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The transport potential of Cryptosporidium parvum through intermittent, unsaturated, sand filters used for wastewater treatment was investigated using a duplicated, 2 2 factorial design experiment performed in bench-scale, sand columns. Eight columns (dia=15 cm, L=60 cm) were dosed 8 times daily for 61 days with 65,000 C. parvum oocysts per liter at 15 o C. The effects of media grain size and hydraulic loading rates on oocyst transport were examined. Surrogate wastewater was prepared following published methods. Effluent samples were tested for pH, turbidity, and oocyst content. C. parvum effluent concentrations were determined by staining oocysts on polycarbonate filters and enumerating using epifluorescent microscopy. At completion, the columns were dismantled and sand samples were taken at discrete depths within the columns. These samples were washed in a surfactant solution and the oocysts were enumerated using immunomagnetic separation techniques. The fine-grained sand columns (d50=0.31 mm) effectively removed oocysts under the conditions examined with low concentrations of oocysts infrequently detected in the effluent. Coarse-grained media columns (d50=1.40 mm) yielded larger numbers of oocysts which were commonly observed in the effluent. Factorial design analysis indicated that grain size was the variable which most affected oocyst effluent concentrations in these intermittent filters. Loading rate had a significant effect when coarse-grained media was used and lesser effect with fine-grained media. No correlations between turbidity, pH, and effluent oocyst concentrations were found. Pore-size calculations indicated that adequate space for oocyst transport existed in the filters. It was concluded that processes other than physical pore size exclusion are mainly responsible for C. parvum removal from wastewater in intermittent sand filters used under the conditions studied in this research.