|Arsenic Removal From Drinking Water by Iron Removal Plants (EPA/600/R-00/086) August 2000
This report documents the results of sampling and analysis at two iron removal plants (referred to as Plants A and B). The objective of sampling and analysis was to evaluate the effectiveness of the water treatment plants to consistently remove arsenic from source water. In addition, data were collected to evaluate the chemical characteristics of residuals produced by the treatment processes.
The study was divided into three phases:
Plant personnel conducted all sampling during the long-term evaluation phase and Battelle coordinated sampling logistics. Sludge samples also were collected at Plant A during a single sampling event from an outdoor settling pond in November 1998. Samples of supernatant discharge (Plant A) and recycled supernatant (Plant B) were collected monthly from November 1998 until June 1999 at Plant A and until January 1999 at Plant B.
Results from the long-term evaluation phase were varied regarding the ability of the iron removal process to consistently achieve low-level arsenic concentrations (i.e., less than 5 micrograms per liter [µg/L] in the finished water). The total arsenic concentrations at Plant A were reduced by an average of 87 percent, which represents a decrease in average arsenic concentration from 20.3 µg/L to 3.0 µg/L. Adsorption and coprecipitation with iron hydroxide precipitates are believed to be the primary arsenic removal mechanisms. The total arsenic concentrations at Plant B were reduced by an average of 74 percent, which represents a decrease in average arsenic concentration from 48.5 µg/L to 11.9 µg/L. At Plant B, it appeared that only the particulate arsenic in the source water was removed. This particulate arsenic was most likely associated with the oxidized iron particles present in the source water (i.e., arsenic sorbed onto iron particles). The primary difference in arsenic removal efficiency at Plants A and B is believed to be the amount of iron in the source water. Source water at Plant A averaged 2,284 µg/L of iron, while Plant B averaged 1,137 µg/L. Increasing the iron in the source water at Plant B using a coagulant, such as ferric chloride, would likely enable Plant B to consistently achieve lower levels of arsenic.
None of the sludge samples collected at Plant A qualified as a hazardous waste based on the Toxicity Characteristic Leaching Procedure (TCLP) test for metals. Therefore, nonhazardous waste landfills should be able to accept the sludge generated by this treatment facility. Stricter hazardous waste classification regulations in some states, such as California, on total arsenic concentrations in solid waste also were met at Plant A. Sludge samples were not collected at Plant B; however, analytical results were provided from a 1994 sludge sampling event.
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