AWWA WQTC58867

With the recent reduction in the arsenic maximum contaminant level (MCL), utilities out ofcompliance must modify current treatment processes. Enhanced coagulation is generally consideredto be the least-cost modification option when a ferric coagulation system already exists.Options for implementing enhanced coagulation as a modification to conventional sweep-floctreatment include: increased ferric dose; addition of an acid dosing system; and, acombination of the individual options. This study determines the least-cost enhanced coagulationmodification for three model waters (soft, moderate, and hard, based on increasingbackground inorganic concentration) by incorporating a mathematical representation of arsenicsorption to hydrous ferric oxide, and subsequent removal through sedimentation and filtrationwithin an optimization algorithm for minimizing treatment cost. The optimization frameworkexplicitly includes influent water quality and flow rate variability, and model parameter uncertaintyto 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 satisfythe MCL by 23.6% - 63.0% relative to a deterministic approach. As for the three modificationoptions, the FERRIC ONLY option was always the least-cost treatment modification. TheACID ONLY option had a relatively small feasible region due to the limited sorption sites fromthe existing ferric dose. The FERRIC AND ACID option was always slightly more expensivethan the FERRIC ONLY option, and, in many cases, the resulting optimal acid dose was atthe minimum allowable dose. By including variability and uncertainty, the relative importanceof the individual parameter distributions can be determined. The variability of influent arsenicconcentration was always critical. The variability of influent pH was critical, or important, atlower pH values for the soft and hard waters, and for all influent pH values for the moderatewater. Other important parameter distributions include the uncertainty in the filter removalefficiency of entrapped particles (soft water at higher influent arsenic concentrations), variabilityin sulfate concentration (low influent pH and arsenic concentrations), and uncertainty in theequilibrium sorption coeficient for the Fe(III)SOHCa²⁺ complex (moderate and hard water athigh influent pH). Includes 25 references, tables, figures.

Product Details

Edition:

Vol. - No.

Published:

11/02/2003

Number of Pages:

14

File Size:

1 file , 430 KB