AWWA ACE59827

The City of Phoenix in Arizona is currently in the process of updating its Water Quality Master Plan in order to continue development of strategies to meet water quality goals, including reduction of disinfection byproducts (DBP), which are expected to be an area of concern for upcoming water quality regulations. During the first phase of the Water Quality Master Plan Update, it was determined that City of Phoenix Water Treatment Plants will need to undergo enhanced coagulation to achieve adequate removal of total organic carbon (TOC) to ensure compliance with the Stage 2 D/DBP rule. Even though the extent of TOC removal by enhanced coagulation has already been established for the City of Phoenix surface water sources through numerous bench-scale evaluations, the operational complexity with respect to limits of coagulant dose and residuals handling has not been sufficiently evaluated. For this reason, a plant-scale enhanced coagulation test with aluminum sulfate and ferric chloride was performed. The study was designed to assess the advantages and disadvantages of enhanced coagulation in terms of natural organic removal, and to evaluate impacts on residuals handling and operational cost. The plant-scale enhanced coagulation testing was performed at the City of Phoenix Val Vista Water Treatment Plant (WTP). The Val Vista WTP is a conventional water treatment plant consisting of two parallel treatment trains (East Plant and West Plant) with a total capacity of 220 mgd, and includes a Solids Handling Facility. The Solids Handling facility is made up of two process groups: the Used Water Recovery System (UWRS), and the Solids Dewatering System. The Used Water Recovery System includes equalization basins, backwash clarifiers, and a gravity thickening complex to recover water from residuals process streams. The Solids Dewatering System includes centrifuges, solar drying beds, and evaporation lagoons. During the plant-scale testing, residuals produced at the water treatment process (i.e. sedimentation basin blow-downs and filter backwash water) were routed to the gravity thickening complex. After the gravity thickening process, thickened sludge was transferred to the Solids Dewatering System for further separation using centrifuges. During the study, the effects of enhanced coagulation on filter performance, gravity thickening operation, and the dewatering process were assessed. In addition, a comparison of the costs associated with thickening, dewatering, and disposal between ferric and alum residuals, was performed. Includes 3 references, tables, figures.