Operational effects on pathogen passage: Assessing the risk of Cryptosporidium passage through drinking water filters

摘要

Recent investigations of operational effects on Cryptosporidium passage through granularmedia filters have demonstrated that, relative to optimized operating conditions, end-ofrunfiltration is a particularly vulnerable period of operation during which increasedCryptosporidium passage through filters may be expected. In the context of filteroperation, system failure can be considered the passage of pathogens through a filter ormultiple filters at levels that exceed a specified level or regulatory criterion. Informationregarding the risk of system failure and how this risk changes with respect to time iscritical to facilitating decision making and cost/benefit analyses for optimizingoperational strategies. Using previously reported pilot-scale Cryptosporidium removaldata obtained at the City of Ottawa pilot plant, the risk of an infectious dose ofCryptosporidium oocysts being released into a filter effluent and the subsequentdistribution system was modelled. The model results demonstrated a considerableincrease in risk when a filter is operated beyond 0.3NTU compared to operating the filteruntil 0.3NTU or to only 0.1NTU of filter effluent turbidity. The model also demonstratedthat operating multiple filters that are equally offset from one another is an alternativemethod to reducing the risk of Cryptosporidium passage through the filtration step of thetreatment process.