Arsenic removal from groundwater by alum

Abstract

Widespread arsenic contamination of shallow aquifers in Bangladesh has posed a major public health concern as most of its population uses the aquifers as the sources of water supply. There is an urgent need to supply arsenic-safe drinking and cooking water to the millions of arsenic affected people in Bangladesh. The present study is focused on removal of arsenic from groundwater by alum coagulation. An arsenic removal unit based on co-precipitation-adsorption-sedimentation and filtration processes was used to study the arsenic removal efficiency under a variety of conditions both in the laboratory and field. Optimum alum dose for As(V) removal was found to be 100 ppm and the percent removal efficiency was better for higher initial arsenic concentration. A moderate mixing of the coagulant was required for satisfactory removal of arsenic from water. The optimum sorbate/sorbent ratio was 75 flg As I mg AI. Under normal concentrations of pH, alkalinity, hardness, chloride, nitrate, ferric iron, silicate in Bangladesh groundwater, these parameters individually had no significant effect on arsenic removal efficiency. Fe(II) enhanced the efficiency slightly. Phosphate decreased the removal efficiency significantly. At low concentrations, anions had no synergistic effect on arsenic removal efficiency, but at medium to high concentrations, the anions reduced the removal efficiency very significantly. At low permanganate dose (1.0 ppm) and medium iron and phosphate concentrations (5.0 ppm and 3.0 ppm, respectively), the removal efficiency varied little with the form of arsenic. Higher concentration of Fe(II) in the feed water required higher dose of potassium permanganate for effective As(III) removal. Sand filtration was very effective in removing residual color and residual arsenic. One hour settling time in the top bucket was found enough for the alum based arsenic removal unit. It was found that the adsorption capacity of the iron present in the sand filter greatly enhanced the arsenic removal efficiency. Performance of arsenic removal unit was evaluated in the field in order to determine their suitability at household levels. The variations of raw water arsenic and phosphate concentrations with time were found significant. The residual arsenic concentrations in the treated water of 3 field units were found to be mostly below 20 ppb, much below the Bangladesh standard at alum dose of 100 ppm and permanganate dose of 1 ppm. These doses were enough to remove arsenic effectively from natural groundwater if its iron content was high (~ 9 ppm) and phosphate content was low «1.0 ppm). Alum dose of 100 ppm and permanganate dose of 1 ppm could not produce arsenic-safe water of two field units where moderate amount of iron (4.0 - 5.0 ppm) and high concentration of phosphate (4.84 - 5.54 ppm) was present in the raw water. Higher permanganate dose was required for effective arsenic removal by the two field units. For arsenic and phosphate concentrations of 190 ppb and 4.84 ppm respectively, the required permanganate dose was 3 ppm. The permanganate dose requirement was 6 ppm for arsenic and phosphate concentrations of 278 ppb and 5.54 ppm respectively. Under favorable conditions (low phosphate concentration and high iron concentration), the unit removed arsenic satisfactorily without addition of any chemical. All the five tubewells were bacteriologically contaminated. So chlorine dose was introduced and it was found that 0.45 ppm was satisfactory for disinfections. Filtration removed a substantial amount of arsenic (39-49%) when natural iron content was 9 to 20 ppm. Proper operation of the system was essential to obtain satisfactory performance. Users' acceptability of the units was found to be satisfactory especially among the poor and conscious people.