Occurrence and Levels of Fecal Indicators and Pathogenic Bacteria in Market-Ready Recycled Organic Matter Composts

Abstract

Landfill diversion of organic wastes through composting is making compost products available for agricultural and horticultural crops. On certified organic farms, nonsludge green waste and manure composts are widely used because the use of these products removes harvest date restrictions imposed by the U.S. Department of Agriculture when raw manure is applied. We quantified several pathogens in point-of-sale composts from 94 nonsludge facilities processing 2.2 million m3 year−1 of recycled green waste. Only one compost contained Salmonella (1.8 most probable number [MPN]/4 g), 28% had fecal coliforms exceeding the Environmental Protection Agency 503 sludge hygiene limits (1,000 MPN g−1), and 6% had detectable Escherichia coli O157:H7. In 22 of 47 samples, very low levels of Listeria spp. were found. However, in one sample the Listeria level was very high, coinciding with the highest overall level of all pathogen indicators. Seventy percent of the compost samples were positive for Clostridium perfringens, but only 20% of the samples had levels >1,000 CFU/g. All samples were positive for fecal streptococci, and 47% had >1,000 MPN g−1. Statistical analyses conducted using documented site characteristics revealed that factors contributing to elevated pathogen levels were large facility size, large pile size, and immaturity of compost. Application of the California Compost Maturity Index distinguished compost products that had very low levels of E. coli from those with high levels. Products produced with windrow methods were of higher microbiological quality than were those produced with static pile methods, and point-of-sale bagged composts scored very high. These data indicate that compost that is hygienic by common standards can be produced, but more effort is required to improve hygiene consistency in relation to management practices.