Granular filtration has been incorporated as a major barrier to prevent the dissemination of disease-causing agents by drinking water. Particles and pathogens such as Giardia lamblia and Cryptosporidium parvum retained in the filters are then washed by utilizing clean water. This study was conducted, primarily, to evaluate coagulation as a pretreatment for the Spent Filter Backwash Water (SFBW) treatment by ultrafiltration (UF). SFBW Samples were collected from four different water treatment plants and carefully analyzed. Jar-tests and backwash pilot studies were performed in the laboratory. Depending on the water source, protozoan parasites and viruses were found to be prevalent in SFBW. The results show that turbidity cannot serve as a surrogate for the microbial load of the SFBW. Alum flocculation pretreatment of SFBW was found to be effective in reducing turbidity, particle count, viruses and parasites, consequently it may also reduce membrane fouling. Settling the SFBW prior to flocculation did not enhance the removal of turbidity and particle count as compared to the unsettled SFBW samples. This finding might imply that settling would not be required prior to UF. The largest remaining particle fraction after alum flocculation was 3-10 μm in size, both Cryptosporidium and Giardia are found in this size range. Coagulation enhanced the removal of small size particles and may result in extending the filtration cycle by reducing the SFBW fouling potential.

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