Arsenic removal by coagulation

he current 50-μg/L maximum contaminant level (MCL) for arsenic in drinking water was first set by the US Public Health Service in 1942 and was adopted by the US Environmental Protection Agency (USEPA) in 1975.1 The standard was based primarily on toxic effects resulting from short-term or acute exposure to arsenic.2 Subsequent investigations have linked arsenic in drinking water with skin cancer and, more recently, with cancers of the liver, lung, bladder, and kidney.2–4 These studies have raised the concern that the current arsenic MCL may be too high. According to a February 1994 federal court decision, USEPA is obligated to propose a new arsenic regulation by November 1995.* The new regulation is expected to lower the MCL This study evaluated the removal of naturally occurring arsenic in a full-scale (106-mgd) conventional treatment plant. When the source water was treated with 3–10 mg/L of ferric chloride or 6, 10, or 20 mg/L of alum, arsenic removal was 81–96 percent (ferric chloride) and 23–71 percent (alum). Metal concentrations in the sludge produced during this study were below the state’s current hazardous waste levels at all coagulant dosages. No operational difficulties were encountered. *At press time USEPA had sought to delay the arsenic rulemaking in order to conduct further research before proposing a new rule. For additional information, see Legislation/Regulation, beginning on page 12 of this issue.