Anaerobically Digested Biosolids Beyond Class B

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 Hampton Roads Sanitation District (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 (4) digesters operating in parallel 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. The results of testing performed in 2006 and early 2007 were previously summarized (Williams and Bowen, 2007). This paper summarizes additional testing and the resultant management strategy used to ensure all land applied biosolids achieve USEPA Class B standards and fecal coliform concentrations below 2,000,000 MPN/g TS.