in softwoods is largely apolymer of coniferyl alcohol The solvent extractable compounds of wood termed as

“extractives” include aliphatics such as fats, waxes, and phenolics that include tannins,flavonoids, stilbenes, and terpenoids Extractives comprise 1 – 5% of wood depending upon thespecies and age of the tree Terpenoids that include resin acids are found only in soft wood andare derived from the “pitch” component of wood Compared to wood, the structures of nonwoodspecies are not well studied Grasses usually contain higher amounts of hemicelluloses, proteins,silica, and waxes [9] On the other hand, grasses contain lower lignin content compared to woodand the bonding of lignin to cellulose is weaker and therefore easier to access

The steps involved in pulping are debarking, wood chipping, chip washing, chip crushing/digestion, pulp screening, thickening, and washing (Fig 1) The two major pulping processesthat are in operation worldwide are mechanical and chemical processes Mechanical pulpingmethods use mechanical pressure, disc refiners, heating, and mild chemical treatment to yieldpulps Chemical pulping involves cooking of wood chips in pulping liquors containingchemicals under high temperature and pressure Other pulping operations combine thermal,mechanical, and/or chemical methods Characteristic features of various pulping processes aresummarized inTable 3and are further described shortly in the following subsections [3,10 – 12]

Figure 1 Steps involved in the pulping and pulp bleaching processes (from Ref 2).

Name of the pulping process

Pulping

mechanism

Grinding stone,double disc refiners,steaming, followed

by refining in TMPprocess

Chemical treatmentusing NaOH orNaHSO3þsteaming followed

by mechanicalrefining

Continuous digestion in

Na2SO3þ Na2CO3liquor using steamfollowed bymechanical refining

Cooking at 340 – 3508F,

100 – 135 psi for

2 – 5 hours inNaOH, Na2S, and

Na2CO3; efficientrecovery of chemicals

Sulfonation at 255 – 3508F,

90 – 110 psi for

6 – 12 hours in H2SO3and Ca, Na, NH4,Mg(HSO3)2Cellulosic

raw material

Hard woods likepoplar and soft woodslike balsam, fir,hemlock

Hard andsoft woods

Hard woods like aspen,oak, alder, birch, andsoft wood sawdustand chips

Any type of hard andsoft wood, nonwoodfiber sources

Any hard wood andnonresinous soft woods

Pulp

properties

Low-strengthsoft pulp, lowbrightness

Moderatestrength

Good stiffness andmoldability

High-strength brownpulps, difficult tobleach

Dull white-light brownpulp, easily bleached,lower strength thanKraft pulpTypical

yields of pulp

pulps, 47 – 50% forbleachable pulps,

43 – 45% afterbleaching

48 – 51% for bleachablepulp, 46 – 48% afterbleaching

Paper

products

Newspaper, magazines,inexpensive writingpapers, moldedproducts

Newspaper, magazines,inexpensive writingpapers, molded products

gumming paper,white papers frombleached Kraft pulp,cartons, containers,corrugated board

Fine paper, sanitarytissue, wraps, glassinestrength reinforcement

in newsprint

TMP, thermomechanical pump; CTMP, chemi-thermomechanical pump; NSSC, neutral sulfite semichemical pulp.

Source: Refs 3, 10, and 12.

Nonconventional pulping methods such as solvent pulping, acid pulping, and biopulping arediscussed in subsection 10.9.1.

10.3.1 Mechanical Pulps

Stone-Ground Wood Pulp

Wood logs are pushed under the revolving grindstone and crushed by mechanical pressure toyield low-grade pulps Lignin is not removed during this process and therefore imparts a darkcolor to the pulp and paper product

Refiner Mechanical Pulp

Wood chips are passed through a narrow gap of a double-disc steel refiner consisting ofstationary and rotating plates having serrated surfaces This process results in the mechanicalseparation of fibers that are subsequently frayed for bonding The strength of the refiner pulp isbetter than that of ground-wood pulps

Thermomechanical Pulp (TMP)

Wood chips are preheated in steam before passage through disc refiners Heating is meant forsoftening the lignin portion of wood and to promote fiber separation This pulp is stronger thanthat produced by the ground-wood process

10.3.2 Semichemical Pulp

Wood chips are processed in mild chemical liquor and subjected to mechanical refiningusing disc refiners Semichemical pulping liquors have variable composition ranging fromsodium hydroxide alone, alkaline sulfite (sodium sulfite þ sodium carbonate), mixtures ofsodium hydroxide and sodium carbonate, to Kraft green or white liquors [3] Sodium sulfite/sodium carbonate liquor is most commonly used and the pulp product obtained thereafter isreferred to as neutral sulfite semichemical (NSSC) pulp

10.3.3 Chemithermo Mechanical Pulp (CTMP)

This process involves a mild chemical treatment of wood chips in sodium hydroxide or sodiumbisulfite before or during steaming Chemically treated chips are passed through mechanical discrefiners

Sulfite Pulping

The sulfite process solubilizes lignin through sulfonation at 255 – 3508F under 90 – 110 psi Thepulping liquors are composed of mixture of sulfurous acid (H2SO3) and bisulfites (HSO322) ofammonium, sodium, magnesium, or calcium, and lignin is separated from the cellulose aslignosulfonates [3] Bisulfite pulping is performed in the pH range of 3 – 5 while acid sulfitepulping is carried out with free sulfurous acid at pH 1 – 2 Sulfite pulping mills frequently adaptmethods for the recovery of SO2, magnesium, sodium, or ammonium base liquors [3]

10.4.1 Kraft Pulping Liquors (Black Liquors)

During Kraft pulping, about 90 – 95% of the reactive biopolymer, namely lignin, becomessolubilized to form a mixture of lignin oligomers that contribute to the dark brown color andpollution load of pulping liquors Lignin oligomers that are released into the spent liquorsundergo cleavage to low-molecular-weight phenylpropanoic acids, methoxylated and/orhydroxylated aromatic acids In addition, cellulose and hemicelluloses that are sensitive to alkalialso dissolve during the pulping processes [13] Black liquors generated from the Kraft pulpingprocess are known to have an adverse impact on biological treatment facilities and aquatic life.Emissions of total reduced sulfur (TRS) and hazardous air pollutants (HAP) are also generated.Black liquors typically consist of the following four categories of compounds derived fromdissolution of wood [3]:

ligninolytic compounds that are polyaromatic in nature;

saccharic acids derived from the degradation of carbohydrates;

Evaporator Condensate

Kraft black liquor characteristics

Compounds that inhibit

anaerobic metabolism

Reduced sulfur, resin acids, fatty acids,volatile terpenes

COD, chemical oxygen demand.

Source: Refs 3 and 6.

solvent extractives that include fatty acids and resin acids;

low-molecular-weight organic acids

Table 4shows the typical ranges of black liquor constituents and characteristics of Kraftevaporator condensates The composition of liquors may vary significantly, depending upon thetype of raw material used Inorganic constituents in black liquor are sodium hydroxide, sodiumsulfate, sodium thiosulfate, sodium sulfide, sodium carbonate, and sodium chloride [11]

10.4.2 Sulfite Pulping Liquors (Red Liquors)

Table 5 summarizes the composition of ammonia, sodium, magnesium, and calcium base sulfitepulping liquors In general, spent ammonia base liquors have higher BOD5, COD, and dissolvedorganics and exhibit more toxicity as compared to sodium, calcium, or magnesium base liquors.Higher toxicity is attributed to ammoniacal compounds in the spent liquors The sulfite-spentliquors contain COD values typically ranging from 120 – 220 g/L and 50 – 60% of these arelignosulfonates [6] The sulfite-spent liquor evaporator condensates have COD values in therange of 7500 – 50,000 mg/L The major organic components in the condensates are acetic acid(30 – 60% of COD) and methanol (10 – 25% of COD) Anaerobic biodegradability of thecondensates is typically 50 – 90% of COD and sulfur compounds are the major inhibitors ofmethanogenic activity [6]

Parameter

Ammoniabase milla

Sodiumbase millb

Magnesiumbase millc

Calciumbase milldPulp liquor volume

Source: Refs 3 and 10.

10.4.3 Thermomechanical Pulp (TMP), CTMP, and

Semichemical Pulping Liquors

Thermomechanical pulp (TMP) and CTMP pulping liquors exhibit COD values in the ranges of

1000 – 5600 mg/L and 2500 – 13,000 mg/L, respectively [6] Lignin derivatives can constituteanywhere from 15 to 50% of the soluble COD values in these spent liquors The composition ofspent NSSC pulping liquors and evaporator condensates are shown in Table 6 In general,anaerobic biodegradability of semichemical pulping and CTMP effluents are low as well asinhibitory to methanogenic metabolism [6]

10.4.4 Spent Liquors from Agro-Residue Based Mills

Agro-residue mills typically employ a soda or alkaline sulfite pulping process [14] Typicalcompositions of the spent liquors generated from the small-scale, agro-residue utilizing pulp andpaper mills are shown inTable 7 It is evident from the table that 45 – 50% of the total solids isrepresented by lignin Most of the lignin present in the black liquor is the high-molecular-weightfraction, a key factor contributing to low BOD/COD ratio

10.5 TOXICITY OF PULPING LIQUORS

A number of studies have evaluated the toxicity of pulping liquors, in particular the black liquorsgenerated from Kraft mills.Table 8shows a partial representation of toxicity data compiled bythe NCASI (National Council of the Paper Industry for Air and Stream Improvement) andMcKee and Wolf for Kraft mill pulping wastewaters [15,16] The table indicates that hydrogensulfide, methyl mercaptan, crude sulfate soap, and salts of fatty and resin acids are particularly

Spent NSSC pulping liquor characteristics

Compounds that have the potential to

inhibit anaerobic process

Tannins, sulfurcompoundsNSSC pulping liquor condensate characteristics

NR, not reported; COD, chemical oxygen demand; BOD, biochemical oxygen demand.

Source: Refs 3 and 6.

toxic to Daphnia and fish populations Among the toxic pollutants, compounds such as sodiumhydroxide, hydrogen sulfide, and methyl mercaptan fall under the EPA’s list of hazardoussubstances Extractive compounds such as resin acids are known to contribute up to 70% of thetotal toxicity of effluents generated from chemical and mechanical pulping processes [17] Theconcentrations of resin acids in the pulp mill discharges are two to four times higher than their

LC50 values (0.5 – 1.7 mg/L) [17] Some reports suggest that the transformation products ofresin acids such as retene, dehydroabietin, and tetrahydroretene induce mixed functionmonooxygenases (MFO) in fish populations [17,18] Hickey and Martin in 1995 found acorrelation between the extent of resin acid contamination in sediments and behaviormodification in benthic invertebrate species [19] Johnsen et al in 1995 reported that TMP milleffluents containing resin acids were lethal to rainbow trout following 2 – 4 weeks exposure at200-fold dilution [20] McCarthy et al in 1990 demonstrated that resin acids are toxic tomethanogens, thereby inhibiting the performance of these bacteria in anaerobic reactors [21]

Parameter

Mill 1 (bagasse,wheat straw, andlake reed used asraw material)

Mill 2 (wheat strawused as the rawmaterial)

Mill 3 (rice strawused as theraw material)

BOD, biochemical oxygen demand; COD, chemical oxygen demand.

Courtesy of MNES and UNDP India websites, Ref 14.

Minimum lethal dose (ppm)

Source: Refs 15 and 16.

10.6 PULP BLEACHING PROCESSES

About 5 – 10% of the original lignin cannot be removed from the pulp without substantialdamage to the cellulosic fraction Removal of the residual lignin, which is responsible forimparting dark color to the pulp, and the production of white pulp, requires a series of stepsemploying bleach chemicals (Fig 1) Pulp bleaching is normally accomplished by sequentialtreatments with elemental chlorine (C1), alkali (E1), chlorine dioxide (D1), alkali (E2), andchlorine dioxide (D2) The C stage consists of charging a slurry of the pulp (at 3 – 4% con-sistency) with elemental chlorine (60 – 70 kg/ton of pulp) at 15 – 308C at pH 1.5 – 2.0 [2] Thechlorinated pulp slurry (at 10% consistency) is treated with alkali (35 – 40 kg/ton of pulp) at 55 –708C and pH 10 – 11 An optional hypochlorite (H) stage is introduced between the E1and D1stages for increasing the brightness of pulp During the conventional bleaching, approximately

70 kg of each ton of pulp is expected to dissolve into the bleaching liquors [2] The largestquantity of pulp is dissolved during the C1and E1stages Alternate pulp bleaching techniquessuch as the elemental chlorine free (ECF), total chlorine free (TCF), and biobleaching aredescribed in subsection 10.9.2

10.6.1 Compounds Formed during Chlorine Bleaching Process

During pulp bleaching, lignin is extensively modified by chlorination (C stage) and dissolved

by alkali (E stage) into the bleaching liquor The E stage is intended for dissolving thefragmented chloro-lignin compounds and removal of noncellulosic carbohydrates The mostimportant reactions are oxidation and substitution by chlorine, which lead to the formation ofchlorinated organic compounds or the AOX (Table 2) Chlorine bleaching liquors exhibitCOD values ranging from 900 – 2000 mg/L and 65 – 75% of this is from chlorinated ligninpolymers [6] The types of chlorinated compounds found in the spent bleach liquors and theirconcentrations depend upon the quantity of residual lignin (Kappa number) in the pulp, nature

of lignin, and bleaching conditions such as chlorine dosage, pH, temperatures, and pulpconsistencies The spent liquors generated from the conventional pulping and bleachingprocesses contain approximately 80% of the organically bound chlorine as high-molecular-mass material (MW above 1000) and 20% as the low-molecular-mass (MW of less than 1000)fraction [22]

The high-molecular-mass compounds, referred to as chlorolignins, cannot be transportedacross the cell membranes of living organisms and are likely to be biologically inactive.Nevertheless, these compounds are of environmental importance because they carrychromophoric structures that impart light-absorbing qualities to receiving waters Long-termand low rates of biodegradation may generate low-molecular-weight compounds, causingdetrimental effects on biological systems

Efforts have been made to characterize the nature and content of individual componentsthat are present in the low-molecular-mass fraction of the total mill effluents, which include thespent chlorination and alkali extraction stage liquors [2,4] Approximately 456 types ofcompounds have been detected in the conventional bleach effluents, of which 330 arechlorinated organic compounds [22] The compounds may be lumped into three main groups,namely, acidic, phenolic, and neutral (Table 2) Acidic compounds are further divided into thefive categories of acids: fatty, resin, hydroxy, dibasic, and aromatic acids The most importantfatty acids are formic and acetic acids The dominant resin acids are abietic and dehydroabieticacids Among the hydroxy acids identified, glyceric acid predominates Dibasic acids such asoxalic, malonic, succinic, and malic acids are derived from the lignin and carbohydrate fraction

of wood and are present in significant amounts in the mill bleach effluents Aromatic acids areformed from residual lignin through the oxidation of phenylpropanoid units and comprise fourmajor categories: monohydroxy (phenolic), ortho-dihydroxy (catecholic), methoxy-hydroxy(guaiacolic), and dimethoxy-hydroxy (syringic) acids The principal phenolics are chlorinatedphenols, chlorinated catechols, chlorinated guaiacols, and chlorinated vanillin, derived from thechlorination and oxidative cleavage of lignin The major neutral compounds are methanol,hemicellulose, and trace concentrations of aldehydes, ketones, chlorinated acetones, di-chloromethane, trichloroethene, chloropropenal, chlorofuranone, chloroform, chlorinated sulfurderivatives, and 1,1-dichloromethylsulfone In addition to the abovementioned compounds, thespent bleaching liquors have been reported to contain about 210 different chlorinated dioxinsthat belong to the two families: polychlorinated dibenzodioxins (PCDDs) and polychlorinateddibenzofurans (PCDFs) [22].

Compounds responsible for imparting toxicity to the spent bleach effluents originate during thechlorination (C) stage and caustic extraction (E) stages The major classes of toxic compoundsare resin acids, fatty acids, and AOX Fatty and resin acids in bleach liquors often originate fromthe washing of unbleached pulps They are recalcitrant to biodegradation as well as inhibitory tothe anaerobic process Adsorbable organic halides are the products of lignin degradation formedexclusively during the C stage of pulp bleaching and dissolved into the bleaching liquors duringthe E stage About 1 – 3% of the AOX fraction is extractable into nonpolar organic solvents and

is referred to as extractable organic halide (EOX) This extractable fraction poses greaterenvironmental risks than the remaining 99% of the AOX and comprises compounds that arelipophilic with the ability to penetrate cell membranes and potential to bioaccumulate in thefatty tissues of higher organisms Dioxins, in particular 2,3,7,8-tetrachlorodibenzodioxin(2,3,7,8-TCDD) and 2,3,7,8-tetrachlorodibenzofuran (2,3,7,8-TCDF) are highly toxic, bio-accumulable, carcinogenic, and cause an adverse impact on almost all types of tested species[2,22,23] Additionally, the abovementioned dioxins and the other unidentified components ofbleach liquors are also endocrine disrupting chemicals (EDC) that decrease the levels andactivity of the estrogen hormone, thereby reducing reproductive efficiency in higher organisms[24] However, limited information is available regarding these undesirable, genetically active,and endocrine-disrupting pollutants in receiving waters; further research is essential in thisdirection.Table 9summarizes some findings related to the toxicity and impact of bleach milldischarges on selected aquatic organisms [25 – 31]

AND PAPER INDUSTRIES

Traditionally, discharge limits have been set for lumped environmental parameters such asBOD5, COD, TSS, and so on However, on account of the adverse biological effects ofchlorinated organics coupled to the introduction of stricter environmental legislation, pulp andpaper mills are faced with the challenges of not only reducing the BOD and suspended solids, butalso controlling the total color as well as AOX in the effluents prior to discharge In recent years,

the pulp and paper industry has taken great strides forward in recognizing and solving many ofthe environmental problems by adopting two strategies:

1 Pollution reduction measures within plants that include minimization of spills andmodifications in the process through adaptation of cleaner technologies as alternatives

to conventional technologies

2 End-of-pipe pollution treatment technologies, which are essential either as asupplement or as backup measures to pollution reduction techniques in order to meetthe effluent regulation standards

Effluents

Bleach process adopted

by the mill

Organismstudied

Physiological/biochemicaleffect(s)/ levels of toxicity

ResearchgroupKraft mill using 100%

chlorine dioxide

Coastal fishcommunity

High levels of mortalityand low embryo quality

Sandstrom,

1994 [25]New and old wood pulp

bleaching employing

various bleach

sequences

Mesocosm andfish biomarkertests

Elemental chlorinecontaining bleachsequence, CEHDEDwas the most toxic

Kankaanpaaetal.,

1995 [27]Kraft bleach mill effluent

produced by oxygen

delignification or

100% chlorine dioxide

oxidase (MFO) enzymesfollowing exposure to 4%

and 12% effluent in artificialstreams

Bankey et al.,

1995 [28]

Kraft mill using

100% ClO2

effluents in the bioassay was:

Untreated ECF untreatedTCF secondary treatedECF secondary treatedTCF

in lake sedimentssampled from

2 – 8 cm depths

Drop in the ATP content,depressed butyrate-esteraseactivity indicating toxicity tomicroorganisms, andreduction in diatom speciesrichness

Mika et al.,

1999 [31]

These two approaches are equally important in meeting environmental regulations and areaddressed in separate sections.

PROCESS MODIFICATIONS

10.9.1 Nonconventional Pulping Technologies

Industries have developed alternate pulping techniques that do not use the conventional cookingmethods Some of these techniques are described briefly in the following subsections Readersmay note that some of these processes have not yet reached commercial stages

Organic Solvent Pulping

Organic solvents such as methanol, ethanol, and other alcohols are used for pulping Thisprocess is economical for small- to medium-scale mills with significant recovery of chemicalsfor reuse However, pulping must be conducted in enclosed containers to prevent the loss ofvolatile solvents and for workers’ safety Additionally, some of these processes are more energyintensive than traditional methods Major benefits include the elimination of odorous sulfur-containing compounds in the effluents and air

Acid Pulping

Wood chips are treated with acetic acid at pressures that are significantly lower than those usedfor Kraft pulping Drawbacks include loss of acid, although recovery is possible through theenergy-intensive distillation process

Biopulping

This method utilizes whole cells of microorganisms and microbial enzymes such as xylanases,pectinases, cellulases, hemicellulases, and ligninases, or their combinations, for pulping her-baceous fibers and improving the properties of pulp derived from wood [32] Pretreatment ofwood chips with lipases is known to reduce the problematic oily exudates during the pulpingprocess as well as improving the texture of paper through the specific degradative action of theseenzymes on pitch-derived extractives such as fatty acids and waxes The innovative approach ofusing microorganisms or microbial enzymes to reduce the consumption of chemicals in the pulpand paper industry is known as biopulping Biopulping has generated much interest among thepulp and paper industries because of the following advantages:

Reduction in the chemical and energy requirements per unit of pulp produced Thus,the process is expected to be cost effective and more affordable for medium- andsmall-scale mills

Reduction in the pollution load due to reduced application of chemicals

The yield and strength properties of the pulp are comparable (sometimes even better)

to those obtained through conventional pulping techniques

Nonwood fibers are more responsive to the action of pulping enzymes compared to wood,presumably due to lower lignin content and weaker hemicellulose – lignin bonds This is clearlyadvantageous for developing countries, which are faced with the problem of shrinking forestwood resources However, further research is required to optimize the conditions required forenzymatic pulping of herbaceous fibers and commercialization of the process

10.9.2 Cleaner Pulp Bleaching Technologies

The use of elemental chlorine in pulp bleaching has been gradually discontinued in severalcountries to prevent the toxic effects of chlorinated organics in receiving waters and to meetregulatory requirements Most nations have imposed stringent regulatory limits on AOX,ranging from 0.3 to 2.0 kg/ton of pulp [22] Cleaner bleaching methods have been developed byindustries based on elemental chlorine free (ECF), total chlorine free (TCF), microbial systems(bio-bleaching), extended delignification, and methods for monitoring and improved control ofbleaching operations Each of these approaches is discussed in the following subsections

Elemental Chlorine Free (ECF) and Total Chlorine Free (TCF) Bleaching

Elemental chlorine has been replaced by chlorine dioxide and hypochlorite in the ECF bleachsequence, while oxygen, ozone, caustic soda, and hydrogen peroxide have been advocated forTCF bleaching of softwood and hardwood Kraft pulps Benefits include significant reduction inthe formation of chlorinated organics or their elimination and lower ecological impacts TwoFinnish mills eliminated elemental chlorine from the bleach sequence and substituted chlorinedioxide, thereby sharply reducing the concentration of chlorinated cymenes [33] In anotherFinnish example, levels of chlorinated polyaromatic hydrocarbons in mill wastes were substantiallyreduced during production of bleached birch Kraft pulp without the use of elemental chlorine ascompared to pine pulp bleached with elemental chlorine [34,35] Research has also been conducted

on the optimal usage of agents such as ozone and hydrogen peroxide [36,37] However, alternativessuch as ozonation, oxygenation, and peroxidation are not economically viable for medium- orsmall-capacity mills due to higher capital investments and plant operation costs

Biobleaching

Biobleaching processes based on the pretreatment of pulp with microbial whole cells or enzymeshave emerged as viable options A number of studies examined the direct application of white rotfungi such as Phanerochaete chrysosporium and Coriolus versicolor for biobleaching of softwoodand hardwood Kraft pulps [38 – 44] It has been found that fungal treatment reduced the chemicaldosage significantly as compared to the conventional chemical bleach sequence and enhanced thebrightness of the pulp Specific features of the fungal-mediated biobleaching processes are: Action through delignification that commences at the onset of the secondary metabolic(nitrogen starvation) phase in most fungi

Delignification is an enzymatic process mediated through the action of extracellularenzymes

The growth phase of the fungus has an obligate requirement for a primary substratesuch as glucose

The major drawbacks of the fungal bleaching process are that it is extremely slow forindustrial application and requires expensive substrates for growth To overcome theseproblems, enzyme preparations derived from selected strains of bacteria or fungi arerecommended The enzymatic method of pulp bleaching is being increasingly preferred by anumber of pulp and paper industries, especially in the West, because it is a cost-effective andenvironmentally sound technology [32] The distinct advantages of enzyme-mediated pulpbleaching are:

minimal energy input;

specificity in reactivity, unlike that of chemicals;

reduced dosage of bleach chemicals in the downstream steps;

improved quality of pulp through bleach boosting;

reduced load of AOX in the effluents

Two categories of enzymes, namely xylanases and peroxidases (lignin degrading), havebeen identified in the pulp bleaching processes Of the two classes, the use of xylanases hasachieved enormous success in aiding pulp bleaching [45] Xylanase enzymes apparently causehydrolytic breakdown of xylan chains (hemicellulose) as well as the cleavage of the lignin –carbohydrate bonds, thereby exposing lignin to the action of subsequent chemical bleachingsteps [46] However, most biobleaching studies using xylanases have been carried out witheither hardwood or softwood, while nonwood resources are being increasingly used as the chiefagricultural raw material for pulp production Therefore, further research with regard to enzymeapplications for nonwood pulp bleaching is warranted

It is unlikely that xylanase treatment alone will completely replace the existing chemicalbleaching technology, because this enzyme does not act directly on lignin, a crucial color-imparting polymer of the pulp Nonspecific oxido-reductive enzymes such as lignin peroxidase,manganese peroxidase and in particular, laccases, which are lignolytic, are likely to be moreeffective in biobleaching [47] The abovementioned enzymes can also act on a wide variety ofsubstrates and therefore have significant potential for applications to pulp and paper effluenttreatment [48,49] The applicability of the laccase mediator system for lignolytic bleaching ofpulps derived from hard wood, soft wood, and bagasse has been reviewed and compared by Calland Mucke [47] The major advantage of enzymatic bleaching is that the process may beemployed by the mills over and beyond the existing technologies with limited investment.Furthermore, there is ample scope for the improvement of the process in terms of cost andperformance

Extended Delignification

The key focus of this process is on the enhanced removal of lignin before subjecting the pulp tobleaching steps [50,51] Such internal process measures also imply cost savings during thesubsequent chemical bleaching steps and have a positive impact on the bleach effluent qualityparameters such as COD, BOD, color, and AOX Extended delignification may be achievedthrough:

Extended cooking This can be done by enhancing cooking time or temperature or bymultiple dosing of the cooking liquors

Oxygenation The pulp is mixed with elemental oxygen, sodium, and magnesiumhydroxides under high pressure An example is the PRENOX process [50] According

to Reeve, about 50% of the world capacity for Kraft pulp production incorporatedoxygen delignification by the year 1994 [52]

Ozonation Ozone and sulfuric acid are mixed with the pulp in a pressurized reactor Addition of chemical catalysts Compounds such as anthraquinone or polysulfide or amixture of the two are introduced into the Kraft cooking liquor

Improved Control of Bleaching Operations

Installation of online monitoring systems at appropriate locations and controlled dosing ofbleach chemicals can aid in the reduction of chlorinated organics in effluents

10.10 TREATMENT OF PULP AND PAPER WASTEWATERS

Plant process modifications and cleaner technologies have the potential to reduce the pollutionload in effluents However, this approach cannot eliminate waste generation End-of-pipepollution treatment technologies are essential for meeting the prescribed limits for dischargedpollutant concentrations such as color, AOX, BOD, COD, and so on Assessment of the waterquality of receiving ecosystems and periodic ecological risk assessments are required to validatethe effectiveness of various treatment methods [53] The most common unit processes employed

by the pulp and paper mills during preliminary, primary, secondary, and tertiary (optional)stages of effluent treatment are listed in the flow sheet shown in Figure 2 Process technologiesthat are currently applied can be broadly classified as the physico-chemical and biologicaltreatment methods These technologies are discussed in the following subsections

10.10.1 Physico-Chemical Processes

Several physico-chemical methods are available for the treatment of pulping and pulp bleachingeffluents The most prominent methods are membrane separation, chemical coagulation, andprecipitation using metal salts and advanced oxidation processes

Membrane Separation Techniques

Membrane processes operate on the basis of the following mechanisms:

pressure driven, which includes reverse osmosis (RO), ultrafiltration (UF), andnanofiltration (NF);

concentration driven, which includes diffusion dialysis, vapor permeation, and gasseparation;

electrically driven, which includes electrodialysis;

temperature difference driven, including membrane distillation

Membrane filtration (UF, RO, and NF) is a potential technology for simultaneouslyremoving color, COD, AOX, salts, heavy metals, and total dissolved solids (TDS) from pulp milleffluents, resulting in the generation of high-quality effluent for water recycling and finaldischarges The possibility of obtaining solid free effluents is a very attractive feature of thisprocess Ultrafiltration was used by Jonsson et al [54] for the treatment of bleach plant effluents

Figure 2 Flow sheet showing the unit processes employed by pulp and paper mills for effluent treatment

Sierka et al [55] described a study that compared the efficiencies of UF alone and UF incombination with RO for the removal of color and total organic carbon (TOC) in the Do(acidstage) wastewaters discharged from the Weyerhaeuser Grande Priare pulp mill, which produces300,000 tons of bleached Kraft bleached pulp per year The bleach plant of this Kraft milltypically employs five stages (ECDoEopDED sequence) for the production of tissue and specialtygrade paper Dostage wastewaters were sterilized using 0.45 mm filters and subsequently passedthrough Amicon-stirred UF cells fitted with membranes having cutoff values of 500 Daltons (D)(YCO5), 1000 D (YM1), 3000 D (YM3), or 10,000 D (YM10) Table 10 summarizes thecharacteristics of the permeate and concentrates obtained following the ultrafiltration of Dowastewater using various membranes Based on these results, Sierka et al concluded that most

of the color (50%) is due to organic compounds with molecular size above 3000 D.Table 11presents the results of additional studies conducted on Do stage effluents that involvedpretreatment by UF followed by RO Clearly, the combination of the UF and RO steps gaveexcellent results by removing 99% of the color and more than 80% of the TOC from the Dostageeffluent

Koyuncu et al [56] presented pilot-scale studies on the treatment of pulp and paper milleffluents using two-stage membrane filtrations, ultrafiltration and reverse osmosis [56] Thecombination of UF and RO resulted in very high removals of COD, color, and conductivity fromthe effluents At the end of a single pass with seawater membrane, the initial COD, color andconductivity values were reduced to 10 – 20 mg/L, 0 – 100 PCCU (platinum cobalt color units)and 200 – 300 ms/cm, respectively Nearly complete color removals were achieved in the ROexperiments with seawater membranes

A distinct advantage of the membrane technology is that it can be utilized at the primary,secondary, or tertiary phases of water treatment Some membranes can withstand highconcentrations of suspended solids, which presents a possible direct application for separatingmixed liquor suspended solids (MLSS) in an activated sludge plant (membrane bioreactor) toreplace the conventional sedimentation tank Key variable parameters of membrane technologyinclude variation in membrane pore size, transmembrane pressure, cross-flow velocity,temperature, and back flushing The major disadvantages are high capital and maintenancecosts, accumulation of reject solutes and decrease in the membrane performance, membranefouling, and requirement for the pretreatment of discharges

Chemical Coagulation and Precipitation

This method relies on the addition of metal salts to cause agglomeration of small particles intolarger flocs that can be easily removed by settling The effectiveness of this process is dependentupon the nature of coagulating agent, coagulant dosage, pH, ionic strength, and the nature andconcentration of compounds present in wastewaters The not-so-easily biodegradable fraction ofpulping and bleaching effluents consists of polar and hydrophobic compounds, notably resinacids, long-chain fatty acids, aromatic acids and phenols, lignin, and terpenes Almost all ofthese toxic compounds can be effectively removed through coagulation using chloride andsulfate salts of Fe3þand Al3þ Typically, these trivalent cations remain in solution at acidic pHand form metal hydroxides that aggregate rapidly at higher pH conditions Hydrogen bonding,electrostatic and surface interactions (adsorption) between the metal hydroxides and organicanions (containing hydroxyl and carboxyl groups) lead to the formation of metal hydroxide –organic compound precipitates [57,58] Dissolved organics are also removed by physicaladsorption to flocs

Chemical precipitation of mill effluents from CTMP, BKME (bleached Kraft milleffluent), NSSC, and E & C bleach discharges have been extensively studied by Stephenson andDuff [59] using alum, lime, ferric chloride, ferrous sulfate, magnesium hydroxide, polyimine,polymers, and alum in combination with lime They observed removal of 88% of total carbonand 90 – 98% of color and turbidity from mechanical pulping effluents using Fe3þ/Al3þ salts

In another publication, Stephenson and Duff reported significant reduction in the toxicity ofwastewaters subsequent to the chemical coagulation process [60] Ganjidoust et al [61]compared the effects of a natural polymer, chitosan, and synthetic polymers, namely hexa-methylene diamine epichlorohydrin polycondensate (HE), polyethyleneimine (PEI), polyacryl-amide (PAM), and a chemical alum coagulant, alum on the removal of lignin (black liquor colorand total organic carbon) from alkaline pulp and paper industrial wastewater They observed thatPAM, a nonionic polymer, had a poor effect, whereas HE and PEI, which are cationic polymers,coagulated 80% of the color and 30% of the total organic carbon from alkaline black liquorwastewater by gravity settling in 30 min Alum precipitation removed 80% of the color and 40%

of total organic carbon By comparison, the natural coagulant chitosan was the most effective; iteliminated up to 90% of the color and 70% of the total organic carbon, respectively

Ultrafiltration and Reverse Osmosis

Input/output of

TOC(mg/L)

Color(PCCUa)

a Platinum cobalt color units.

Experimental conditions: Pressure ¼ 1104 kPa; temperature ¼ 408C; batch

volume ¼ 4 L; cutoff value of the UF membrane ¼ 8000 D.

Source: Ref 55.

The major disadvantages of coagulation and precipitation are the generation of chemicalsludge and the need for subsequent treatment of the sludge to eliminate the adsorbed toxicpollutants prior to disposal.

Advanced Oxidation Processes

Destruction of chromophoric and nonchromophoric pollutants in pulp and paper effluents may

be achieved by advanced oxidation methods such as photocatalysis, photo-oxidation usinghydrogen peroxide (H2O2)/UV or ozone (O3)/UV systems, Fenton-type reactions, wetoxidation, and by employing strong oxidants such as ozone

Photocatalysis has gained attention for its application to aqueous phase and wastewatersfor near total oxidation and elimination of organic compounds [62] The process involves mixingwastewater with aeration in a reactor at 20 – 258C and the introduction of titanium dioxide (TiO2)followed by irradiation using a UV lamp Irradiation by UV light generates an electron hole onTiO2 surface, which reacts with the adsorbed organic compounds or water molecules TiO2can

be provided as a suspension or as covered supports (immobilized on beads, inside tubes of glass/teflon, fiberglass, woven fibers, etc.) Various research groups have shown that photocatalysis isnonselective and that there is a nearly parallel reduction in the color, lignosulfonic acids, andother organic compounds in the treated pulp and paper mill effluents Balcioglu et al [63]observed enhanced biodegradability (increase in BOD5/COD ratio) of raw Kraft pulp bleachingeffluents and improved quality of the biologically pretreated effluents following TiO2photocatalytic oxidation Yeber et al [64] described the photocatalytic (TiO2 and ZnO)treatment of bleaching effluents from two pulp mills Photocatalysis resulted in the enhancedbiodegradability of effluents with concomitant reduction in the toxicity

Photo-oxidation systems using H2O2/UV or O3/UV combinations generate hydroxylradicals that are short lived but extremely powerful oxidizing organics through hydrogenabstraction The result is the onsite total destruction of refractory organics without generation ofsludges or residues Wastewater is injected with H2O2or saturated with O3and irradiated with

UV light at 254 nm in a suitable reactor with no additional requirement for chemicals The rate

of oxidative degradation is generally much higher than systems employing UV or O3alone.Legrini et al [62] have extensively reviewed the experimental conditions used by variousresearchers for conducting the photo-oxidation process as well as their application for removal

of various types of organic compounds

Fenton’s reactions involving hydrogen peroxide (H2O2) and ferrous ion as the solutioncatalyst are an effective option for effluent treatment Fenton’s reaction as described byWinterbourn [65] requires a slightly acidic pH and results in the formation of highly reactivehydroxyl radicals (†

OH), which are capable of degrading many organic pollutants Rodriguez

et al [66] evaluated Fenton-type reactions facilitated by catecholic compounds such as dihydroxybenzoic acid, 3,4-dihydroxybenzoic acid, and 1,2-dihydroxybenzene for treating pulpbleaching effluent Their research indicated that 2,3-dihydrobenzoic acid was the most effectivecompound in enhancing hydroxyl radical formation in the iron – hydrogen peroxide reactionsystem at pH 4.0 with the concomitant reduction in the AOX concentration and toxicity of thebleach effluents

2,3-Wet oxidation is a process where organic contaminants in liquids or soils are extracted into

an aqueous phase and reacted with an oxidant at high temperature (220 – 2908C) and pressures(100 – 250 bar) to promote rapid destruction Laari et al [67] evaluated the efficiency of wetoxidation for the treatment of TMP processing waters The major objective of this research was

to reduce the concentration of lipophilic wood extractives (LWE) and to treat concentratedresidues from evaporation and membrane filtration by low-pressure catalytic wet oxidation

The wet oxidation of membrane and evaporation concentrates was effective in reducing 50% ofthe COD at 1508C and enhancing the biodegradability of wastewater.

Oxidants such as chlorine, oxygen, ozone, and peroxide have been proposed for thetreatment of pulp bleach effluents Ozonation has been reported to reduce the toxicity of CTMPand bleached Kraft mill (BKM) effluents at low dosages [22] Hostachy et al [68] reporteddetoxification and an increase in the biodegradability of bleach effluents by ozonation at lowdosages [0.5 – 1 kg/ADMT (air-dried metric ton)] of pulp The researchers observed significantelimination of the residual COD by catalyzed ozone treatment of hardwood and softwood pulpand paper mill final discharges Such a treatment method may allow for reutilization of treatedprocess waters and reduce consumption of freshwater during pulping steps Helbe et al [69]described a tertiary treatment process involving ozonation in combination with a fixed-bedbiofilm reactor for the reuse of treated effluent in a pulp and paper industry Sequential ozonationand bioreactor treatment gave maximum elimination of COD, color, and AOX from biologicallytreated effluent with minimum dosage of ozone Further, the authors suggested that two-stageozonation with intermediate biodegradation is more effective in terms of achieving higherremoval of persistent COD

The advantages of the various oxidation processes include nonselective and rapiddestruction of pollutants, absence of residues, and improved biodegradability of the effluents.Some of the disadvantages are extremely short half-life of the oxidants and high expense of theirgeneration

10.10.2 Biological Processes

The most commonly used biological treatment systems for the pulp and paper mill dischargesare activated sludge plants, aerated lagoons, and anaerobic reactors Sequential aerobic-anaerobic systems (and vice versa) are a recent trend for handling complex wastewaters ofpulp and paper mills that contain a multitude of pollutants The application of various types ofbiological reactor systems for treating pulp and paper mill effluents are discussed in thefollowing subsections

Activated Sludge Process

This conventional aerobic biological treatment train consists of an aeration tank with completemixing (for industrial discharges) followed by a secondary clarifier and has been typically usedfor the reduction of COD, BOD, TSS, and AOX in pulp and paper mill waste effluents Oxygen

is provided to the aerobic microorganisms through aeration or by using pure oxygen as in thedeep shaft systems Bajpai [22] has reviewed the efficiencies of activated sludge plants andreported that the overall removal of AOX can range from 15 to 65%, while the extents ofremoval of individual chlorinated organics such as chlorinated phenols, guaiacols, catechols,and vanillins can vary from 20 to 100% Biotransformation and biodegradation seem to be theimportant mechanisms for reduction in the overall AOX concentrations and hydraulic retentiontime (HRT) is the key operating parameter

There are a number of full-scale activated sludge plants that are in operation in countriessuch as the United States, Canada, and Finland, which treat effluents from Kraft, sulfite, TMP,CTMP, and newsprint mills [22] Schnell et al [70] reported the effectiveness of a conventionalactivated sludge process operating at an alkaline-peroxide mechanical pulping (APMP) plant atMalette Quebec, Canada The full-scale plant achieved 74% reduction in filterable COD andnearly complete elimination of BOD5, resin acids, and fatty acids in the whole mill effluent Thetreated effluent tested nontoxic as measured by a Microtox assay Saunamaki [71] reported