4.8. The Results of the Bench-Scale Treatability Study for Arsenic Removal from Groundwater

     The effect of chlorinated lime (a locally available oxidant) and ferric chloride hexahydrate (an effective coagulant) on the removal of arsenic from groundwater was evaluated at the bench-scale (see Table 4). The tubewell water used in this experiment was fortified to 2.0 mg of As(III)/L, approximately 3 times the total arsenic concentration of the most severely impacted tubewell found during this project.

Table 4. The effect of oxidant [chlorinated lime, aCa(OCl)2·bCaCl2·c Ca(OH)2·dH2O)] and coagulant [FeCl3·6H2O] on the removal of 2.0 mg of As/L.

Oxidant
(mg/L)		 Coagulant
(mg/L)		 Final arsenic
concentration
(mg/L)
4
6
10
15
20		 250
250
250
250
250		 0.075
0.037
0.006a
0.003a
< 0.002a
20
20
20
20		 100
150
200
250		 0.06
0.03
0.02a
< 0.002a

a The method detection limit for arsenic could not be determined in the time allotted for this project; therefore, all arsenic concentrations less than the most dilute standard used for calibration (0.028 mg/L) are estimated.

The results shown in Table 4 suggest that oxidation followed by coagulation can reduce relatively large arsenic concentrations in tubewell water to below the more stringent WHO drinking standard of 0.01 mg/L. The addition of water treatment chemicals can yield pH values outside of the 5.5 to 8.5 drinking water range recommended by WHO (20); therefore, pH adjustment may be required after coagulation. Limestone (CaCO3) offers inexpensive and effective pH control after coagulation, often without the need of expensive dosing equipment.