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San Diego County Water Authority (Water Authority) is planning to procure a new watertreatment 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 constructionof the new treatment plant. Giving due consideration to source water quality, treated waterquality goals, site constraints and potential environmental impact, two alternative treatmenttrains were identified and evaluated under conceptual design: low-pressure membranes(microfiltration (MF)/ultrafiltration (UF)) with granular activated carbon (GAC) as polishingtreatment; and, conventional treatment with intermediate ozonation. These two alternativetrains would produce water that would consistently meet the water quality goals but would resultin finished water of varying quality; however, the ultimate decision about process selection is leftup to the proposing companies.Conceptual designs consisted of plant hydraulic profiles, design criteria, site layout and costopinions for the two alternative treatment trains. The purposes of the conceptual designs were toserve as the basis for the Environmental Impact Report, and to provide preliminary cost opinionsfor the project. Because there is no sewer system at the proposed plant site, the conceptualdesigns had to be developed giving consideration to zero liquid discharge from the treatmentplant. This paper focuses on the process and planning considerations for the conceptual designof the zero liquid discharge membrane treatment plant.The membrane treatment plant conceptual design consisted of two stages of submergedmembrane treatment systems. The backwash water from Stage 1 (which is approximately 7percent 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 treatmentwas assumed to be returned to the front of the treatment plant. It may be possible to get theState'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 mechanicaldewatering and solids handling facilities. The membrane clean-in-place (CIP) chemicals wastewould be neutralized and blended with Stage 2 backwash water. The dewatering and solidshandling facilities would thicken and dewater the Stage 2 membrane reject, CIP waste andintermittently treat the solids from the equalization basins that hold the spent backwash waterfrom the GAC contactors. The solids handling facilities would include gravity thickener andcentrifuges. The centrate and thickener return would be returned to the front of the treatmentplant. The dewatered solids would be hauled away to a landfill. Includes table, figures.

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Edition: Vol. - No. Published: 06/17/2005 Number of Pages: 11File Size: 1 file , 1.7 MB