Development and field testing of a low-cost household arsenic removal unit

Abstract

In this study, efficiency of arsenic removal from groundwater by alum and iron coagulation was evaluated. Experimental results showed that vigorous mixing for 10- 15 seconds followed by 90 slow turns yielded optimum floc formation for the coagulants. This method of mixing was adopted in all coagulation experiments conducted in this study. Results of coagulation experiments suggest that ferric chloride is much more efficient than alum in removing arsenic from groundwater. As expected, removal of arsenate was much more efficient than arsenite. Potassium permanganate was used as an oxidizing agent for oxidation of arsenite to arsenate for effective removal of arsenic. Experimental results suggest that a dose of potassium permanganate twice the stoichiometric requirement is sufficient for oxidation of arsenite to arsenate. However, use of potassium permanganate produced slight pink color in the treated water, which would be objectionable to the users. This color was removed using a sand filter. Laboratory test results suggest that a sand filter 20-cm deep was sufficient for removal of color. In this study, effect of phosphate on arsenic removal efficiency was evaluated in batch sorption experiments. Results show that presence of high level of phosphate can reduce the efficiency of arsenic removal by ferric chloride to some extent. From the results of batch experiments, a household arsenic removal unit (ARU) based on ferric chloride coagulation was designed and the ARU was tested in an arsenicaffected village in Comilla district. The unit is similar in design to the two-bucket treatment unit developed by DPHE-Danida. About 25 liters of arsenic-affected groundwater is treated in the unit in one batch, in which a chemical packet containing 2.5 gm of ferric chloride and 35 mg of potassium permanganate are added to the groundwater in the upper bucket. The cost of the unit is Tk. 5201- and chemical cost for groundwater treatment is about Tk. O.I0 per liter. Field testing of J 5 units at Adda village in Barura thana of Comilla district showed very good arsenic removal efficiency. Arsenic concentrations in the treated water were found to be mostly below 20 ppb; while maximum arsenic concentration in the tubewell water was about 400 ppb. Forsome of these units, presence of fecal coliform was detected in the treated water. In order to eliminate this problem, I mg of bleaching powder was added to the chemical packet. Continued use of bleaching powder (along with ferric chloride and potassium permanganate for a period of about 15 days) eliminated fecal coliform. Analysis of the performance of the different parts of the unit suggest that over 70 percent of total arsenic is. removed the upper bucket, while the rest is removed in the sand filter in the lower bucket. The cloth strainer does not appear to contribute in arsenic removal, and hence should no longer be used in the . ARU system. The developed ARU appears to be widely accepted and in great demand at the village.