Transport and fate of Cryptosporidium parvum oocysts in intermittent sand filters

The transport potential of Cryptosporidium parvum (C. parvum) through intermittent, unsaturated, sand filters used for water and wastewater treatment was investigated using a duplicated, 2³ factorial design experiment performed in bench-scale, sand columns. Sixteen columns (dia=15 cm, L=60 cm) were dosed eight times daily for up to 61 days with 65,000 C. parvum oocysts per liter at 15°C. The effects of water quality, media grain size, and hydraulic loading rates were examined. 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 variety of 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 regardless of operating conditions. Factorial design analysis indicated that grain size was the variable which most affected the 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 while the effect of feed composition was inconclusive. 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 therefore concluded that processes other than physical straining mechanisms are mainly responsible for the removal of C. parvum oocysts from aqueous fluids in intermittent sand filters used under the conditions studied in this research.