Effects of filter operation on Cryptosporidium removal

Physicochemical removal of protozoan pathogens is receiving increased attention because of the difficulty of chemically inactivating these organisms, particularly Cryptosporidium parvum. Most research examining the removal of these and other pathogens by filtration has been conducted under steady-state conditions with optimized pretreatment. This study evaluated the removal of Cryptosporidium and changes in surrogate parameters at various points in the filter cycle and under nonoptimal conditions at two pilot plants with different coagulation regimes. The study found a reproducible 2-log difference in Cryptosporidium removals between the two locations under optimal conditions, with similar low effluent turbidity levels and particle counts. Either suboptimal coagulation or the early stages of breakthrough at the end of a filter run produced substantial deterioration of Cryptosporidium removal capability. Filter ripening or the imposition of a hydraulic step generally had much less effect on removals.