Effect of total solids on fecal coliform regrowth in anaerobically digested biosolids

Fecal coliform (FC) concentrations in anaerobically digested biosolids can increase during centrifugal dewatering and afterwards in storage of dewatered cake. The immediate increase after centrifugation (reactivation) has been demonstrated to be the revitalization of fecal coliforms that had become non-culturable. The increase during storage (regrowth) has been regarded as a subsequence of reactivated bacteria growing in a favorable environment. In this paper, however, regrowth is demonstrated without preceding reactivation, using intensive laboratory centrifugation to duplicate the levels of regrowth seen in full-scale centrifugation. Higher total solids (TS) levels of the dewatered biosolids lead to greater magnitudes of FC increase. The final TS level appears much more important than the level of shear imposed during centrifugation, based on comparison of different centrifugation/dilution procedures used to obtain similar TS levels. The greater TS levels also reduce methane production, suggesting that methanogens compete with, or inhibit, the fecal coliforms. The addition of bromoethanesulfonate as a methanogen-specific inhibitor decreased the production of methane gas, and also increased the number of fecal coliforms.