Full Description

The majority of the drinking water provided to Watertown, South Dakota, is treated at the 8million gallons per day (MGD) Town Water Treatment Plant (WTP), which employs aconventional groundwater treatment system, comprised of aeration, lime softening, recarbonation,media filtration and disinfection. The plant has experienced a few occurrences of elevated levelsof disinfection byproducts (DBPs) in the distributed water, with concentrations of TotalTrihalomethanes (TTHM) up to 120 g/L. A nanofiltration membrane system is proposed asenhanced treatment for the existing softened water. The purpose of the NF membrane treatmentsystem is to further treat a stream of the softened water in order to reduce the concentration ofdisinfection byproduct (DBP) precursors. The NF permeate will be blended with the remaining softened water, which is notrouted through the NF system, as well as with additional 4 MGD from a new conventionalgroundwater softening plant. Final blend will represent a total of 12 MGD of drinking watermeeting Stage 1 and Stage 2 of D/DBP Rules.One of the main challenges of this project is selecting the NF membrane which can provide themost economical solution while satisfying several competing goals: high TOC rejection to reducethe level of DBP precursors; low calcium and alkalinity removal to avoid the need for final blendstabilization treatment; and, low inorganic rejection in the concentrate water for cost-effective localdisposal. Selection of the NF membrane is performed based on the results of a pilot study programcomposed of three segments. First, a membrane screening study was performed with a single 4"element pilot unit. This allowed screening of six candidate NF membranes in a quick but effectivemanner. The screening criteria were high TOC removal, low simulated distribution system (SDS)-DBP formation in the product water and in the blend, and low total dissolved solids (TDS) passingrates. In no preferred order, the selected membranes were Koch TFC-SR3, Trisep XN45, andHydranautics ESNA1-LF.These membranes underwent a second phase of testing which was performed with a two stagepilot unit, modeling the full scale plant design, and which was just completed. One membrane willbe selected based on operating performance parameters as well as quality of product water (TOC,SDS-DBP, TDS, alkalinity and calcium). A third and last phase of the piloting will focus onfouling and cleaning requirements and will begin in January 2009. The results of this pilot studywill not only allow selection of the most suitable NF membrane for this application but will alsoprovide a set of hydraulic parameters for the design of the full scale NF membrane plant. Thispaper focuses on the results and the selection process for the first two phases of the pilotingprogram. Includes tables, figures.

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Edition: Vol. - No. Published: 11/01/2009 Number of Pages: 20File Size: 1 file , 1 MB