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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 HRSD, have traditionally used the time and temperature method to demonstrate compliance with state and federal regulations.

The recently (June 2006) published Water Environment Research Foundation (WERF) study entitled Examination of Reactivation and Regrowth of Fecal Coliforms in Centrifuge Dewatered, Anaerobically Digested Sludges 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 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. These biosolids are typically land applied after Class B mesophilic digestion and high solids centrifugation. Initial test results indicated an increase in fecal coliform bacteria concentrations after dewatering. Although HRSD remains in compliance with state and federal operational standards and EPA has re-affirmed that the time and temperature operational standards of the existing regulations are adequately protective of public health when biosolids are land applied in accordance with their regulations, HRSD wants to assure the public that their land application program meets both regulatory 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 December 2006 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 high solids centrifuge vs. belt filter press cake solids as well as extended storage methods and combinations of operating changes were evaluated in the field. This paper presents the finding of this testing and reports on the methodologies used, the options tested, and the efficiency of the low lime dosing process in meeting the operating objectives.

Document Type: Research Article


Publication date: 2007-01-01

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