Brackish Water RO and NF Operations on High-TDS Feedwaters
By Wayne T. Bates, Craig Bartels, Ph.D., and Kirk Lai
An increasing number of brackish water reverse osmosis (RO) and nanofiltration (NF) systems are being designed and operated on high salinity brackish feedwater sources which can range from 2,000 to 12,000 mg/L total dissolved solids (TDS). Increased feed TDS can dramatically impact the design of the RO/NF in terms of hydraulic design, feed pressure requirements and in permeate quality. Issues discussed in this article include hybrid RO/NF membrane system designs for improved hydraulic flux design, the use of energy recovery devices (ERDs), and the impact of higher salinities on the rejection of ions. Systems with improved hydraulic flux designs can reduce the rate of fouling and cleaning, be more energy efficient, and improve permeate quality. Systems with either feed-side or interstage energy recovery devices can be more energy efficient, but the payback time period has to be reviewed. A discussion will be presented that the industry has not widely addressed and that is the observation of increased system salt passage with increased feed TDS and increased concentrate TDS at higher recoveries.
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