AWWA JAW56601

Arsenic (As) is common in US raw water sources, particularly in groundwater of the midwestern and western states. Most water utilities meet the current As maximum contaminant level (MCL) of 50 ug/L, but if the MCL is lowered to 10 ug/L, cost-effective As removal will be critical to an increased number of water utilities. Groundwaters that have high As concentrations sometimes also contain moderate to high concentrations of ferrous iron (Fe). When conventional water treatment methods are used to remove the soluble Fe by oxidizing it to hydrous ferric oxide, As is also removed. Thus, this paper discusses optimizing As removal at water treatment plants that remove Fe. Common dissolved substances, including bicarbonate, silica (Si), and phosphate (PO4 3-) interfere with As removal by iron oxide. This study was undertaken because the effects of bicarbonate and Si on As removal were not well known. However, these substances are present in all source waters and may affect As removal at many water treatment plants. Holm shows how a widely used chemical-equilibrium computer model can be easily modified to predict such interferences. The article also describes experiments that were designed to simulate the effects of bicarbonate, Si, and PO4 3- on concurrent Fe and As removal. According to the author, if a source water contains As and Fe, it may be possible to improve As removal by lowering the pH. He also points out that the computer model may be useful in designing processes such as Fe removal or ferric chloride coagulation to remove As. Includes 50 references, table, figures.