Many compost producers' finished product would not be saleable without screening. Leaf and yard waste compost is routinely screened to remove large amounts of woodchips, sticks etc. Many compost operations who accept food waste (particularly super market waste) as a raw material have to contend with significant amounts of non compostable materials – predominantly plastics. These materials must be removed by a screen if the waste generators cannot improve their source separation.
Screened compost is the end result of loading finished compost into a large piece of diesel engine-operated equipment complete with conveyors and a long rotating cylindrical screen which removes large quantities of contaminants such as plastic, woodchips, sticks, stones etc.
Compost screens are very expensive to own and even rent – particularly if they are used infrequently. The screening process is time-consuming and is not 100% effective, particularly for removing small pieces of plastic. Compost operations who tub grind raw materials at the beginning of the composting process are much more likely to have lots of small, uncompostable contaminants in their screened compost. Very moist finished compost (which tends to be the norm) is difficult to effectively screen because it tends to develop clumps, which end up in the reject pile with contaminants.
To effectively minimize contaminants in the finished product, Bear Path Compost has adopted a low tech approach. All suppliers of raw materials (food waste, horse bedding from fairs etc.) are instructed about the need to keep contaminants to a minimum. If particular contaminants become a problem, the waste generator is informed and asked to take corrective action. But even still there will always be a very small amount of contaminants to contend with. Our low tech method of removing 99.9% of the contaminants is not the most pleasant activity, but is effective. During the first three or so turnings, contaminants are removed by hand, one turned bucket at a time. A little time consuming but cost-effective for our operation where the incoming materials have minimal contaminants to begin with. So if you find a plastic fork, or other “artifact” in your compost, please remember the environmental service that we are providing and that we try our very best to keep the contaminants in our finished compost as close to zero as is practical.
For most applications the unscreened compost that Bear Path Compost provides is ideal. This compost is slightly coarser in texture and is excellent as a soil amendment, a top dressing around trees and shrubs and as a nutrient and microbe rich mulch for perennial beds.