Anaerobic Conversion of Biopolymers to Methane and CO2

The principle of the anaerobic metabolism of biopolymers was outlined by the pioneering work of Wolin (1976) and Bryant (1979). There is an essential requirement for syntrophic interaction between different metabolic groups for complete anaerobic degradation (Wolin, 1976, 1982). The most sensitive switch of the carbon flow of substrates to biogas is the H2 partial pressure. The substrate supply for biomethana-tion processes must be limited so that the most slowly growing group in the food chain, the obligate proton-reducing acetogens, can still excrete hydrogen at a maximum rate, but at the same time hydrogen accumulation >10-5 atm is prevented by active methanogenesis or sulfate reduction (Bryant, 1979). Whereas hydrogen seemed to be the most sensitive regulator of anaerobic degradation, formate (Bleicher and Winter, 1994), acetate, or other fatty acids accumulated to much higher concentrations (McInerney, 1988) but did not repress anaerobic degradation. If hydro gen accumulated due to an oversupply or to inhibition of methanogens, anaerobic biodegradation was disturbed successively in different stages. Initially, in the aceto-genic stage p oxidation of fatty acids and alcohols failed, leading to an accumulation of these acid metabolites. Later, the spectrum of metabolites of the fermentative flora changed toward more reduced products like ethanol, lactate, propionate, and n-butyrate, leading to an even higher concentration of volatile fatty acids and a further decrease in pH. At pH values below 6.5, methanogenic reactions were almost completely prevented. At this stage acidification with a rich spectrum of reduced products still proceeded.

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