Conversion Processes in Anaerobic Digestion

Abstract

Six different conversion processes are identified in the degradation of particulate organic material (biopolymers) to methane. Hydrolysis of particulate material is followed by the degradation of the hydrolysis and intermediary products by five independent groups of microorganisms. Process and growth kinetic data for the six processes are reviewed. The kinetic data are applied to the design of an anaerobic digester for raw domestic sludge. Variations in loading rates primarily affect acetate decarboxylation and, thereby, may cause shifts in pH which, in turn, cause the digester to operate in the acidic regime.

A mathematical model of a high sulphate wastewater anaerobic treatment system

“…The rate of hydrolysis increases exponentially with an increase in temperature (Gujer and Zehnder, 1983). …”

“…An odd numbered molecule will result in the formation of both acetic and propionic acids (McInerney and Bryant, 1981). The general stoichiometry for beta oxidation as given by Gujer and Zehnder (1983) is: …”

Biogas process parameters—energetics and kinetics of secondary fermentations in methanogenic biomass degradation

“…For the first time, we also included measurements of formate, together with assays of hydrogen and carbon monoxide concentrations in the headspace. Whereas pool sizes of short-chain fatty acids have been reported repeatedly in the past and have been used as indicators of reactor stability and efficiency (Gujer and Zehnder 1983), an assessment of the energetic situation especially of the secondary fermenters in the system requires measurement also of hydrogen and formate which are considered as important co-products of secondary fermentations (Schink 1997). Since the redox potential of the H + /H 2 pair and the CO 2 /formate pair at pH 7.0 are nearly identical (E 0 ' = −414 vs. −430 mV) and most methanogenic partners can use both hydrogen and formate as electron donors, it has remained a matter of discussion whether actually hydrogen or formate is the electron carrier between secondary fermenters and their methanogenic partners (Bryant et al 1967;McInerney et al 1979).…”

“…The overall process is found naturally in freshwater sediments and is being employed technically in biogas reactors and in anaerobic steps in wastewater treatment plants (Gujer and Zehnder 1983;Kaspar and Wuhrmann 1978). Biogas production can help to remove waste material, especially wastes of high water content that are not suited for incineration or pyrolysis, and provides a storable chemical energy carrier that can be applied also in a Bpower-ondemand^strategy in a network of renewable energy sources (Nationale Akademie der Wissenschaften L 2012).…”

“…Acetate and hydrogen typically contribute to methane formation at approximately a two-to-one ratio (Gujer and Zehnder 1983). This ratio may be shifted under specific conditions (enhanced temperature, high nitrogen loads) by homoacetogenesis (Ragsdale and Pierce 2008) or syntrophic acetate oxidation (Schnürer et al 1996).…”