| Recent studies have reported significant increases in the counts of indicator bacteria for anaerobically digested sludge samples following centrifugal dewatering. As bacterial enumeration at the treatment plant is done immediately following digestion, this could cause samples to exceed regulatory standards for land disposal. The study was divided into two phases. In the first phase it was found that the centrifuge shear, as well as the polymer type and dose did not significantly affect the fecal coliform counts through a laboratory centrifuge. In the second phase, although immediate reactivation of fecal coliforms was not observed, significant regrowth was detected for cake samples at one of two treatment plants. This regrowth was only possible for samples in the absence of centrate. Chemical characterisation showed that the full-scale centrate had significantly higher sulphide levels and provided a more stressful environment for the fecal coliforms than centrate from a laboratory centrifuge. Counts of total bacteria in the samples were not consistent with the counts of the culturable fecal coliforms. |
