Environmental Engineering Reference
In-Depth Information
components and water vapor. The final stages mark the end of the reaction and a return to
aerobic conditions. Hydrogen sulphide gas may also form, derived from sulphate-reducing
micro-organisms, in wastes with a high concentration of sulphate.
Figure 6. Changes in composition of LFG and leachate during stages of waste decomposition (modified
from Pohland and Harper, 1986)
1.2. Factors affecting LFG generation
Solid waste disposal sites are by nature heterogeneous. Microbiological investigations
into site characteristics have shown that there are considerable differences between different
SWDSs and even different regions within the same SWDS. This makes it very difficult to
extrapolate from observations on single SWDSs to predictions of global CH
4
emissions.
Nevertheless, a better understanding of the factors thought to most significantly influence the
generation of CH
4
from land disposal of solid waste can reduce the uncertainty associated
with emissions estimates.
1. Site characteristics
Landfill sites with waste depths exceeding 5m tend to develop anaerobic conditions and
greater quantities of landfill gas. Shallower sites allow air interchange and lower anaerobic
activity, and consequently lower landfill gas production. However, if the site is well capped,
anaerobic conditions will be created. Similarly, rapid covering of the waste will reduce the aerobic
phase, and since this is the increasing temperature phase, this will tend to keep waste temperatures
down. Also, rapid covering of the waste will reduce the chance of rainfall increasing the moisture
content of the waste, which in turn reduces the initial rate of biodegradation.
2. Waste characteristics
The major components of municipal solid waste include the biodegradable fraction, that
is, the paper and board, food and garden waste, and non-biodegradable components, plastics,
glass and textiles. The amount of gas produced will vary depending on the proportion of
biodegradable components in the waste. This fraction has been shown to vary depending on a
number of factors, for example, higher concentrations of garden waste are produced in spring
and autumn, and more industrially developed countries produce more paper. In addition, the
bacteria that break down the waste require small amounts of certain minerals such as calcium,
