166 Organic waste recycling: technology and management
gas pressure in the dome of between 1 and 1.5 m of water. The high pressure
exerted on the construction is partially balanced by the weight of soil which
covers the digester's top.
This type of digester is common in developing countries. Approximately
seven millions of this type is in use in China. Most of the operational parameters
of this digester are semi-continuous, although some digesters are operated as
batch operation. Figure 4.6 shows fixed-dome digesters in a rural village in
Thailand.
Figure 4.6 Fixed-dome digesters treating piggery wastewater, central Thailand
Typical feed to these digesters is usually a mixture of animal manure,
nightsoil, water hyacinth and agricultural residues, depending on their
availability and C/N ratios. The rate of gas production is reported to be 0.15 -
0.2m^3 /day per m^3 of digester capacity, but in tropical areas it can be as much as
0.3-0.4m^3 /day per m^3 of digester capacity.
Mixing of the digester contents is accomplished through feeding of the
influent materials and withdrawal of effluent slurry. An improvement on mixing
efficiency of the digester contents is shown in Figure 4.7. In this digester design,
mixing is accomplished by pulling the plastic rope back and forth at the inlet
and outlet, causing the plastic blades to agitate the slurry to provide better
contact between the organic matter and the anaerobic bacteria. This method of
mixing prevents both blocking of the inlet and outlet pipes and accumulation of
the digested residues in the digestion chamber.