Environmental Engineering Reference
In-Depth Information
4. Temperature
Similar to other microbial processes, the biodegradation rate of microorganisms involved
in the MSW decomposition is highly affected by temperature. Methane production increases
with an increase in temperature. The optimum temperature for methane production in
mesophilic waste decomposition is in the range of 30 to 40ºC, whereas 60 is the optimum
temperature for thermophilic waste decomposition (Ham et al. 1989; Barlaz et al. 1990).
Hartz et al. (1982) recommended that the optimum temperature for methane generation is in
the range of 36 to 41˚C. Ham and Barlaz (1989) concluded that gas production rates at 30, 35,
and 40˚C are much higher than rates at 20 and 25˚C.
Anaerobic bacteria produce only small amounts of heat and may not be able to maintain
the temperature of a shallow landfill when external temperatures fall so landfill gas
generation may show pronounced diurnal or seasonal variations. Likewise, waterlogged
landfills may not attain optimum temperatures because the bacteria do not generate sufficient
heat to raise the temperature of the excess water. Higher temperatures promote volatilisation
and chemical reactions within the waste so the trace gas component of landfill gas tends to
increase with higher landfill temperatures.
Chaiampo et al. (1996) have monitored the temperature changes with depth throughout a
20 m deep municipal solid waste landfill in Italy. They showed that the first 1-2m were in the
temperature range of 10-15 °C, but the temperature increased to 35-40 °C at the 3-5m depth
and to 45-65 °C in the 5-20 m depth region. They equated the temperature regions with the
mesophilic bacteria in the 1-5m range and thermophilic bacteria in the deeper layers.
5. Pressure
Atmospheric pressure can have a minor affect on the rate at which landfill gas is released
to the atmosphere. It can also influence the operation of gas extraction systems. A decrease in
barometric pressure results in a temporary increase in LFG flow and an increase in barometric
pressure will cause LFG flow to temporarily decrease. This is because the pressure within the
landfill changes at a slower rate than the atmosphere and a pressure gradient temporarily
develops between the inside and outside of the landfill until these pressures equalize.
6. Moisture content and movement
Moisture is essential for the activity of all microorganisms in the landfill. The moisture
content therefore is one of the most critical factors controlling the biodegradation of MSW.
Many researches have shown that the methane production rate increases by increasing the
moisture content of the MSW. Rees (1980) found from existing literature that by increasing the
water content from 25% to 60% (wt), the rate of gas production and the percentage of methane
in the gas are increased. Baldwin et al. (1998) studied the moisture content in three landfills
over 1-6 years and found that wastes with high moisture content are more quickly decomposed.
The increase in moisture content affects the limitation of oxygen diffusion from the atmosphere
into the landfill, the exchange of substrate, nutrients and microorganisms, and the dilution of
inhibitors and improved distribution of enzymes and microorganisms within the landfill (Klink
et al. 1982; Christensen et al. 1996). Furthermore, Klink et al. (1982) concluded that moisture
movement through the MSW increased the methane production rate from 25% to 50%,
compared to no movement of moisture at the same moisture content levels. Compaction of the
waste and the presence of layers of poorly permeable material such as clay used for covering
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