Predation and transport of persistent pathogens in GAC and slow sand filters: A threat to drinking water safety?

• Internalized (oo)cysts in rotifers from pilot GAC and SSF were enumerated.
• Survival and transport ratios for internalized (oo)cysts were estimated.
• A predation/transport conceptual model was developed for ingested (oo)cysts.
• Probability of infection from internalized (oo)cysts was estimated.
• Health risk from internalized (oo)cysts in filtered water is discussed.

Zooplankton has been shown to transport internalized pathogens throughout engineered drinking water systems. In this study, experimental measurements from GAC and SSF filtration tests using high influent concentrations of Cryptosporidium (1.3 × 106 and 3.3 × 104 oocysts L−1) and Giardia (4.8 × 104 cysts L−1) are presented and compared. A predation and transport conceptual model was developed to extrapolate these results to environmental conditions of typical (oo)cyst concentrations in surface water in order to predict concentrations of internalized (oo)cysts in filtered water. Pilot test results were used to estimate transport and survival ratios of internalized (oo)cysts following predation by rotifers in the filter beds. Preliminary indications of lower transport and survival ratios in SSF were found as compared with GAC filters. A probability of infection due to internalized (oo)cysts in filtered water was calculated under likeliest environmental conditions and under a worst-case scenario. Estimated risks under the likeliest environmental scenario were found to fall below the tolerable risk target of 10−4 infections per person per year. A discussion is presented on the health significance of persistent pathogens that are internalized by zooplankton during granular filtration processes and released into treated water.

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