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INCREASE OF INDICATOR ORGANISMS FOLLOWING ANAEROBIC DIGESTION AND CENTRIFUGE DEWATERING.
MECKES, M. C. AND R. STEVENS. INCREASE OF INDICATOR ORGANISMS FOLLOWING ANAEROBIC DIGESTION AND CENTRIFUGE DEWATERING. Presented at ANNUAL BIOSOLIDS CONFERENCE/WORKSHOP, POTOMAC, MD, October 08 - 11, 2007.
to inform the public
The Water Environment Research Foundation (WERF) recently published a report titled “Examination of Reactivation and Regrowth of Fecal Coliforms in Anaerobically Digested Sludges”. Seven full-scale publicly owned treatment facilities were sampled several times to determine if bacterial indicator organisms (e.g., Fecal coliforms and E. coli) could become reactivated or if regrowth occurs after anaerobic digestion and high-solids centrifugation. Wastewater treatment facilities often use dewatering processes after anaerobic digestion to reduce the water content of sewage sludge for more efficient handling and transport. Four facilities demonstrated higher levels of fecal coliforms after the anaerobically digested sludge was processed using high-solids centrifuges. The other three facilities did not show an increase in fecal coliforms following high-solids centrifugation. Researchers theorize that some bacteria populations increase after prolonged stress experienced during anaerobic digestion, and that the high shear during high-solids centrifuge dewatering may release compounds that induce reactivation. Other possibilities include a change in environmental conditions or removal of bacterial growth inhibitors.
From EPAs perspective, the WERF study raises some concerns over the potential for pathogen reactivation following anaerobic digestion and centrifugation. However, we cannot assess how widespread this occurrence is because of the small sample size. Since Class B biosolids may contain some viable pathogens, site restrictions that restrict food crop harvesting, animal grazing, and public access are required. This allows environmental stress factors to further reduce pathogens in Class B biosolids.
The study was not able to identify all the conditions that caused some plants to have reactiviation while others did not. The ability to estimate the level of pathogens in sewage sludge (through testing for indicator organisms) at various points in the treatment process is important to assure the protection of public health. This is particularly true for land-applied sewage sludge. The WERF study does not call into question the protection of the two million fecal coliform treatment standard of the 40 CFR Part 503 standards. Rather, the study questions the efficacy of treatment using anaerobic digestion followed by high-solids centrifugation.