(620e) Coupled Membrane Distillation-Crystallization for Water and Salt Recovery from High-Strength Brine

Authors: 
Hiibel, S. R., University of Nevada Reno
Balis, E., University of Nevada, Reno
The need for alternative freshwater sources is increasing in the U.S. and around the world due to rising populations, expanding urbanization, and climate events. Although many coastal regions rely on seawater desalination for potable water, inland regions cannot rely on the oceans as a feed source for water treatment due to the high cost of transporting water over long distances and against an elevation gradient. Inland brackish waters are a promising alternative water source for non-coastal regions, and desalination of these waters has potential to provide high-quality product water. Unfortunately, most desalination techniques developed for coastal regions cannot be directly translated to their full extent inland due to differences in water chemistry between seawater and brackish waters. Additionally, concentrated brine disposal is a challenge in non-coastal areas as the ocean cannot serve as a repository for the brines.

This work evaluates Membrane Distillation-Crystallization (MDC) as a method for effectively treating reverse osmosis (RO) brine resulting from desalinating inland brackish waters to minimizing concentrate volume. The process combines membrane distillation (MD) with crystallization. MD is a non-pressure-driven separation process that involves the transport of water through a hydrophobic, microporous membrane. Compared to conventional distillation methods, MD requires only small temperature differences and has a simple configuration that makes it highly suitable for coupling to other processes. The driving force of MD is the vapor pressure of the brackish water, which is relatively independent of salt concentration, thus it can be used to treat brines to near saturation. When coupled with a crystallization unit, the overall system can remove solid salts crystals away from the membrane surface to maximize membrane longevity and produce high-quality distillate water.

The bench-scale system uses a novel, multi-stage, cascading crystallizer designed by the University of Nevada team in conjunction with a flat-sheet MD unit with 140 cm^2 of active membrane area. The MDC system has been validated using a 290 g/L solution of NaCl as an initial feed, with MD operating at 50 / 25 C as the feed / distillate side temperatures. After 10 hours of batch operation, the MDC system recovered 2.5 times more water than MD alone, while also capturing 150 g of solid salt in the crystallizer. When cooling was added to the crystallizer, 3.3 times more water was recovered compared to MD alone, and 195 g of solid salt were recovered. Further testing with additional salts is currently underway and will be presented.