Environmental Engineering Reference
In-Depth Information
potassium and magnesium and other micronutrients. If these are present the bacteria thrive
and gas is produced rapidly. If they are lacking or if substances that inhibit bacterial growth
are present, gas production will proceed more slowly and in extreme circumstances may stall
completely. In addition, the composition of the organic components, that is, the proportion of
cellulose, proteins and lipids, will similarly influence the degradation pathway. The rate at
which gas is produced depends on the proportions of each type present in the waste.
Shredding or pulverisation of the waste prior to landfilling results in increased available
surface area and consequent increased homogeneity and increased rates of biological
degradation. The density or degree of compaction of the waste in the landfill will increase the
amount of biodegradable material available for degradation and therefore increase the
production of landfill gas per unit volume of void space in the landfill. Too high a degree of
compaction, however, may limit the percolation of water through the site, which is necessary
for the free flow of nutrients for the micro-organisms.
3. Age of the waste
Landfill gas production begins as soon as waste has been deposited, but anaerobic
methane production only occurs when all of the available oxygen has been absorbed. Peak
landfill gas production generally occurs about a year after deposit and thereafter gradually
declines. Significant gas production is generally completed within about 20 years of
deposition, but every site is different. Where gas production is slow, the period of significant
gas production may extend for 40 or 50 years. Also, older landfill sites have been shown to
contain lower proportions of biodegradable waste than modern sites due to the changing
nature of waste over the last few decades.
The pattern of gas production for an entire site is the sum of the performance of all of the
individual components of waste. Some will be rapidly enter the gas generation stage, others
will be slower, particularly where the period over which waste has been deposited, has been
many years. Similarly, the period of significant gas production will vary, and for an entire site
most often extends over several decades. Figure 7 below shows an idealised and fairly typical
sequence of LFG production, although actual durations will vary greatly site by site.
Figure 7. Variations in Rate of Landfill Gas Production with Time
Source: UK Department of Energy (1992)
