Characterization of Man-made Foreign Matter And its Presence in Multiple Size Fractions From Mixed Waste Composting

Compost Science & Utilization, (2005), Vol. 13, No. 4, 274-280

William F. Brinton, Jr.

Woods End Research Laboratory, Mt Vernon, Maine

Municipal solid waste (MSW) has for decades been processed into compost. Community collection of residential trash of various degrees of separation followed by some form of presorting, tumbling or batch processing along with aerated static or mechanical-turned windrowing are typical in the industry. A significant portion of MSW is plastic or synthetic material including woven polyester and polyethylene materials that may easily reduce into fine fragments. European studies in the 1980's indicated significant contamination as a result of mixing and grinding which caused metals and glass to become reduced and enter the fine compost fraction. The results of these early studies provided impetus towards source separation,
nongrinding of initial MSW and later introduction of novel biodegradable plastics and natural polymers to replace plastic content for collection of household organics. The present study was conducted using several mixed waste compost streams from archived lab samples taken recently from across North America. The objective is to determine the extent that laboratory sieving and microscopy techniques may fully characterize compost with regard to foreign, man-made content. Results indicated that foreign matter comprised principally of glass and plastic fibers occupied all size fractions from 25 mm down to 420-micron sieving. Owing to the very small dimensions of certain foreign material, fragments are not easily measured by classical sorting procedures. We employ various separation techniques including dry and wet ultrasonic sieving, optical microscopy and FTIR to separate and classify foreign matter fines. The origin of fine
nondegradable materials as observed in the compost was likely building materials, carpets, textiles and diapers. Fragments of PET, polyester, polystyrene and foam were identified by simple and polarized-light microscopy and estimated to occupy up to 5% for all fractions. The study suggests that refined and improved characterization techniques for MSW compost may be important for better understanding of the fate and content of inerts. The study did not evaluate the potential risks of accumulation of nonbiodegradable fragments within sustainable soil systems nor ecological toxicity of components of these fractions. These data support the concept that in order to attain a high degree of cleanliness of finished MSW compost, early sorting and separation steps must be implemented prior to composting.

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