AWWA MTC64565

The Olivenhain Municipal Water District (the District) has been a pioneer in the use of ultrafiltration (UF) membranes for the treatment of drinking water. The Olivenhain Water Treatment Plant (OWTP), which began operation in 2002, utilizes immersed UF to produce up to 34 mgd of finished water from the treatment of State Project and Colorado River water, both characterized by low and consistent levels of total organic carbon (TOC) and turbidity. In response to projected population growth and increasing water demand, the District began evaluating an expansion of the OWTP in early 2006. The District also faced introduction of new source water, Lake Hodges, into Olivenhain Reservoir, which currently stores State Project and Colorado River water blends for use at OWTP. Lake Hodges water presents a significant change in water quality and new challenges for treatment, including higher levels of TOC, taste and odor, iron and manganese, and disinfection byproduct formation. The existing OWTP utilizes UF and chlorine disinfection and is not equipped to adequately address these water quality challenges. The District's objective was to identify the optimum approach for expanding the OWTP, including addressing water quality changes from Lake Hodges, while continuing to use the existing UF system. The evaluation considered 26 water treatment alternatives including membrane pretreatment, UF, membrane post-treatment, and residuals handling and disposal, with an emphasis on effective control of organics and taste and odors and on achieving a sustainable membrane flux and cleaning frequency interval, despite variations in anticipated source water quality. Laboratory-scale treatability testing was conducted to supplement the evaluation by filling identified data gaps as part of the decision process. A key challenge was to make optimum use of older generation UF modules having lower packing density that are not compliant with LT2ESWTR requirements for direct integrity testing, which will take effect in 2012. This paper provides an overview of the decision evaluation, including a brief summary of the alternatives considered, implications for the existing UF system, distinguishing characteristics of each alternative, key lab-scale test results, and the option identified as offering the optimum solution from a benefit-cost perspective. The results will be valuable to utilities that are interested in implementing membrane treatment and will have to consider the need to address new challenges in their source water, while meeting more stringent drinking water regulations. The results will be important to improve the water treatment community's understanding of how integrated treatment strategies coupled with UF (or microfiltration) can be used to provide effective pre- or post treatment approaches for addressing these additional challenges for existing UF facilities that may not be able to address changes in raw water quality or regulatory requirements. Includes table, figures.