PCR was used to detect Cryptosporidium parvum oocysts in secondary effluent samples from activated sludge systems. Serial dilutions of the purified nucleic acid extract from the samples were made and PCR was conducted to estimate the C. parvum oocyst concentration via a Poisson distribution based most probable number (PCR/MPN). The degree of oocyst association with wastewater particles was also evaluated. The sensitivity of the PCR was 20 oocysts whereas the detection limit of the wastewater samples ranged from 5 to 1000 oocysts/L depending on the extent of inhibition in each sample. The recovery efficiency of the oocysts ranged from 48 to 59% in most samples. Oocysts were found in two out of seven samples with concentrations of 120 and 160 oocysts/L, as estimated by the PCR/MPN method. The oocysts were found only in the filtrate of the grab samples, particle associated oocysts were not detected. Association of spiked C. parvum oocysts with particles in secondary effluent drawn from wastewater plants with varying operating conditions indicated a weak correlation between the degree of association and the mean cell residence time (MCRT) of the system. A model developed to describe the degree of oocyst association with wastewater particles in the activated sludge system indicated that the fraction of particle associated oocysts in the effluent typically would be less than 1% and increase as MCRT increased.
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