Arsenic removal from drinking water by a household sand filter in Vietnam - Effect of filter usage practices on arsenic removal efficiency and microbiological water quality

Abstract

Household sand filters are applied to treat arsenic- and iron-containing anoxic groundwater that is used as drinking water in rural areas of North Vietnam. These filters immobilize poisonous arsenic (As) via co-oxidation with Fe(II) and sorption to or co-precipitationwith the formed Fe(III) (oxyhydr)oxides. However, information is lacking regarding the effect of the frequency and duration of filter use aswell as of filter sand replacement on the residual As concentrations in the filtered water and on the presence of potentially pathogenic bacteria in the filtered and stored water.We therefore scrutinized a household sand filter with respect to As removal efficiency and the presence of fecal indicator bacteria in treatedwater as a function of filter operation before and after sand replacement. Quantification of As in the filtered water showed that periods of intense daily use followed by periods of non-use and even sand replacement did not significantly (p b 0.05) affect As removal efficiency. The As concentration was reduced during filtration from115.1±3.4 μg L−1 in the groundwater to 5.3±0.7 μg L−1 in the filteredwater (95% removal). The first flush of water fromthe filter contained As concentrations belowthe drinking water limit and suggests that this water can be used without risk for human health. Colony forming units (CFUs) of coliform bacteria increased during filtration and storage from 5 ± 4 per 100 mL in the groundwater to 5.1 ± 1.5 × 103 and 15 ± 1.4 × 103 per 100 mL in the filtered water and in the water from the storage tank, respectively. After filter sand replacement, CFUs of Escherichia coli of b100 per 100 mL were quantified. None of the samples contained CFUs of Enterococcus spp. No critical enrichment of fecal indicator bacteria belonging to E. coli or Enterococcus spp. was observed in the treated drinking water by qPCR targeting the 23S rRNA gene. The results demonstrate the efficient and reliable performance of household sand filters regarding As removal, but indicate a potential risk for human health arising from the enrichment of coliform bacteria during filtration and from E. coli cells that are introduced by sand replacement.