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.
