391Arsenic Removal from Drinking Water by Coagulation/filtration
and Lime Softening Plants
EPA 600-R-00-063
This report documents treatment plant information as well as 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 (As) from source water. Additionally, data were collected in this study to
evaluate the characteristics of the residuals produced by the treatment processes.
The study was divided into three phases: source water sampling, preliminary sampling,
and long-term evaluation. The first phase, source water sampling, was conducted to
evaluate source water characteristics at each plant.
The second phase, preliminary sampling, consisted of a four-week sampling period to
refine procedures prior to implementing the long-term evaluation phase. The third phase,
long-term evaluation, consisted of weekly sample collection and analysis for
approximately 1 year. Sludge samples also were collected at each facility during a single
sampling event from settling lagoons/ponds during a two-month period.
Samples of recycle supernatant water (Plant A) and supernatant discharge water (Plants
B and C) were collected monthly beginning in November 1998 and continuing until June
- Long-term evaluation of Plants A and B demonstrated that conventional
coagulation/filtration can consistently achieve low levels of arsenic in the treated water
(i.e., less than 5 μg/L).
The total arsenic concentrations at Plant A were reduced by an average of 52%, which
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%, 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 As(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, and the lime softening plant reduced the average total arsenic
concentration to 16.6 μg/L in the finished water, which equals a 45% removal efficiency.
As(III) was the primary species of soluble arsenic in the raw water and was almost
completely oxidized to As(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
As(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% (Sorg and Logsdon, 1978; McNeill and Edwards, 1997b).