[PDF Notes] The process of anaerobic digestion can be described in three phases

The process of anaerobic digestion can be described in three phases.

1. Hydrolysis: The breakdown of high molecular compounds to low molecular compounds as in lipids to fatty acids, polysaccharides to monosaccharides, proteins to amino acids, etc.

2. Acidogenesis: In these process low molecular compounds of fatty acids, amino acids and monosaccharides are converted to lower molecular intermediate compounds such as organic fatty acids.

3. Methanogenesis: The organic acids are in turn degraded into the final products of methane and carbon dioxide.

In each of these three phases, a different group of bacteria are active and are called the hydrolysing bacteria, the acidogenic bacteria and methanogenic bacteria respectively.

Process description

In most processes where methane is to be produced from solid wastes by anaerobic digestion, three steps are involved. The first step is the preparation of organic fraction of the solid wastes for anaerobic digestion and usually includes receiving, sorting, separation and size
reduction. The second step is the addition of moisture and nutrients, blending, pH adjustment to 6.7-7.5, heating of the slurry to between 55-60°C and anaerobic digestion in a reactor with continuous flow, in which the contents are well mixed for a period of 5-10 days. In most operations, the required moisture content and nutrients are added to the processed solid wastes in the form of sewage sludge. Depending on the chemical characteristics of the sludge, additional nutrients may also have to be added. The third step involves capture, storage and separation of the gas components evolved during the degradation process. The disposal of the digested sludge is an additional task that must be accomplished.

The maintenance of an environment that keeps the acidogens and methanogens in dynamic equilibrium is required to:

(i) be oxygen free

(ii) should not contain toxic compounds

(iii) pH should be maintained in neutral range

(iv) the alkalinity should be in the range 1500-7500 mg/1 as CaC03

(v) have sufficient nutrients such as nitrogen and phosphorus

(vi) be temperature steady at either mesophilic or thermophilic conditions

(vii) have a constant solids input.

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