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A review of the kinetics of anaerobic treatment and the reported values of such kinetic parameters as the maximum specific substrate utilization rate (k), the half-saturation constant (Ks), the microbial growth yield (Y), and the microorganism decay rate constant (b) are presented. The available kinetic information is presented for each subprocess: (a) hydrolysis of complex, paniculate organic materials; (b) fermentation of amino acids and sugars; (c) anaerobic oxidation of long-chain fatty acids and alcohols; (d) anaerobic oxidation of intermediary products (such as short-chain fatty acids); (e) homoacetogenesis; and (f) methanogenesis. The intrinsic rates of each step as well as mass transfer limitations and their effect on the intrinsic kinetics are discussed and areas requiring further research are also identified. Substantial variation exists in the reported values of the kinetic coefficients. This variation is due in part to the variability in mode of operation, environmental and operational conditions in the various studies as well as to the lack of a widely accepted standard procedure for measuring and expressing the biokinetic coefficients. The hydrolysis step is usually assumed to follow first-order kinetics. Whenever the kinetics of the hydrolysis step were studied, they were generally found to be the limiting-step in the overall conversion of complex substrates to methane. With the exception of the hydrolysis step, all other subprocesses of anaerobic treatment have been successfully modeled by following Monod kinetics. The Contois and Chen & Hashimoto model has also been used quite extensively to account for the effect of influent substrate concentration on effluent quality. Based on a brief overview of the observed phenomena related to the kinetics of mass transfer in methanogenesis, it is concluded that with but few exceptions, the evidence for the significance of mass transfer effects in the different reactor configurations is circumstantial and, in some cases, contradictory. Our understanding of the kinetics of paniculate substrate removal in biofilms is still incomplete for engineering applications, and more research is necessary.

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There is strong evidence in the literature supporting the existence of significant mass transfer limitations on the kinetics of exogenous H(2) consumption by methanogens. The half saturation constant for H (2) uptake by a mixed-culture, CH(4) producing enrichment was measured using an experimental protocol that avoided internal mass transfer limitations. The value obtained was two orders of magnitude smaller than any other previously reported. A mathematical model for acetogenic syntrophic associations was developed to check the capacity of H(2) as electron transporter between syntrophic partners. It was found that H(2) diffusion could account for the rate of transport of electrons between the syntrophic microorganisms and that formate is not a necessary intermediate. The possibility that formate may be an intermediate in this system was not ruled out. A Monod-type kinetic equation was modified to include the observed H(2) threshold effect. This modified equation was used to predict the CH(4)-production rate in a batch-fed digester. The results show that the external and internal H(2) pools are kinetically coupled.

The ability of hydrogen diffusion to account for the rates of methane production in microbial aggregates was studied in a defined coculture consisting of a sulfate reducer grown as a syntrophic hydrogen producer in the absence of sulfate and a methanogen. The hydrogen uptake kinetics of the methanogen were determined using the infinite dilution technique. The maximum hydrogen uptake velocity was 7.1 nmol/min/μg protein and the half saturation constant for hydrogen uptake was 386 nmol/liter. A threshold of 28 nmol/liter below which no further hydrogen consumption occurred was observed. The reconstituted co-culture was shown to produce methane at rates similar to mixed culture enrichments grown on lactate. The diffusion model demonstrated that for the particular system studied, the rates of hydrogen diffusion could account for the overall rate of methane production.