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5.3 Anaerobic digestion

Anaerobic digestion is a bacterial decomposition process which stabilises organic wastes and produces a mixture of methane and carbon dioxide gas (biogas). The heat value of methane is the same as natural petroleum gas, and so biogas is valuable as an energy source.

Anaerobic digestion is usually carried out in a specially built digestor, where the content is mixed and the digestor maintained at 35 °C by combusting the biogas produced. After digestion the sludge is passed to a sedimentation tank where the sludge is thickened. Biogas is collected from the digestor (Figure 2.41). The thickened sludge requires further treatment prior to reuse or disposal.

Figure 2.41: Simple anaerobic digestion process

Anaerobic digestion can also be carried out at a slower rate in an unmixed tank or pond. Covering is usually by a UV resistant plastic sheet, because of the large area needed to be covered, and biogas is collected from the top of the sheet.

Storage of biogas can be in a cylindrical tank with a floating roof. The cylindrical roof floats on water and its position is determined by the volume of the gas stored under the pressure of the roof. Biogas can also be stored in a balloon, but only under low pressure.

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