EFFECTS OF DEWATERING ON BACTERIA INACTIVATION: Centrifuge Simulation And Field Tests at The Hyperion Treatment Plant

The Hyperion Treatment Plant produces Class A biosolids by thermophilic anaerobic digestion according to the EPA 40 CFR Part 503 Biosolids Rule. Additional studies were performed to determine the effect of post-digestion processing on the regrowth of pathogens in biosolids. Potential sources of contamination by pathogens during postdigestion processing are in-plant transfer (pipelines), de-watering (diluted polymer addition and centrifugation), and storage. Temperature profiles along the plant's postdigestion train showed that cooling of biosolids was sufficient to allow for regrowth of pathogens. Laboratory experiments focused on re-growth patterns of Salmonella and fecal coliform in digested sludge (immediately after digester) and in wetcake (immediately after centrifuge) at ambient temperature. Salmonella regrowth was not observed in any of the experiments, nor in the field. Fecal coliform regrowth was observed in wetcake stored in silos at the truckloading facility, but not in laboratory experiments with digested sludge and centrifuge wetcake. Fecal coliform regowth studies were performed by centrifuging digested sludge in the laboratory under conditions that simulated the field dewatering system. These experiments showed that polymer and secondary effluent additions prior to dewatering of digested sludge are probably not significant factors that stimulate fecal regrowth.