AWWA MTC53709

AWWA MTC53709 Membrane Replacement: Realizing the Benefit of Low Pressure RO in Existing Infrastructure
Conference Proceeding by American Water Works Association, 05/01/2001

Seacord, Thomas;Frank, Kurt;Crozes, Gil;Hill, Greg

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The Mount Pleasant Waterworks (MPW) of Mount Pleasant, South Carolina, operatesthree reverse osmosis (RO) water treatment plants, with a total capacity ofapproximately 5 mgd, using the Middendorf Aquifer as a source. These three plantswere constructed in the early 1990s and the membranes they use may be reachingthe end of their useful lives. In an effort to reduce costs and maximizeproduction capacity, the MPW evaluated new low pressure RO membranes for use intheir existing water treatment plants (WTPs). Recent advances in the productionof composite membranes have led to the development of new low pressure ROmembranes. These membranes offer similar salt rejection and an increasedpermeability (i.e., 0.30 gfd/psi compared to 0.19 gfd/psi) compared to theirpredecessors. Additionally, new 400 ft 2 , 8-inch by 40-inch elements are nowavailable. Implementing these membranes offers the ability to increase productioncapacity if hydraulics are not limiting. Increased permeability is a complicatedissue associated with the use of these new low pressure RO membranes and results in poor flow distributions and requires careful hydraulicconsideration. Excess product flow from first stage membrane elements (i.e., in amulti-stage array) results in lower flows to subsequent stages. Elementrecoveries and concentration polarization factors must be less than 18% and 1.2,respectively in order to prevent membrane fouling. Hydraulic remedies fordistributing flow evenly between stages in a low pressure RO membrane arrayinclude: permeate throttling, interstage boosting, and hybrid membraneinstallations. In the case of Mount Pleasant, infrastructure is existing, pumpsare sized for traditional (high pressure) brackish water membranes, and membraneequipment may be difficult to alter. This study consisted of pilot testingvarious low pressure RO membranes from multiple suppliers. Modeling and costanalyses were conducted to determine the most economical way to implement lowpressure RO. This approach to low pressure RO implementation and competitiveprocurement is anticipated to save Mount Pleasant approximately $1 million overthe next ten years in operations and maintenance savings. Includes 11 references, tables, figures.