WERF ROSI Project – Do We Need a Revised Time-Temperature Requirement to Achieve Class A Biosolids

Sudden increase (SI) of indicator bacteria, such as E. coli, in dewatered cake has been reported in processes that utilize thermophilic pretreatment followed by digestion or thermophilic digestion. The SI typically occurs when centrifuge dewatering is utilized. Increases of up to 4 or 5 orders of magnitude have been measured. The objective of this research was to investigate if increased time-temperature would better inactivate the indicator organisms and prevent SI. The work examined a range of time and temperature both in the lab and full-scale systems. The results suggested that an increased time-temperature for thermal treatment was associated with plants that did not have SI. In addition, a field trial with a pre-pasteurization plant showed that increasing the pre-pasteurization temperature from about 68 to the mid 70s decreased the extent of SI. Overall the results are supportive of revising the time-temperature for complete inactivation of indicator bacteria. However, this increased time-temperature regime is likely not necessary for the inactivation of Salmonella, as other research has shown that SI did not occur for Salmonella.