Biological decomposition takes place in four stages, each of which has its own
environmental and substrate requirements (Fig. l).
1. phase: Aerobic. Oxygen entrained at the time of refuse placement is. consumed during
aerobic phase. Therefore, because additional oxygen supply is limited after refuse placement,
this aerobic phase of biodegradation takes only few days or weeks. The aerobic decomposition
generates heat. This temperature increase can he much higher if the moisture content is also high.
2. phase: Anoxic, Nonmethanegenic. During this phase is produced CO2 as a result of acid
fermentation. Some hydrogen is also produced. Under saturated conditions more than 70 % CO2
by volume can be produced in 11 days.
3. phase Anaerobic, Methanegenic, Unsteady. Methanegenesis begins in 3. phase Indications
of the time required to reach 50% of CH4 by volume during this phase range from 3 months in
wet refuse to much longer, and, perhaps, never in dry refuse.
4. phase: Anaerobic, Mothanegenic. Throughout 4. phase the production of methane remains
stead at 40-70% by volume. The production of methane declines as refuse organics get depleted
on a unit basis, but the slowly biodegradable organics yield CH4 for decades.