AWWA ACE61849

San Diego County Water Authority (Water Authority) is planning to procure a new water treatment plant up to 100 million gallons per day (mgd) capacity using the design-build-operate (DBO) approach. The Water Authority identified the Twin Oaks Valley site for the construction of the new treatment plant. Giving due consideration to source water quality, treated water quality goals, site constraints and potential environmental impact, two alternative treatment trains were identified and evaluated under conceptual design: low-pressure membranes (microfiltration (MF)/ultrafiltration (UF)) with granular activated carbon (GAC) as polishing treatment; and, conventional treatment with intermediate ozonation. These two alternative trains would produce water that would consistently meet the water quality goals but would result in finished water of varying quality; however, the ultimate decision about process selection is left up to the proposing companies. Conceptual designs consisted of plant hydraulic profiles, design criteria, site layout and cost opinions for the two alternative treatment trains. The purposes of the conceptual designs were to serve as the basis for the Environmental Impact Report, and to provide preliminary cost opinions for the project. Because there is no sewer system at the proposed plant site, the conceptual designs had to be developed giving consideration to zero liquid discharge from the treatment plant. This paper focuses on the process and planning considerations for the conceptual design of the zero liquid discharge membrane treatment plant. The membrane treatment plant conceptual design consisted of two stages of submerged membrane treatment systems. The backwash water from Stage 1 (which is approximately 7 percent of the feed flow) would be sent to Stage 2. Stage 1 would have 6-8 membrane trains. Stage 2 would have 3 membrane trains. The product water from Stage 2 membrane treatment was assumed to be returned to the front of the treatment plant. It may be possible to get the State's approval to blend Stage 2 product water with Stage 1 product water. The backwash water from Stage 2 membranes and GAC would be sent through mechanical dewatering and solids handling facilities. The membrane clean-in-place (CIP) chemicals waste would be neutralized and blended with Stage 2 backwash water. The dewatering and solids handling facilities would thicken and dewater the Stage 2 membrane reject, CIP waste and intermittently treat the solids from the equalization basins that hold the spent backwash water from the GAC contactors. The solids handling facilities would include gravity thickener and centrifuges. The centrate and thickener return would be returned to the front of the treatment plant. The dewatered solids would be hauled away to a landfill. Includes table, figures.