Making desalination more eco-friendly: New membranes could help eliminate brine waste

Desalination plants, a major and growing source of freshwater in dry regions, could produce less harmful waste using electricity and new membranes made at the University of Michigan.

The membranes could help desalination plants minimize or eliminate brine waste produced as a byproduct of turning seawater into drinking water. Today, 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, providing ample opportunities to contaminate groundwater.

Space is also an issue. For every liter of drinking water produced at the typical desalination plant, 1.5 liters of brine are produced. Over 37 billion gallons of brine waste is produced globally every day, according to a UN study. When space for evaporation ponds is lacking, desalination plants inject the brine underground or dump it into the ocean. Rising salt levels near desalination plants can harm marine ecosystems.

“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 a study published in Nature Chemical Engineering. “Our technology could help desalination plants be more sustainable by reducing waste while using less energy.”

To eliminate brine waste, desalination engineers would like to concentrate the salt such that it can be easily crystallized 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 fertilizer.

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

Here’s how the process works. Water flows into many channels separated by membranes, and each membrane has an electrical charge opposite that of its neighbors. The entire stream is flanked by a pair of electrodes.

The positive salt ions move toward the negatively charged electrode, and are stopped by a positively charged membrane. Negative ions move toward 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.

But, 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 overcame 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, researchers can produce membranes that are ten times more conductive than relatively leak-proof membranes on the market today.

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 customizability 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.”