Technologies and Costs for Removal of Arsenic From Drinking Water

Vickers et al. (1997) reported that microfiltration exhibited excellent arsenic removalcapability. This report is corroborated by pilot studies conducted by Clifford (1997), which foundthat coagulation assisted microfiltration could reduce arsenic levels below 2 Fg/L in waters with apH of between 6 and 7, even when the influent concentration of Fe(III) is approximately 2.5 mg/L.These studies also found that the same level of arsenic removal could be achieved by this treatmentprocess even if source water sulfate and silica levels were high. Further, coagulation assistedmicrofiltration can reduce arsenic levels to an even greater extent at a slightly lower pH (approximately5.5).Addition of a coagulant did not significantly affect the membrane cleaning interval, althoughthe solids level to the membrane system increased substantially. With an iron and manganese removalsystem, it is critical that all of the iron and manganese be fully oxidized before they reach themembrane to prevent fouling (Muilenberg, 1997).2.2.4 Enhanced CoagulationThe Disinfectant/Disinfection Byproduct (D/DBP) Rule requires the use of enhancedcoagulation treatment for the reduction of disinfection byproduct (DBP) precursors for surface watersystems which have sedimentation capabilities. The enhanced process involves modifications to theexisting coagulation process such as increasing the coagulant dosage, reducing the pH, or both.Cheng et al. (1994) conducted bench, pilot, and demonstration scale studies to examine As(V)removals during enhanced coagulation. The enhanced coagulation conditions in these studies includedincrease of alum and ferric chloride coagulant dosage from 10 to 30 mg/L, decrease of pH from 7 to5.5, or both. Results from these studies indicated the following:# Greater than 90 percent As(V) removal can be achieved under enhanced coagulationconditions. As(V) removals greater than 90 percent were easily attained under all conditionswhen ferric chloride was used.# Enhanced coagulation using ferric salts is more effective for arsenic removal than enhancedcoagulation using alum. With an influent arsenic concentration of 5 µg/L, ferric chlorideachieved 96 percent As(V) removal with a dosage of 10 mg/L and no acid addition. Whenalum was used, 90 percent As(V) removal could not be achieved without reducing the pH.# Lowering pH during enhanced coagulation improved arsenic removal by alum coagulation.With ferric coagulation pH does not have a significant effect between 5.5 and 7.0.2-9