(11d) Case Study: the Coagulation Filtration Process for Groundwater Arsenic Removal

As a response to 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 facilities, which were designed by Damon S. Williams Associates (DSWA), treat a combined maximum flow of approximately 24 MGD.

Since the C/F process is capable of achieving high arsenic removal, a portion of the raw water flow from the wells bypasses the treatment system and is recombined prior to distribution. The facilities have been designed to meet a finished water arsenic goal of 8 ppb, which provides a factor of safety to ensure that the facilities comply with the 10 ppb 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 adsorbs to the surface of the ferric hydroxide particle and is co-precipitated. Following coagulation, solids flocculate and then the water passes through horizontal dual media pressure filters. Treated water recombines 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, which contain arsenic from the raw water. 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 processing equipment selection and design considerations imposed by local conditions. Because multiple series of C/F tests were conducted to validate the process design, the facilities can be used to compare literature and pilot scale testing results to demonstrated full-scale arsenic removal results obtained during facility operation. Besides helping to bridge the gap between theoretical and observed C/F arsenic removal results, this project presents two full scale C/F arsenic removal facilities to the chemical engineering community.