Hydrolysis of biopolymers and fermentation or respiration of monomers can be catalyzed by strictly anaerobic, facultative anaerobic, and aerobic microorganisms. With some exceptions (e.g., small protein molecules, dextran), biopolymers are insoluble and form fibers (cellulose), grains (starch) or globules (casein after enzymat ic precipitation) or can be melted or emulsified (fat). Henze et al. (1997) reported hydrolysis constants kh for dissolved organic polymers of 3-20 d-1 under aerobic conditions and of 2-20 d-1 under anaerobic conditions, whereas for suspended solids the hydrolysis constants kh were 0.6 to 1.4 d-1 under aerobic conditions and 0.3 to 0.7 d-1 under anaerobic conditions. For a kinetic description of hydrolysis and fermentation, a substrate-limited first-order reaction was assumed by Buchauer (1997), who deduced that the temperature-dependent reaction rate for hydrolysis is a little lower than the reaction rate for fermentation of the hydrolysis products. The rate-limiting step is therefore hydrolysis of particles and not fermentation of solubilized material (Buchauer, 1997). Since hydrolysis is catalyzed not only by freely soluble exoenzymes, diluted in the bulk mass of liquid, but to a much higher extent by enzymes that are excreted in the neighborhood of bacterial colonies growing attached to the surface of the particles, the above description of complex fermentation processes is not always valid. Cellulases can be arranged in cellulosomes, which attach to the particles, which in turn serve as carriers until they themselves are solubilized. For this reason, Vavilin et al. (1997) included biomass in their description of the hydrolysis of cellulose, cattle manure, and sludge. Shin and Song (1995) determined the maximum rates of acidification and methanation for several substrates. For hydrolysis of particulate organic matter, the ratio of surface area to particle size is important. They found that for glucose, starch, carboxymethyl cellulose, casein, and food residues from a restaurant, hydrolysis proceeded faster than methanogenesis, whereas for newspaper and leaves hydrolysis was the rate-limiting step.
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