The production of compost is both a mechanical and a biological process. The raw materials must first be separated, collected, and shredded by mechanical means before the biological decomposition process can begin. In some cases, the decomposition process itself is aided by mechanical agitation or aeration of the materials. After decomposition, the finished compost is mechanically screened and bagged for distribution.
Compost made from yard wastes, such as leaves and grass clippings, rarely contains any harmful materials. Problems can occur, however, when compost is made from partially sorted municipal refuse, certain industrial wastes, or sewage sludge. In those cases, unacceptable levels of toxic metals, chemicals, or harmful bacteria may be present.