|Treatment of Arsenic Residuals From Drinking Water Removal Processes (96 pp, 1.14 MB) (EPA/600/R-01/033) June 2001
The drinking water maximum contaminant level (MCL) for arsenic was recently lowered from 0.05 milligrams per liter (mg/L) to 0.01 mg/L. One concern was that a reduction in the Toxicity Characteristic Leaching Procedure (TCLP) arsenic limit in response to the drinking water MCL could be problematic with regard to disposal of solid residuals generated at arsenic removal facilities.
This project focused on developing a short list of arsenic removal options for residuals produced by ion exchange, reverse osmosis, nanofiltration, activated alumina, and iron removal processes. Both precipitation and adsorption processes were evaluated to assess their arsenic removal effectiveness.
In precipitation tests, ferric chloride outperformed alum for removal of arsenic from residuals by sedimentation, generally resulting in arsenic removals of 88 to 99 percent. Arsenic removal from the high-alkalinity ion exchange samples was poorer. The required iron-to-arsenic molar ratio for best removal of arsenic in these screening tests varied widely from 4:1 to 191:1, depending on residuals type; the best arsenic removal using ferric chloride typically occurred between pH 5.0 and 6.2. Polymer addition typically did not significantly improve arsenic removal using either coagulant. Supernatant total arsenic levels of 0.08 mg/L or lower were attained with ferric chloride precipitation for membrane concentrates and residuals from iron removal facilities compared to an in-stream arsenic limit of 0.05 mg/L being used in some states. Settling alone, with no coagulant, also effectively removed arsenic from iron removal facility residuals. Even with ferric chloride dosages of 50 to 200 mg/L applied to ion exchange regenerants, supernatant arsenic levels after treatment were 1 to 18 mg/L. Required iron-to-arsenic molar ratios developed in precipitation work could be used by utilities as guidelines for establishing coagulant dose needs to meet in-stream standards and to develop preliminary treatment costs.
Adsorption tests demonstrated the potential for different types of media and resins to remove arsenic from liquid residuals but did not assess ultimate capacity. Overall, the iron-based granular ferric hydroxide media outperformed the aluminum-based media and ion exchange resin for removal of arsenic. However, activated alumina and the iron-based media provided comparable arsenic removals of close to 100 percent, with an empty bed contact time of three minutes for most of the membrane concentrates and the settled iron removal facility residuals. Removal of suspended solids was key to the success of adsorption for spent-filter backwash water and clarifier flush residuals. Arsenic breakthrough occurred very rapidly for the ion exchange samples and for one reverse osmosis concentrate, all of which had an alkalinity of more than 1,000 mg/L (as calcium carbonate). This again suggests that alkalinity significantly interferes with adsorption of arsenic. Based on this work, use of adsorption media for treatment of arsenic-laden water plant residuals merits further exploration.
Of all of the residuals streams tested, ion exchange regenerants were the most difficult to treat using precipitation or adsorption. Disposal of supernatant streams resulting from treatment of arsenic-laden residuals from ion exchange plants could pose a major challenge. TCLP arsenic levels in all residuals generated in this work and in full-scale solid media samples were far below the regulatory limit of 5 mg/L and, in fact, were below 0.5 mg/L.
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