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.