AWWA WQTC62545

The City of Prescott, Arizona is one of many Southwestern Community Water Systems that must comply with the new Arsenic MCL of 10 µg/L by January 2006. This paper presents the technical challenges the City of Prescott faced during treatment process selection and design of their Arsenic treatment system. The City of Prescott operates the water production and distribution system as an enterprise fund. The City of Prescott has 6 drinking water wells, which have Arsenic concentrations from 9-53 µg/L and a combined capacity of 14.4 MGD. All of the wells are located in the Town of Chino Valley, a community 20-miles to the northeast of Prescott. Water is also served to a portion of Chino Valley residents on its way to the City's main pumping station (Chino Forebay) and the 20-mile pipeline serving the City of Prescott. The individual wells are spread throughout the central portion of Chino Valley and are all less than 1.5 miles away from the Chino Forebay. The initial goal of the study was to determine which Arsenic treatment technology out of four (e.g., adsorption, coagulation/filtration (C/F), coagulation/membrane filtration (C/MF) and ion exchange (IX)) would be most appropriate for Prescott's specific water quality, and whether centralized treatment at the Chino Forebay or individual wellhead treatment would be more cost effective. Specific challenges included a lack of sewers for waste discharge; constrained space at several of the individual well sites; a separate supply water piping system requirement to serve Chino Valley residents, after treatment, if centralized treatment is selected; all of the well capacity required in the summer months to meet demand; and power is limited only to the Chino Forebay site. The Conceptual Design results led to a recommendation that design of a 14.4 MGD centralized treatment facility would be most cost effective. Two technologies were selected for further evaluation based on conceptual design cost assessments: adsorption and C/F. Bench testing was conducted for adsorption and C/F to determine the lowest cost solution based on Prescott's specific water quality conditions. C/F jar tests were conducted to examine nineteen conditions with varying ferric chloride coagulant doses and pH conditions. Rapid Small Scale Column Tests (RSSCTs) were conducted on three adsorption media (Bayoxide E33, GFH and As:Xnp) to determine the approximate number of bed volumes that can be anticipated with this water and whether any deviations in performance between media should be anticipated under full scale conditions. Results from both tests were evaluated and C/F was selected as the most cost effective technology. The results of this study and design will help other water systems to understand the importance of verifying treatment performance through bench testing when conducting Arsenic treatment technology selection. While meeting the January 2006 compliance deadline is important, it is also critical that water systems verify that the technology they are selecting is appropriate for the specific water quality being treated before investing in costly treatment systems. Includes tables, figures.