The Brackish Water Challenge: Adequate Pretreatment to the RO Units

摘要

The Swansea Water District (Massachusetts) is planning to construct a plant to treat arnblend of water from the Palmer River and a groundwater wellfield. The river, approximately 5rnmiles upstream of Narragansett Bay, is tidally influenced, and seawater from the bay will intruderninto the planned river intakes during high tides. At high tide, when salinity is highest, the waterrnhas 21,300 mg/L of total dissolved solids, 14 mg/L total organic carbon (TOC), and 33 PCU ofrncolor. To treat this water, a system comprising of coagulation-assisted membrane pretreatmentrnfollowed by reverse osmosis has been proposed. To determine the optimum coagulant type andrndose for Palmer River pretreatment, a series of bench-scale jar tests was conducted using ferricrnchloride (FeCl3) and a medium-basicity polyaluminum chloride (PACl) to simulate therncoagulation, sedimentation, and membrane filtration processes. Coagulant doses up to 150 mg/Lrn(as coagulant) were tested at various pH levels to determine an optimum dose and pH. Differentrnpolymers and a powdered activated carbon (PAC) were then tested at the optimum coagulantrndose and pH to determine if the pretreated water quality could be improved.rnFeCl3 was found to be very effective at removing TOC and color and reducing UVrnabsorbance. The optimum dose was 40 mg/L FeCl3. Higher FeCl3 doses produced water withrnbetter pre- and post-filtration quality, but the improved water quality was outweighed by thernsubstantially higher chemical use and residuals solids handling. PACl was not effective; evenrndoses as high as 150 mg/L PACl failed to achieve results comparable to the optimum FeCl3 dose.rnDecreasing the pH to as low as pH 5.0 offered substantial improvements to the water quality.rnUse of PAC and polymers did not improve the performance of the pretreatment step.