The anaerobic breakdown of organic material has only been studied in detail in the case of the degradation of sewage sludge and an outline of the process is given in Fig. 5.4. A consortium of microorganisms that develop under anaerobic conditions degrade the organic materials in series of stages.

In the first stage is the hydrolysis phase. Here complex organic materials consist­ing of carbohydrates, lipids, proteins, DNA and RNA are broken down by hydrolytic bacteria such as Clostridium sp., Eubacteria sp. and bacteroids. The result of the hydrolysis is simple sugars, acids, ketones and amino acids. In the next stage, acido — genesis, the simpler compounds are then broken down to acetate, lactate, propionate, ethanol, carbon dioxide and hydrogen. In the next stage, acetogenesis, the simple compounds are converted into acetate. Acetate is then combined with carbon dioxide

Fig. 5.4. Stages of the anaerobic breakdown of organic materials.

and hydrogen to form methane by a group of bacteria known as the methanogens in the last stage, methanogenesis. These bacteria are some of the most oxygen — sensitive bacteria found and include Methanobacterium sp., Methanobacillus sp., Methanococcoides sp. and Methanosarcina sp. The methanogens function in close contact with the acetogenic bacteria in time and space. This allows any hydrogen formed to be transferred without loss to the atmosphere. Strict anaerobes only grow and metabolize slowly so that anaerobic metabolism is much slower than aerobic metabolism and this is why the process can take up to 30 days. There are other pro­cesses that can lead to the production of methane, carbon dioxide and hydrogen.

Methanococcoides and Methanolobus form methane from acetate which can be the preferred substrate in cold wet anaerobic soils found in wetlands:

CH3COOH ^ CH4 + CO2 (5.1)

Another reaction which can occur converts carbon monoxide produced from acetate to carbon dioxide and hydrogen:

CH3COOH ^ CH3 + CO (5.2)

CO + H2O ^ CO2 + H2 (5.3)

Other methanogens produce methane by a complex series of reactions involving hydrogen and carbon dioxide:

4H2 + CO2 ^ CH4 + 2H2O (5.4)

The exact proportion of gases varies depending on the materials broken down and the process conditions. It is likely that similar stages occur in landfill and anaerobic digestion of animal slurries: