AWWA MTC57545

The City of Sweetwater, Texas, (population 12,000) shut down their surface water treatment plant in December, 2001. Originally constructed in the 1920s, the conventional treatment plant (rapid mix - flocculation - sedimentation - filtration) could not meet the finished water turbidity requirements that became effective January 1, 2002. Additionally, monitoring at the plant conducted prior to shutdown revealed an inability to meet a 25% reduction in total organic carbon (TOC) as required under provisions of the Interim Enhanced Surface Water Treatment Rules also effective for Sweetwater on January 1, 2002. Planning for the future, the City of Sweetwater conducted pilot studies from October, 1999, through March, 2000, to test the treatment capabilities of membrane technologies. Pilot objectives included: demonstrating organic removal performance; demonstrating turbidity/particle removal capabilities; demonstrating the overall treatment capability for feed water from three surface water sources; and, determining the design and operating parameters for a full-scale plant. The size of natural organic matter present in surface waters is smaller than the pore size of membranes. The pilot study focused on optimizing TOC removal by enhanced coagulation upstream of the membranes. The goal was to generate pin-sized floc particles large enough to be removed by ultrafiltration. The ZeeWeed Immersed Membrane System from ZENON Environmental Systems, Inc. met all of the study objectives. TOC reductions of over 32% were achieved when the ZENON pilot unit was operated for enhanced coagulation. Design parameters developed from the pilot study included provisions for rapid mixing of the coagulant, followed by a limited flocculation stage of 12.5 minutes prior to entering the membrane tanks. By controlling system recovery and maintaining a high solids concentration in the membrane process tanks, TOC reductions meeting or exceeding the regulatory standards will be achievable in the full-scale plant. The City of Sweetwater's new 8.0 MGD (30.28 MLD) surface water treatment plant is under construction, with startup anticipated in August 2003. In addition to combining enhanced coagulation for TOC removal with ultrafiltration, design of the full-scale plant includes a secondary UF process tank to achieve a 99% recovery of feed water. These two design characteristics (enhanced coagulation and 99% recovery) are key elements of the first immersed, vacuum-driven membrane system in Texas. Includes 6 references, tables, figures.