Reduction of MS2 bacteriophage and rotavirus in biosand filters

Abstract

Diarrheal diseases caused by pathogens remain a significant cause of death in developing areas, especially for children under the age of five. Biosand filters (BSFs) are a promising technology implemented worldwide that can effectively reduce levels of bacteria and bacteriophages. However, besides echovirus, the efficacy of enteric virus reduction in BSFs has not been studied. Furthermore, how divalent cation concentrations in the source water used in BSFs influences virus reduction is not clearly understood. In this study, three bench-scale BSFs were fed daily with groundwater containing divalent cations or cation-free buffered solution to determine MS2 or rotavirus reduction as a function of filter depth, residence time, media ripening, and water source. An integrated cell culture and RT-qPCR assay was developed to quantify rotavirus reduction in water samples collected from the filters. Rotavirus reduction obtained by experiments performed in groundwater increased with depth and reached a cumulative average of 5 log₁₀ (99.999%) reduction after 31 days. Experiments with 1 mM NaHCO₃ spiked with MS2 averaged 1.2 log₁₀ reduction after 42 days, and there was not an increasing trend of reduction as a function of depth. Finally, MS2 experiments performed in groundwater reached a cumulative average of 5.36 log₁₀ reduction by a BSF that had been in operation for 684 days, but the reduction also did not increase as a function of depth. Overall conclusions include that 1) at the same filter age and using the same water source, rotavirus reduction was higher than what was previously seen with MS2, indicating that MS2 is a conservative surrogate for rotavirus, 2) MS2 reduction efficacy was related to the divalent cation concentrations of the influent water for an unripened filter, and 3) residence time was crucial for increasing virus reduction in all experiments. This is the first study to determine the efficiency of rotavirus reduction in BSFs, which is an essential first step in understanding the extent to which BSFs can reduce human enteric viruses, and hence decrease diarrheal disease incidence.