Impacts of Process Enhancements on Fecal Coliform Concentrations in Anaerobically Digested Biosolids

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Approximately 60 percent of wastewater utilities across the nation land apply biosolids for agricultural use. The US EPA approved methods most commonly used to measure pathogens killed and to ensure biosolids are safe for land application are (1) monitoring time and temperature during the treatment process (operational standards) or (2) testing for the presence of fecal coliform bacteria (indicator organisms) after treatment. Most utilities, including Hampton Roads Sanitation District (HRSD), have traditionally used the time and temperature method to demonstrate compliance with state and federal regulations.

In June, 2006, the Water Environment Research Foundation (WERF) released a report entitled Examination of Reactivation and Regrowth of Fecal Coliforms in Centrifuge Dewatered, Anaerobically Digested Sludges. This study evaluated levels of fecal coliform bacteria in dewatered biosolids at seven wastewater treatment facilities. Four of the facilities tested reported increased fecal coliform levels after anaerobic digestion and after high speed centrifuge dewatering. The WERF research, while extremely limited in scope, suggests the possibility that some dewatering processes following digestion may exhibit re-activation and/or re-growth of fecal coliform bacteria.

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