Figure 2. Anaerobic degradation of organic matter to methane in relation to microbial trophic groups.
lation process (138), and exopolysaccharides produced by these methanogens, specifically Methanobacterium formi-cicum, M. mazei, and Methanosaeta sp. seem to be responsible for the stability of granular structure (139). Industrial wastes are often unreliable in terms of their composition. Inhibition of methanogens caused by toxic compounds present in industrial wastewaters can be alleviated by adding activated carbon directly in the anaerobic reactors (140). Addition of GAC to UASB reactors treating textile wastewater prevented the chronic intoxication of anaerobic sludge that takes place in the absence of the GAC (134).
Anaerobic treatment systems are gaining in popularity and finding new applications. As revealed in papers presented at the 8th International Conference on Anaerobic Digestion, Sendai, Japan, in May 1997, the databank of anaerobic reactors presently includes 1066 operating anaerobic reactors worldwide. Of these reactors, 956 (89.7%) are for the treatment of industrial effluents, while 78 (7.3%) treat domestic sewage, and only 32 (3.0%) of the total are anaerobic-treatment systems for organic solid waste (excluding the vast number of biogas plants for manure treatment that have been installed worldwide). It is estimated that there are about two million biogas plants for manure treatment just in India alone. The largest num ber of municipal anaerobic treatment plants presently exist on a moderate, but remarkably growing, scale for sewage treatment in countries such as Brazil, Colombia, Mexico, India, and China. In industrial countries, only sewage sludge is treated anaerobically on a large scale due to climatic factors. For industrial wastewater treatment, countries such as India, China, Thailand, Brazil, and Mexico have a growing interest in anaerobic treatment for the local industry; in industrialized regions the leading countries, especially The Netherlands and Belgium, have almost already saturated their home markets. For anaerobic solid-waste treatment (and cofermentation), Germany and Denmark have significant numbers of plants so far. In the near future, anaerobic treatment of solid wastes will become a viable option provided that appropriate collection systems are introduced.
Toxic Organics. In recent years, AD has been used to degrade xenobiotics present in wastewater, soils, and river sediments. Complex xenobiotics often require more than one species to be completely mineralized. Microorganisms need to adapt, maybe for several months, before the maximal conversion rate of a new xenobiotic compound can be achieved. Anaerobic tunnel reactors are used to treat soils and sediments polluted with chloroethene, BTEX, or TNT (141). Efficiency of removal of adsorbable organic halogens by a UASB treating kraft-mill bleach wastewater varied between 27-65%, depending on the residence time (142). Desulfomonile tiedjei, which can rapidly transform 3-chlorobenzoate (3-CB), has been shown to become established in an UASB reactor. This indicates that a specific strain or desired species can be incorporated in microbial granular culture of an UASB reactor. Incorporation of an adapted microbial sludge culture into syntrophic biome-thanation granules has been demonstrated to treat PCP (14). These studies were further validated by developing PCE/TCE-dechlorinating methanogenic granules (143) using a similar protocol. In this, the anaerobic granules, using methanol as an electron donor for reductive dehalogen-ation, completely converts highly oxidized PCE to ethylene. In contrast, PCBs biodegrade very slowly and persist in the environment for decades. An effective micro-bial enrichment that was shown to remove ortho-, meta-, and para-chlorine was incorporated into the PCP-degrad-ing methanogenic microbial granules to produce PCB-dechlorinating anaerobic microbial granules in a manner similar to achieving PCP-granules in the first place (31). The PCB-dechlorinating anaerobic methanogenic granules have shown PCB-dechlorination of defined PCB-congeners (29) and Aroclor-contaminated river sediments (30). These granules are stable, culturable, and can be produced on a mass scale for bioaugmentation for application in both in situ and ex situ processes. These granules have also demonstrated degradation and mineralization of biphenyl, a non-chlorinated end-product of PCB-dechlorination process reductively (Natarajan, Wu, and Jain, unpublished data). Since PCBs are a complex mixture of various PCB compounds, and each PCB compound varies in its degree of dechlorination, it is expected that a mixture of bacteria capable of removing chlorines from different positions on the PCB molecule must be involved in achieving a com plete dechlorination of PCB. A similar assumption, however, is no longer true for dechlorination of perchloroeth-ylene (PCE). Recently, Zinder and his group isolated a pure culture of an anaerobic bacteria, tentatively named De-halococcoides strain 195, that dechlorinates PCE and TCE in a stepwise manner to ethene (144). This novel eubac-terium is not methanogenic, acetogenic, or sulfidogenic.
2,4,6-Trinitrotoluene (TNT), one of the most widely used explosives, occurs as a pollutant of soil and ground water, especially at sites of ammunition factories. There have been several reports of degradation or transformation of nitro and aminoaromatic compounds under anaerobic conditions. A nonspecific reduction of the nitro group to the corresponding amine was assigned to a variety of meth-anogenic bacteria, sulfate-reducing bacteria, and clostridia (145). In contrast, it was reported that 2,4-dinitrophenol, 2,4- and 2,6-dinitrotoluene (146), and TNT (147) were used as sources of nitrogen by sulfate-reducing bacteria. Thus, it is clear that many nitroaromatic compounds can serve as growth substrates for anaerobic bacteria. Anaerobic treatment systems have been proposed as a means of avoiding the accumulation of partially reduced intermediates during degradation of TNT. Under strictly anaerobic conditions, TNT can be completely reduced to triaminoto-luene (147). C. acetobutylicum transformed 2,4,6-TNT to undetermined end products via monohydroxylamino derivatives (148). In contrast to solventogenic cells, acidogenic cultures showed rapid transformation rates and the ability to transform TNT and its primary reduction products to below detection limits. Anaerobic treatment of explosive (primarily TNT)-contaminated soil was demonstrated in open bulk containers containing soil, phosphate buffer, and potato starch (149). The potato starch served as a rapidly degradable carbon source that allowed the rapid establishment of anaerobic conditions. Cresols and small organic acids were observed as end products. Similarly, Dinoseb, a very persistent herbicide that is highly toxic to virtually all living systems, has also been shown to be degraded by anaerobic microbial consortia using potato starch as a carbon source (150).
Inorganics. Microorganisms degrade certain toxic constituents used in mineral processing, and concentrate and immobilize soluble heavy metals released as a result of mining and mineral-processing activities. The initial mechanism of metal binding by microorganisms is electrostatic attraction between charged metal ions in solution and charged functional groups on microbial cell walls. The cell walls are composed of macromolecules with functional groups (principally carboxylate, amine, imidazole, phosphate, sulfhydryl, and sulfate) that contribute a net negative charge to the surface of the microorganism. These functional groups remain active even when the microorganism is not viable. Sulfate-reducing bacteria (SRBs) are used in highly controlled reactor systems and in constructed anaerobic wetlands for removal of sulfate and heavy metals from acid-rock drainage and other aqueous, metal-contaminated, streams. SRBs such as Desulfovibrio and Desulfatomaculum oxidize organic matter or H2 by using sulfate as an electron acceptor to produce hydrogen sulfide and bicarbonates. The sulfide immediately reacts with soluble heavy metal ions to form highly insoluble metal sulfides. A system to evaluate anaerobic and aerobic treatment of mine effluents containing excessive sulfate and heavy metals has been described (151). In the continuous system, the H2S is produced by the SRB biofilm on the dolomite pebbles in the anaerobic stage, which precipitates metal sulfides from a waste stream amended with an organic energy source. In the aerobic stage, a completely mixed reactor and a settling tank facilitate oxidation of residual H2S and biodegradation of residual organ-ics from the primary anaerobic column. Sulfate reduction has been considered as a method for permanent stabilization of sulfidic mine tailings.
High sulfate concentrations are common in wastewa-ters from paperboard industries, molasses-based fermentation industries, edible oil refineries, and in acidic leach-ates of pyritic waste rock and tailings (152). Sulfidogenic UASB reactors have attracted some attention for treating such polluted streams, but the biological processes to remove high SO4~ contents have not yet been optimized. A new bioprocess in which sulfate and heavy metals are removed simultaneously from groundwater has been developed (153). In this two-step process, SO;j~ bacteria produce sulfides that precipitate with the heavy metals after eth-anol, an electron donor, is added to the first reactor (BIO-PAQ UASB). In the second reactor (THIOPAQ-submerged fixed-film reactor), sulfur bacteria reoxidize the excess sulfide selectively to solid S° under controlled dosage of O2. With HRT of 4 h in the BIOPAQ reactor and 30 min in THIOPAQ reactor, removal efficiencies of 99% for heavy metals and of 85% for sulfate have been achieved. This technology has also been adapted for desulfurization offlue gas from power plants (132). In the modified process for flue-gas desulfurization, H2 can be used as an electron source in lieu of ethanol.
Minewaters and industrial effluents containing high sulfate concentrations create a disposal problem that requires an urgent solution in order to avoid excess mineralization of surface waters. Sulfate can be converted quantitatively to hydrogen sulfide by D. desulfuricans (154). Further conversion to elemental sulfur can be effected by the photosynthetic bacteria Chlorobium limicola and Chromatium cinosum. Two separate reactors were used for hydrogen sulfide and sulfur production. A possible way to increase the sulfate reduction rate is by making use of a packed-bed, instead of a completely mixed, reactor.
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