Wastewater samples collected from ten wastewater facilities across the US were analyzed to determine the occurrence of indigenous Cryptosporidium oocysts using methods based on modifications of USEPA Method 1622. Wastewater facilities participating in this study ranged in size from 0.6 to 193 mgd average daily flow. A total of 289 wastewater samples were analyzed over a 15-month period. ColorSeed is a commercial product containing gamma-irradiated Cryptosporidium oocysts that have been permanently stained with a Texas Red dye. ColorSeed was used as an internal control with each sample to assess method performance. In 500 ml sample volumes, mean ColorSeed recoveries in raw influents and primary effluents were 26.1±17.7% and 33.0±17.9%, respectively. In 10 liter volumes of secondary effluent, mean ColorSeed recovery was 25.0±16.6%. Volumes analyzed for tertiary effluent samples ranged from 14.8 l to 131.3 l resulting in a mean ColorSeed recovery of 48.8±14.5%. Indigenous oocysts were detected in 30% of raw influents, 46% of primary effluents, 58% of secondary effluents and 19% of tertiary effluents analyzed. Indigenous oocyst concentrations ranged from <2 to 86 /liter across all wastewater matrices tested.

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