AWWA MTC61073

The Long Beach Water Department, EE&T, Inc., and the University of Nevada (UNR), Reno are engaged on an 18-month long project to better understand and optimize a dual-staged nanofiltration (NF) process that could potentially be a lower-energy alternative to the traditional reverse osmosis (RO) desalination process. The UNR focus in this research project is on the theoretical basis for the dual NF concept and includes membrane characterization and bench-scale membrane performance. The objectives of this project are to evaluate three NF membranes for their permeate water flux and ion rejection for both single salt feed water solutions as well as for synthetic seawater solutions. This data will provide the basis for a predictive model and allow the formation of a selection protocol to evaluate possible candidate NF membranes. The three commercially available NF membranes selected for this study include the NF- 90 membrane (FilmTec, Midland, MI), the TS-80 membrane (TriSep, Goleta, CA), and the NE- 90 membrane (Saehan, Seoul, Korea). All membranes were supplied as flat sheets and stored in doubly deionized water (DDW) at 5°C. The flux and rejection properties of these membranes were systematically investigated using stirred-cell and bench-scale membrane test units. The water flux and solute rejection of the three membranes under various solution chemistries are being evaluated using a bench-scale membrane test unit. Prior to each set of experiments, the membranes are compacted at 575 psi (the highest experimental operating pressure) for 8 hours using DDW. After compaction, the salt solution is added to the feed reservoir. The solution is recirculated through the system at a low pressure for the equivalent of two tank volumes to ensure adequate mixing. During this time, the temperature is equilibrated to 15°C. The pressure is then increased to the initial operating pressure, and the system is allowed to reach equilibrium over 30 min. This time was found to be sufficient for the flux to reach a stable value. The permeate flux, J, is then measured continuously for 20 min using an electronic balance (Model PB 303-3, Mettler Toledo, Columbus, OH) interfaced with a computer. The permeate water is returned to the feed tank after each experiment. At the end of the 20-min experiment, two 100 mL samples are taken for analysis. This process is repeated for the remainder of the operating pressures. The temperature is then raised to 20°C, and the entire set of experiments is repeated. The temperature range of 15 to 20 °C was based on the seasonal range of temperatures from the Pacific Ocean near Long Beach (i.e., 60 to 70 °F). The different molarities for the tests of individual salts bracket the anticipated individual salt concentrations in the feed and permeate of Stage 1. Sodium and potassium are measured using flame emission spectrophotometry; calcium and magnesium using flame atomic absorption spectrometry (AAS); boron with inductively coupled plasma spectrophotometry (ICP); sulfate, bromide, and chloride with ion chromatography (IC); and bicarbonate with an alkalinity titration. Includes tables, figures.