ABSTRACT
Anaerobic degradation occurs in the absence of oxygen. The basic difference between aerobic and anaerobic oxidation is that in the aerobic system oxygen is the ultimate hydrogen acceptor with a large release of energy, but in anaerobic systems the ultimate hydrogen acceptor may be nitrate, sulphate or an organic compound with a much lower release of energy (see ‘ Introduction ’ in Chapter 5) ( Fig. 19.1 ). The process of anaerobic decomposition involves four discrete stages ( Fig. 19.2 ). First is the hydrolysis of high-molecular-weight carbohydrates, fats and proteins, which are often insoluble, by enzymatic action into soluble polymers. The second stage involves the acid-forming bacteria that convert the soluble polymers into a
range of organic acids (acetic, butyric and propionic acids), alcohols, hydrogen and carbon dioxide. Acetic acid, hydrogen and carbon dioxide are the only end products of the acid production, which can be converted directly into methane by methanogenic bacteria. So a third stage is present where the organic acids and alcohols are converted to acetic acid by acetogenic bacteria. In the fi nal phase, which is the most sensitive to inhibition, methanogenic bacteria convert the acetic acid to methane. Although methane is also produced from hydrogen and carbon dioxide, in practice about 70% of the methane produced is from acetic acid. Obviously the methanogenic stage is totally dependent on the production of acetic acid and so it is the third stage, the acetogenic phase, which is the rate-limiting step in any anaerobic process. In practice, however, as the methanogenic bacteria have a much lower growth rate
than the acid-producing bacteria, the conversion of volatile acids to biogas is generally considered to be the rate-limiting step of the overall reaction with the methanogenic phase normally used for modelling purposes.
