Reactivation and/or growth of fecal coliform bacteria during centrifugal dewatering of anaerobically digested biosolids

Fecal coliform bacteria are used as indicator organisms for the presence of pathogens. In sludges, it has often been assumed that the counts of fecal coliforms after digestion (where the sludges may also be called biosolids) are representative of the counts when the sludge is disposed or recycled, such as by land application. The possibility has been raised, however, that dewatering processes can lead to increased counts of fecal coliforms and, by inference, human pathogens. This paper presents data from previous studies of this possibility; the results were inconsistent but showed observable increases in fecal coliforms at one treatment plant. Additional studies were then performed at another treatment facility, which showed statistically significant increases in fecal coliform counts after dewatering and two days of aging. The increases exceeded two orders of magnitude and included two centrifuge types and two biosolids types. Artifacts of media type and enumeration method have been excluded, and shearing of the material by commercial blender did not produce the same effects.