Increased Selectivity for Desalination Membranes (Not Increased Water Permeability)

Achievement date: 
2016
Outcome/accomplishment: 

Using thermodynamic and modeling considerations, researchers at the NSF-funded Nanosystems Engineering Research Center (NERC) for Nanotechnology Enabled Water Treatment (NEWT) Systems at Rice University have demonstrated that improving a desalination membrane’s ability to separate water from dissolved salts has great potential to enhance the efficacy of reverse osmosis (RO) processes. Increased membrane selectivity can reduce the energy consumption of RO processes beyond the gains possible from increased membrane permeability.

Impact/benefits: 

The United Nations forecasts that two thirds of the global population will live in water-stressed regions by the end of the next decade.Highly selective RO membranes will reduce the extent of post-treatment required to ensure desalinated water is suitable for human consumption or use in agriculture, thereby reducing the chemical and energy consumption associated with desalination. Highly selective membranes will also be vital in treating highly contaminated waters, such as wastewater or water produced during the hydraulic fracturing of shale rock formations.

Explanation/Background: 

An estimated 1.8 billion people already live in countries experiencing absolute water scarcity. If left unchecked, current trends in water consumption will lead to a 40 percent shortfall in global water supplies by 2030.

Desalination, the extraction of clean water from salty water, plays an important role in augmenting freshwater supplies and tackling water scarcity. In the energy-efficient membrane-based process of RO, salty seawater or brackish water is pressurized before it is contacted with a semi-permeable membrane. The membrane is able to separate clean water out from salty water by allowing pure water to pass through it more easily than dissolved salts.

The water permeability coefficient of a RO membrane represents the ease with which water can pass through it. Historically, increasing the water permeability of desalination membranes has driven large reductions in the energy consumption of RO. But the studies at NEWT show that current membranes used to desalinate seawater and brackish water are now reasonably effective at allowing water to pass through them. Increasing the desalination membrane’s water permeability further will not reduce the energy consumption of RO processes. Instead, future research should prioritize potential gains made available from increasing the desalination membrane’s selectivity.