IMPACTS OF PROCESS ENHANCEMENTS ON FECAL COLIFORM CONCENTRATIONS IN ANAEROBICALLY DIGESTED BIOSOLIDS

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.When the HRSD became aware of the WERF study results in May 2006, a series of tests were initiated to measure fecal coliform levels in biosolids cake generated at the Atlantic Treatment Plant where mesophilic anaerobic digestion is practiced using four digesters operating in series at a temperature of 37°C with an average detention time of 20–22 days. The resultant Class B biosolids are typically land applied after conventional centrifugation. Initial test results indicated an increase in fecal coliform bacteria concentrations after dewatering. Although in compliance with state and federal operational standards, HRSD wanted to assure the public that their biosolids land application program meets both the time and temperature and fecal coliform density pathogen control methods. In response to this finding, HRSD temporarily suspended biosolids land application until tests could verify that both the time and temperature and fecal coliform criteria were being consistently met.HRSD conducted additional tests in June through March 2007 to determine methods of preventing the fecal coliform re-activation/re-growth phenomenon. The addition of low doses of lime to biosolids after dewatering is one proven strategy mentioned in the WERF report that was tested at HRSD and found to be dependable in consistently achieving this goal. However, increased odor production was noted with the biosolids cake that was treated with low level lime dosages. Additional testing of dewatering methods comparing centrifuge dewatering and belt filter press dewatering, as well as extended storage methods and combinations of operating changes were evaluated in the field. Further, odor generation evaluations and digestion evaluations were performed. This paper presents the finding of this testing and reports on the methodologies used, the options tested, and the final strategy employed by HRSD to achieve compliance with either USEPA Class B criteria.