With the recent reduction in the arsenic maximum contaminant level (MCL), utilities out of
compliance must modify current treatment processes. Enhanced coagulation is generally considered
to be the least-cost modification option when a ferric coagulation system already exists.
Options for implementing enhanced coagulation as a modification to conventional sweep-floc
treatment include: increased ferric dose; addition of an acid dosing system; and, a
combination of the individual options. This study determines the least-cost enhanced coagulation
modification for three model waters (soft, moderate, and hard, based on increasing
background inorganic concentration) by incorporating a mathematical representation of arsenic
sorption to hydrous ferric oxide, and subsequent removal through sedimentation and filtration
within an optimization algorithm for minimizing treatment cost. The optimization framework
explicitly includes influent water quality and flow rate variability, and model parameter uncertainty
to ensure the arsenic MCL is satisfied with a 95% probability. For the moderate water,
incorporating variability and uncertainty increased the minimum required ferric dose to satisfy
the MCL by 23.6% - 63.0% relative to a deterministic approach. As for the three modification
options, the FERRIC ONLY option was always the least-cost treatment modification. The
ACID ONLY option had a relatively small feasible region due to the limited sorption sites from
the existing ferric dose. The FERRIC AND ACID option was always slightly more expensive
than the FERRIC ONLY option, and, in many cases, the resulting optimal acid dose was at
the minimum allowable dose. By including variability and uncertainty, the relative importance
of the individual parameter distributions can be determined. The variability of influent arsenic
concentration was always critical. The variability of influent pH was critical, or important, at
lower pH values for the soft and hard waters, and for all influent pH values for the moderate
water. Other important parameter distributions include the uncertainty in the filter removal
efficiency of entrapped particles (soft water at higher influent arsenic concentrations), variability
in sulfate concentration (low influent pH and arsenic concentrations), and uncertainty in the
equilibrium sorption coeficient for the Fe(III)SOHCa<sup>2+</sup> complex (moderate and hard water at
high influent pH). Includes 25 references, tables, figures.
| Edition : | Vol. - No. |
| File Size : | 1
file
, 430 KB |
| Note : | This product is unavailable in Ukraine, Russia, Belarus |
| Number of Pages : | 14 |
| Published : | 11/02/2003 |
