AWWA WQTC56992

Recent investigations of operational effects on Cryptosporidium passage through granular media filters have demonstrated that, relative to optimized operating conditions, end-of- run filtration is a particularly vulnerable period of operation during which increased Cryptosporidium passage through filters may be expected. In the context of filter operation, system failure can be considered the passage of pathogens through a filter or multiple filters at levels that exceed a specified level or regulatory criterion. Information regarding the risk of system failure and how this risk changes with respect to time is critical to facilitating decision making and cost/benefit analyses for optimizing operational strategies. Using previously reported pilot-scale Cryptosporidium removal data obtained at the City of Ottawa pilot plant, the risk of an infectious dose of Cryptosporidium oocysts being released into a filter effluent and the subsequent distribution system was modeled. The model results demonstrated a considerable increase in risk when a filter is operated beyond 0.3NTU compared to operating the filter until 0.3NTU or to only 0.1NTU of filter effluent turbidity. The model also demonstrated that operating multiple filters that are equally offset from one another is an alternative method to reducing the risk of Cryptosporidium passage through the filtration step of the treatment process. Includes 12 references, tables, figures.