Nadh

Ferredoxin/flavodoxin Ferredoxin/flavodoxin Ferredoxin/flavodoxin Ferredoxin/flavodoxin

Lipoamide/NADH

C. paraputrificum C. bifermentans C. thermohydrosulfuricum C. pasteurianum C. tyrobutyricum

C. sporogenes C. kluyveri

C. sporogenes

C. sporogenes C. tyrobutyricum

C. sporogenes

C. acetobutylicum

C. kluyveri

may include acetate- and H2-CO2-consuming methano-gens. Anaerobic microorganisms in general show a high degree of metabolic specialization. The success of the anaerobic digestion process therefore depends upon cooperative interactions between microorganisms with different metabolic capabilities. Figure 2 illustrates the anaerobic food chain that converts complex organic matter into methane. Since the rate of VFA production can be significantly higher than that of VFA conversion to methane, an imbalance between these two rates can occur in a single-stage digestor, resulting in VFA accumulation, a concomitant pH drop, cessation of methane fermentation, and ultimate process failure. A two-stage AD process that separates the acid-forming and methane-forming stages has been practiced with a variety of wastes. Alternatively, a high-rate syntrophic methanogenic microbial consortium can be developed (127,128). The leach-bed two-phase digestion process, consisting of an acid-phase, solid-bed reactor operated in tandem with a separate, packed-bed, methanephase digestor (anaerobic filter), for high solids waste has been described (129). Low-strength wastewaters can now be treated, even under psychrophilic conditions, by using specific rheologic conditions in the expanded granular sludge bed (EGSB) reactor (130). Solid wastes can be treated anaerobically using the thermophilic "high-solids" fermentation technology (131). New reactor designs permit S° recovery from SO2-rich waste gases (132).

Many human-made organic compounds are degraded during sewage treatment. Since 1986, full-scale UASB reactors have been used to treat municipal wastewater throughout the world. Treatment costs are halved when anaerobic treatment (e.g., UASB reactor) is applied instead of just aerobic processes (133). In warm climates, a simple upflow sludge blanket (USB) septic tank with an aerobic posttreatment (e.g., a trickling filter) can be combined to remove the bulk of its suspended solids (134). AD is expected to reduce most of the pathogenic bacteria. Thus, the treated water may be considered reclaimed for use in crop production in the next decade. Organic slurries containing particulate organic matter, such as animal manures and primary or secondary sewage sludges, are normally digested in completely mixed reactors. Because the hydrolysis rates of certain solids is slow, separation of hydraulic retention time (HRT) and solids retention time (SRT) may improve performance of digestors treating slurries. AD does not remove NH4 In a new NH4-removal pathway observed in the methanogenic reactors, NH4 was oxidized anaerobically to N2 in the presence of NO;T, with a lab-scale reactor achieving a removal rate of 0.7 kg NH4+-N/m3 per day (135).

The UASB technology is now widely used to anaerobi-cally digest industrial wastewaters with concentrations in the range 2-20 g COD/L. UASB reactors are usually implemented when wastewater is rich in carbohydrates and relatively poor in other contaminants. Biomass retention through adequate granulation is of utmost importance in UASB technology, first to obtain a good effluent quality and, second, in order to ensure a minimal cell residence time of 7-12 day that is required to avoid the wash-out of the slowest-growing anaerobic bacteria. The onset of sludge granulation after start-up of an anaerobic-sequencing batch reactor was enhanced by adding a cationic polymer, divalent cations, and granulation nuclei such as clay minerals (136). Consideration of surface thermodynamics is important in granulation (137). Some key methanogenic species have been identified and implicated in the granu-

Organic matter

Methanogenesis Wastewater

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