New membranes could help eliminate brine waste at desalination plants

New membranes made at the University of Michigan (U-M) could help desalination plants minimise or eliminate brine waste produced as a by-product of turning seawater into drinking water.

Currently, liquid brine waste is stored in ponds until the water evaporates, leaving behind solid salt or a concentrated brine that can be further processed. But brine needs time to evaporate, giving it a chance to potentially contaminate the groundwater.

For every litre of drinking water produced at a typical desalination plant, 1.5 litres of brine is produced. According to a UN study, over 37 billion gallons of brine waste is produced globally every day. When there is no space for evaporation ponds, desalination plants inject the brine underground or dump it into the ocean. Increasing the salt levels near desalination plants is a problem as it can harm local marine ecosystems.

To eliminate brine waste, desalination engineers would like to concentrate the salt so it can be easily crystallised in industrial vats rather than ponds that can occupy over a hundred acres. The separated water could be used for drinking or agriculture, while the solid salt could then be harvested for useful products.

Seawater not only contains sodium chloride — or table salt — but valuable metals such as lithium for batteries, magnesium for lightweight alloys and potassium for fertiliser.

“There’s a big push in the desalination industry for a better solution,” said Jovan Kamcev, U-M Assistant Professor of Chemical Engineering and the corresponding author of the study published in Nature Chemical Engineering. “Our technology could help desalination plants be more sustainable by reducing waste while using less energy.”

Desalination plants can concentrate brines by heating and evaporating the water, which is energy intensive, or with reverse osmosis, which only works at relatively low salinity. Electrodialysis is said to be a suitable alternative because it works at high salt concentrations and requires relatively little energy. This process uses electricity to concentrate salt, which exists in water as charged atoms and molecules called ions.

Water flows into many channels separated by membranes, and each membrane has the opposite electrical charge of its neighbours. The entire stream is flanked by a pair of electrodes. The positive salt ions move towards the negatively charged electrode, and are stopped by a positively charged membrane. Negative ions move towards the positive electrode, stopped by a negative membrane. This creates two types of channels — one that both positive and negative ions leave and another that the ions enter, resulting in streams of purified water and concentrated brine.

However, electrodialysis has its own salinity limits. As the salt concentrations rise, ions start to leak through electrodialysis membranes. While leak-resistant membranes exist on the market, they tend to transport ions too slowly, making the power requirements impractical for brines more than six times saltier than average seawater. 

The researchers overcome this limit by packing a record number of charged molecules into the membrane, increasing their ion-repelling power and their conductivity — meaning they can move more salt with less power. With their chemistry, the researchers can produce membranes that are 10 times more conductive than relatively ‘leak-proof’ membranes on the market.

The dense charge ordinarily attracts a lot of water molecules, which limits how much charge can fit in conventional electrodialysis membranes. The membranes swell as they absorb water, and the charge is diluted. In the new membranes, connectors made of carbon prevent swelling by locking the charged molecules together.

The level of restriction can be changed to control the leakiness and the conductivity of the membranes. Allowing some level of leakiness can push the conductivity beyond today’s commercially available membranes. The researchers hope the membrane’s customisability will help it take off.

“Each membrane isn’t fit for every purpose, but our study demonstrates a broad range of choices,” said David Kitto, a postdoctoral fellow in chemical engineering and the study’s first author. “Water is such an important resource, so it would be amazing to help to make desalination a sustainable solution to our global water crisis.”

Image caption: Jovan Kamcev, Assistant Professor of Chemical Engineering, places a membrane into an electrodialysis device. Credit: Marcin Szczepanski, Michigan Engineering.