The promulgation of the 10 µg/L arsenic MCL will require some utilities to implement new, or to modify
existing, treatment processes. Arsenate removal is commonly performed by adsorption to hydrous ferric
oxide during sweep-floc conventional treatment, and can be improved through enhanced coagulation
modification options. An integrated process model including detailed process chemistry was developed
and used to investigate the effect of water composition, influent pH, and silica competition on operating
conditions for arsenic removal to produce a 10 µg/L effluent arsenic concentration. Additionally, two different
sets of arsenate and silica adsorption constants were used to assess qualitatively the impacts of
model structure uncertainty. In general, the background inorganic concentrations, influent pH, and (for
one model) silica competition increased the range of conditions under which treatment modification is required
to improve removal efficiency. The treatment modifications considered include additional ferric
chloride dose, acid addition, or both. For additional treatment cases, an optimization approach was used
to determine the least-cost modification option to improve removal efficiency. When silica competition
was not considered, the additional ferric chloride dose option was determined to be the least-cost modification
for both models. When silica competition was considered, the model that predicts significant silica
competition selects acid addition or both modification options at high pH and background inorganic
concentrations; the other model continued to suggest only the additional ferric chloride dose option.
