10 Treatment of Pulp and Paper Mill Wastes Suresh Sumathi Indian Institute of Technology, Bombay, India Yung-Tse Hung Cleveland State University, Cleveland, Ohio, U.S.A. 10.1 POLLUTION PROBLEMS OF PULP AND PAPER INDUSTRIES Pulp and paper mills are a major source of industrial pollution worldwide. The pulping and bleaching steps generate most of the liquid, solid, and gaseous wastes (Table 1) [1]. Pulping is a process 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 papermaking properties of fibers. Bleaching is a multistage process to whiten and brighten the pulp through removal of residual lignin. Pulping and bleaching operations are energy intensive and typically consume huge volumes of fresh water and large quantities of chemicals such as sodium hydroxide, 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 compounds found in spent liquors generated from pulping and bleaching steps is shown in Table 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 of light, thereby affecting benthic growth and habitat. The color responsible for causing aesthetic 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 of chlorinated organics to wastewater particulates and sludge. Significant solid wastes from pulp and paper mills include bark, reject fibers, wastewater treatment plant sludge, scrubber sludge, lime mud, green liquor dregs, and boiler and furnace ash. The bulk of the solid wastes is generated during wastewater treatment. Sludge disposal is a serious environmental problem due to the partitioning of chlorinated organics from effluents to 453 © 2006 by Taylor & Francis Group, LLC solids. The major air emissions are fine and coarse particulates from recovery furnaces and burners, sulfur oxides (SOx) from sulfite mills, reduced sulfur gases and associated odor problems 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 load from hardwood is lower than softwood. On the other hand, the spent liquor generated from pulping of nonwood fiber has a high silica content. Volumes of wastewater discharged may vary from near zero to 400 m 3 per ton of pulp depending on the raw material used, manufacturing process, and size of the mill [6]. Thus, the variability of effluent characteristics and volume from one mill to another emphasizes the requirement for a variety of pollution prevention and treatment technologies, tailored for a specific industry. Table 1 Types of Pollutants Generated During Chemical (Kraft) Pulping and Bleaching Steps Pollution generating step Pollution output phase Nature of pollution Wood debarking and chipping, chip washing Solid Bark, wood processing residues Water SS, BOD, color, resin acids Chemical (Kraft) pulping, black liquor evaporation, and chemical recovery steps Air Total reduced sulfur (hydrogen sulfide, methyl mercaptan, dimethyl sulfide, dimethyl disulfide), VOC Wood chip digestion, spent pulping liquor evaporator condensates Water High BOD, color, may contain reduced sulfur compounds, resin acids Pulp screening, thickening, and cleaning operations Water Large volume of waters with SS, BOD, color Smelt dissolution, clarification to generate green liquor Solid Green liquor dregs Recausticizing of green liquor, clarification to generate white liquor Solid Lime slaker grits Chlorine bleaching of pulp Water BOD, color, chlorinated organics, resin acids Air VOC Wastewater treatment Solid Primary and secondary sludge, chemical sludge Air VOC Scrubbing for flue gases Solid Scrubber sludge Recovery furnaces and boilers Air Fine and coarse particulates, nitrogen oxides, SO 2 Solid Ash SS, suspended solids; VOC, volatile organics; BOD, biochemical oxygen demand. Source: Ref. 1. 454 Sumathi and Hung © 2006 by Taylor & Francis Group, LLC Table 2 Low-Molecular-Weight Organic Compounds Found in the Spent Liquors from Pulping and Bleaching Processes Class of compounds Acidic Wood extractives Lignin/carbohydrate derived Phenolic Neutral Miscellaneous Category: Fatty acid Formic acid (S) Acetic acid (S) Palmitic acid (S) Heptadecanoic acid (S) Stearic acid (S) Arachidic acid (S) Tricosanoic acid (S) Lignoceric (S) Oleic (US) Linolenic acid (US) Behenic acid (S) Category: Resin acid Abietic acid Dehydroabietic acid Mono and dichloro dehydrabietic acids Hydroxylated- dehydroabietic acid Levopimaric acid Pimaric acid Sandracopimaric acid Category: Hydroxy Glyceric acid Category: Dibasic Oxalic acid Malonic acid Succinic acid Malic acid Category: Phenolic acid Monohydroxy benzoic acid Dihydroxy benzoic acid Guaiacolic acid Syringic acid Category: Phenolic Monochlorophenols Dichlorophenols Trichlorophenols Tetrachlorophenol Pentachlorophenol Category: Guaiacolic Dichloroguaiacols Trichloroguaiacols Tetrachloroguaiacol Category: Catecholic Dichlorocatechols Trichlorocatechols Category: Syringic Trichlorosyringol Chlorosyringaldehyde Hemicelluloses Methanol Chlorinated acetones Chloroform Dichloromethane Trichloroethene Chloropropenal Chlorofuranone 1,1-dichloro- methylsulfone Aldehydes Ketones Chlorinated sulfur Reduced sulfur compounds Category: Dioxins 2,3,7,8-tetrachloro- dibenzodioxin (2,3,7,8-TCDD) 2,3,7,8-tetrachloro-dibenzofuran (2,3,7,8-TCDF) Wood derivatives Monoterpenes Sesquiterpenes Diterpenes: Pimarol Abienol Juvabiones Juvabiol Juvabione Lignin derivatives Eugenol Isoeugenol Stilbene Tannins (monomeric, condensed and hydrolysable) Flavonoids S, saturated; US, unsaturated Source: Refs 2–4. Treatment of Pulp and Paper Mill Wastes 455 © 2006 by Taylor & Francis Group, LLC The focus of this chapter is to trace the origin and nature of the major pollution (especially water) problems within the pulp and paper industries and to present an overview of the pollution mitigation strategies and technologies that are currently in practice or being developed (emerging technologies). 10.2 NATURE AND COMPOSITION OF RAW MATERIALS USED 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 of b -D-glucose units linked by 1–4 glucosidic bonds. Hemicelluloses are branched and varying types of this polymer are found 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% by weight) and glucomannan (1–5% by weight) are found [2,3]. Lignin is a complex heterogeneous phenylpropanoid biopolymer containing a diverse array of stable carbon–carbon bonds with aryl/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 most organisms and must be so in order to impart resistance to plants against many physical and environmental stresses. This recalcitrant biopolymer is formed in plant cell walls by the enzyme- catalyzed coupling of p-hydroxycinnamyl alcohols, namely, p-coumaryl, coniferyl, and sinapyl alcohols that make up significant proportion of the biomass in terrestrial higher plants. In hardwoods, lignin is composed of coniferyl and sinapyl alcohols and in softwoods is largely a polymer 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 the species and age of the tree. Terpenoids that include resin acids are found only in soft wood and are derived from the “pitch” component of wood. Compared to wood, the structures of nonwood species 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 wood and the bonding of lignin to cellulose is weaker and therefore easier to access. 10.3 PULPING PROCESSES 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 processes that are in operation worldwide are mechanical and chemical processes. Mechanical pulping methods use mechanical pressure, disc refiners, heating, and mild chemical treatment to yield pulps. Chemical pulping involves cooking of wood chips in pulping liquors containing chemicals under high temperature and pressure. Other pulping operations combine thermal, mechanical, and/or chemical methods. Characteristic features of various pulping processes are summarized in Table 3 and are further described shortly in the following subsections [3,10–12]. 456 Sumathi and Hung © 2006 by Taylor & Francis Group, LLC Figure 1 Steps involved in the pulping and pulp bleaching processes (from Ref. 2). Treatment of Pulp and Paper Mill Wastes 457 © 2006 by Taylor & Francis Group, LLC Table 3 Comparison of Various Pulping Processes Name of the pulping process Process features Mechanical CTMP NSSC Kraft Sulfite Pulping mechanism Grinding stone, double disc refiners, steaming, followed by refining in TMP process Chemical treatment using NaOH or NaHSO 3 þ steaming followed by mechanical refining Continuous digestion in Na 2 SO 3 þ Na 2 CO 3 liquor using steam followed by mechanical refining Cooking at 340–3508F, 100–135 psi for 2– 5 hours in NaOH, Na 2 S, and Na 2 CO3; efficient recovery of chemicals Sulfonation at 255 – 3508F, 90–110 psi for 6–12 hours in H 2 SO 3 and Ca, Na, NH 4 , Mg(HSO 3 ) 2 Cellulosic raw material Hard woods like poplar and soft woods like balsam, fir, hemlock Hard and soft woods Hard woods like aspen, oak, alder, birch, and soft wood sawdust and chips Any type of hard and soft wood, nonwood fiber sources Any hard wood and nonresinous soft woods Pulp properties Low-strength soft pulp, low brightness Moderate strength Good stiffness and moldability High-strength brown pulps, difficult to bleach Dull white-light brown pulp, easily bleached, lower strength than Kraft pulp Typical yields of pulp 92–96% 88–95% 70–80% 65–70% for brown pulps, 47–50% for bleachable pulps, 43– 45% after bleaching 48–51% for bleachable pulp, 46–48% after bleaching Paper products Newspaper, magazines, inexpensive writing papers, molded products Newspaper, magazines, inexpensive writing papers, molded products Corrugating medium Bags, wrappings, gumming paper, white papers from bleached Kraft pulp, cartons, containers, corrugated board Fine paper, sanitary tissue, wraps, glassine strength reinforcement in newsprint TMP, thermomechanical pump; CTMP, chemi-thermomechanical pump; NSSC, neutral sulfite semichemical pulp. Source: Refs. 3, 10, and 12. 458 Sumathi and Hung © 2006 by Taylor & Francis Group, LLC Nonconventional pulping methods such as solvent pulping, acid pulping, and biopulping are discussed 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 to yield low-grade pulps. Lignin is not removed during this process and therefore imparts a dark color to the pulp and paper product. Refiner Mechanical Pulp Wood chips are passed through a narrow gap of a double-disc steel refiner consisting of stationary and rotating plates having serrated surfaces. This process results in the mechanical separation of fibers that are subsequently frayed for bonding. The strength of the refiner pulp is better than that of ground-wood pulps. Thermomechanical Pulp (TMP) Wood chips are preheated in steam before passage through disc refiners. Heating is meant for softening the lignin portion of wood and to promote fiber separation. This pulp is stronger than that produced by the ground-wood process. 10.3.2 Semichemical Pulp Wood chips are processed in mild chemical liquor and subjected to mechanical refining using disc refiners. Semichemical pulping liquors have variable composition ranging from sodium hydroxide alone, alkaline sulfite (sodium sulfite þ sodium carbonate), mixtures of sodium 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 is referred 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 sodium bisulfite before or during steaming. Chemically treated chips are passed through mechanical disc refiners. 10.3.4 Chemical Pulps Chemical pulping of wood is commonly carried out according to the Kraft (sulfate) or sulfite processes [13]. These methods are described in the following subsections. Kraft Pulping Kraft pulping involves the cooking of wood chips at 340– 3508F and 100–135 psi in liquor that contains sodium hydroxide, sodium sulfide, and sodium carbonate. This process promotes cleavage of the various ether bonds in lignin and the degradative products so formed dissolve in alkaline pulping liquor. The Kraft process normally incorporates several steps to recover chemicals from the spent black liquor [3]. Treatment of Pulp and Paper Mill Wastes 459 © 2006 by Taylor & Francis Group, LLC Sulfite Pulping The sulfite process solubilizes lignin through sulfonation at 255–3508F under 90 –110 psi. The pulping liquors are composed of mixture of sulfurous acid (H 2 SO 3 ) and bisulfites (HSO 3 22 )of ammonium, sodium, magnesium, or calcium, and lignin is separated from the cellulose as lignosulfonates [3]. Bisulfite pulping is performed in the pH range of 3–5 while acid sulfite pulping is carried out with free sulfurous acid at pH 1–2. Sulfite pulping mills frequently adapt methods for the recovery of SO 2 , magnesium, sodium, or ammonium base liquors [3]. 10.4 COMPOSITION OF SPENT PULPING LIQUORS 10.4.1 Kraft Pulping Liquors (Black Liquors) During Kraft pulping, about 90 –95% of the reactive biopolymer, namely lignin, becomes solubilized to form a mixture of lignin oligomers that contribute to the dark brown color and pollution load of pulping liquors. Lignin oligomers that are released into the spent liquors undergo cleavage to low-molecular-weight phenylpropanoic acids, methoxylated and/or hydroxylated aromatic acids. In addition, cellulose and hemicelluloses that are sensitive to alkali also dissolve during the pulping processes [13]. Black liquors generated from the Kraft pulping process 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 from dissolution of wood [3]: . ligninolytic compounds that are polyaromatic in nature; . saccharic acids derived from the degradation of carbohydrates; Table 4 Components of Kraft Black Liquor and Characteristics of Kraft Evaporator Condensate Kraft black liquor characteristics Component Weight %, dry solids basis Lignin 30–45 Hemicellulose and sugars 1 Hydroxy acids 25–35 Extractives 3–5 Acetic acid 2–5 Formic acid 3–5 Methanol 1 Sulfur 3–5 Sodium 17–20 Kraft liquor evaporator condensate characteristics COD 1000–33,600 mg/L Major organic component Methanol, 60–90% of COD Anaerobic degradability 80–90% of COD Compounds that inhibit anaerobic metabolism Reduced sulfur, resin acids, fatty acids, volatile terpenes COD, chemical oxygen demand. Source: Refs 3 and 6. 460 Sumathi and Hung © 2006 by Taylor & Francis Group, LLC . solvent extractives that include fatty acids and resin acids; . low-molecular-weight organic acids. Table 4 shows the typical ranges of black liquor constituents and characteristics of Kraft evaporator condensates. The composition of liquors may vary significantly, depending upon the type of raw material used. Inorganic constituents in black liquor are sodium hydroxide, sodium sulfate, 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 sulfite pulping liquors. In general, spent ammonia base liquors have higher BOD 5 , COD, and dissolved organics 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-spent liquors contain COD values typically ranging from 120–220 g/L and 50 –60% of these are lignosulfonates [6]. The sulfite-spent liquor evaporator condensates have COD values in the range 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 the condensates is typically 50–90% of COD and sulfur compounds are the major inhibitors of methanogenic activity [6]. Table 5 Composition of Ammonia, Sodium, Magnesium, and Calcium Base Sulfite Pulping Liquors Parameter Ammonia base mill a Sodium base mill b Magnesium base mill c Calcium base mill d Pulp liquor volume (m 3 /ODT) 9.46 7.10 6.08 9.28 pH range 1.5–3.3 2.1– 4.8 $3.4 5.3 BOD (kg/ODT) 413 235 222 357 COD (kg/ODT) 1728 938 975 1533 Dissolved organics (kg/ODT) 1223 595 782 1043 Dissolved inorganics (kg/ODT) 12.5 226 126 250 Lignin as determined by UV absorption (kg/ODT) 892 410 501 800 Total sugars (kg/ODT) 288 137 129 264 Reduced sugars (kg/ ODT) 212 74 106 238 Toxicity emission factor e (TEF) 3663 714 – 422 a Average data based on 4 mills; b Average data based on 12 mills; c Average data based on 2 mills; d Composition of one mill; e Toxicity emission factors are based on static 96 hour bioassays and factored to the volume of liquor production. ODT ¼ Oven dried ton of pulp. Source: Refs. 3 and 10. Treatment of Pulp and Paper Mill Wastes 461 © 2006 by Taylor & Francis Group, LLC 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 constitute anywhere from 15 to 50% of the soluble COD values in these spent liquors. The composition of spent 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 as inhibitory 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]. Typical compositions of the spent liquors generated from the small-scale, agro-residue utilizing pulp and paper mills are shown in Table 7. It is evident from the table that 45– 50% of the total solids is represented by lignin. Most of the lignin present in the black liquor is the high-molecular-weight fraction, 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 liquors generated from Kraft mills. Table 8 shows a partial representation of toxicity data compiled by the NCASI (National Council of the Paper Industry for Air and Stream Improvement) and McKee and Wolf for Kraft mill pulping wastewaters [15,16]. The table indicates that hydrogen sulfide, methyl mercaptan, crude sulfate soap, and salts of fatty and resin acids are particularly Table 6 Composition of Spent NSSC Pulping Liquor Spent NSSC pulping liquor characteristics Parameter Average value Total solids (%) 12 Volatile solids (% of total solids) 48 COD (mg/L) 40,000 BOD 5 (mg/L) 25,000 Wood sugars (mg/L) 7000 Lignin (mg/L) 45,000 Acetate (mg/L) 18,000 pH range 6.5–8.5 Anaerobic degradability NR Compounds that have the potential to inhibit anaerobic process Tannins, sulfur compounds NSSC pulping liquor condensate characteristics COD 7000 mg/L Major organic component Acetic acid, 70% of COD Anaerobic degradability NR Inhibitors of anaerobic degradation process Sulfur compounds NR, not reported; COD, chemical oxygen demand; BOD, biochemical oxygen demand. Source: Refs. 3 and 6. 462 Sumathi and Hung © 2006 by Taylor & Francis Group, LLC [...]... total chlorine free (TCF), and biobleaching are described 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 the fragmented chloro-lignin compounds and removal of noncellulosic carbohydrates The most... located in the top portion of the reactor The gas – liquid –solid separating device consists of a gas collection hood and a settler section Most of the treatment occurs within the blanket of granules The gas collected in the hood area of the bioreactor is continually removed while the liquid flows into the settler section for liquid –solid separation and settlement of solids back into the reactor The combined... classified as the physico-chemical and biological treatment 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 bleaching effluents The most prominent methods are membrane separation, chemical coagulation, and precipitation using metal salts and advanced oxidation processes... nature of lignin, and bleaching conditions such as chlorine dosage, pH, temperatures, and pulp consistencies The spent liquors generated from the conventional pulping and bleaching processes contain approximately 80% of the organically bound chlorine as high-molecularmass material (MW above 100 0) and 20% as the low-molecular-mass (MW of less than 100 0) fraction [22] The high-molecular-mass compounds,... to reduce the problematic oily exudates during the pulping process as well as improving the texture of paper through the specific degradative action of these enzymes on pitch-derived extractives such as fatty acids and waxes The innovative approach of using microorganisms or microbial enzymes to reduce the consumption of chemicals in the pulp and paper industry is known as biopulping Biopulping has generated... Two Finnish mills eliminated elemental chlorine from the bleach sequence and substituted chlorine dioxide, thereby sharply reducing the concentration of chlorinated cymenes [33] In another Finnish example, levels of chlorinated polyaromatic hydrocarbons in mill wastes were substantially reduced during production of bleached birch Kraft pulp without the use of elemental chlorine as compared to pine pulp... Francis Group, LLC Daphnia Fish 100 10 1.0 1.0 5 – 10 1.0 3.0 – – – 100 3.0 0.5 1.0 5.0 5.0 1.0 5.0 10. 0 3.0 464 10. 6 Sumathi and Hung PULP BLEACHING PROCESSES About 5– 10% of the original lignin cannot be removed from the pulp without substantial damage to the cellulosic fraction Removal of the residual lignin, which is responsible for imparting dark color to the pulp, and the production of white pulp,... pilot-scale studies on a Canadian integrated sulfite pulp and paper mill effluent using an aerobic biofilter (1 m2, 3 m depth) as the main unit and aerated stabilization basins (3 m3) as the polishing stage The biofilter unit treated the most concentrated sulfite mill effluent and the resulting effluent was mixed with remaining mill wastewaters to be treated in the polishing ASB unit Characteristics of the. .. In addition to the abovementioned compounds, the spent bleaching liquors have been reported to contain about 210 different chlorinated dioxins that belong to the two families: polychlorinated dibenzodioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs) [22] 10. 7 TOXICITY OF SPENT BLEACH LIQUORS Compounds responsible for imparting toxicity to the spent bleach effluents originate during the chlorination... compounds The anaerobic reactor was efficient in dechlorination, thereby eliminating most of the toxicity and improving biodegradability of the subsequent aerobic reactor at shorter retention times The researchers identified two species of Rhodococcus bacteria that were capable of degrading polychlorinated phenols, guaiacols, and syringols in the bleaching effluents Wang et al [90] examined continuous-flow . and therefore easier to access. 10. 3 PULPING PROCESSES The steps involved in pulping are debarking, wood chipping, chip washing, chip crushing/ digestion, pulp screening, thickening, and washing. chlorine free (TCF), and biobleaching are described in subsection 10. 9.2. 10. 6.1 Compounds Formed during Chlorine Bleaching Process During pulp bleaching, lignin is extensively modified by chlorination. bleaching is that the process may be employed by the mills over and beyond the existing technologies with limited investment. Furthermore, there is ample scope for the improvement of the process in