Water desalination is energy-intensive. A typical seawater reverse osmosis (RO) plant requires 1.5 to 2.5 kilowatt hours (kwh) of electricity to produce 1 m3 of treated water; a thermal distillation plant requires five to ten times more. The principal objective of this project is to develop a new hybrid ion exchange-nanofiltration (HIX-NF) process that has the potential to reduce energy consumption of water desalination by two to three times for a wide range of feed water salinities. The proposed process is innovative in its approach for it partially alters the feed water chemistry prior to the membrane treatment. Specifically, it converts chloride, the predominant anion in salt water, into sulfate through a reversible anion exchange process without requiring any regenerant. As a result, RO membranes can be replaced altogether by NF membranes resulting in a marked reduction in energy requirement. Also, NF membrane is more robust and less susceptible to fouling compared to RO, the HIX-NF process has the potential to concentrate the reject stream further, thus reducing the volume of disposable brine.
The primary challenge of this research lies in engineering and perfecting the reversible sulfate-chloride anion exchange process, a significant component of the HIX-NF process. For every brackish/sea water composition, there remains a unique sulfate/chloride selectivity (i.e., separation factor value) for anion exchangers to sustain the HIX-NF process. The present body of knowledge on ion exchange fundamentals and our related work strongly suggest that by changing the sizes of the amine functional groups, the relative affinity between divalent sulfate and monovalent chloride can be greatly manipulated for an optimal process performance. In this project, rigorous experimental studies will be conducted with the three following feed water salinities to cover the entire range for the HIX-NF process: 6,000 mg/l, 12,000 mg/l and 30,000 mg/l NaCl concentrations.
Two graduate students (one doctoral and one MS) will work on the project. The P.I. plans to recruit one female student from the graduating class. The P.I. gives water related lectures in science classes of Freedom High School in the Lehigh Valley. Due to its global appeal, desalination will continue to be a relevant and interesting topic for high school students.
Research Outcome The issues of calcium-based scaling in desalination processes are well documented. Current measures such as chemical dosing or operating the process at reduced efficiency are not sustainable solutions to this problem. Previous work on this issue studied the selective removal of Ca2+ though the use of reversible cation exchange (RCIX) desalination. These works used sodium - loaded cation exchange resin to selectively replace Ca2+ with Na+. However, the 2:1 replacement of divalent cations with Na+ results in feed solutions with increased osmotic pressures. As such, instead of using Na+ as the replacement ion, Mg2+ - based RICX process was studied. Results show that the Mg2+ - based RCIX process, using strong acid cation exchange resin, was able to consistently provide a desalination feed water with 50% Ca2+ removal. Produced water was low in TDS, and the process was able to be operated at 50% recovery. Process efficiencies between strong acid and weak acid cation exchange resins were also studied. Results show that weak acid cation exchange resins are not well-suited for the process due to their inability to be regenerated by the desalination concentrate stream. While strong acid resins do provide significant removal, there still exists room for improvement on the process. Therefore, future work will focus on the development and testing of tailor-made resins which have these desirable qualities. Broader Impacts Throughout the United States, but mainly in the southwest, California, and Florida, desalination processes are used for providing drinking water. Continued operation of these drinking water treatment plants requires the constant addition of anti-scalants in order to prevent Ca2+ scaling. The disposal of these anti-scalants, mainly organophosphate- and polyacrylate-based, is an emerging environmental issue due to the potential for algal blooms. An Mg2+ - based RCIX process would allow for the elimination of the need for anti-scalants addition and yield higher product water recoveries. Due to the increased process efficiency, there will be less concentrated brine to dispose of – this further minimizes the impact the desalination plant would have on the surrounding environment. Furthermore, due to increased process efficiencies, the cost of water, per unit, will be cheaper.
