For anaerobic wastewater or sludge treatment, oxygen must be excluded to maintain the low redox potential that is required for survival and metabolic activity of the acet-ogenic, sulfidogenic, and methanogenic populations. Hydrolysis of polymers, uptake of soluble or solubilized carbon sources, and the primary metabolic reactions of glycolysis up to pyruvate and acetate formation seem to proceed identically or at least analogously in aerobic and anaerobic bacteria. Whereas aerobes oxidize acetate in the TCA cycle and respire the reducing equivalents with oxygen, anaerobes, such as Ruminococcus sp., Clostridium sp., or Eubacterium sp., either release molecular hydrogen or transform pyruvate or acetate to highly reduced metabolites, such as lac-tate, succinate, ethanol, propionate, or n-butyrate. For further degradation within the anaerobic food chain, these reduced metabolites must be oxidized anaerobically by acetogenic bacteria. Since the anaerobic oxidation of propionate or n-butyrate by acetogenic bacteria is obligately accompanied by hydrogen production but is only slightly exergonic under conditions of a low H2 partial pressure (Bryant, 1979), ace-togens can grow only when hydrogen is consumed by hydrogen-scavenging organisms such as methanogens or sulfate reducers.
During anaerobic degradation of 1 mol glucose, approximately 95% of the glucose carbon is used for biogas formation (171 g = 127.7 L CH4 + CO2), and only about 5% of the substrate carbon (9 g) converted to biomass (Table 1.1). Much less heat energy is released during anaerobic metabolism than during aerobic respiration (131 kJ mol-1 versus 890 kJ mol-1, respectively), and the biogas contains almost 90% of the energy of the fermented substrate (Fig. 1.2). Due to the heat energy requirement to warm the wastewater and due to heat losses via irradiation from pipes and reactor walls, heat generation is not nearly sufficient to maintain a constant mesophilic fermentation temperature. For this reason, anaerobic digesters must be heated.
In sewage sludge with 36 g L-1 organic dry matter content and 50% biodegradabil-ity, 0.9 g surplus sludge and 17.1 g biogas (equivalent to 12.75 L) are formed during anaerobic stabilization. Only 13.1 kJ of heat energy per mol of glucose is released, leading to a self-heating potential of 3.1 °C. Since the heat energy is released only during the hydraulic residence time of the wastewater in the reactor, which is usually more than 10 d (except for high-rate industrial wastewater treatment, where in special situations the HRT may be shorter than one day), much more heat energy is lost by irradiation via the reactor walls than is required to maintain the temperature. If highly concentrated wastewater streams of the food and beverage industry are stabilized anaerobically at hydraulic retention times of <1 d (high space loading), more heat energy is generated within a much shorter time. But even then, process energy from external sources has to be supplied to maintain a temperature of 30-37 °C. Proper insulation of anaerobic reactors can minimize but not replace external heating of the reactor. The methane in the biogas, generated during anaerobic sludge or wastewater stabilization processes, contains about 90% of the energy of the fermented substrate. Since methane is a climate-relevant gas it may not be emitted into the atmosphere but must be combusted to CO2. Methane from anaerobic reac-
tors can be used as a fuel for gas engines to generate electricity and/or heat energy (Eq. 2):
CH4 + 2 O2 ^ CO2 + 2 H2O + heat energy (AG0' = -891.6 kJ mol-1 CH4) (2) 1.2.2
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