AWWA WQTC60668

Sweetwater Authority (SWA) owns and operates the Robert A. Perdue Water Treatment Plant (WTP), a 30 mgd conventional filtration plant located in San Diego County, California. This paper documents the results of SWA's pilot testing of chlorine dioxide (ClO₂), dissolved air flotation (DAF), and submerged microfiltration/ultrafiltration (MF/UF) membranes, begun in early 2004, as a reliable and cost-effective treatment strategy for meeting future disinfection byproduct (DBP) regulations. The Perdue WTP treats two source waters, local surface water from Sweetwater Reservoir (SWR) (average total organic carbon (TOC) = 6.5 mg/L, bromide up to 0.5 mg/L) and San Diego County Water Authority (CWA) aqueduct water (average TOC = 2.5 mg/L, low bromide). SWA currently applies free chlorine and potassium permanganate (KMnO₄) to the raw SWR water and uses chloramines in the distribution system. However, excessive trihalomethanes (THMs) formation is a potential concern for compliance with the Stage 2 Disinfection Byproducts Rule using the Locational Running Annual Average (LRAA) method. The current testing program was preceded by pilot testing of intermediate-ozone with enhanced coagulation (30 mg/L ferric chloride) and bench-scale testing of ClO₂ and ferric chloride doses up to 80 mg/L, results of which are also highlighted in this paper. CWA was responsible for the first phase of DAF and MF/UF membrane testing at the Perdue WTP site, using the CWA Aqueduct water as the pilot feedwater. With DAF as pretreatment, both of the submerged membrane systems tested provided reliable water quality at fluxes above 30 U.S. gallons per square foot per day (gfd) (51 liters per hour per square meter), with recoveries around 95%, for at least 30 days before the need for membrane clean-in-place (CIP). For the SWR water testing, the full-scale WTP influent is being treated with ClO₂ at doses up to 1.5 mg/L, and chlorite is controlled by applying ferrous chloride (FeCl₂) at a 3.3:1 ratio of ferrous ion to chlorite. The first two weeks of the SWR water testing used DAF pretreated water as the feedwater to the membrane pilots while the following six weeks used settled water from the full-scale plant as the membrane feedwater to compare the effects of these two pretreatment options on membrane operation. Membrane fluxes were set initially based on the results from the CWA water testing and optimized to provide a minimum 30 day CIP interval. ClO₂ was evaluated as the plant primary disinfectant with chloramines downstream of the membranes for distribution system residual. In addition to monitoring the performance of the DAF and membrane systems, simulated distribution system (SDS) testing was conducted on the membrane filtrate on a weekly basis to estimate disinfection byproduct (DBP) formation. The process combination of ClO₂, DAF, and submerged membranes represents a promising process combination for agencies seeking to meet the Stage 2 DBP Rule with source waters with high bromide and highly reactive TOC. Includes 3 references, figures.