Removal of inorganic contaminants and natural organic matter by enhanced alum coagulation : defluoridation at the pilot scale and application to arsenic
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The removal of inorganic contaminants is a primary concern in drinking water treatment. Fluoride and arsenic both naturally occur in ground and surface waters throughout the world. Recent health concerns regarding extended exposure to high levels of fluoride in drinking water have prompted the United States Environmental Protection Agency (USEPA) to review the fluoride maximum contaminant level (MCL). Arsenic is one of the most well-known and toxic inorganic contaminants regulated by the USEPA. While typically associated with groundwater, arsenic could be an increasing concern for surface water sources in the future. Small water systems may not have the resources to adjust their treatment scheme to accommodate a lower fluoride MCL or target arsenic removal. In this study, enhanced alum coagulation was investigated as a treatment strategy for both fluoride and arsenic. Facilities with surface water sources face a particular challenge, as the presence of natural organic matter (NOM) may interfere with the removal of fluoride and/or arsenic during coagulation. This work builds upon previous investigation of the interactions between fluoride, NOM, and aluminum during coagulation by comparing two pilot studies in Texas and Colorado. Each pilot study confirmed that enhanced alum coagulation was able to remove fluoride during continuous flow experiments using natural source water; the comparison of the studies revealed that source water composition impacts the maximum efficacy of alum coagulation for fluoride removal. A higher influent organic concentration appears to reduce the maximum efficacy of fluoride removal. However, the use of pH control may also be a contributing factor to the discrepancy in fluoride removal between pilot studies. The pilot study with a lower maximum fluoride removal had a higher influent DOC concentration, but was run without pH control. Synthetic water jar testing confirmed the ability of alum coagulation to remove arsenic (V). A maximum arsenic removal of 99% was observed for As(V) at alum doses of 100 mg/L and above, and an alum dose of 20 mg/L achieved an As(V) removal of 97%. Aluminum residuals suggest that the presence of As(V) lowers the point of zero charge for aluminum hydroxide solid.