Arsenic Removal from Groundwater by Household Sand Filters: Comparative Field Study, Model Calculations, and Health Benefits

Arsenic removal efficiencies of 43 household sand filters
were studied in rural areas of the Red River Delta in
Vietnam. Simultaneously, raw groundwater from the same
households and additional 31 tubewells was sampled to
investigate arsenic coprecipitation with hydrous ferric iron
from solution, i.e., without contact to sand surfaces.
From the groundwaters containing 10−382 μg/L As, <0.1−48 mg/L Fe, <0.01−3.7 mg/L P, and 0.05−3.3 mg/L Mn,
similar average removal rates of 80% and 76% were found
for the sand filter and coprecipitation experiments,
respectively. The filtering process requires only a few
minutes. Removal efficiencies of Fe, phosphate, and Mn
were >99%, 90%, and 71%, respectively. The concentration
of dissolved iron in groundwater was the decisive factor
for the removal of arsenic. Residual arsenic levels below 50
μg/L were achieved by 90% of the studied sand filters,
and 40% were even below 10 μg/L. Fe/As ratios of ≥50 or
≥250 were required to ensure arsenic removal to levels
below 50 or 10 μg/L, respectively. Phosphate concentrations
>2.5 mg P/L slightly hampered the sand filter and
coprecipitation efficiencies. Interestingly, the overall
arsenic elimination was higher than predicted from model
calculations based on sorption constants determined
from coprecipitation experiments with artificial groundwater.
This observation is assumed to result from As(III) oxidation
involving Mn, microorganisms, and possibly dissolved
organic matter present in the natural groundwaters. Clear
evidence of lowered arsenic burden for people consuming
sand-filtered water is demonstrated from hair analyses. The
investigated sand filters proved to operate fast and
robust for a broad range of groundwater composition and
are thus also a viable option for mitigation in other
arsenic affected regions. An estimation conducted for
Bangladesh indicates that a median residual level of 25
μg/L arsenic could be reached in 84% of the polluted
groundwater. The easily observable removal of iron from
the pumped water makes the effect of a sand filter immediately
recognizable even to people who are not aware of the
arsenic problem.