AWWA WQTC60682

To meet the anticipated requirements of the Stage II Disinfectant/Disinfection Byproducts Rule (D/DBPR), the Cobb County-Marietta Water Authority conducted a pilot study at the Hugh A. Wyckoff Water Treatment Plant (WTP) to examine the effects of ozone/biofiltration, chlorine dioxide preoxidation, magnetic ion exchange (MIEX®), and post-filter nanofiltration (NF) on the removal of disinfection byproduct (DBP) precursor material. The study showed that ozone/biofiltration was ineffective in reducing total trihalomethane (TTHMFP) and haloacetic acid formation potential (HAA5FP), and that chlorine dioxide preoxidation would be sufficient to meet the Stage I D/DBPR requirements. The MIEX and post-filter NF alternatives demonstrated incremental improvement in the removal that was already being achieved by conventional treatment; therefore, MIEX and post-filter NF were examined further as options to ensure that the Authority can achieve the Stage II requirements. The two treatment alternatives were then compared to determine which would be the best alternative to implement at the Wyckoff WTP. The examination included both economic and non-economic factors, including construction cost, operations and maintenance (O&M) cost, system reliability, process performance, design, and implementation factors. Key process design considerations included split treatment, coagulant reduction, concentrate treatment and disposal, flux and recovery rate optimization, and membrane fouling. The major difference between the two alternatives was that post-filter NF was significantly less expensive to build and operate. This was due to the fact that a post-filter NF plant could treat only 26.5 percent of the flow and still achieve the DBP treatment goals. A MIEX facility large enough to treat the entire 104 million gallons-per-day (mgd) capacity of the plant, after the next plant expansion, was required to meet the same goals. Therefore, the ability to treat a fraction of the flow made post-filter NF the most economic option of the two examined. The estimated construction cost for post-filter nanofiltration was approximately $33 million versus the MIEX process that was estimated at approximately $47 million. Additionally, post-filter NF was found to be significantly less expensive to operate, with an estimated annual O&M cost of $1.8 million versus the MIEX process at $4.2 million. Based on an average daily flow of 70 mgd, the cost per 1,000 gallons (including the amortized capital cost) was estimated to be $0.20 for post-filter NF versus $0.34 for MIEX. Includes tables, figure.