Abstract
The manufacture of pulp and paper is a resource-intensive industry that requires a large quantity of water, energy, and lignocellulosic materials of plant origin. Presently, the high cost of energy inputs, and realization of increased environmental concerns are the driving forces behind the need for the pulp and paper industry to seek more cost-effective and environmental-friendly alternatives for pollution abatement. This industry currently constitutes one of the largest contributors to air and water pollution.
1 Introduction
The manufacture of pulp and paper is a resource-intensive industry that requires a large quantity of water, energy, and lignocellulosic materials of plant origin. Presently, the high cost of energy inputs, and realization of increased environmental concerns are the driving forces behind the need for the pulp and paper industry to seek more cost-effective and environmental-friendly alternatives for pollution abatement. This industry currently constitutes one of the largest contributors to air and water pollution.
The three main component groups of lignocellulosic plant materials are cellulose, hemicelluloses, and lignin. Pulp and paper mill effluent is highly colored and imparts a dark brown/black appearance to the receiving waterbody. The color of these wastewaters primarily results from the presence of lignin and its derivatives that are released from the substrate, and discharged in such effluents. The color mainly derives from various processing steps undertaken during paper manufacture (Prouty 1990; Esposito et al. 1991; Bergbauer and Eggert 1992). The major source of phenolic compounds in pulp and paper mill wastewater is lignin (Amat et al. 2005), whereas pentachlorophenol is generated in the effluent as a by-product from chlorine bleaching of pulp (Steinle et al. 1998; Vallecillo et al. 1999). The chlorinated phenols, chlorinated dibenzo-p-dioxin and dibenzofuran, chlorinated hydrocarbon, etc., that are released from lignin during chlorine bleaching of pulp are largely responsible for the toxic compounds released in pulp mill effluents. In addition, extractives are also present in such effluents. Although the concentration of extractives is lower than that of the phenolic compounds, some are relatively more toxic, and significantly reduce water quality and degrade the aquatic habitat of lakes and rivers receiving paper mill effluents.
In general, colored effluents are not only aesthetically unacceptable, but also reduce the transmission of light in contaminated waterways, thereby reducing aquatic plant photosynthesis. This, in turn, reduces the dissolved oxygen content and ultimately causes the death and putrefaction of aquatic fauna (Sahoo and Gupta 2005).
The decolorization and detoxification of paper mill effluents has been the subject of several studies performed in recent years (Raj et al. 2007). Several physical and chemical methods (e.g., precipitation, sorption, ozonation, ultrafiltration, reverse osmosis, and electrochemical treatment) have been attempted to decolorize pulp and paper mill effluents. Other physicochemical treatment methods, viz., incineration of black liquor lignin (Harila and Kivilinna 1999), titanium oxide oxidation systems (Chang et al. 2004; Yeber et al. 2007), Fenton and photo-Fenton reactions (Perez et al. 2002; Kazmi and Thul 2007; Zahrim et al. 2007), and chemical coagulation of lignin (Ganjidoust et al. 1997) have also been reported to be effective for reducing color in, and toxicity of paper mill wastewaters.
Other treatment procedures have also been evaluated. Garg et al. (2004) studied the removal of nonbiodegradable and toxic compounds in pulp and paper mill effluent by wet air oxidation (WAO), using a heterogeneous catalyst (CuO–ZnO) supported on alumina and ceria. The authors recorded a maximum chemical oxygen demand (COD) reduction of 83% for this (CuO–ZnO)/CeO2 catalyst; the concentration of the pulp–paper mill wastewater was 5 kg/m3 (2-h retention time), and the initial pH was 3.0. Recently, Mishra et al. (2009) compared the efficacy of activated charcoal and heat-treated coal for decolorizing pulp and paper mill effluent.
However, these latter methods have not been implemented at an industrial scale, mainly because they are energy intensive, and too expensive per unit volume of the effluent. Therefore, alternative biotreatment processes are being considered (Boman et al. 1988). Physical and chemical processes are capable of removing only coloring agents, toxicants, suspended solids, COD, and high molecular weight lignins. But biological oxygen demand (BOD) is not reduced and low molecular weight compounds are not efficiently removed by such physical and chemical treatments (Singh and Singh 2004). The biological color-removal processes are particularly efficient, attractive, and cost and energy effective, because they reduce color, COD, BOD, and toxic low molecular weight chlorinated lignin derivatives (Barton et al. 1996; Nagarathnamma et al. 1999).
At present, conventional biological treatment methods employed in the industry include use of aerated lagoons and activated sludge processes. These biological treatment methods are employed to ensure that effluents meet the BOD discharge limits, and pass fish toxicity tests. Such treatment processes usually remove 85–90% of the readily biodegradable fraction of the BOD from these effluents. However, such systems are generally less effective in removing color, COD and chlorinated phenolic compounds (Saunamaki 1989; Raj et al. 2007). Therefore, more advanced alternative biological wastewater treatment strategies will be required to meet the new and more stringent discharge limits set for absorbable organic halogens (AOX). Such new biological treatment technologies must be designed to degrade halogenated chemicals in effluents that pose the greatest threat to human health. Dealing effectively with the AOX chemicals is important, because they are toxic and/or mutagenic, can bioaccumulate in primary and secondary consumers of the food chain, and are difficult to degrade.
Microorganisms, such as fungi, bacteria, and algae, are suitable biological candidates for treating wastewaters. The role of white-rot (Basidiomycetes) and other fungi in lignin/phenolics-laden wastewater treatment is gaining momentum, because of their potential for degrading lignin and its derivatives (Eriksson et al. 1980; Eaton et al. 1981; Driessel and Christov 2001; Christian et al. 2005; Saritha et al. 2010). Biodegradation of lignin is of ecological significance and also has wide industrial application (Bhoominathan and Reddy 1992). Since fungi cannot utilize lignin either as a carbon or energy source, basic nutrients (i.e., carbon and nitrogen) are added to the medium to stimulate fungal growth and to advance the breakdown of lignin in the effluent (Kirk et al. 1976; Keyser et al. 1978; Archibald et al. 1990; Wang et al. 2003; Sahoo and Gupta 2005; Sukumar et al. 2006; Jaganathan et al. 2009). However, an increased carbon supply stimulates, whereas increased nitrogen inhibits, lignin degradation. Therefore, the C:N ratio is considered to be a better predictor of lignin degradation than are the absolute levels of carbohydrates and nitrogen (Reid 1979). However, fungi are able to remove chromophoric constituents and lignin compounds during aerobic treatment but are not efficient in degrading chloroorganic compounds. Vora et al. (1988) have reported that many bacteria, viz., Pseudomonas, Flavobacterium, Xanthomonas, Nocardia, Aeromonas, and Arthrobacter are able to utilize several lignocellulosic compounds of the bleached plant effluent, including organochlorine constituents.
Anaerobic biological treatment of effluents also plays a pivotal role in cleaning wastewaters. Anaerobic treatment efficiently destroys chlorophenolic compounds, and reduces mutagenicity and toxicity caused by the effluents (Hickey et al. 1995). Therefore, the development of advanced biological treatment processes, particularly hybrid or dual systems may offer advantages, wherein both anaerobic and aerobic digestion approaches are used.
In the near future, the paper industry will face more severe legal restrictions, having mainly to do with the toxicity and level of environmental pollution caused by their effluents. New regulations are expected that will improve the environmental quality, and will reduce human health associated with the pulp and paper industry. The present laws around the world have already forced paper manufacturing companies to implement relevant manufacturing improvements and end-of-pipe biological treatment processes.
It is the aim of this paper to present an overview of the advanced technologies that are used by the pulp and paper mill industries, in some developed countries, and are suggested for use by other nations, to decolorize and remediate the toxic pollutants that exist in mill effluents. In addition, we endeavor to summarize and consolidate the scattered literature on removing harmful constituents and color from paper mill effluents.
2 The Structure of Woody Materials and Other Lignocellulosics
Insights into the primary substrate from which pulp and paper are made will help to understand the sources of the resulting manufacturing pollutants. Paper is made from waste paper, agricultural residues, and wood. The respective amounts of paper made from each of these is 29, 28, and 43%. In earlier times, small paper mills were dependent upon linen and cotton rags and recycled rope, jute cloth, flax wastes, hemp, ramie, kenaf, straw, bagasse, etc., for their raw feedstocks in paper making. All such substrates either had the advantage of being already processed cellulosic materials, or were raw substrates from which cellulosic fibers could be easily extracted (Van Roekel 1994). The mills of yesteryear were not equipped with the machines, or economically viable alternative technology, for grinding wood, and therefore wood was not preferred as the substrate for paper manufacturing. The use of wood as a cellulosic feedstock for paper making was therefore a technological breakthrough. This occurred in 1850 by Don Valley Paper Mill, Toronto, Canada, which almost completely transformed the paper making industry. However, the use of wood-based paper manufacture has gradually declined, particularly in India, from the constraints of cost and raw material availability. The share of waste paper and agro-residue-based technology will increase as future feedstocks in paper manufacturing, because they require less energy input.
Although the proportion of the three main components of wood, cellulose, hemicellulose, and lignin is about 50, 25, and 25%, respectively, the content of these constituents varies among wood species. Cellulose is a very long molecule and is composed of repeating d-glucose units joined through a linear linkage of β, 1-4 glycosidic units. The extent of polymerization varies from species to species. This long natural polymer is variously folded, bundled, and stabilized by hydrogen bonding (Ranby 1969). The degree of order created within the bundle determines whether it is in a crystalline or amorphous cellulosic form. The cellulose units are assembled to first form microfibrils, then fibrils, and finally through hydrogen bonding between linear molecules produces a strong microcrystalline structure. This structure for cellulose is amenable to biodegradation by a variety of microorganisms. However, in their natural state, cellulose fibrils are associated with lignin, hemicellulose, and other materials in a complex and heterogeneous structure. Therefore, many bacteria and mold species are generally discouraged by the presence of the hemicellulose and lignin, which effectively provide a protective covering to cellulosic fibers.
Hemicellulose is composed of five-carbon sugars, which are randomly arranged to form long branched molecules. These hemicellulosic molecules cover the cellulosic fibers and also enter the pores of cellulose. This layer acts as an anchoring agent between cellulose and lignin, because hemicellulose forms chemical bonds with the adjoining layer of lignin. Lignin is both the most abundant aromatic (phenolic) polymer and the second most abundant raw material (Li et al. 2009). The lignin layer is a complex phenylpropanoid polymer responsible for imparting strength to cellulose–hemicellulosic structures and is also described as a heterogeneous polymer comprising substituted cinnamyl alcohols.
A large portion of the lignin in woody plants is always associated with cellulose and hemicellulose, not only in intimate physical admixture, but through ether, ester, acetyl, ketal, and hydrogen bonds. Lignin forms a matrix that surrounds cellulose, the most abundant natural polymer. Lignin is basically composed of three randomly polymerized phenol-based building blocks, which is very difficult to degrade because of its random arrangement. The biodegradative recalcitrance of lignin is believed to result from its high molecular weight and three dimensional structure (Zeikus et al. 1982). The presence of this intractable polymer creates an obstacle to the efficient use of cellulose for a wide range of industrial applications. However, lignin may be slowly degraded to release toxic phenolic compounds. The aromatic content of lignin, expressed as monomeric phenol, is ∼51%. It is the phenolic compounds released from lignin during the chlorine bleaching of pulp that are responsible for a large percentage of the toxic compounds released into pulp mill effluents. Only a limited number of microorganisms derive benefit from the biological breakdown of lignin.
In addition to cellulose, hemicellulose, and lignin, which constitute the major components, wood contains approximately 1.5–5.0% of compounds that are extractable with organic solvents. Some of these extractable compounds include resin acids, fats, waxes, terpenoids, tannins, flavanoids, stilbenes, and troppolines.
3 Hazardous Colored and Toxic Compounds Released During Pulping and Bleaching
The sole purpose of pulping is to separate cellulose fibers from other hemicellulosic and lignin components that comprises the lignocellulosic substrate. Certain pulping procedures, such as mechanical stone-grinding and chemical sulphite pulping, were employed until the Kraft chemical pulping process was introduced. Kraft pulping involves cooking woodchips in a high concentration of sodium hydroxide and sodium sulfide for 2–4 h at 170°C (Gellerstedt 2001). The advantage of Kraft pulping is to produce a high quality cellulose fiber. In addition, fatty and resin acids, turpentine, bioenergy, etc., collectively called tall oil, can be recovered from the spent-liquors. The flow chart for a Kraft pulping process is illustrated in Fig. 1.
Approximately 55% of the original wood is dissolved in the Kraft pulping process and is released as “black liquor.” The black liquor contains 90–95% lignin, nearly all hemicellulose, wood extractives, and a small quantity of soluble cellulosic oligo- and polysaccharides, derived from the original wood. In most developed countries that have stringent emission standards, by-products are recovered from the black liquor, remaining amounts of the black liquor is evapoconcentrated, and the solids are burned for energy, although some inorganic chemicals are sometimes also recovered from the solids (Fig. 1).
The pulping process is terminated when about 5–10% lignin remains in the pulp. Further delignification at this stage is avoided because it damages fiber quality (Kringstad and Lindstorm 1984). The process is often performed differently in some underdeveloped and developing countries. In these cases, the black liquor is indiscriminately mixed with bleachery effluent (pooled wastewater released from various bleaching steps) and is discharged into nearby streams or other waterbodies. Such release of untreated material threatens aquatic organisms and depending on how local water supplies for human use are handled, may threaten human health as well. Some lignin-degradates that are released in the black liquor during the pulping stage are toxic, and more specifically may be mutagenic; moreover, these compounds may be bioaccumulated in the tissues of animals, particularly fish, and in humans (Bajpai et al. 2000). Currently, many mills do not opt to utilize recovery plants, and thereby release a load of lignin waste directly to local streams or water bodies. The rivers or streams that receive pulp and paper mill wastes usually acquire a blackish or coffee color, and display a considerable amount of surface foam (Bharati et al. 1992).
In the bleaching process, a slurry (3%) of pulp is made and is usually serially treated with chlorine, alkali, hypochlorite, chlorine dioxide, alkali, and finally, with chlorine dioxide (Fig. 2). Each treatment process is followed by filtration, and ultimately all filtrates are combined and discharged as bleachery effluent (Kringstad and Lindstorm 1984). The chlorine present during the pulp bleaching process reacts with organic wastes (wood extractives, polysaccharides and remaining amounts of lignin), to form several organochlorine compounds. Such compounds are toxic and include simple monoaromatic lignin derivatives (e.g., chlorinated phenol, quaiacols, catechols, and chlorolignins). The effect of chlorolignins in receiving water systems is not well understood. Therefore, chlorolignins are considered not to be acutely toxic, because they are of large molecular size and do not penetrate cytoplasmic membranes. However, it is possible that they are degraded to release smaller compounds that are toxic. There is a real danger to human health from the presence in drinking water of chlorinated organic compounds that come from pulp–paper mill effluents. Such compounds not only pollute drinking water but are bioaccumulated by and contaminate fish, which, if consumed, could be another source of human exposure (Neilson et al. 1983; Eriksson et al. 1985).
4 Pulp and Paper Mill Effluent
Pulp and paper mills are one of the five major contributors to environmental pollution by industrial wastewaters. Approximately 160 million t of wood pulp was produced worldwide in 2006 (Metso Corp. 2006); production was expected to increase to ∼260 million t for the current year 2010. For every t of paper produced, the mills are estimated to generate ∼220–380 m3 of highly colored and potentially toxic wastewater (Eriksson and Kohler 1985; Chuphal et al. 2005). In the year 1951, there were only 17 paper mills in India, and they had a combined production capacity of ∼0.13 million t paper per annum. The number of paper mills has increased to 406 in 2002, and their annual combined capacities now equal ∼1.9 million t of paper (Singh and Thakur 2006). The world’s population now uses a minimum of 300 million t of paper annually, and this is likely to increase in the coming years.
In the pulping process, the debarked wood chips are treated at 160–180°C in a “white liquor” solution of sodium sulfide (Na2S) and sodium hydroxide (NaOH), which dissolves 90–95% of lignin, nearly all hemicellulose and wood extractives and a small amount of cellulose-derived polysaccharides (Garg et al. 2004). Approximately 55% of the original wood is dissolved in the form of “black liquor.” The pulp is then mixed with water and is filtered. The resultant unbleached pulp is made into a slurry, which is then treated with chlorine at the charging rate of 60–70 kg/t, and at a pH of 1.5–2.0. After filtration, the bleached pulp is treated with alkali @ 35–40 kg/t, at 55–70°C, followed by series of bleaching treatments with hypochlorite, chlorine dioxide, alkali, and chlorine dioxide. After each treatment step, the pulp is filtered and the resultant liquids are pooled as bleachery effluent. During the bleaching process, ∼50 kg of lignin, ∼19 kg of polysaccharides, and ∼1 kg of extractives are dissolved, and thereby released, from 1 t of the softwood pulp. Chlorine reacts with these organic wastes to produce structurally diverse organochlorine compounds. Chlorinated lignin is formed during the bleaching of pulp with chlorine chemicals. Most of the remaining lignin in pulp is dissolved during the first two chlorine and alkali extraction steps. Reactions at these stages lead to the formation of chlorolignin (as mentioned above) that has a high content of carboxyl, hydroxyl, and conjugated carbonyl groups, and a low content of aromatic moieties (Lindstorm and Osterberg 1984; Osterberg and Lindstrom 1985). Such effluents impose coloration and toxicity problems in the receiving waters that can cause serious environmental hazards. This problem of toxicity and color, and its removal from pulp–paper mill effluents, has been a subject of much study during the last few decades. Although the pollution load derived from other factors can be satisfactorily reduced, color removal is more difficult and has been a matter of great concern.
Most of the chlorinated organics that are randomly synthesized during pulp bleaching are toxic xenobiotics and tend to persist. The reason is that many native microorganisms have not evolved the enzymatic machinery to rapidly degrade these chlorinated organics. If they did, or do develop such capacity, the conditions favoring degradation of lignin and its chlorinated derivatives would also be suitable to decolorize the lignin-based effluents.
Several color-removal strategies have been developed which rely on either physicochemical or biological techniques (Fig. 3). The former includes ultrafiltration, lime coagulation, rapid land filtration, adsorption on activated carbon and polymeric adsorbants (Springer 1985). However, such methods have operational difficulties and are prohibitively expensive for application on an industrial scale (Royer et al. 1991). Further, the conventional biological methods employed in paper industries, such as use of aerated lagoons, and activated sludge process are usually able to remove 85–90% of the readily biodegradable fraction that contributes to BOD. However, these biological systems are generally less efficient and scarcely effective in removing color, COD, and chlorinated phenolic compounds (Saunamaki 1989). This may be attributed to the low content of nutrients (nitrogen and phosphorus) in these wastewaters, and to the toxicity caused by the phenolic compounds present. It is, therefore, necessary to develop new, more advanced treatment technologies that degrade such chemicals.
The colored and potentially toxic chemicals are also known to possess mutagenic and carcinogenic characteristics, have a tendency to bioaccumulate, and are difficult to degrade. Therefore, the focus of future treatment processes should also effectively address the degradation of the mutagenic/carcinogenic compounds, resin acids and chlorinated phenols, guaiacols, catechols, and chlorinated aliphatic hydrocarbons.
Saravanan and Sreekrishnan (2005) suggested that the problem of sludge disposal, cost, and toxicity can be reduced if a biological treatment step is combined with appropriate physicochemical treatment. The authors reported that, when pulp and paper mill effluent was treated with Trichoderma sp. in batch studies, 72% color reduction was achieved within 24 h. When the effluent was treated in continuous mode by this fungus in a fluidized bed reactor, ∼27% color reduction was achieved. Color reduction further increased to 81%, when the biological and physicochemical treatments (using poly-electrolyte (potash alum)) were combined.
5 Microbial Decolorization of Effluent
Attempts have been made to decolorize or degrade toxicants in pulp and paper mill effluents with variety of microorganisms, including bacteria, fungi and algae (Table 1). Wood degrading white-rot fungi have been found to be effective in treating effluent. Their effectiveness derives from their ability to degrade lignin and its chlorinated compounds, which are mainly responsible for the color and toxicity of pulp and paper mill effluent (Prouty 1990; Bergbauer et al. 1991; Sayadi and Ellouz 1992; Bajpai et al. 1993; Martin and Manzanares 1994; Prasongsuk et al. 2009; Saritha et al. 2010). Garg and Modi (1999) and Tripathi et al. (2007) have reviewed various aspects of papermill wastewaters, including decolorization by white-rot and other fungi. Fukuzumi and coworkers (1977) were first to use white-rot fungi for wastewater treatment. They inoculated effluent with a variety of fungi, and discovered that Tinctoporia sp. was the best fungus for wastewater treatment. Eaton et al. (1982) performed similar experiments using Phlebia brevispora, P. subserialis, Poria cinerascens, and Tremetes versicolor. All of these were capable of efficiently decolorizing the effluent. Livernoche et al. (1983) isolated and screened 15 strains of white-rot fungi and reported that several of them, viz., T. versicolor, Phanerochaete chrysosporium, Pleurotus ostreatus, Polyporus versicolor, and one unidentified strain were capable of decolorizing pulp–paper mill effluent.
White-rot fungi, such as P. chrysosporium (Pellinen et al. 1988; Mittar et al. 1992; Celal and Filiz 1994; Sukumar et al. 2006), Trametes versicolor (Manzanares et al. 1995; Pallerla and Chambers 1995; Modi et al. 1998; Garg et al. 1999; Srinivasan and Murthy 1999), P. radiata (Lankinen et al. 1991; Hatakka et al. 1992), Marulius tremellosus (Lankinen et al. 1991), were found to have the ability to decolorize and detoxify effluent from the bleaching stage of pulp and paper mills. Moreover, the white-rot fungus P. chrysosporium was shown to degrade a wide variety of xenobiotic compounds, in addition to its natural substrate, lignin. Recalcitrant xenobiotics that P. chrysosporium can degrade include aromatic compounds, polycyclic aromatics, chlorinated aromatics, polycyclic chlorinated aromatics and nonaromatic chlorinated compounds, as well as some naturally occurring biopolymers (Bumpus and Aust 1987; Mileski et al. 1988; Bumpus 1989; Aust 1990; Fernando et al. 1990; Yadav and Reddy 1993). The ability of white-rot fungi to detoxify and decolorize bleach plant effluents is believed to derive from their production of ligninolytic enzymes. These exoenzymes exhibit substrate specificity for solubilized lignin and chlorinated lignin derivatives in the effluent (Nagarathnamma and Bajpai 1999). The greater emphasis for use in biological treatment processes has been laid on the use of P. chrysosporium and T. versicolor (Chang et al. 1987; Archibald et al. 1990, Eriksson 1990; Terron et al. 1991, 1992).
Sahoo and Gupta (2005) isolated several ligninolytic microorganisms from the environment of small pulp and paper mills and evaluated their ability to decolorize effluents of an agriresidue-based effluent that were dark brown in color. The promising Aspergillus fumigatus and A. flavus isolates were more efficient in color removal than was the lignin-degrading T. versicolor. Taseli (2007) studied the capability of Penicillium camemberti to dechlorinate and decolorize wheat straw-based pulping and bleaching effluents. They reported that the rates of highest color removal for AOX were 65% and, under no-shaking conditions 84%; these values compare to 60% or 79% color removal for AOX in shake-flask experiments. The fungus was effective in treating softwood pulping and bleaching effluents (Taseli and Gokcay 2006; Taseli 2007) and chlorinated model compounds like pentachlorophenol and 2-chlorophenol (Taseli and Gokcay 2005). Similarly, Apiwattanapiwat et al. (2005) screened 64 fungal strains for their ability to decolorize pulp and paper mill effluent. Only three strains, identified as Trichoderma sp., Datronia sp., and Tremetes sp., decreased the effluent color (by 54.4, 54.9, and 53.7%, respectively).
Malaviya and Rathore (2007) used a novel consortium of white-rot and soft-rot fungi for bioremediation of pollutants from pulp–paper mill effluent, and reported reduction in color, lignin, and COD by 78.6, 79.0, and 89.4%, respectively, within 4 days. Jaganathan et al. (2009) used ligninolytic fungus P. chrysosporium for aerobic pollution abatement of pulp–paper mill effluent. The average removal of color, COD, and BOD was 86.4, 78.8, and 70.5%, respectively, after 3 days in the shake-flask batch experiments. The entire fungal mycelial mass was fragmented from the shaking action. The fragmentation of mycelial mass, which increased the contact area, was probably responsible for better performance in batch experiments. Prasongsuk et al. (2009) isolated three thermotolerant ligninolytic enzyme-producing fungi that were capable of paper mill effluent decolorization. Daedaleopsis sp., a new fungal isolate, exhibited the highest effluent decolorization efficiencies of 52 and 86%, respectively, in wastewaters from pulping, and from combined pulping and paper recycling processes. The thermotolerant fungi are particularly suitable for decolorizing wastewater in tropical environments.
The main constraint in using a fungal degrading system is the requirement to maintain growth and/or enzyme (ligninases) activity at the prevailing low pH (4–5). However, at low pH the solubility of high molecular weight fragments that are derived from lignin is reduced. Furthermore, the natural pH of pulp and paper mill effluents generally remains alkaline (in a range of pH 8–9). Therefore, any requirement to reduce the pH to the acidic range of 4–5 prior to fungal augmentation would be uneconomical (Raj et al. 2007). Further, Garg et al. (1999) have reported that a pH adjustment of paper mill effluent to lower values, and/or sterilization of effluent, are unnecessary, because such treatments resulted in sedimentation of the chromophoric compounds, rendering the effluent light colored. Readjustment of effluent pH to normal (pH 8–9) redissolved the sediments and restored the effluent color. Livernoche et al. (1983) also reported similar findings, in which the color of effluent was pH sensitive, and the pH effect on the color of effluent was reversible. In view of these observations, Garg et al. (1999) suggested that effluent decolorization studies should be performed that employ unsterilized and unfiltered effluent without pH adjustment. Considering the foregoing, bacterial treatment systems that have an optimum pH range of 7–9 may play a pivotal role in decolorizing pulp and paper mill effluents, without any requirement for pH adjustment.
Lignin is decomposed by bacteria in the natural environment. Because of their immense environmental adaptability and biochemical versatility, both anaerobic and aerobic bacteria have been studied for their ligninolytic potential (Chandra et al. 2007). Several bacterial species capable of metabolizing various industrial pollutants have been isolated from the natural environment, viz., Bacillus sp., B. subtilis (IS13) (Niazi et al. 2001; Andretta et al. 2004), Pseudomonas sp., P. veronii, P. fluorescens, P. aeruginosa (Premlatha and Rajkumar 1994; Thakur et al. 2002; Nam et al. 2003; Shah and Thakur 2003), Flavobacterium (Saber and Crawford 1985), Sphingomonas chlorophenolicum ATCC 39723 (Xun et al. 1999), Desulfomonile tiejei DCB-1 (Mohn and Kennedy 1992), Arthrobacter chlorophenolicus A6 (Agneta et al. 2004). Many researchers have evaluated Pseudomonas sp., P. putida, Flavobacterium sp., Xanthomonas sp., Nocardia sp., Aeromonas sp., Arthrobacter sp., Ancylobacter sp., Methylobacterium sp., Acinetobacter calcoaceticus, etc., for effluent color and toxicity reduction (Vora et al. 1988; Fulthrope and Allen 1995; Jain et al. 1996). Blair and Davis (1980) treated Kraft mill effluent with P. aeruginosa under aerobic conditions, and reported a color reduction of 26–54%. A mixed population of bacteria and yeast, including P. putida, Nocardia coralina, and Torula sp., were employed to degrade lignin (Bajpai and Bajpai 1997). Chauhan and Thakur (2002) treated pulp–paper mill effluent in a fixed-film bioreactor by P. fluorescens, and noted reductions of color (75%), phenol (66%), COD (79%), and lignin (45%) within 15 days. Raj et al. (2007) examined three lignin-degrading bacterial isolates, viz., Paenibacillus sp., Aneurinibacillus aneurinilyticus, and Bacillus sp. for the treatment of effluent, and observed reduction in color (39–61%), lignin (28–53%), BOD (65–82%), COD (52–78%), and total phenols (64–77%) within 6 days, with highest reduction achieved by Bacillus sp.
In this same experiment, the maximum reduction in total phenol (77%) was recorded with Paenibacillus sp. (Raj et al. 2007). The authors asserted that a significant reduction in color and lignin by the three bacterial strains was noted at the second day of incubation. This indicated that bacterial strains initially utilized growth supportive substrates that consequently cometabolized chromophoric compounds, ultimately reducing lignin and the color of the effluent. Chandra et al. (2009) employed two pentachlorophenol-degrading bacterial strains B. cereus ITRC-S6 and Serratia marcescens ITRC S-7 for pulp–paper mill effluent treatment. The effective reduction in color (45–52%), lignin (30–42%), BOD (40–70%), COD (50–60%), total phenol (32–40%), and PCP (85–90%) was noted. However, maximum reduction in various study parameters, viz., color (62%), lignin (54%), BOD (70%), COD (90%), total phenol (90%), and PCP (100%) was recorded by a mixed culture of the above two bacterial strains (Table 1). The reduction of COD and total phenol may have resulted from the degradation of lignin and chlorinated organic compounds (Singh and Thakur 2006; Latorre et al. 2007). Further, the bacterial degradative capability of lignin and PCP was apparent, because the aromatic compounds (e.g., 2-chlorophenol, 2,4,6-trichlorophenol, tetrahydroquinone, 6-chlorohydroxyquinol, and tetrachlorohydroxyquinone) that were absent in untreated effluent were detected in the treated effluent.
Singhal and Thakur (2009) studied the decolorization and detoxification of pulp–paper mill effluent by Cryptococcus sp. This bacterial isolate, designated as PF7, reduced the color (27%) and lignin content (24%) of the effluent on the fifth day under unoptimized conditions. However, enhanced reduction in color (50–53%) and lignin (35–40%) were noted to occur after optimum treatment conditions were reached during the 24 h incubation: temperature (35–40°C), shaking (125 rpm), dextrose (1.0% w/v), tryptone (0.1% w/v), inoculum size (7.5% v/v), and pH (5.0).
The biological treatment strategy was slightly changed by Singh and Thakur (2004) to improve the efficiency of effluent decolorization and detoxification by microorganisms in a two-step aerobic bioreactor. In the first step, the effluent was treated by eight individual fungal isolates, among which Paecilomyces sp. exhibited reduction in phenol (40%), lignin (66%), COD (81%) (performed in a 100 L sequential bioreactor at 6 h retention time). When this fungus-treated effluent was subsequently treated by the bacterial isolate Microbrevis luteum, the reduction in phenol (77%), color (84%), COD (83%), and lignin (72%) further increased. The authors concluded that, although Paecilomyces sp. was more efficient than was M. luteum in removing color, lignin, and COD, the bacterial strain M. luteum was capable of removing significant amounts of chlorinated phenols and their metabolites.
In another study, Chuphal et al. (2005) applied fungal (Paecilomyces sp.) and bacterial (P. syringae pv myricae) isolates to treat pulp–paper mill effluent in a two- and three-step fixed-film sequential bioreactor. The two-step aerobic treatment (aerobic fungus + aerobic bacteria) was slightly better in decreasing color (88.5%), lignin (79.5%), COD (87.2%), and phenol (87.5%), than was the three-step anaerobic–aerobic treatment (anaerobic + aerobic fungus + aerobic bacteria), in which color was reduced by 87.7%, lignin by 76.5%, COD by 83.9%, and phenol by 87.2%. The advantage of anaerobic treatment, in the three-step process, is that biogas is produced that is utilized for energy generation. Singh and Thakur (2006) performed sequential anaerobic and aerobic treatment in a two-step bioreactor to remove color in pulp and paper mill effluent. All pollution parameters such as color (70%), lignin (25%), COD (42%), AOX (15%), and phenol (39%) decreased slowly over 15 days, without appreciable increase in biomass. To further reduce pollutants, the 7 days anaerobically treated effluent was separately applied to a bioreactor, in the presence of the fungus Paecilomyces sp. and the bacterium M. luteus, in step two and three. The reduction in color (76%), lignin (69%), COD (75%), AOX (82 %), and phenol (93%) by day 3 was achieved. Thus, degradation of the pollutants and associated parameters was relatively faster under aerobic than under anaerobic treatment conditions. This was attributed to the unique ability of aerobic microbes to secrete enzymes that efficiently degraded chromophoric compounds and toxic chlorinated phenols from the effluent (Livernoche et al. 1983; Pokhrel and Viraraghavan 2004).
6 Anaerobic vs. Aerobic Biological Treatment Strategies
Many highly chlorinated compounds are known to be quite stable and difficult to degrade. However, anaerobes can sometimes catalyze biotransformation reactions in which chloride ions (Cl−) of the chlorinated compounds are displaced by protons (H+). The more chloride ions that are thus removed, the more reactive the resultant compounds become, thereby rendering them susceptible to conventional activated sewage sludge treatment (Mikesell and Boyd 1986; Haggblom 1990).
Anaerobic treatment is cost effective and has become the most commonly used method for treating medium and high strength effluents. Different anaerobic technologies have been applied for treating less concentrated effluents, such as domestic wastewater and some industrial effluents. Such anaerobic technologies provide good treatment efficiencies at low hydraulic retention times (Hickey et al. 1995). Accordingly, the establishment of sequential anaerobic–aerobic two-step wastewater treatment facilities at Kraft pulp–paper mills would probably help to reduce color and toxic contaminants. Haggblom and Salonen (1992) studied the biodegradability of chlorinated organics and conventional pollutants in Kraft bleaching effluents, using a two-stage anaerobic fluidized bed/aerobic trickling filter treatment system. The anaerobic stage removed >65% of AOX, and chlorinated phenolics removed >75%. COD and BOD reduction was greatest in the aerobic process, whereas dechlorination was significant in the anaerobic process. The concept of sequential treatment is very important, because both anaerobic and aerobic fungi and bacteria can be used to treat effluent at different stages in the bioreactor. However, Thakur (2004) reported that bacteria are more potent for degrading aromatic compounds. Furthermore, two or more types of microbes may be attempted sequentially, in which one organism may transform the original organic pollutant by initial catabolic reactions to products that are then mineralized by (an)other organism(s). Pokhrel and Viraraghavan (2004) and Thakur (2004) have developed such consortia for mineralizing bicyclic aromatics, viz., chlorinated biphenyls, chlorinated dibenzofurans, and naphthalene sulphonates. Singh and Thakur (2006) indicated that sequential anaerobic and aerobic treatment was more efficient in removing color and chlorinated compounds, because anaerobic microorganisms degraded highly chlorinated organics more efficiently than did aerobic microbes, although the latter microbes removed the last remaining chlorine atom. The combined treatment typically removed 82% of AOX, COD, and chlorinated phenolics and completely eliminated chlorate.
Mutagenic and chlorinated aliphatic compounds in effluent, many of which are known mammalian carcinogens, are also effectively degraded in sequential anaerobic–aerobic treatment processes. However, resin acids, which are excessively present in certain wastewaters, such as chemothermo-mechanical pulping (CTMP), are highly toxic to anaerobic bacteria, ultimately causing them to fail (Sierra-Alvarez and Lettinga 1990). But, resin acids can be degraded by aerobic microorganisms. Therefore, a three-step process of aerobic–anaerobic–aerobic degradation of effluent pollutants has been suggested as a means to successfully degrade both resin acids and organochlorine compounds (Welander 1988).
7 Other Strategies for Reducing Effluent Color and Toxicity
The major constraints of usual chemical pulping and the bleaching process are (1) excessive use of chemicals and electrical energy and (2) generation and release of enormous colored and toxic wastewater, thus contributing to environmental pollution. These shortcomings can be largely overcome by using a pulping process, in which cooking time is reduced. The biopulping process removes lignin, along with some wood extractives, thereby reducing effluent toxicity (Ali and Sreekrishnan 2001). Moreover, biopulping and biobleaching hold enormous potential in the pulp–paper industry for rendering it more energy efficient, more economical, and more environmentally friendly. The exposure of wood chips to steam treatment prior to biopulping also eliminates another risk, to wit, that it destroys certain indigenous sporulating fungi that are capable of causing respiratory problems in mill workers (Agbiotech Bulletin 2003). Biopulping is often followed by biobleaching. Biobleaching employs white-rot fungi and/or ligninolytic enzymes and is effective in removing unwanted constituents in the pulp, including residual lignin. One new strategy that could add to pollutant removal efficiency in the industry is to use a new pulping/bleaching process, in which delignification by oxygen, hydrogen peroxide, and ozone is undertaken.
7.1 Biopulping and Biobleaching
The fungal and/or enzymatic treatment of woodchips and pulps, respectively referred to as biopulping and biobleaching, offer promising alternatives to alkali and chemical treatments (Keller et al. 2003; Camarero et al. 2007). Such new environment-friendly technologies with elemental chlorine-free (ECF) and totally chlorine-free (TCF) bleaching strategies are necessary to achieve the following: (1) minimize hemicellulose in the pulp, (2) achieve a high level of paper brightness, and (3) improve effluent quality in terms of reduced toxicity and AOX. In biobleaching, pulp can be bleached with white-rot fungi and their ligninolytic enzymes, to reduce chemical use, and to establish a chlorine-free bleaching process (Shukla et al. 2004).
Biopulping with the use of various species of white-rot fungi have been reported, and Ceriporiopsis subvermispora has proved suitable both for soft and hard woods (Milagres et al. 2005; Ferraz et al. 2007). Yaghoubi et al. (2008) employed C. subvermispora for biochemical pulping of agriresidues, and the results were comparable to those achieved with the chemical pulping process. Moreover, the paper quality produced by biochemical pulping of straws was excellent. Singh and Chen (2008) reported that pretreatment with P. chrysosporium could reduce fiberizing and refining energy costs by 30% (Singh et al. 2010). Arias et al. (2009) studied the suitability of Streptomyces strains for biochemical pulping of spruce wood (Picea abies). The engineering and economic analyses indicate that the biopulping process is technologically feasible and economically beneficial. However, the major constraints of the process are that the biological pretreatment step has a slow reaction rate, and the process control is complex (Chen et al. 2010).
7.2 In Plant Processing
Another strategy is to develop new pulping processes that emphasize improved delignification and complete or partial replacement of chlorine in bleaching processes. The use of elemental chlorine for bleaching causes environmental problems by releasing toxic and recalcitrant chlorinated aromatic compounds. In recent years, TCF pulp production is gaining momentum. Alternative chemicals, e.g., oxygen, hydrogen peroxide, and ozone for bleaching, have been successfully used. The new process, called oxygen delignification, has proved quite effective in removing lignin and hemicellulose fractions from cellulose, without damaging the fibers. This process releases less lignin than does traditional chlorine bleaching. Moreover, partial replacement of elemental chlorine with chlorine dioxide would greatly reduce the amount of organochlorine compounds in the effluent, while complete substitution with H2O2, would totally eliminate these toxic compounds. H2O2 also assists in oxidizing other organic contaminants in the effluent. Unfortunately, these alternative compounds are not free from shortcomings, and have the disadvantage of producing lower quality paper, during the bleaching process.
8 Summary
The potential hazards associated with industrial effluents, coupled with increasing awareness of environmental problems, have prompted many countries to limit the indiscriminate discharge of untreated wastewaters. The pulp and paper industry has been among the most significant of industrial polluters of the waterways, and therefore has been one of the industries of concern.
The pulp and paper industry produces large quantities of brown/black effluent that primarily result from the pulping, bleaching, and paper-making production stages. The dark color and toxicity of pulp–paper mill effluent comes primarily from lignin and its chlorinated derivatives (e.g., lignosulphonic acid, resins, phenols, and hydrocarbons) that are released during various processing steps of lignocellulosic materials. The color originates from pulping and pulp bleaching stages, while adsorbable organic halides (AOX) originate exclusively from chlorine bleaching. Discharge of untreated effluent results in increased BOD/COD, slime growth, thermal problems, scum formation, discoloration, loss of aesthetic quality and toxicity to the aquatic life, in the receiving waterbodies.
The dark brown color of pulp–paper effluent is not only responsible for aesthetic unacceptability, but also prevents the passage of sunlight through colored waterbodies. This reduces the photosynthetic activity of aquatic flora, ultimately causing depletion of dissolved oxygen. The pulp–paper organic waste, coupled with the presence of chlorine, results in the generation of highly chlorinated organic compounds. These toxic constituents of wastewater pose a human health risk through long-term exposure via drinking water and/or through consumption of fish that can bioaccumulate certain pollutants from the food chain. Therefore, considerable attention has been focused by many countries on decolorization of paper mill effluents, along with reduction in the contaminants that pose human health or other environmental hazards.
Various physicochemical remediation treatments in the pulp–paper industry are now used, or have been suggested, but often are not implemented, because of the high costs involved. More recently, the paper and pulp industry has been investigating the use of biological remediation steps to replace or augment current treatment strategies. Certain biological treatments offer opportunities to reduce cost (both capital and operating), reduce energy consumption, and minimize environmental impact. Two primary approaches may be effective to curtail release of toxic effluents: first, development of pulping and bleaching processes that emphasize improved oxygen delignification or biopulping, plus partial or complete replacement of chlorine treatment with hydrogen peroxide or with biobleaching; second, implementation of biological processing that involves sequential two-step anaerobic–aerobic or three-step aerobic–anaerobic–aerobic treatment technologies at end of pipe. The selection of the specific process will depend upon the type of pollutants/toxicants/mutagens present in the effluent.
The use of environmental-friendly technologies in the pulp and paper industry is becoming more popular, partly because of increasing regulation, and partly because of the availability of new techniques that can be used to economically deal with pollutants in the effluents. Moreover, biotechnology research methods are offering promise for even greater improvements in the future. The obvious ultimate goal of the industry and the regulators should be zero emission through recycling of industrial wastewater, or discharge of the bare minimum amount of toxicants or color.
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Garg, S.K., Tripathi, M. (2011). Strategies for Decolorization and Detoxification of Pulp and Paper Mill Effluent. In: Whitacre, D. (eds) Reviews of Environmental Contamination and Toxicology Volume 212. Reviews of Environmental Contamination and Toxicology, vol 212. Springer, New York, NY. https://doi.org/10.1007/978-1-4419-8453-1_4
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