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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.

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