anaerobic treatment in pulp- and paper-mill

Bioresource Technology 47 (1994) 1-18

ANAEROBIC TREATMENT IN PULP- A N D PAPER-MILL WASTE MANAGEMENT: A REVIEW

Jukka A. Rintala & Jaakko A. Puhakka*

Institute of Water and Environmental Engineering, Tampere University of Technology, PO Box 527, FIN-33101 Tampere, Finland

(Received 20 March 1993; accepted 24 March 1993)

Abstract BOD The pulp- and paper-industry generates large volumes of COD highly heterogenous wastewaters containing compounds CTMP from wood or other raw material, process chemicals and HC compounds formed during processing. The wastewaters HMW from mechanical pulping and secondary fiber pulping as KEC well as the condensates from chemical and semi- MLVSS chemical pulping are typically non-toxic to methano- MW genie degradation and contain easily degradable organic N compounds. Consequently, anaerobic digestion is an NSSC attractive treatment alternative for these effluents. In P addition, both primary and secondary sludges from PM pulp- and paper-industry wastewater treatment-plants PS are amenable to anaerobic digestion. In contrast, the SEC bleaching effluents from chemical pulping, the debark- SS ing effluents as well as the CTMP effluents are likely to TMP be inhibitory to methanogenic degradation; also their TOCI biodegrability is relatively low. Dilutiol~ with other UASB wastewater streams or detoxification by various pretreat- VFA ments have been used to facilitate anaerobic treatment of VS these inhibitory wastewaters. The potential of the anaer- VSS obic systems for reductive dechlorination and sulfur WAS recovery is unique and of great interest. In almost all pulp- and paper-industry full-scale applications, anaerobic treatment is followed by aerobic post-treatment. The suitability and the cost of the anaerobic-aerobic and aerobic treatment-systems are largely affected by a variety of mill-specific factors.

Key words: Anaerobic, anaerobic biodegradability, pulp- and paper-industry, wastewater characteristics, wastewater treatment.

NOTATION

adt air dry tonne AOX Adsorbable organic halogens

*Present address: Department of Civil Engineering, Univer- sity of Washington, Seattle, Washington 98195, USA.

Bioresource Technology 0960-8524/94/S07.00 © 1994 Elsevier Science Limited, England. Printed in Great Britain

Biological oxygen demand Chemical oxygen demand Chemithermomechanical pulping Hemicellulose High molecular weight Kraft evaporator condensate Mixed liquor volatile suspended solids Molecular weight Nitrogen Neutral sulfite semichemical Phosphorus Paper machine Primary sludge Sulfite evaporator condensate Suspended solids Thermomechanical pulping Total organic chlorine Upflow anaerobic sludge blanket Volatile fatty acid Volatile solids Volatile suspended solids Waste activated sludge

INTRODUCTION

The estimated total annual world capacity of pulp production in 1992 will amount to 261 million tonnes and that of paper and paperboard production to 196 million tonnes (FAO, 1988). For each tonne of manu- factured pulp the wastewater discharge volume ranges from 30 to 180 m 3 (e.g. Puhakka, 1990; Gullichsen, 1991) while 20-70 m 3 is discharged per tonne of paper and paperboard (e. g. Miner & Unwin, 1991). The quantities and characteristics of the generated pulp- and paper-wastewaters are highly dependent on raw materials, processes and process conditions used in the mill. The pulp- and paper-industry wastewaters may cause oxygen deficiency, acute or chronic toxicity, mutagenicity or eutrophication in recipient water bodies. In some countries the emission standards for pulp- and paper-industry discharges include besides

2 Jukka A. Rintala, Jaakko A. Puhakka

conventional BOD and suspended solids also nutrients and organochlorine compounds expressed as AOX (e.g. Puhakka etal., 1991).

In the pulp and paper industries, both internal and external measures have been undertaken to reduce the wastewater discharges. As external methods, mechanical treatment with primary clarifiers is usually applied to remove the wastewater solids and the organic load thereof while dissolved organics are removed in aerated lagoons and activated-sludge systems, the most commonly applied biological processes so far. In some cases, tertiary treatments, such as chemical precipitation and flotation, have been employed to improve the effluent quality from biological treatment. Both mechanical and activated-sludge treatments applied in the pulp and paper industries generate large quantities of sludges.

Anaerobic technology has been applied in sewage sludge treatment for over one hundred years (reviewed by McCarty, 1982). Retention times of several weeks are typical in anaerobic sludge-digestion. In the 1980s, anaerobic systems became increasingly common in the treatment of industrial wastewaters containing easily degradable, non-toxic compounds. The use of anaerobic systems was made possible by the development of high-rate processes and the increased understanding of factors controlling the anaerobic degradation. This enabled compact, and therefore relatively low-cost, reactor structures. Anaerobic wastewater treatment is usually followed by aerobic treatment to meet the effluent-quality criteria. In many cases, anaerobic pretreatment has resulted in improved economy of overall waste management as compared to systems based on the conventional aerobic processes. This is partially due to the resource recovery through methane gas.

The first anaerobic systems in the pulp- and paper- industries were anaerobic lagoons, introduced towards the end of the 1970s together with the contact reactor in 1981 (Anon., 1987). The first high-rate anaerobic process was the UASB reactor, which was introduced in 1983 (Habets & Knelissen, 1985a,b). In 1989, at least 33 fullZscale anaerobic external treatment plants were in operation in the pulp and paper mills (Pearson, 1989). The full-scale applications and intensive pilot and laboratory studies have demonstrated the suitability of anaerobic processes for the treatment of several types of pulp- and paper-industry wastewaters. On the other hand, they have also demonstrated that these wastewaters contain compounds recalcitrant and/or toxic to anaerobic degradation. Recently anaerobic process have been studied in order to incorporate them into the pulp- and paper-mill water circuits (Huster et al., 1991; Barascud etal., 1992).

This paper surveys recent literature on the anaerobic treatment of wastewaters from the wood-utilizing pulp- and paper-industry. The effluent characteristics are summarized and their potential for anaerobic treatment is discussed based on laboratory and pilot studies and full-scale experiences. Finally, a comparison is made between anaerobic and aerobic treatments.

PULP- AND PAPER-INDUSTRY WASTEWATERS

Waste loads In general, the pulping and bleaching processes are

the main sources of wastewater loads in the pulp and paper mills, while minor loads are generated in the paper machines. Approximately 89% of the total world pulp capacity is produced from wood while the rest comes from non-wood materials such as bagasse, straw, bamboo, and grasses (FAO, 1988). Chemical, mechanical, and chemimechanical pulps were estimated to account for 68, 26, and 6%, respectively, of the total world wood pulp capacity in 1992 (FAO, 1988). Mechanical and chemimechanical pulping are in most cases integrated with paper manufacturing while chemical pulps are often produced in separate mills.

The quantity and quality of the wastewaters discharged from the actual mill depend on the manufacturing units integrated to the mill. The main wastewater sources in the pulp and paper manufacturing are shown in Fig. 1. The specific wastewater loads in typical production stages are summarized in Table 1. Table 2 shows a typical share of various streams of the total flow and organic load in mills built in the 1970s and 1980s.

The pulp- and paper-industry wastewaters contain compounds originating from the raw material of the pulp and the chemicals added during the manufacturing process. The main components of wood are cellulose (40-45%), hemicellulose (20-30%), lignin (20-30%), and extractives (2-5%) (Sj6str6m, 1981). The chemical compositions of the wastewaters are greatly affected by the pulping and bleaching chemicals used.

Large quantities of sludges are generated in the mechanical and activated-sludge treatment of pulp- and paper-mill wastewaters. PS production varies widely being equivalent to about 2% (w/w) of the quantity of pulp and paper products. These sludges are partially recycled to the process. Activated sludge accounts for 10-50% of total sludge from individual mills (Saunam~iki, 1988).

Effluent characteristics Debarking

The first stage in the manufacture of pulp is wood debarking, usually wet drum debarking. In this process, water is used to thaw the frozen logs, remove the bark, and wash the logs after debarking. The contribution of wet debarking water to a Finnish chemical pulp mill total effluent is about 5% as BOD 7 or COD, and 16% as SS (Virkola & Honkanen, 1985). The soluble COD, of approximately 1000 mg/litre of various debarking effluents, consists of tannins (30-55%), monomeric phenols (10-20%), simple carbohydrates (30-40%) and resin compounds (5%), as reviewed by Field et al. (1988). Dry debarking, introduced especially in chemical pulp mills has decreased the B O D 7 load to 10%

Anaerobic treatment in pulp- and paper-mill waste management 3

Tree Dobarking

I w~r DEBARKING ~ Wastewater V

Wood Y J MECHANICAL PULPING ~ Mechanical Pulping

Wastewater or ]

Kraft Black Liquor _ _ _ [ CHEMICAL PULPING ~ Spent Sulflte Liquor

Y Pulp

~v Bleaching [ BLEACHING ~ Wastewater [

V Bleached Pulp

t Paper Mill [_ PAPERMAKING ~ Wastewater

V Paper

7

I y

CHEMICAL RECOVERY V

Condensates

Fig. 1. The main wastewater sources in the pulp- and paper-industry (Sierra-Alvarez, 1990).

Table 1. Specific loads of pulp- and paper-industry wastewaters (Virkola & Honkanen, 1985; Mehner et al., 1988;

reviews by Puhakka, 1990; Sierra-Alvarez, 1990)

Process Water consumption COD SS (m3/adt pulp or paper)

(kg/adt pulp)

Wet debarking 5-25 5-20 nr Groundwood 10-15 15-32 nr

pulping TMP

unbleached 10-30 40-60 10-40 bleached 10-30 50-120 10-40

CTMP unbleached 10-15 70-120 20-50 bleached 10-15 100-180 20-50

NSSC 20-80 30-120 3-10 Kraft

unbleached 40-60 40-60 10-20 bleached 60-90 100-140 10-40

Ca-sulftte unbleached 80-100 nr 20-50 bleached 150-180 120-180 20-60

Mg sulfite unbleached 40-60 60-120 10-40

Papermaking 10- 50 nr nr

nr, Not reported in references; adt, air dry tonne.

from that of conventional wet debarking (Hynninen, P., pers. comm., 1990).

Kraft pulping In the kraft process, wood chips are cooked under

pressure with a mixture of hot caustic soda and sodium sulfide to promote cleavage of various ether bonds in the lignin. The inorganic chemicals are recovered from the spent black liquor and the pulp washing liquor in the recovery boiler where the organic residue is corn- busted to generate steam for the process. In the kraft

Table 2. The share (%) of the different pulp- and paper- industry effluents of the total flow and COD load of typical mills (Benjamin et al., 1984; Virkola & Honkanen, 1985;

Mehner etal . , 1988)

Effluent Chemical pulp mill Pulp and paper integrate

Flow BOD Flow BOD

Wet debarking 3.1 4.7 5'5 25 KEC nr 25-90 a nr nr SEC 15 30-50" nr nr Bleaching nr 50-60 nr nr TMP -- -- 20 50

aLower and upper ranges are for bleached and unbleached pulp, respectively. nr, Not reported in references.

process, approximately 55% of the total weight of the wood is dissolved in the pulping liquor which contains degradation products of its constituents; i.e. lignin, polysaccharides, and wood extractives (Rydholm, 1965).

The composite kraft pulp mill effluent consists of waste streams from debarking, cooking, washing and screening, evaporation, and bleaching (Fig. 2).

Gas relief and digester blows are sources of foul condensates in the cooking process. Methanol is the main organic compound in digester foul condensates (Blackwell et al., 1979). Ethanol plus minor amounts of other alcohols from an aerobic fermentation of wood during storage (Wilson & Hrutfiord, 1971), terpenes present in the wood, ketones, phenols and reduced sulfur compounds such as hydrogen sulfide and organic sulfides are typical digester-condensate constituents.

The pulp is washed to remove the spent black liquor from the fibers. In screening, partially cooked fibers,


HIGH DENSITY

TOWER

Fig. 2. Scheme of kraft pulping process.

shives, and other materials, are removed. The washing and screening waters are recycled as much as possible to reduce the discharge load. The BOD load from washing is 6-10 kg/tonne pulp (Virkola & Honkanen, 1985) which leaves the process through bleaching and partially open washing.

The evaporation of spent black liquor produces condensates containing methanol and reduced-sulfur compounds as the main components (as reviewed by Blackwell et al., 1979). Resin and fatty acids, terpenes and ethanol are present in lower concentrations. Most of the BOD is concentrated in a small volume fraction of evaporator condensates. Digester and evaporator condensates represent about 5% of the mill effluent volume and account for 5-10% and 10-20% of the total BOD discharge of a mill producing bleached and unbleached sulfate pulp, respectively (Hynninen, P., pers. comm., 1990).

Sulfite and semichemical pulping The active chemical in sulfite process cooking liquor

is calcium, magnesium, ammonium or sodium sulfite which in acidic or neutral conditions solubilizes the wood lignin as lignosulfonic acids. Semichemical pulping involves the cooking of chips with a neutral or slightly alkaline sodium sulfite solution and mechanical separation of fibers. After chemical recovery, the remaining spent sulfite liquor flows with the pulp and finally leaves the process with bleaching and pulp-dewatering effluents.

The wastewater sources for the sulfite process include spills from the digester area, digester relief and blow condensate, washing and screening, and from the recovery system as evaporator condensate.

Spent sulfite liquor mainly consists of lignosulfonates and carbohydrates. In acid spent liquor, carbohydrates appear as monosaccharides (Pfister & Sjrstrrm, 1977) and in bisulfite liquor as oligo or polysaccharides (Jurgensen & Patton, 1979).

Evaporator condensates may account for about 15% of sulfite pulp mill total effluent volume and 30-50% of total BOD load (Benjamin et al., 1984). Various evaporator condensates contain acetic acid (1.6-8.2 g/litre), methanol (0"2-1-2 g/litre), and furfural (0.2-1.0 g/litre) as the major organic components with smaller concentrations of formaldehyde, formic acid, acetaldehyde, and methylglyoxal (Ruus, 1964). The sulfur is mainly present as free SOz, loosely bound SOz (hydroxysulfonic acids), and sulfonates

including lignosulfonates and other organic sulfur (Rexfelt & Samuelson, 1970).

Chlorine bleaching In chemical pulping about 5-10% of the lignin

remains in the pulp which is subsequently depolymer- ized and removed in multistage bleaching (Rydholm, 1.965). The lignin is converted to alkaline-soluble compounds by treatment with oxidizing agents (chlorine, chlorine dioxide and hypochlorite), and then washed out with sodium hydroxide. Recently, the use of small amounts of oxygen and/or hydrogen peroxide in the alkali stages has been introduced for partial substitu- tion of other oxidizing agents. A commonly used multistage bleaching sequence for softwood is as fol- lows: chlorine/chlorine dioxide-sodium hydroxide-chlorine dioxide-sodium hydroxide-chlorine dioxide. For example, the COD and AOX load from kraft pulp bleaching varies from 39 to 80 kg COD per tonne and from 2-8 to 7 kg AOX per tonne, respectively, depending on the delignification method applied (Gullichsen, 1991 ).

Bleachery effluents mainly contain degradation products of lignin. Smaller amount of polysaccharide and wood-extractive degradation products are generated. Methanol and various hemicelluloses are dominant organic compounds (over 90%) in bleaching liquors. A vast variety of organochlorines are created as reviewed e.g. by Dellinger (1980), Voss et al. (1980), Kringstad and Lindstrrm (1984) and McLeay (1987). About 70 and 95% of the organically bound chlorine has been reported to be as high-relative-molar-mass material (MW> 1000) in spent chlorination and alkali extraction liquors, respectively, and the major part of this material consists of cross-linked aliphatic compounds (Kringstad & Lindstrrm, 1984). Jokela and Salkinoja- Salonen (1992) showed that the molecular sizes of pulp bleaching organic halogens were smaller than generally assumed. They concluded that over 85% of these compounds were of low molecular size (< 1000 g/mol). The low-molar-mass compounds containing organically bound chlorine consist of acidic, phenolic, and neutral compounds. Typical chlorophenolic bleaching waste constituents are presented in Fig. 3. About 10% of the low-molar-mass chlorinated compounds have been identified so far. The BOD load of a conventional bleachery accounts for 50-60% of the total BOD load of a pulp mill (Virkola & Honkanen, 1985).


OH OH OH OH OH

Cl Cl CI Cl ~ y "CI

Cl CI CI CI Cl

2,4-dichloro - 2,4,6-trichloro- 3,4,5-trichloro- 4,5,6-trichloro - tetrachloro- phenol phenol catechol gualacol gualacol

Fig. 3. Typical chlorophenolic compounds in bleaching effluents.

I I STEAMING AND (CHEMICAL ADDITION) Fig. 4. Scheme of mechanical pulping (TMP, CTMP) process.

Mechanical and chemithermomechanical pulping In mechanical pulping, the wood is converted into

fibers by physical or mechanical grinding, aided in some processes by heat or chemicals. Figure 4 presents the scheme of mechanical pulping processes.

The main dissolved compounds in groundwood pulping effluents are carbohydrates (80-90%), extrac- rives and acids (10-20%) (J~irvinen et al., 1980). TMP involves heating wood chips under pressure prior to mechanical refining. The organic compounds in TMP effluents consist of lignin (40%), carbohydrates (40%), and extractives (20%) (J~irvinen et al., 1980). In CTMP pulping, the wood chips are impregnated with sodium sulfite and steamed before refining. The CTMP effluents contain 1-5 g/litre BOD or 2"5-13 g/litre COD as reviewed by Cornacchio and Hall (1988). Polysaccha- rides (10-15%), organic acids (35-40%), and lignin (30-40%) are principal constituents of CTMP effluents (Pichon et al., 1987). Resin and fatty acid concentrations are high in CTMP effluents (Walden & Howard, 1981; Cornacchio & Hall, 1988). Sulfur is present mainly as sulfate and sulfite with minor amounts of lignosulfonates (Pichon et al., 1988). In dithionite bleaching of mechanical pulp the discharge load is low (2-8 kg BODv/tonne pulp), whereas effluent dissolved compounds from alkaline peroxide bleaching (5-20 kg/tonne pulp) consist of carbohydrates (60%) and acetic acid and formic acid and methanol (40%) (J~irvinen et al., 1981 ). Typical COD/BOD ratios for TMP and CTMP effluents are between 2.2 and 3 (Cornac- chio & Hall, 1988).

Sludges The principal organic constituents in pulp-mill PS

include cellulose, hemicellulose, lignin, and other components of wood fiber. In addition, primary sludges may contain process chemicals, compounds produced

during pulping, bark, and sand. The proportions of fiber and ash in the solid matter of pulp- and paper-mill primary sludge range from 40 to 95% and from 5 to 60%, respectively (Kyll6nen, 1986). Kraft-miU sludges have a low ash and resin content, whereas sulfite-mill sludges contain over 5% of resin (Fineman et al., 1978). For comparison, a typical municipal PS contains protein (20-30%), grease and fat (6-35%), cellulose (8-15%) and lignin (approximately 6%) as its main constituents (Process Design Manual for Sludge Treat- ment and Disposal, 1979).

Activated-sludge flocs are formed from a wide variety of bacteria, a few fungi, protozoa, some meta- zoa and extracellular polymers. The community is dominated by heterotrophic bacteria (e.g. Dias & Bhat, 1964; Tabor, 1976). Microorganism viability in the flocs is in the range of 5-20% (Weddle & Jenkins, 1971). The WAS solids contain the same chemical components as a typical cell: carbohydrates, protein, lipid, nucleic acid, and inorganic matter. Besides microbiota, activated-sludge flocs contain organic and inorganic particles from the wastewater.

Kyll6nen et al. (1988) reported four activated sludges from pulp-mill effluent treatment plants to consist mainly of protein (22-52%), lignin (20-58%), carbohydrate (0-23%), lipid (2-10%), and cellulose (2-8%). The lignin in pulp mill WAS is due to sorption, oxidative polymerization and condensation from wastewater (Ganczarczyk & Obiaga, 1974). Kraft lignin in wastewater contains a large number of hydroxyl groups which can facilitate chemisorption on biomass. AOX and chlorinated phenolic compounds have also been found in WAS from bleached pulp effluent treatment (Gergov et al., 1988). Bleaching of kraft pulps with chlorine chemicals results in formation of polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans which concentrate in wastewater


CI x Cl y

polychlorodibenzo-p-dioxi n

C l x ~ Cly

polychlorodibenzofuran

Fig. 5. Chemical structures of polychlorodibenzo-p-dioxin and polychlorodibenzofuran.

100

80 z _O

(:3

20

0 0

n

[ ] 12

[3 o

i i i [[]

1 2 3 4 5 6 7 kraft high MW lignin lignin

APPROXIMATE POLYMER LENGTH (NO. OF MONOMERIC UNITS)

Fig. 6. The role of polymer size on anaerobic degradabilit of fignin and its building blocs (from Field, 1989).

sludges in parts per trillion levels (Amendola et al., 1989; Clement et al., 1989). Examples of Chemical structures of these compounds are presented in Fig. 5.

ANAEROBIC BIOTRANSFORMATIONS AND TOXICITY OF PULP- AND PAPER-MILL WASTE CONSTITUENTS

The activity of a complex mixture of microorganisms involving degradation, transformation, and synthesis reactions of organic matter, and finally lead- ing to mineralization, is called anaerobic digestion. In the first stage of digestion, organic polymers are hydro- lyzed, then fermented into short-chain organic acids, neutral compounds, and gases. Methane production, sulfate reduction, and denitrification are processes responsible for terminal electron removal in anaerobic decomposition. The different biochemical possibilities and strategies for anaerobic degradation of organic matter have been reviewed e.g. by McInerney and Bry- ant (1981), Gujer and Zehnder (1983), Zinder (1984), Hungate (1985), Harper and Pohland (1986), Zehnder and Svensson (1986) and Zehnder and Stumm 1988).

Wood constituents Lignin

Of the wood constituents (also occurring In pulp- mill primary Sludge) high-molar-mass lignin is recalcitrant towards anaerobic degradation (Hackett et al., 1977; Zeikus et al., 1982; Odier & Monties, 1983). However, some studies indicate slow but detectable anaerobic biodegradation of lignin to methane and carbon dioxide in sediments (Benner et al., 1984) and thermophilic laboratory conditions (Benner & Hod- son, 1985). Anaerobic methanogenic mixed cultures can decompose monomeric (e.g. Kaiser & Hansel- mann, 1982; Grbic'-Galic', 1983), dimeric (e.g. Chen et al., 1985) and oligomeric (e.g. Colberg & Young, 1985) lignin substructure model compounds. Field (1989) reviewed the previous literature to show the rela- tionship between lignin polymer length and its anaerobic degradability (Fig. 6). This indicates that the share of polymeric and oligomeric lignin compounds in the pulp mill wastewater COD mainly determines its recalcitrance towards anaerobic degradation. Wastewater lignins are either non-toxic or may show some inhibi-

tion towards methanogenesis at high concentrations (3300-6000 mg/liter)(Sierra-Alvarez & Lettinga, 1991).

Cellulose In anaerobic systems, cellulose is easily degradable.

The rumen is an example of an efficient cellulolytic ecosystem for fatty acid production (Hungate, 1966). In anaerobic digestion of e.g. municipal sludge (Edberg & Hofsten, 1975) or agricultural wastes (Hobson, 1982), cellulose is converted into methane and carbon dioxide. The degradation of cellulose and hemicellulose (the main components of PS) decreases when the polysaccharides are complexed with lignin (Benner et al., 1984; Benner & Hodson, 1985).

Resin Resin acids, as natural microbe inhibitors in wood,

may also inhibit anaerobic treatment of wastes containing these compounds at high concentrations (Welander et al., 1988). The hydrolyzable tannin, gallotannic acid, is toxic to methanogens (Field et al., 1988). Sierra- Alvarez (1990) studied the role of natural wood constituents on the anaerobic treatability of forest-industry wastewaters. In her study, a variety of resin compounds including volatile terpenes, resin acids and apolar phenols were tested for methanogenic toxicity and were shown to cause 50% inhibition in concentrations rang- ing from 20 to 330 mg/litre (Sierra-Alvarez & Lettinga, 1990).

Compounds formed during pulping and wastewater treatment Low-molecular-weight organics

Low-molecular-weight organics such as volatile fatty acids, sugars and alcohols are produced in large quantities during pulping. They are anaerobically biodegradable and account for most of the methanogenic potential of pulping wastewaters.

Chlorinated phenolics Chlorinated phenolics, resin acids, and chloroform

from bleaching of pulp are of primary environmental concern due to their toxicity (Kozak et al., 1979; McLeay, 1987). Anaerobic degradation of chlorophe-


nols has been demonstrated in sewage sludge, bio- reactors, and mixed cultures (e.g. Salkinoja-Salonen et aL, 1984; Boyd & Shelton, 1984; Mikesell & Boyd, 1986; Krumme & Boyd, 1988). Anaerobic bacterial consortia first dechlorinate both mono and poly- chlorophenols and further mineralize them to methane and carbon dioxide (Suflita et al., 1982; Mikesell & Boyd, 1986; Tiedje et aL, 1987; Wu et al., 1993). Other reductive transformations of chlorinatedaro- matic bleach-waste constituents, such as chloroquiaia- cols and chlorosyringols include de-o-methylations and dehydroxylations (Puhakka et aL, 1993).

The microbiological details of reductive dechlorination are under intensive study (for review, see Mohn & Tiedje, 1992). Desulfomonile tiedjei is the first known anaerobic pure culture capable of catalyzing aryl reductive dehalogenations (Shelton & Tiedje, 1984; DeWeerd et al., 1990). With, D. tiedjei, chlorobenzoate serves as an electron acceptor for hydrogen consumption (DeWeerd et al., 1991). This represents a novel type of anaerobic respiration. This bacterium also dechlorinates pentachlorophenol to 2,4,6-trichlorophenol (Mohn & Kennedy, 1992). The results of, for example, Madsen and Aamand (1992) and Perkins et aL (1993) indicate that reductive dechlorination reactions are carried out rather by acidogenic bacteria than methanogens. Zhang and Wiegel (1990) showed that anaerobic conversion of 2,4-dichlorophenol to methane and carbon dioxide involved at least five different organisms.

Chloroaliphatics Recent reports show reductive dehalogenation of

chlorinated aliphatic compounds such as chloroform (trichloromethane) by many anaerobes, i.e. Acetobacte- rium woodii (Egli et al., 1988), Clostridium sp. (Gfilli & McCarty, 1989) and Methanosarcina (Mikesell & Boyd, 1990). The mechanism of reductive dehalogenation is often abiotic catalysis by transition metal coenzymes (B~2 , F4.~0) (Krone et al., 1989; Gantzer & Wackett, 1991 ).

Waste activated sludge Most of the building blocks of WAS (amino acids,

purines, pyrimidines, ribose sugars and glucose) are biodegradable under anaerobic conditions (Stuckey & McCarty, 1984). However, when these compounds are combined and organized in structures of bacterial cells their overall biodegradability decreases. Conventional municipal WAS digestion usually results in 20-50% COD and VS reduction (e.g. Garrison et aL, 1978; Haug et al., 1978; Process Design Manual for Sludge Treatment and Disposal, 1979; Parkin & Owen, 1986). The biodegradable fraction of WAS is comprised pri- marily of active bacterial cells in which the biodegradable fraction is below 70% (Gossett & Belser, 1982). WAS also contains refractory portions of decayed cells and influent organics. Hence, anaerobic degradation of WAS depends on the activated sludge process performance. For example, decreasing the solids retention

Solids

Cellulose WAS Constituents etc.

Hydrolysis and

Fermentation

I I~ VFA's

Oxidative + Fermentation ~

Acetate

Acetoclastic Methanogenesis

Pulp Mill Wastes ] \ [ Uquids

~'-~,~ A~o.o~, Phenols ~N"~ VFA's etc.

( 'x f ~

' I Fermentat on

Hydrogen Utilizing Hydrogen Utilizing Acetogenesis Methanogenesis

Fig. 7. The fate of pulp- and paper-mill-waste constituents in mesophilic anaerobic treatment.

time in an activated-sludge process resulted in increased sludge-solids production, but these solids were of increasing anaerobic digestibility (Gossett & Belser, 1982).

The main pathways in mesophilic anaerobic degradation of different pulp- and paper-mill waste components are summarized in Fig. 7.

Pulping chemicals Sulfur compounds

Inorganic sulfur can appear as sulfate, sulfite or dithionite in pulping wastewaters. Sodium sulfate (kraft process) and sulfur dioxide (sulfite process) are principal agents added in the chemical pulping. Dithionite is used to bleach mechanical pulp.

Sulfate in some pulp- and paper-industry wastewaters has caused problems in methanogenic anaerobic treatment (reviewed by Rinzema & Lettinga, 1988). Sulfate reduction occurs also in thermophilic anaerobic reactors (Rintala et al., 1991; Rintala & Lettinga, 1992). On the other hand, anaerobic processes have been used to recover sulfur from sulfate- rich pulp- and paper-industry wastewaters (S~irner, 1990). In anaerobic treatment the terminal oxidations are coupled to methane production and dissimilatory reduction of oxidized sulfur compounds present in these wastes. The competition for available substrates, such as hydrogen, acetate and methanol, by sulfate- reducing bacteria and methanogenic bacteria affects the carbon flow. In addition, sulfate reducers compete with acetogenic bacteria for compounds such as prop- ionate. Thermodynamics (Thauer et al., 1977; Gotts- chalk, 1986) and substrate-consumption kinetics (Widdel, 1988) indicate an advantage for sulfate reducers over their acetogenic and methanogenic compe- titors. Hydrogen sulfide production is generally


undesirable since it reduces the removal efficiency measured as COD and the methane yield (e.g. Eis et al., 1983; Frostell, 1984). In addition, H2S is toxic, corro- sive and contributes to the chemical oxygen demand of the effluent. Sulfide either stimulates or inhibits methanogenesis depending on its concentration and form in the environment. Undissociated hydrogen sulfide may penetrate cell membranes and thereby be toxic to methanogens. The distribution of sulfide species in the methanogenic pH range is presented in Fig. 8.

The sulfide transport in an anaerobic digester is affected by chemical dissociation and the distribution of hydrogen sulfide between gas and liquid phase. The transfer of H2S from liquid to gas depends on the difference in partial pressures of HeS between these phases and is further driven by biogas produced in the digestion process.

Sulfide inhibition depends on the methane-production and sulfate-reduction potentials of the wastewater COD constituents (Puhakka et al., 1989). The use of molybdate, a stereochemical analog of sulfate has been proposed for inhibition of sulfate reduction in anaerobic waste-treatment (Karhadkar et al., 1987; Hilton & Archer, 1988). However, molybdate also inhibits methanogenesis and, therefore, the use of molybdate will not be beneficial (Puhakka et al., 1990). In addition to hydrogen sulfide, some partially oxidized forms of sulfur such as sulfite and dithionite may inhibit anaerobic processes (Puhakka et al., 1985, 1989).

ANAEROBIC TREATMENT OF PULP- AND PAPER-INDUSTRY WASTEWATERS AND SLUDGES

Full-scale anaerobic reactors are used to treat numerous types of pulp- and paper-industry wastewaters including condensates from chemical and semichemical pulping and effluents from mechanical and secondary fiber pulping (Anon., 1987, 1990; Pearson, 1989; Lee et al., 1991 ). Most of the full-scale plants are UASB and contact processes. In UASB reactors the hydraulic retention times are generally less than 1 day while those in anaerobic contact processes vary from 1 to 3 days (Pearson, 1989). So far, there are apparently no full-scale reactors for treatment of chlorination stage effluents from sulfite- and kraft-pulp bleaching, for alkaline extraction stage effluent from kraft-pulp bleaching, and or for wet-debarking effluents. These wastewaters often contain compounds with limited anaerobic degradability and/or toxicity. Various measures have been studied and applied in order to enable their anaerobic treatment.

The following discusses the literature on the experiences obtained in laboratory-scale, pilot-scale as well as in full-scale anaerobic treatment of pulp- and paper- industry wastewater streams. Table 3 summarizes the COD concentrations and the main organics in the pulp- and paper-industry wastewaters, the anaerobically achieved COD removals as well as potential inhibitory compounds.

Hydrogen peroxide Toxicity by hydrogen peroxide (Welander &

Andersson, 1985) may interfere with the anaerobic treatment unless these toxicants are elimi, ated from the effluents. Decomposition of hydrogen peroxide into water and oxygen by catalase-containing aerobic biomass has been proposed as a pretreatment step to reduce toxicity of CTMP effluents (Welander, 1988).

100

80

d z 0 60 o "5 _D rr" co 4O ..J

20

0 5

H2s ,

6 7 8 9 10

pH

Fig. 8. Prevalence of sulfide forms at different pH values (Weast, 1974-1975). e, The pH range of anaerobic digestion; e, the optimum pH for anaerobic digestion are also

shown (Mah & Smith, 1981).

Debarking Wet-debarking effluents inhibited methanogenesis

(Latola, 1985; Rekunen et al., 1985; Rekunen, 1986; Field et al., 1988, 1991 ). This was attributed to tannins contributing 30-50% of the COD of the wastewater (Field et al., 1988, 1991 ). Dilution of the wastewater or polymerization of toxic tannins to HMW compounds by autoxidation at high pH as the only treatment (Field et al., 1991) or as combined with fungal treatment by Aspergillus niger (Field & Lettinga, 1991 ) were shown to enable anaerobic treatment of debarking effluents. About 40% COD removal was obtained with the UASB treatment of autoxidized debarking effluents at loading rates up to 40 kg COD/m 3 per day (Field et al., 1991). Hakulinen and Salkinoja-Salonen (1982) reported about 50% BOD 5 removal in anaerobic fluidized-bed treatment of debarking effluents at loading rates of 0"66 m3/m 3 per day.

Mechanical pulping TMP effluents are well suited for anaerobic treat-

ment because of their high anaerobic biodegradability and low methanogenic toxicity (Jurgensen et al., 1985; Sierra-Alvarez et al., 1990b, 1991 ): loading rates up to 12-31 kg COD/m 3 per day with about 60-70% COD- removal efficiency have been obtained in mesophilic anaerobic processes (Rintala & Vuoriranta, 1988; Schnell etal., 1990; Sierra-Alvarez etal., 1990b, 1991; Habets & de Vegt, 1991). About 60% COD removals

A n a e r o b i c t r e a t m e n t in p u l p - a n d p a p e r - m i l l was te m a n a g e m e n t

Table 3. The pulp- and paper-industry wastewater characteristics relevant to their anaerobic treatment

9

Wastewater COD Organic composition Anaerobic Potential (mg COD/liter) (% of COD) degradability inhibitors for

(% of COD) anaerobic treatment

R~. (seefootnote)

Wet debarking 1 300-4 100

TMP 1000-5 600

CTMP 2 500-13 000

NSSC spent liquor 40000 NSSCcondensate 7000 KEC 1 000-33 600

SEC 7 500-50 000

Chlorine bleaching 900-2000

Sulfite spent liquor 120 000-220 000

Tannins 30-55, 44-78 carbohydrates 30-40, monomeric phenols 10-20, resin acids 5 Carbohydrates 25-40, 60-87 lignin 16-49, extractives 20, acids < 10 Polysaccharides 10-15, lignin, 40-60 30-40, organic acids 35-40 nr nr Acetic acid 70 nr Methanol 60-90 83-92

Acetic acid 33-60, methanol 50-90 10-25, fatty acids < 10 Chlorinated lignin 30-50 polymers 65-75, methanol 1-27, carbohydrates 1-5, VFA 3 Lignosulfates 50-60, nr carbohydrates 15-25

Tannins, resin acids 1, 2

Resin acids 3-5

Resin acids, fatty 6-10 acids, sulfur, DTPA Tannins 11 Sulfur, ammonia 12 Sulfur, resin acids, 13-15 fatty acids, volatile terpenes Sulfur, organic 16-22 sulfur Chlorinated 23-25 phenols, resin acids

nr 25

References: 1, reviewed by Field et al. (1988); 2, Field et al. (1991); 3, Jurgensen et al. (1985); 4, Sierra-Alvarez et al. (1991 ); 5, Sierra-Alvarez et al. (1990b); 6, Welander (1988); 7, Pichon et al. (1987); 8, Pichon et al, (1988); 9, Eeckhaut et al. (1986); 10, Richardson et al. (1991); 11, Habets et al. (1985); 12, Perttula et al. (1991); 13, Yamaguchi et al. (1990); 14, Blackwell et al. (1979); 15, Norrman et al. (1983); 16, Eis et al. (1983); 17, Benjamin et al. (1982); 18, Saslawsky et al. (1988); 19, Frostell ( 1984); 20, Ney et al. ( 1991); 21, Kroiss et al. (1985); 22, Geller & G6ttsching (1985); 23, Rintala & Lepist6 (1992a); Sj6str6m ( 1981); 25, reviewed by Sierra-Alvarez (1990); nr, not reported in references.

were obtained in 55 and 70°C UASB reactors at the loading rates of 80 and 13 kg COD/ni 3 per day, respectively (Rintala & Lepist6, 1992b). Apparently other mechanical pulping effluents, such as groundwood pulping effluents, are also well suited to anaerobic treatment because of their higher carbohydrate content compared with TMP effluents (J~irvinen et al.,

1980). The hydrogen peroxide in the bleached pressur- ized groundwood-TMP-paper-integrate effluent was found to inhibit methanogenesis, but anaerobic treatment of this stream was possible after eliminating the hydrogen peroxide in aerobic pretreatment (Rekunen, 1986; Lehtom~iki, 1988).

CTMP The high lignin and wood extractive contents of the

CTMP effluents cause the generally observed lower COD removal in their anaerobic treatment than in treatment of TMP effluents. Maximum COD removals of about 60% have been achieved in the treatment of predetoxified and nontoxified CTMP effluents at loading rates up to 4 (Welander, 1988) and 20 kg COD/m 3 per day (Pichon et al., 1987), respectively. Maximum COD removals ranged only from 35 to 55% at the loading rates of 4-7-22 kg COD/m 3 per day (Eeckhaut et al., 1986; Pichon et al., 1988; Habets & de Vegt, 1991; Richardson et al., 1991 ).

The inhibition in the anaerobic treatment of CTMP has been associated with various compounds in these wastewaters: sulfate and sulfite (Eeckhaut et al., 1986; Pichon et al., 1988), hydrogen peroxide (Welander & Hansson, 1983; Welander & Andersson, 1985; Welan- der, 1988), resin acids and fatty acids (Welander & Andersson, 1985; Andersson et al., 1987; Cornacchio & Hall, 1988; Welander, 1988; MacLean et al., 1990; McCarthy et al., 1990; Habets & de Vegt, 1991; Richardson et al., 1991), and DTPA (Welander & Andersson, 1985). The solids present in the CTMP effluent were found to contribute to 80-90% of the acetoclastic inhibition (Richardson et al., 1991).

The apparent inhibition by resin acids was over- come by diluting the anaerobic reactor influent with water or aerobically treated CTMP effluent which con- tained less than 10% of the resin acids present in the untreated wastewater (MacLean et al., 1990; Habets & de Vegt, 1991). Similarly, the inhibition by sulfur compounds was avoided by diluting the CTMP effluent with water and by aerating the wastewater to oxidize sulfite to sulfate prior to anaerobic treatment (Eeck- haut et al., 1986). Furthermore, biocatalytic removal of hydrogen peroxide, followed by precipitation of long- chain fatty acids and resin acids with aluminum, calcium, and iron salts has been used to detoxify CTMP effluents before anaerobic treatment (Rosen & Gun-


narsson, 1986; Andersson et al., 1987; Welander, 1988, 1989).

Chemical pulping Black liquors

A chemical recovery by evaporation and incinera- tion of the black liquors is a common practice in most kraft mills while it is not the case in sulfite mills. In fact, there is no recovery method for calcium-based sulfite mills.

The methanogenic toxicity of the sulfite spent liquor was suggested to be due to phenolic compounds (Chave et al., 1988). About 20% of the 200-220 g COD/litre in spent liquor was anaerobically biodegradable. About 95% of the biodegradable COD was removed from spent liquor diluted to 60% with water (Chave et al., 1988).

Condensates KEC is readily anaerobically degradable and it

imparts only a slight methanogenic toxicity (Qiu et al., 1988). Toxicity, at least in some cases, could be caused by sulfur compounds (Norrman et al., 1983). In the anaerobic treatment of KEC containing 1.0-1-4 g COD/litre, the COD removals were 70-80% at loading rates of 13-15 kg COD/m 3 per day (Norrman, 1983; Qiu et al., 1988). The COD removals ranged from 83 to 92% at loading rates of 12-38 kg COD/m 3 per day in the thermophilic (53°C) anaerobic treatment of KEC containing 19-20 g COD/litre (Yamaguchi et al., 1990).

In the anaerobic treatment of SEC, COD removals up to 80 and 84-87% have been achieved at loading rates of 4-16 (Benjamin et al., 1982; Salkinoja-Salonen et al., 1985) and 100 kg COD/m 3 per day (Aivasidis, 1985; Aivasidis & Wandrey, 1988), respectively. Fur- thermore, 84% COD removals were obtained at loading rates of 48 kg COD/m 3 per day in the treatment of SEC by a defined anaerobic culture (Ney et al., 1991). The anaerobic treatment of SEC at 60°C has been shown to be feasible (Brune et al., 1982).

Combined treatment of SEC with caustic extraction wastewater from bleaching and the utilization of alkal- inity produced in anaerobic treatment by effluent recycle are methods suggested for reduction of high neutralization costs of SEC (Ferguson et al., 1984; Nitchals et al., 1985).

Anaerobic toxicity of SEC has been attributed to inorganic (sulfur dioxide) (Kroiss et al., 1985) or organic sulfur compounds (difurfuryl disulfide) (Eis et al., 1983; Benjamin et al., 1984). About 30-50% COD removal and over 50% total sulphur reduction were obtained in a pilot contact process treating SEC at loading rates of 5-0 kg COD/m 3 per day (Frostell, 1984). Removal of hydrogen sulfide from SEC-treating anaerobic reactor by scrubbing and recirculating the reactor biogas was reported to stabilize the process performance and enhance methane production (Salkin- oja-Salonen et al., 1985; S~irner, 1988). Sulfur recovery from sulfite mill effluents using the anaerobic process

was suggested by Frostell (1984) and has subsequently been shown to be feasible (Shrner, 1986, 1990; Shrner etal., 1988).

Chlorine bleaching The chlorination and alkaline extraction stage efflu-

ents from kraft and sulfite pulp bleaching are inhibitory to methanogenesis even at low concentrations (Nitch- als et al., 1985; Hall & Cornacchio, 1988; Qiu et al., 1988; Welander et al., 1988; Yu & Welander, 1988; Ferguson et al., 1990; Ferguson & Dalentoft, 1991; Raizer-Neto et al., 1991; Rintala et al., 1992). The toxicity has been attributed to the chlorinated organics, but also resin acids may be present at inhibitory concentrations (Sierra-Alvarez, 1990).

Methanogenic treatment of bleaching effluents has been proved feasible after dilution with water (Nitchals et al., 1985; Ferguson & Dalentoft, 1991; La Fond & Ferguson, 1991) or in combination with condensates (Qiu et al., 1988; Shrner, 1988; Welander et al., 1988). Adaptation periods of several months have been used to enable anaerobic treatment of kraft-bleaching effluents (Salkinoja-Salonen et al., 1985; Parker et al., 1991 ). Pretreatment by ultrafiltration has been studied in order to improve the anaerobic treatment efficiency (Boman etal., 1991).

The COD removals in the anaerobic treatment of bleaching effluents have ranged from 28 to 50% (Nitchals et al., 1985; Salkinoja-Salonen et al., 1985; Hall & Cornacchio, 1988; Qiu et al., 1988; Fitzsimons et al., 1990; La Fond & Ferguson, 1991; Raizer-Neto et al., 1991; Rintala & Lepistr, 1992a). Low removals have been claimed to be due to the high amount of HMW chlorinated lignins in bleaching effluents. This explanation is probably not valid since, as already discussed in this paper, Jokela and Salkinoja-Salonen (1992) recently showed that the amount of HMW chlorinated compounds is much smaller than generally reported. About 40-65% AOX or TOC1 removal has been obtained under methanogenic conditions, but the role of the methanogens in AOX removal has not been demonstrated (Yu & Welander, 1988; Fitzsimons et al., 1990; Ferguson & Dalentofl, 1991; La Fond & Fer- guson, 1991). Bleaching effluents are chemically unstable (Eriksson et al., 1985) and part of the AOX removal under anaerobic batch conditions has been found to result from chemical depolymerization (Fitzi- mons etal., 1990).

Anaerobic treatment has been shown to remove AOX and chlorophenols of bleaching effluents by up to 60-65% (Boman et al., 1991; Parker et al., 1991) and 70-80% (Salkinoja-Salonen et al., 1983; Boman et al., 1991), respectively. An addition of easily degradable cosubstrate stimulated AOX removal by approximately 5% (Parker et al., 1991 ). Combined anaerobic-aerobic treatment has been reported to remove chlorinated phenols from bleaching effluents (e. g. Salkinoja-Sal- onen et al., 1983, 1985; Hhggblom & Salkinoja-Sal- onen, 1991). In this system, the anaerobic unit removed 30 and 41% of the AOX and chlorinated


phemols, respectively, while only 16% of the COD was removed. The AOX, chlorinated phenols and COD removals for the combined process were 68, 77, and 61%, respectively (H~iggblom & Salkinoja-Salonen, 1991).

Semichemical pulping NSSC pulping is the most widely used semichemical

pulping process. Chemical recovery in semichemical pulping is not practised in all mills and thus, there is a need to treat the spent liquor. The anaerobic treatability of the NSSC spent liquor together with other pulp- and paper-mill wastewater streams has been demonstrated (e. g. Hall et al., 1986; Wilson et al., 1987). The methanogenic inhibition by NSSC spent liquor (Habets et al., 1985; Hall et al., 1986) was apparently the effect of the tannins present in these wastewaters (Habets et al., 1985). Formation of H2S in the anaerobic treatment of the NSSC spent liquor has been reported but not related to methanogenic toxicity (Velasco et al., 1985). Apparently, the evaporator condensates from the NSSC production are amenable to anaerobic treatment (Lehtom~iki, 1988) because of their high volatile fatty acid, mainly acetate, content (Perttula et al., 1991).

Paper and board manufacturing Effluents generated in paper and board making are

commonly dilute. Therefore, anaerobic treatment as such is not attractive. However, in several full-scale plants paper-machine effluents are treated together with more concentrated streams from the pulping. In these cases the paper making effluents, on the one hand, allow dilution to possible non-toxic levels but, on the other hand, increase the reactor volume requirements and may contain inhibitory paper additives (Jopson, 1986).

Primary and secondary sludges WAS is especially difficult to dewater and eliminate

with the current methods available. Today, these sludges are generally combined with PS, mechanically dewatered, and disposed of in landfills or incinerated.

Boman and Bergstrrm (1985) reported a 74% VS reduction for kraft pulp mill PS in anaerobic treatment. A two-phase anaerobic digestion of paper mill PS resulted in 55-65% VS reduction (Gijsen et al., 1988). Anaerobic degradability of pulp and paper mill WAS or sludge mixtures was 40-50% (Takeshita et al., 1981; Boman & Bergstrrm, 1985; Puhakka et al., 1988b) and may in some cases be improved by thermochemical pretreatment (Puhakka et al., 1988a). In a pilot study, the optimum and maximum volatile solids load- ings for anaerobic treatment of bleached kraft pulp mill WAS were 2-2 and 5"2 kg VS/m 3 per day (Puhakka et al., 1992a). Anaerobic digestion of this sludge resulted in deteriorated solid/liquid separation in a belt filter press (Puhakka et al., 1992b). The results of this study also indicated that digested sludge filtrate could be used to recycle nutrients for the treatment of phospho-

rus- and nitrogen-deficient pulp-mill wastewater. The existing treatment practices for pulp- and paper-industry wastewater sludges do not include anaerobic digestion. The demonstration of anaerobic digestion in reductive dehalogenation of chlorinated organics present in the sludges would disclose further advantages for the process.

ANAEROBIC-AEROBIC VERSUS AEROBIC TREATMENT

In pulp- and paper-industry wastewater treatment, an anaerobic process is usually followed by an aerobic post-treatment. The actual treatment systems depend on mill-specific conditions and especially on energy and sludge treatment and disposal costs. In many cases, only some of the waste streams of the pulp and paper integrate are treated in the anaerobic unit while the other streams are directly conducted into aerobic treatment, which often serves as post-treatment for the anaerobically treated streams (Fig. 9). The role of the aerobic post-treatment is to degrade compounds which are non-degradable anaerobically and to reduce the effluent concentrations of the other compounds. For example, resin acids are aerobically degradable (Leach et al., 1978; Junna et al., 1982) while only low degradation has been achieved in the anaerobic process (Sierra-Alvarez et al., 1990a). Aerobic post-treatment of some anaerobically treated pulp- and paper-industry wastewater streams eliminates acute toxicity not removed in the anaerobic treatment (Schnell et al., 1990).

The obvious advantages and disadvantages of the anaerobic over the aerobic processes for the treatment of the pulp- and paper-industry wastewaters are listed in Table 4.

The methane production in anaerobic processes ranges from 0"1 to 0.33 m 3 methane per kg COD removed (reviewed by Anon., 1990) with respective heat values of 1.0 and 3" 2 kWh. On the other hand, the removal of 1 kg COD in the activated-sludge treatment of pulp- and paper-industry wastewaters requires about 0"5 k w h aeration energy (Halttunen, S., pers. comm., 1992).

The reported figures for the sludge production in the full-scale anaerobic treatment of pulp and paper industry wastewaters vary greatly: 0"04 kg solids per kg BOD removed (Paasschens et al., 1991), 0" 1 kg sludge per kg COD removed (Habets & Knelissen, 1985a), and one tenth of that in the aerobic system (Maat, 1990). In the pilot-scale anaerobic-aerobic treatment of various pulp and paper industry wastewaters, the sludge production has been 0.07-0.12 kg VSS/kg CODrem, (Huss et al., 1986). In the activated sludge treatment of the pulp- and paper-industry wastewaters, the excess sludge production has been 0.2-0"6 and 0"8-1"2 kg solids/kg BOD7 removed in the normal (<0.65 kg BOD/kg MLVSS day) and in the high loaded (>0.65 kg BOD/kg MLVSS day) plants, respectively (Saunamiiki, 1988).


TMP/CTMP

PM 2 Gas holder Gas flame Lime kiln

Fig. 9. The anaerobic process as a part of a pulp- and paper-industry wastewater-treatment plant (Habets & de Vegt, 1991 ).

Table 4. The advantages and disadvantages of anaerobic over aerobic systems for the pulp- and paper-industry

effluent treatment

Advantages: -- no energy for aeration -- low excess sludge production - - low nutrient demand -- methane production - - compact installations -- sludge can be stored unfed without serious loss of

activity -- possibility for sulfur recovery -- reductive dechlorination

Disadvantages: -- slow first start-up -- inhibited by several compounds -- high threshold concentrations

In general, the pulp- and paper-industry effluents are nutrient deficient. The addition of phosphorus and nitrogen is a common practice in activated-sludge treatment. A commonly used COD:N:P ratio in the activated-sludge plants is 100:5:1 while in the anaerobic plants a ratio of 350:5:1 has been used (Maat, 1990).

With chlorine-bleached effluents, anaerobic treatment could be advantageous. Reductive dechlorination has been shown to remove efficiently polychlorinated compounds (Woods et al., 1989). Furthermore, sulfate can be converted anaerobically to sulfide which can then be recovered as elemental sulfur by using either a chemical (S~irner, 1990) or a biological (Buisman et al., 1988, 1991 ) method.

The susceptibility of methanogens and acetogens to several inhibitors has been thought to limit the applic-

Table 5. The relative operation costs (%) of anaerobic treatment and activated-sludge plants (calculated from Rekunen

et al., 1985)

Anaerobic Activated sludge

Neutralization 39"6 39"5 Nutrients 7"8 81.3 Polyelectrolyte -- 49"6 Electricity 18"6 103.9 Operation personnel 7.7 15"5 Maintenance 26.3 29.4

Total 100.0 319"2 Biogas - 71.3 --

Total, biogas included 28.7 319.2

ability of anaerobic systems for the pulp- and paper- industry wastewaters (reviewed by Lettinga et al., 1991). Both long-term and short-term toxicity problems have been associated with some failures in full- scale anaerobic-plant performances (Saslawsky et al., 1988; Paasschens et al., 1991). Furthermore, technical problems such as clogging of the reactor feed inlets or the biomass support material have occurred during the introduction of the anaerobic system (Saslawsky et al., 1988; MacLean et al., 1990; Paasschens et al., 1991). However, it must be noted that the aerobic treatment systems used in the pulp and paper industries have also had operational problems, and, for example, large equalization basins have been built to insure constant wastewater characteristics.

Both the investment and operation costs of treatment systems as well as their technical suitability greatly depend on each case of application. In general, anaerobic-aerobic systems have been reported to have


lower operations costs than aerobic treatment because of the reduced aeration-energy requirements and excess sludge production, and because of the methane recovery (Rekunen et al., 1985; Minami et al., 1991; Garvie, 1992). Table 5 gives an example of an annual cost estimate of anaerobic and aerobic treatment of pulp- and paper-industry wastewaters in a mill with an already existing aerated lagoon and proposed extended treatment capacity to meet the stringent discharge limits (Rekunen et al., 1985). When the biogas was uti- lized, the operation costs of the anaerobic systems amounted to about one tenth of those of an activated- sludge plant.

ACKNOWLEDGEMENTS

This work was financially supported by the Nordic Ministerial Council (J.A.R.) and The Academy of Fin- land (J.A.R and J.A.P).

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