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Disposal of human biosolids is a source of concern for public health and the environment. Composting appears to be an interesting alternative to traditional disposal methods as it can decrease the load of human pathogenic microorganisms often present in biosolids and yield an end-product rich in nutrients for use as a soil supplement. Assessing the exact microbial content of biosolids, both for biosafety and operational reasons, has traditionally relied on the use of standard microbiological methods. Recent developments in molecular-based technologies now offer more rapid and specific monitoring of microorganisms in biosolids than culture-based methods. In this study, denaturing gradient gel electrophoresis (DGGE) was adapted to monitor the succession of bacteria in composted biosolids through different steps of compost production. Secondly, a TaqMan quantitative real time PCR (qPCR) approach was developed to detect and quantify the presence of Salmonella species, a model human pathogenic bacterium, susceptible to be found in biosolids. DGGE results indicated that the bacterial content of composted biosolids of different ages belongs to various taxa and significantly changes with age. qPCR results indicated that the quantity of Salmonella species found in composted biosolids ranging from 1 to 24 months significantly decreases with composting time.

Composting is increasingly used to transform biosolids, obtained following wastewater treatment, into a more stable organic product that can be released in the environment. The process must however be closely monitored to assure that the end product meets the regulations set by environmental agencies with regards to the amount of pathogenic microorganisms present. In this study, a TaqMan qPCR approach targeting the invA gene was developed to monitor the presence of Salmonella spp. in composted biosolids. A validation step was first performed to evaluate the effect of compost age on the quantification of various concentrations of seeded Salmonella typhimurium. Secondly, qPCR was used to investigate the effect of composting time, varying from 1 month to 24 months, on the presence of Salmonella spp. naturally present in biosolids samples. Culture media were used in parallel to corroborate the results obtained by qPCR. The detection limit of the invA gene obtained experimentally from composts seeded with S. typhimurium was 5.8 copies or the equivalent of 5.8 CFU per qPCR reaction. Although the results indicated that compost age had a marginal effect on the detection of seeded S. typhimurium, the TaqMan qPCR approach was efficient at detecting and quantifying the amount of Salmonella spp. present in naturally contaminated composted biosolids of different ages. Results showed that there was a significant decrease in the amount of Salmonella DNA present in composted biosolids over time, which was also corroborated by the CFU counts obtained on the BSA culture medium. However, qPCR was more specific, robust and rapid to execute than performing counts on culture media. qPCR shows promise for routine examination of composted biosolids to ascertain that pathogenic microorganisms, including Salmonella spp., are decreased below acceptable limits before their application in the environment.