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Several Investigators have reported higher densities of indicator bacteria after dewatering of anaerobically digested biosolids. The increases appear to occur at two points in the biosolids process: the first, referred to as "sudden increase", occurs immediately after dewatering; the second, "r'J|^wth", occurs during storage over longer periods. The objectives of digestion and dewatering fecal coliform and £, coll. and five mesophiiic digesf ilter press dewatering, Suj thermophilic processes witj in the mesophilic processj dewatering, Regrowth was processes with centrifuge dŴ ater Environ. Res., 83, 7' [is study were to examine the effect of resses on sudden increase and regrowth of i)les were collected from five thermophilic lorocesses, with either centrifuge or belt increase typically was observed in the Ktrifuge dewatering and was not observed fith either centrifuge or belt filter press feerved in both thermophilic and mesophilic latering but not belt filter press dewatering, 12011),

In August of 2001, the largest known installation of a phased temperature anaerobic process at a 60-MDG wastewater treatment plant was placed into operation. The facility met the time and temperature requirement for Class A biosolids. Testing of the biosolids following thermophillic/anaerobic digestion followed by mesophilic/anaerobic digestion revealed no detectable levels of fecal coliform bacteria in the treated biosolids. However, subsequent testing of the biosolids following dewatering by high solid centrifugation revealed high levels of fecal coliform bacteria. These biosolids, following high solid centrifugation, did not meet Class B requirements.This study indicated a very serious reactivation of fecal coliform bacteria following high solid centrifugation. Fifty-three percent of the fecal coliforms isolated were identified as Eschericia coli with two of the isolated organisms identified as E.coli 0157:H7. E.coli 0157:H7 has been shown to be capable of formation of an autoinducer in the presence of norephinephrine. The autoinducer triggers the growth of gram-negative bacteria or the conversion of gram-negative bacteria such as fecal coliforms from a non-culturable to culturable state. It is, therefore, hypothesized that the presence of E.coli 0157:H7 may be involved in the reactivation of fecal coliform bacteria.

The objectives of this research were to evaluate the potential for sudden increase and/or regrowth of alternative bacteria as either indicators or pathogens after dewatering of tiiermophilic and mesophilically digested biosolids. The results showed that, in general, for thermophilic processes, even when a statistically significant {p<0.05) sudden increase and regrowth occurred for fecal coliforms, Escherichia coli, and Enterocoeci, it did not occur for Salmonella or Aeromonas. For the mesophilic process evaluated, sudden increase did not occur, but regrowth occurred for fecal coliforms, £, coli, Enterocoeci, and Salmonella. The results have implications for Class A and B biosolids regulations, as both fecal coliform and Salmonella are part of the regulatory limits. The results also suggest that the public health risks are minimal, as a result of the potential sudden increase and regrowth that may occur.

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