Getting the Salt Out of Seawater at the Same Price as Fresh Water from the Ground


(T) One of the most critical resources for life is water. Without drinking water, there is no survival. Without water for crops, there is no food. Without water for the industry, there are no new products. In many parts of the world, we have exceeded sustainable limits on the withdrawals of water from groundwater aquifers. We have disrupted water flows with the damming of rivers. Global warming is only going to accelerate the scarcity of water, in particular in Africa and South Asia. Less than 1% of the Earth’s water is accessible freshwater; the rest for around 97% is sea water and 2% are the ice caps and glaciers. And, the amount of fresh water has stayed at the same level while the human population will reach 6.9 billion in 2010 and 9.2 billion in 2050 (according to the projections of the United Nations).

This year a strong El Nino has brought some welcome relief to California water crisis. Most of the water supplies in California come from the Sierra Nevada reservoirs and is carried over 2,000 miles of canals, pipelines, and aqueducts, that are vulnerable to an earthquake, to the driest and most populated Southern California. 70% of the water in California comes from the north, while 80% of the demand is in the midsection and south parts of the state.

Like many other parts in the world, the solution to the water crisis in Sunny California will have to get the salt out seawater. According to the International Desalination Association, there are now around 14,000 desalination plants in operation around the world. In California alone, some 20 seawater desalination plants have been proposed, including a $300 million facility near San Diego. However, only 0.3% of clean water is presently obtained from water desalination because plants are expansive to build and expansive to operate due to the energy required to desalinate water.

The first generation of desalination plants, deployed in particular in the Middle East, heats seawater until it turns to steam, leaving its salt behind, then condensing it.

The second and present generation of desalination plant filters water from seawater by using a process called Seawater Reverse Osmosis (SWRO). SWRO forced seawater through a semi-permeable membrane, producing pure water on one side and concentrated brine on the other. By pumping seawater to pressures over more than a thousand pounds per square inch, SWRO takes less energy than boiling seawater. But SWRO is still very expensive because of the required energy to achieve the high pressures that must be attained for it to work effectively.

Recent technological advances from Energy Recovery Inc (ERI), a public-traded company based in San Leandro, California, has pioneered the development of energy recovery devices, that improve the energy efficiency and reduce the cost of SWRO. Early energy recovery devices were only 50% to 75% efficient but newer ones such as the PX product line from ERI can recover up to 98% of the energy from the high-pressure membrane reject stream.

According to ERI, “the core of PX technology is an isobaric energy recovery device, made of uniquely engineered ceramics which captures the hydraulic energy from the high pressure reject stream of seawater reverse osmosis (SWRO) processes. It transfers this energy to low-pressure feed water with an efficiency of nearly 98%. Pressurized seawater from the PX device is sent to the membrane feed, merging with the pressurized water from a high-pressure pump. This significantly reduces the amount of water pressurized by the high-pressure pump. Because the PX device itself consumes no electrical power, overall energy consumption of an SWRO process is drastically reduced”.

In addition, a number of new disruptive technologies could further cut the required energy to desalinate water. The first one, close to commercialization, is forward osmosis, that pushes water to the membrane by “natural osmosis” into a solution that contains salt that can be easily evaporated. Natural osmosis occurs when the water molecules pass through the cell membrane from an area of low solute concentration (outside the cell) to one of high solute concentration (inside the cell).

Other research methods attempt to redesign the permeable membrane such as carbon nanotubes that separate positively charged salt molecules from uncharged water molecules and biometrics that leverage the same proteins that usher water molecules through the membranes of living cells.

With the scarcity of fresh water being more and more a challenge to overcome for many populations and many countries, we need to research more and invest more on how to get the salt out of seawater at the same price as groundwater.


Note 1: After I learned about Energy Recovery from listening to one of its board members, I bought some company shares. ERI is traded on the NASDAQ, symbol ERII.

Note 2: The picture above is the Afareaitu waterfall on the island of Moorea in Tahiti.

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Categories: Sustainability