Transport and fate of Cryptosporidium parvum oocysts in intermittent sand filters

The transport potential of Cryptosporidium parvum (C. parvum) through inter mittent, unsaturated, sand filters used for water and wastewater treatment was investigated using a duplicated, 2(3) factorial design experiment perfo rmed 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 oocys ts per liter at 15 degreesC. The effects of water quality, media grain size , and hydraulic loading rates were examined. Effluent samples were tested f or pH, turbidity, and oocyst content. C. parvum effluent concentrations wer e determined by staining oocysts on polycarbonate filters and enumerating u sing epifluorescent microscopy. At completion, the columns were dismantled and sand samples were taken at discrete depths within the columns. These g samples were washed in a surfactant solution and the oocysts were enumerate d using immunomagnetic separation techniques. The fine-grained sand columns (d(50) = 0.31 mm) effectively removed oocysts under the variety of conditions examined with low concentrations of oocyst s infrequently detected in the effluent. Coarse-grained media columns (d(50 ) = 1.40 nim) yielded larger numbers of oocysts which were commonly observe d in the effluent regardless of operating conditions. Factorial design anal ysis indicated that grain size was the variable which most affected the ooc yst 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 inconclusi ve. No correlations between turbidity, pH, and effluent oocyst concentratio ns were found. Pore-size calculations indicated that adequate space for ooc yst transport existed in the filters. It was therefore concluded that proce sses other than physical straining mechanisms are mainly responsible for th e removal of C. parvum oocysts from aqueous fluids in intermittent sand fil ters used under the conditions studied in this research. (C) 2001 Elsevier Science Ltd. All rights reserved.