Purdue graduate students Abhimanyu Das, left, and Akshay Rao adjust a piston tank

Feeling salty? Engineers develop most energy-efficient desalination process ever

People may wonder why desalination isn’t a more widely used option to help areas that face fresh water insecurity but have ample access to saltwater. The leading reasons are that desalination is very energy- and cost-intensive, so most communities don’t view it as a viable option.

Purdue University engineers are working on what could be the most energy-efficient desalination process ever achieved. The process also can handle water of much higher salinity and the equipment lasts longer.

Building on the past

The process is derived from the most commonly used desalination method: reverse osmosis, which sends seawater over a membrane at high pressure to remove minerals. Desalination plants have to use a lot of pumps and other tools to maintain this high pressure, and that is energy- and emissions-intensive. 

“About a third of the lifetime cost of a desalination plant is energy,” David Warsinger, Purdue mechanical engineering assistant professor, said in a news release.

“Even small improvements to the process — a few percentage points of difference — can save hundreds of millions of dollars and help to keep CO2 out of the atmosphere.”

The Purdue engineers’ process is called batch reverse osmosis. Instead of maintaining a constant flow of water under high pressure, this process takes in a fixed amount of water, processes it, discharges it, and then repeats the process with the next batch.

Each batch is processed for one to two minutes. By ramping up pressure and reducing volume over time, the researchers say they use much less energy to produce the same amount of fresh water.

Some desalination plants have tried batch techniques, but the lag time in between batches when new water is pumped into the chamber lengthens the process and typically negates efficiency gains. “That’s why we developed a solution called ‘double-acting batch reverse osmosis,’” Warsinger said.

The new process uses a high-pressure tank with a piston in the middle. One side of the piston pushes seawater into the processing loop while the other side fills with the next batch of seawater. The piston continuously repeats this process to keep the water flowing and eliminate the need to repressurize and refill the chamber, removing nearly all downtime.

“Downtime is really something you want to avoid,” said Sandra Cordoba, a Purdue master’s student in mechanical engineering. “If you have to service the system after every cycle, you lose all your energy efficiency. Reducing or eliminating that downtime is the key thing that makes batch reverse osmosis feasible.”

Next steps

The Purdue engineers’ test piston is about the size of a fire extinguisher. The technology can be scaled to provide desalination for different scenarios. For example, the device for providing a single household with fresh water could fit in the palm of your hand, whereas it could be 100 feet long in a full-sized desalination plant.

The team has filed patent applications for this technology.

“These results with batch reverse osmosis are really exciting,” Warsinger said. “If we bring the cost down just a little bit, then desalination becomes a viable option for more places. It could be transformative.” 

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