Implementing a Multi-Barrier Approach using Ultrafiltration and UV Disinfection

The Eastern Municipal Water District (EMWD) is expanding the existing Perris Water Filtration Plant (Perris WFP) located in Perris, CA from 10-million gallons per day (mgd) to 20-mgd in order to meet the drinking water needs of its customers. The disinfection philosophy for the expanded facility will utilize the synergy of membrane ultrafiltration (UF), ultraviolet (UV) light, and chlorine disinfection to comply with treatment requirements, while minimizing disinfection byproducts (DBP) formation.The existing Perris WFP treats Colorado River Water (CRW) by UF followed by chlorine disinfection. California Department of Health Services (CDHS) awards 4-log removal of Giardia and Cryptosporidium and 2-log virus removal for the UF process used at Perris, but also requires post-UF disinfection to provide for 2-log virus inactivation and an additional 0.5-log Giardia to comply with their multiple barrier pathogen control philosophy. The area available at Perris WFP is severely limited at 4.5 acres and would prohibit construction of new chlorine contact basins to provide 0.5-log Giardia inactivation for the ultimate 50-mgd capacity of the facility. For this reason, EMWD chose to move forward with UV disinfection after a thorough evaluation and site visits to existing UV installations.The two main considerations for assessing water quality for UV disinfection are UV transmittance (UVT) and fouling potential. UVT, a measure of the clarity of the water, varies with source water. The Perris WFP will have the flexibility to treat raw CRW, which consistently exhibits UVT values above 90%, raw State Project Water (SPW) which exhibits considerable seasonal variation (75%–90%), or a blend of the two. As the UVT decreases, the necessary number of lamps increases, the necessary UV intensity increases, and the required exposure times lengthen to provide the required UV dose for inactivation. During the design phase, bench scale testing was performed on the two source waters in order to find the optimum coagulant dose and flocculation time to achieve the target UVT level after UF. EMWD will save operating costs by either blending the two source waters or pre-treating only the raw SPW with coagulation to achieve a consistently high UVT.In addition to the new UV disinfection and pretreatment facilities, the existing chlorine contact basins will be converted into membrane tanks, and a new finished water pump station and clearwell will be constructed as part of the expansion. The new clearwell is designed to provide for a minimum 2-log virus inactivation using free chlorine, which is the most cost effective method to meet this aspect of the CDHS disinfection requirement.The existing Perris WFP capacity is critical to EMWD's system in the summer months; therefore full operations must be maintained during the construction period. A staged construction schedule and start-up will be used to minimize downtime.This paper presents the benefits of the multi-barrier approach adopted by EMWD for the Perris WFP, the process of selecting and sizing the UV reactors, including upstream treatment to optimize UV system design, and the proactive coordination with CDHS to gain approval of this approach.