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ecently, increased attention has been focused on the occurrence, origin, and mobility of arsenic (As) and how to remove it from drinking water, because ingested As can harm health. The current maximum contaminant level (MCL) for total As in drinking water remains at the original value set in 1942 (50 µg/L). 1,2 However, recent studies suggest that this MCL may be too high. Dose-response relationships between the concentration of inorganic As in water supplies and cancer risks were established in an area of Taiwan in which the As concentration of well water was 170-800 µg/L. 3 On the basis of this study, it is estimated that the current As MCL of 50 µg/L adopted by the US Environmental Protection Agency (USEPA) would result in a lifetime risk of dying from cancer of the liver, lung, kidney, or bladder as high as 13 per 1,000 people. Of the approximately 200 conta-On-site ion exchange studies investigated the combined removal of arsenic (V) [As(V)] and nitrate from drinking water in McFarland and Hanford, Calif., and Albuquerque, N.M. Whereas previous ion exchange workers had studied removal of high concentrations of As (> 50 µg/L As) without nitrate present, these studies focused on removing 10-15 µg/L As to achieve a product water with < 2 µg/L As while also maintaining nitrate below its maximum contaminant level . Results of 1-in.-(25-mm-) column experiments showed that conventional sulfate-selective resins were better than special nitrate-selective resins for combined As(V) and nitrate removal. The conventional resins yielded longer run lengths and leaked less As and nitrate into the product water. Decreasing empty bed contact time from 3.0 to 1.5 min did not greatly alter As leakage into the product water. Particulate iron in the ion exchange feed increased As leakage in the product water. Two commercially available computer programs were reasonably accurate in predicting both As and nitrate run lengths for various influent nitrate and sulfate concentrations. Generally, predicted As and nitrate run lengths were within ±35 percent of those observed experimentally.For executive summary, see page 182.

Arsenic (As) removal using ferric hydroxide coagulation followed by direct microfiltration without flocculation was investigated for an application in Albuquerque, N.M. Typically, the influent drinking water (unchlorinated) was contacted with ferric hydroxide for ≤20 s in a rapid mixer and passed through a membrane microfiltration unit with a nominal pore size of 0.2 μm. Variables investigated included pH, iron (Fe) dose, mixing time and energy, filtrate flux, and backwash interval. The pH and ferric dose were found to be the most important variables controlling As removal. As removal to low levels (<2 μg/L) was achieved using either a dose of 7 mg/L Fe without deliberate pH reduction or a smaller dose of 1.9 mg/L Fe after sulfuric acid addition to reduce pH to 6.4. Extended operation (three to five days) showed that consistent As removal was obtained without any membrane fouling. Both the backwash water and the dried sludge passed the toxicity characteristic leaching procedure test as a nonhazardous waste.

This research explored options for removing perchlorate from drinking water using commercially available ion exchange resins. Experimental data provided perchlorate selectivity, capacity, and thermodynamic properties of the resins. With these data, computer models using equilibrium multicomponent chromatography theory evaluated treatment options based on different water chemistry and operating characteristics; computer predictions were validated by bench-scale tests. Three treatment approaches emerged as viable options for removing perchlorate to concentrations below proposed maximum concentration limits:(1) partial exhaustion-regeneration using polyacrylic resins, with a process similar to current nitrate treatment techniques, (2) partial exhaustion-regeneration using polystyrene resins with elevated temperature regeneration, and (3) use of highly perchlorate-selective resins with resin replacement following exhaustion.

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