AWWA WQTC64028

As a result of the 2006 arsenic MCL change, Arizona American Water (AAW) was required to construct several arsenic removal facilities within the Phoenix, Arizona metropolitan area. Coagulation/filtration (C/F) was the treatment technology selected for AAW's two largest centralized facilities. The new C/F facilities, which were designed by Damon S. Williams Associates (DSWA), treat a combined maximum average flow of approximately 24 MGD; based on available equipment and plant hydraulics, the maximum treatment capabilities are as large as 28 MGD. Since the C/F process is capable of achieving high arsenic removal, a portion of the raw water flow from the wells is allowed to bypass the treatment system and blend with the lower arsenic water exiting the C/F process. The facilities have been designed to meet a finished water arsenic goal of 8 µg/L, which provides a factor of safety to ensure compliance with the 10 µg/L MCL. The portion of the flow that is fed to the treatment system is dosed with ferric chloride under high rate mixing. During coagulation, arsenic binds to the iron and is co-precipitated with ferric hydroxide. Following coagulation, the water passes through horizontal dual media pressure filters, wherein the arsenic-bearing ferric hydroxide particles are removed from the flow. The filter effluent combines with the treatment system bypass water prior to storage and subsequent distribution to the local potable water supply system. The filters are backwashed periodically to remove accumulated iron solids. Following backwashing, the solids are settled and thickened prior to disposal. In addition to the overall scale of the arsenic removal systems, which are some of the largest C/F arsenic removal facilities in the southwest, the facilities are unique because of process equipment selection and design requirements imposed by local conditions. Because multiple rounds of bench-scale and pilot tests were conducted to validate the process design, the facilities can be used to compare literature and pilot scale testing results to full-scale arsenic removal results obtained during facility operation. Includes 7 references, table, figures.