Technologies and Costs for Removal of Arsenic From Drinking Water

Field StudiesSurveys of lime softening facilities by AWWARF (AWWARF 2000) show that the technologyshould be able to produce water with less than 3 Fg/L–all five of the surveyed plants that operatedat a pH of 10.2 or higher would be able to feasibly achieve these low arsenic levels. However,although lime softening processes are quite effective for arsenic removal when operated at high pH,lime softening facilities may not effectively remove arsenic to low levels when operated at or belowpH 10.0. For example, a field study conducted by Battelle Memorial Institute, with funding from EPA,found that a 10 mgd lime softening plant only reduced average total arsenic concentrations of 32.0Fg/L by about 45 percent to an average of 16.6 Fg/L in the finished water even though the treatmenttrain included pre-softening oxidation and post-softening filtration (EPA, June 2000). None of thesludge samples collected as part of this study qualified as hazardous waste based on TCLP testing.Optimization Hierarchy for Softening FacilitiesMcNeill and Edwards (1997b) developed a simple model for predicting As(V) duringsoftening. Using inputs of calcium carbonate, magnesium, and ferric hydroxide solid concentrationsformed during softening, the model can predict percentage As(V) removal.McNeill and Edwards (1997b) suggested an optimization hierarchy strategy for softeningfacilities which are unable to meet arsenic removal requirements with their existing treatment schemesimilar to optimization of coagulation hierarchy. If As(III) is present, the most cost-effective methodof improving arsenic removal is preoxidation of As(III) to As(V), since As(V) is more readilyremoved by precipitation of calcium carbonate and magnesium and ferric hydroxide. For facilitiesthat are currently precipitating only calcium carbonate, addition of iron can dramatically improvearsenic removal. A final option is to raise the softening pH in order to precipitate magnesiumhydroxide which strongly sorbs As(V). These removal trends should be quantitatively confirmed withjar testing for optimizing arsenic removal.SummarySoftening technology can be implemented by water systems to achieve greater than 90 percentremoval of As(V). As discussed above, a survey of softening facilities by AWWARF found that thosesurveyed facilities operating at a pH of 10.2 or higher could feasibly reduce arsenic concentrationsin the treated water below 3 Fg/L.2-13