Anaerobic digestion is a complex bioprocess consisting of successive, often interactive steps carried out by groups of microorganisms with different growth rates and sensitivity to environmental conditions (pH, partial pressure of hydrogen, etc.). The process can be outlined as consisting of the following steps (Fig. 12.1):

• Disintegration: The complex particulate waste disintegrates to organic polymers such as carbohydrates, proteins and lipids. Disintegration lumps a number of steps such as lysis, non enzymatic decay, phase separation and physical breakdown (e. g. shearing; Batstone et al., 2002).

• Hydrolysis: The organic polymers (carbohydrates, proteins and fats) are hydrolysed (depolymerised) by extracellular enzymes to their respective monomers (sugars, amino acids, lipids), which can be taken up by the microorganisms for further degradation. In the case of particulate complex organic matter consisting of lignocellulosic material (mainly of plant origin),

pretreatment steps are necessary to enhance hydrolysis by rendering the substrate matrix more amenable to enzyme attack.

• Acidogenesis: A versatile group of microorganisms are able to convert the simple monomers to a mixture of volatile fatty acids, alcohols and other simpler organic compounds. This step is also often called fermentation. During acidogenesis, large amounts of carbon dioxide are produced as well as hydrogen. Especially in the case of sugars fermentation, the amount of hydrogen produced can be high and may be harvested for energy recovery. The growth rate of acidogens is quite high (doubling time of the order of one hour or even less) and low pH resistant (5-6) giving them the advantage of prevailing in the anaerobic consortium at adverse conditions. As a result of the rapid acid formation, there is a danger of acid accumulation (and concomitant pH drop) if the acids are not degraded in time in the steps that follow.

• Acetogenesis: The higher volatile fatty acids (propionate, butyrate, valerate, etc.) as well as the other organic molecules produced in the acidogenesis step are transformed to acetic acid, carbon dioxide and hydrogen by the acetogenic bacteria. This step is thermodynamically inhibited by hydrogen, meaning that, unless hydrogen is depleted by the hydrogen-consuming bacteria in other steps, there is an accumulation of mainly propionic and butyric acid. The acetogenic bacteria are slow growing microorganisms doubling time of the order of days.

• Methanogenesis: There are two distinct groups of microorganisms that produce methane and carbon dioxide: (1) the acetoclastic methanogens that grow on acetic acid and produce approximately 70% of methane in the biogas, and (2) the hydrogen utilising methanogens that consume hydrogen and carbon dioxide. The methane content of biogas depends on the oxidation state of the organic carbon in the initial substrate (ranging from -4 for methane to +4 for carbon dioxide); the more reduced the initial substrate is, the more methane will be produced, but on average the biogas contains 60% of methane. Acetoclastic methanogens are slow growing microorganisms (doubling time of the order of days) and are particularly sensitive to a number of factors such as pH, lack of nutrients, certain compounds, etc.