Organic Waste Recycling

176 Organic waste recycling: technology and management

Table 4.11 Design criteria and performance data of dispersed growth biogas digesters
(Continued)

Air tightness

Water tightness

COD removal (%)

VS removal (%)

N reduction (%)

Essential

Strongly required

30-70

40-70

20-35

Conventional digesters

This type of digester is used in conventional sewage treatment plants to treat
sludge. The gas produced is used to augment the energy demand of the
treatment plant or to heat the digester. Figure 4.13 shows a typical design of
cylindrical shape digester. Major components of the digester are for mixing and
recirculation of the digester contents, elimination of scum, gas collection and
digested sludge withdrawal. The operation of the above digesters requires
skilled labor and regular monitoring. Variation of designs is in shape and in
mixing method. Elimination of scum is easier in a dome-shaped digester
because of the low surface area at the top portion of the digester, even though its
construction requires special techniques. The size of these conventional
digesters ranges from 250 m^3 to 12,000 m^3 or more.

(a)