AWWA WQTC69427

The City of Phoenix (City) developed the Water Quality Master Plan Update (WQMPU) in 2005 to identify future treatment requirements and improvements needed to comply with the upcoming Stage 2 Disinfectants/ Disinfection Byproducts (D/DBP) Rule. Based on modeling the distribution system using the worst source water quality and with all Water Treatment Plants (WTPs) practicing enhanced coagulation, the WQMPU estimated the total trihalomethane (TTHM) concentrations to be higher than the MCL in certain areas of the City. A variety of treatment alternatives were evaluated and enhanced coagulation followed by granular activated carbon (GAC) filter adsorbers was recommended for reducing distribution system DBP conditions. Since January of 2005, the source waters for the City WTPs have shown elevated levels of total organic carbon (TOC), thereby increasing TTHM formation in the distribution system. This has led to the concern that enhanced coagulation and GAC filter adsorbers by themselves may not be sufficient for compliance with Stage 2 DBPR requirements. The objective of this study was to consider available options for further enhancing DBP control and to develop a road map for the process changes at the City's WTPs. A bench-scale testing program was undertaken to evaluate several technologies in conjunction with enhanced coagulation and determine the appropriate pretreatment technology for lowering DBP concentrations and for extending GAC media life. The study was also used to develop design criteria for these technologies and determine expected GAC reactivation frequencies under various raw water quality conditions. Jar testing was performed to screen the technologies and identify appropriate design (dose, contact time, etc.) conditions. SDS testing was performed on the filtered water to determine the DBP formation potential of water treated with these technologies. Includes tables, figures.