Discontinuous reverse osmosis, drinking sea water much more efficiently

A breakthrough in reverse osmosis could lead to more energy-efficient seawater desalination.

Getting fresh water from sea water often requires huge amounts of energy. The most common desalination process is called reverse osmosis, which works by running seawater over a membrane at high pressure to remove minerals.

Now engineers at Purdue University have developed a variation of the process called “discontinuous reverse osmosiswhich promises greater energy efficiency, longer equipment life and the ability to handle much higher salinity water.

Reverse osmosis is used in many countries; In places like the Middle East, more than half of the drinking water supply comes from desalination plants. But to maintain the high level of pressure necessary for the process – up to 70 times atmospheric pressure – a desalination plant consumes a lot of energy.

About one-third of the lifetime cost of a desalination plant is energy. Even small process improvements – a difference of a few percentage points – can save hundreds of millions of dollars and help keep CO2 out of the atmosphere.

David Warsinger, associate professor of mechanical engineering at Purdue.

During his PhD at MIT, Warsinger first developed the idea of ​​”batch reverse osmosis”. Instead of maintaining a constant flow of seawater at such high pressure levels, a batch process takes in a set amount of water at a time; the Treaty ; discharge; then repeat the process with the next batch.

Each batch lasts between one and two minutes. We increase the pressure over time, reduce the volume over time, and end up using much less energy to produce the same amount of fresh water.

David Warsinger.

Although some desalination plants have attempted to use semi-batch techniques, none have ever implemented a full-batch system, partly because of pauses between batches.

It takes time and energy to pump each batch of water and then pump the next batch of water for treatment. The cost of this time and energy often outweighs the efficiency gains that would be gained by using the batch process. This is why we have developed a solution called ‘double action reverse osmosis’.

David Warsinger.

This new process uses a piston tank, a high pressure vessel with a piston in the center. While one side of the piston sends seawater through the treatment circuit, the other side of the piston simultaneously fills with the next batch of seawater in the queue. When a batch process is complete, the plunger seamlessly injects the next batch of seawater into the system while simultaneously filling its other side with the next batch of seawater in the tail, and the process continues. repeat continuously.

Instead of completely emptying the piston each time or using another liquid or gas to pressurize it, we fill it with the next batch of seawater. So instead of one side of the piston being essentially a gap dead, we use seawater itself to get a double function out of that piston, so there’s almost no downtime.

According to our models, this proposed system offers the lowest energy consumption ever seen for seawater desalination. This is a milestone in its class.

David Warsinger.

Downtime is something you really want to avoid. If the system has to be repaired after each cycle, all energy efficiency is lost. Reducing or eliminating this downtime is key to making batch reverse osmosis viable.

Sandra Córdoba, Purdue Mechanical Engineering Masters student and first author of the paper.

Córdoba also developed the theoretical hydraulic models used in the work.

Reverse osmosis is a complex process. To measure your success, you need to track many variables: water pressure, volume, salinity, recovery rate, time and energy. With these models, we were able to determine the right amount of pressure over time to get the best results using the least energy.

Sandra Cordoba.

What is the size of the piston tank? It depends on the size of the system.

Reverse osmosis works on a wide range of scales. Indian households often have a micro reverse osmosis system for their own home. For our experiments, we built a model system in which the piston tank is the size of a fire extinguisher. In a full-scale factory, it might be 30 meters long. But the good thing is that it’s not a complex team; it’s basically a pipe, with a sealed piston in the middle. But this piston tank changes everything.”

David Warsinger.

Warsinger’s lab has used this dual action batch development to drive several new advances in desalination. Abhimanyu Das, a mechanical engineering PhD student at Purdue, has published research describing a variation of the process called “batch countercurrent reverse osmosis.” By recirculating certain concentrations of water on both sides of the membrane, the Das process proves to be the most energy efficient desalination process for high salinity water, while requiring fewer components. And Purdue master’s student Michael Roggenburg has published research showing that a combination of batch reverse osmosis and renewable energy could deliver fresh water to the entire 1,954-mile border between the United States and Mexico.

Water safety is a huge issue around the world, which I have dedicated my entire career to. These results with discontinuous reverse osmosis are really exciting. If we reduce the cost a bit, desalination will become a viable option for more places. It could be transformative.

David Warsinger.

More information: www.sciencedirect.com

Going through www.purdue.edu

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