Newsletter and Technical Publications
<International Source Book On Environmentally Sound Technologies
for Wastewater and Stormwater Management>
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.