Screening-level quantitative microbial risk assessments (QMRA) undertaken to assess health risks associated with the reticulation of recycled water have identified distribution pipe biofilms and their ability to accumulate enteric and opportunistic pathogens as potential sources of public health concern. A simplistic model to assess human health risks associated with enteric virions (extracellular viruses) present in recycled water for domestic use was therefore developed. Recycled water biofilms formed on glass and stainless steel coupons in a laboratory-scale distribution system were challenged with model enteric virions (B40-8, MS-2 and FX174 bacteriophages). Approximately 1% of bacteriophages present in the adjacent bulk water was incorporated into 3 month-old biofilms and a persistent sub-population (0.01%) of the model enteric virions remained infectious in biofilms throughout an experimental period of 30 days. Significant potential for virions to accumulate within biofilms was therefore demonstrated, and indeed biofilm uptake could mask treatment failures assessed through analysis of the bulk water (infectious virions would not be detected after 1.5 km even if 10.L-1 virions present in treated effluent). Subsequent sloughing of biofilm into the bulk water phase could therefore release infective virions within mobilized aggregates of biofilm and infect susceptible consumers. During normal operating conditions (one virion in 100 L of water) sufficient virions may accumulate within distribution pipe biofilms that a 50% sloughing event could present a Pi (annual) of 1.9 × 10-4 for the consumption of 1 mL of water, exceeding the US-EPA benchmark of 10-4.

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