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  Arsenic Removal From Drinking Water by Coagulation/Filtration and Lime Softening Plants (PDF) (108 pp, 1.54 MB) (EPA/600/R-00/063) June 2000

This report documents the results of sampling and analysis at two coagulation/filtration plants (referred to in this document as Plants A and B) and one lime softening plant (referred to as Plant C). 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 the residuals produced by the treatment processes.

The study was divided into three phases:

  • Source water sampling – conducted to evaluate source water characteristics at each plant
  • Preliminary sampling – consisted of a four-week sampling period to refine procedures prior to implementing the long-term evaluation phase
  • Long-term evaluation – consisted of weekly sample collection and analysis for approximately one year

Sludge samples also were collected at each facility during a single sampling event from settling lagoons and ponds during a two-month period. Samples of recycled supernatant water (Plant A) and supernatant discharge water (Plants B and C) were collected monthly, beginning in November 1998 and continuing until June 1999.

Long-term evaluation of Plants A and B demonstrated that conventional coagulation/filtration can consistently achieve low levels of arsenic (i.e., less than 5 micrograms per liter [µg/L]) in the treated water. The total arsenic concentrations at Plant A were reduced by an average of 52 percent; this represents a decrease of average arsenic concentrations from 7.5 µg/L in the source water to 3.5 µg/L in the finished water. Average total arsenic removal efficiency at Plant B was 79 percent, with an average source water concentration of 19.1 µg/L and an average finished water concentration of 4.0 µg/L. Adsorption and coprecipitation of arsenic (V) with iron and aluminum flocs are believed to have been the primary arsenic removal mechanisms at these plants.

The lime softening facility, Plant C, was not able to consistently reduce arsenic to low levels in treated water. The average total arsenic concentration in Plant C source water was 32.0 µg/L. The lime softening plant reduced the average total arsenic concentration to 16.6 µg/L in the finished water, which equals a 45 percent removal efficiency. Arsenic (III) was the primary species of soluble arsenic in the raw water and was almost completely oxidized to arsenic (V) as a result of two chlorination steps that occurred prior to softening and prior to filtration. The primary mechanism of arsenic removal was likely adsorption and coprecipitation of arsenic (V) with iron that was present in the source water. Plant C operated at a pH of 9.6, a level at which arsenic removal by coprecipitation with calcium carbonate is reported to be less than 10 percent.

None of the sludge samples collected at Plants A, B, and C qualified as a hazardous waste based on Toxicity Characteristic Leaching Procedure (TCLP) testing for metals. Therefore, nonhazardous waste landfills should be able to accept the sludge generated by these treatment processes.


Thomas Sorg

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