Organic Waste Recycling

Biofuels production 173

Gas pressure High: up to 1000 mm water

column

Varying according to gas use

Automatic release of

excessive gas through

manometer

Low: 70-150 mm water column

Steady, due to floating drum

Automatic release of excessive

gas through drum

Efficiency Low, due to gas escape

through large inlets and

outlets: 0.15-0.30 m^3 gas

produced per m^3 digester per

day

Stable through seasons, due to

good insulation of

underground construction

Higher, gas escape insignificant:

0.30-0.60 m^3 gas produced per

m^3 digester per day

Subjected to seasonal variations,

loss heat through metal drum

Feeding

materials

Mostly mixtures of animal

wastes, human excreta,

household refuse, agricultural

residues

Virtually semi-continuous

loading

Operation Mostly semi-continuous

loading, can be batch

operation

Effluent removal by pump or

bucket

Labor-intensive for batch

loading, emptying pit and

removing effluent

Virtually semi-continuous

loading

Effluent removal by gravity flow

Virtually no attention beyond mix

and feeding influent

Maintenance

cost

Wall lining

Low, because no metal part

Drum painting

High due to metal drum

Figure 4.10 Plug-flow digester

INLET PIT

FLEXIBLE MEMBRANE

GAS

SLURRY

GAS OUTLET

OUTLET PIT

SECTION

COMPACTED

BACKFILL

AND

INSULATION