waste can represent a significant source of methane. The anaerobic
decomposition of livestock and poultry manure, common to manure
heaps and slurry tanks, leads to large amounts of methane production
due to its large organic carbon content. Similarly, the processing
of industrial and domestic waste water and sewage can also produce
significant amounts of methane.
In total, such waste accounts for between 14 and 25 million tonnes
of methane emission per year globally. Historically, methane emission
from this source is likely to have been much lower due to lower
livestock numbers and relatively low livestock densities. Today
though consumer demand, for meat and dairy products in particular,
requires ever higher numbers and densities of livestock.
Clearly, man is directly responsible for methane emission from
livestock manure and human sewage. Where animal manure would,
in the wild, be spread over a wide area and decompose aerobically
in the natural environment, intensive livestock rearing methods
mean high concentrations of manure build up in relatively small
areas. Such conditions lead to a predominance of, methane producing,
anaerobic decomposition of the manure.
In a similar way, greater human numbers and population densities
have led to larger concentrations of waste water and sewage in
collection areas, such as sewage works. Human waste processing
can also produce large amounts of methane if anaerobic decomposition
is allowed to predominate.
Potential for control
The trapping of methane from strong sources of livestock manure
methane, such as slurry tanks, has already proved a very successful
way of reducing methane emissions to the atmosphere from this
source. The recovered methane, often called 'biogas', can be simply
flared off as carbon dioxide or can potentially be used as a fuel.
Other options include a move away from such intensive rearing
methods, with an increase in grazing time for animals and so a
greater dispersal of their manure. For human waste water and sewage,
ensuring aerobic decomposition using aeration methods is an oft
employed strategy, though methane trapping and subsequent burning
is practiced at some sites.