Question: I’m an engineer employed by a municipality in the U.S. who’s involved with overseeing the removal of arsenic from drinking water. I’ve been told in parts of the country much of the groundwater has a pH value which his high enough to significantly reduce the operating capacity of adsorptive media. Is there anything that we can do to solve this problem?

Answer: You’re right that in the western and southwestern United States much of the As-tainted groundwater has a pH value this is high enough to significantly reduce the operating capacity of adsorptive media including the iron, alumina, and titanium-based media commercially available to these municipalities. To lower the treatment cost for these higher pH waters, pH adjustment can be successfully employed to increase the capacities of all of these media. pH adjustment involves the use of an acid, inorganic acids such a sulfuric or hydrochloric, or even gases such as carbon dioxide to depress the pH to a suitable range of 6-7. While typically associated with the large flow rates of centralized water treatment plants, pH adjustment has also been adapted to lower flow rates for small community water supplies and even individual wells. The pH adjustment systems are fully automated and require little operator maintenance or supervision.

Arsenic is a common, naturally-occurring drinking water contaminant that originates from arsenic-containing rocks, minerals, and ores, and is transported to natural waters though weathering, erosion, and dissolution. While occurring in both inorganic and organic forms, arsenic is predominantly inorganic in natural waters and is the most likely form of arsenic at concentrations that cause regulatory concern. Predominately found in groundwater as opposed to surface water, arsenic exists in drinking water in two primary valence forms: arsenate and arsenite. The relative concentrations of each depend primarily on the geology of the surrounding rock formations and the water chemistry at each individual site.

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