Newsletter and Technical Publications
<Municipal Solid Waste Management>
1.4.3 Environmental impacts of composting
Use of compost as a soil conditioner, a fertilizer, or
a growth medium has, of course, significant environmental benefits. In addition
to returning nutrients to the soil and thus permitting the reduction of
artificial fertilizers, compost is waste that does not have to be landfilled.
When it is used as daily cover at landfills, it replaces other materials that
would otherwise be used for that purpose.
However, there are also negative impacts on the environment associated with
making and using compost. These impacts depend both on the technical approach
used and the waste composition of the input streams. Mixed MSW and sewage sludge
composting pose greater risks because these materials typically contain higher
levels of heavy metals than do kitchen or yard wastes.
Environmental impacts of the compost process
Gases released from improperly maintained compost piles are a negative effect
associated with the composting process. When piles are not properly aerated,
colonies of anaerobic bacteria flourish and produce methane gas. The
decomposition process also releases carbon dioxide, volatile organic compounds,
bacteria, and fungi. The release of methane and carbon dioxide contributes to
the problem of greenhouse gases in the atmosphere. Poorly operated composting
facilities also cause unpleasant odors. Other air emissions are generated by the
combustion engines used to power windrow turning machines and grinders.
Leachate production is also common. Leachate from water runoff and
condensation at compost facilities occasionally contains levels of biological
oxygen demand (BOD) and phenols (a byproduct of the decomposition of the lignin
in leaves) that may exceed acceptable discharge limits, but pose few problems if
absorbed into the ground or passed through a sand filter. High concentrations of
BOD in runoff to surface water is a bigger problem, as this can reduce the
amount of dissolved oxygen in lakes and streams that is available for aquatic
life. Sound practice here is to avoid discharge to water and to capture or
direct all leachate to absorption in sand or soil.
Environmental effects of compost use
The most significant potential environmental problem arising from compost use
is its potential to convey heavy metals to the soil. This is a serious concern,
and sound practice requires controlling impacts through:
- analysis of composts;
- development and enforcement of land application standards; and
- research and development on pre-processing and process control mechanisms
to limit or reduce contaminants.