Occurrence and levels of fecal indicators and pathogenic bacteria in market-ready recycled organic matter composts.

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 (1000 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 >1000 CFU/g. All samples were positive for fecal streptococci, and 47% had >1000 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.

Occurrence and detection of viable Listeria in food scrap compost.

Listeria species (L. innocua, L. ivanovii, L. seeligeri, and L. grayi) were readily detected in food scraps by Nucleic Acid Hybridization (NAH) probes using a standard Listeria selective medium (UVM-1) at ambient temperature. Various food scrap compost recipes artificially contaminated with Listeria at 10(7) cells per gram wet weight were composted in thermally insulated bench scale reactor vessels. These Listeria were not detected when the compost temperature became elevated. Different isolation methods for the Listeria showed this result to be a false negative occurring apparently because the heat stressed Listeria were unable to survive in the selective medium (UVM-1). Once incubated at 37 degrees C in Universal Listeria medium (ULM), the Listeria were detectable for a short period in compost at temperatures as high as 64 degrees C.

Survival of E. coli and Salmonella populations in aerobic thermophilic composts as measured with DNA gene probes.

Aerobic, thermophilic composting is a widely practiced method for disposal of organic wastes. The wastes which are composted include biosolids from waste water treatment plants (WWTP), and biowastes (food scraps and yardwaste). Important hygiene issues are involved in composting since many potential pathogens may be present in the fresh wastes. In this study, the survival of Salmonella and Escherichia coli is examined during aerobic composting of municipal solid wastes, municipal wastewater sludge and biowastes. A laboratory compost was prepared by inoculating with 10(7) Salmonella typhimurium Q and Escherichia coli B. In both industrial and laboratory trials, gene probes were used to determine at what time during the composting and at what temperature these bacteria became undetectable. It was observed that Salmonella and E. coli survived for 59 days at about 60 degrees C in an industrial compost. The bacteria became undetectable after the temperature decreased from 62 degrees C to about 40 degrees C in the compost curing. The bench scale trials showed that E. coli B survived for at least 9 days at 60-70 degrees C in a biowaste (food waste) compost or a waste water sludge compost. Salmonella typhimurium Q survived for at least 9 days over 60 degrees C in the food biowaste compost and at least 5 days in the waste water sludge compost. Data collected show that the temperature or the time of high temperature is difficult to correlate to the destruction of the pathogen, Salmonella, or the pathogen indicator, E. coli. These results suggest that the mechanism for removal of these microorganisms during aerobic composting is complex and not simply the result of a thermal physical environment.