Tag Archives: desalination

Could “switchable salts” be a game-changer for desalination market?

 

saltwaterCanada, the land of abundant fresh water, has little need for desalination technologies to quench the thirst of its citizens.

This makes it all the more amazing that Canadians are behind some of the most innovative new approaches to taking salt out of seawater, the need for which is expected to rise substantially over the coming years.

According to a recent report from the National Intelligence Council, which reflects the combined input of 16 U.S. intelligence agencies, global water demand will exceed sustainable supplies by 40 per cent by 2030.

That means certain countries, particularly in the already volatile Middle East region, will need to rely increasingly on the ocean as a source for drinking water and crop irrigation. Just as important, they will need more efficient and low-cost ways of doing it.

Vancouver-based Saltworks Technologies, which has been mentioned many times in this column, is an example of a company responding to the need. Assisted by waste heat or solar heat, it uses specially tuned filters that selectively block the natural flow of sodium, chlorine and other ions as they move through various stages of concentration.

The approach requires little pressure, making it tremendously energy-efficient when compared to conventional methods of salt removal such as distillation and reverse-osmosis.

Now researchers at GreenCentre Canada, the government-funded green chemistry research lab based at Queen’s University in Kingston, have come up with yet another novel and promising approach based on the well-known concept of forward osmosis.

Osmosis, as you might remember from high-school science class, is the natural movement of a solvent through a partially permeable membrane from a low concentration to a high concentration until a balance is reached on both sides. This natural movement is called “osmotic pressure.”

One of the most popular approaches to water desalination today is reserve-osmosis, which is designed to work against osmotic pressure. Seawater is pumped through a salt-blocking membrane to produce purified water on the other side. This uses a lot of energy because it requires high pressure. The membranes also tend to get fouled up with contaminants, boosting maintenance costs.

Forward osmosis, on the other hand, goes with the flow by taking advantage of osmotic pressure. Instead of using electricity to force water through a membrane, a draw solution with much higher salt concentrations than seawater is used to pull the pure water through the membrane.

While efficient and effective, it doesn’t quite do the job. “The problem is, what you have at the end of the day are two buckets of salty water,” said Rui Resendes, executive director of GreenCentre Canada.

But GreenCentre researchers cleverly got around this issue by creating an additive they call “switchable salts,” building on the ground-breaking research of Philip Jessop, an organic chemistry professor at Queen’s University.

When in its salt form, the additive is used to create a super-concentrated brine solution that draws pure water out of the much lower-concentrated seawater on the other side of a membrane. The sea salts and other contaminants are left behind.

It’s at this point that the magic of green chemistry takes over. To end up with pure water, the additive in the super-concentrated solution must be removed. “We simply switch off the salt and turn it into a gaseous byproduct that just leaves the water,” explained Resendes.

Huh? How does one “switch off” salt? That’s the Cadbury secret, but it has something to do with bubbling air through the solution at about 50 degrees C. Somehow it interacts with and alters the chemical properties of the salt additive, turning it into a gas that rises out of the solution.

The beauty of this whole process – which sounds simpler than it is – is that the gas can be collected and reused to create the next batch of super-concentrated brine solution, creating a sustainable loop that depends on relatively little input of outside energy.

“It stands the conventional reverse-osmosis approach on its head,” said Resendes. “The past process has been all about forcing the water out of the system. We leave the water alone and focus on getting the salt out of the system. It’s working with nature, not against it.”

The researchers have proven that it works in the lab. The challenge now is to lead it along the path to commercialization. A company, Forward Water Technologies, was spun out of GreenCentre late last year and is now focused on building a larger tabletop demonstration unit.

“We believe we can achieve that within 12 months,” said Resendes, who has temporarily taken on the job of president and CEO of the new company.

Forward Water already has a major multinational water company on board as a strategic partner. The hope is that in 2014 they’ll be ready to start planning for a larger-scale pilot project.

But the company is determined not to rush. It wants to get the technical work right before marching boldly into the crowded $13-billion desalination market.

Besides, the need for better approaches to desalination isn’t going away. “Think of all the human condition challenges – political strife, drought, famine – all of this is connected to future supplies of fresh water,” Resendes said.

“Technology that can make fresh water sustainably can really take a bite out of the world’s major issues.”

NOTE: I should mention there is one U.K.-based company, Modern Water, having some success in the market with forward osmosis — particularly in the Middle East and China. Here’s what’s posted on their website: In June 2011, the Group won a public tender to construct and operate the world’s first commercial FO plant. Located in Oman, we are now operating the plant after successfully completing construction and commissioning of the plant in September 2012. The Group has a further two proving plants operating in Gibraltar and Oman. They have proven to deliver substantially lower operating costs by reducing energy use by up to 30%, with higher availability than conventional reverse osmosis plants, reducing chemical consumption and carbon footprint. Here’s a link to a fact sheet explaining the process a little better. Boston-based Oasys Water also has a forward-osmosis approach to desalination. Neither Oasys or Modern Water are using anything that resembles Forward Water’s switchable salts.

Tyler Hamilton, author of Mad Like Tesla, writes weekly about green energy and clean technologies.

Vancouver startup Saltworks working on desalination game-changer

A story I wrote last week for MIT Technology Review takes a look at a new energy-efficient approach to desalination developed by a Vancouver-based startup called Saltworks Technologies. Conventional desalination relies on reverse-osmosis and costly membrane technologies. Pumping the water at high pressure through these desalting membranes takes a lot of energy, which drives up the cost of this form of desalination. Another approach is to evaporate and then condense the water, another energy-intensive approach.

Saltworks has a completely different, and quite novel approach. It starts by using the sun or industrial waste heat to evaporate one pool of seawater until it becomes concentrated with 18 per cent salt (compared to 3.5 per cent for regular seawater). This concentrated stream is pumped into a desalting unit along with three other regular seawater streams. The concentrated stream is connected by specially designed “bridges” to two regular streams, and because it has a higher concentration it is compelled to diffuse its salt content — sodium and chloride — into the weaker streams. But the bridge connecting to the one weaker stream only allows sodium ions, which are positive, to flow through; the bridge connected to the second weaker stream only allows chloride ions, which are negative, to flow through. The end result at this stage is that one of the two weaker streams now has surplus positive ions, mostly sodium, and the other has surplus negative ions, mostly chloride. The two streams, now out of balance and eager to pick up ions of opposite charge, are separately “bridged” to the third regular seawater stream. The one stream with surplus positive ions strips the third stream of all its negative ions, and the second stream with surplus negative ions strips the third stream of all its positive ions. This leaves the third stream completely stripped of all ions — i.e. it’s de-ionized, or pure drinking water.

It’s a brilliant process because most of the energy that’s required comes at the front end through evaporation, which is accomplished in a low-tech way with abundant solar energy, or waste heat from a neighbouring industrial facility. The rest is accomplished through electrochemical reactions requiring no outside energy source. If Saltworks can scale this approach up, it could bring cheap desalination to the many parts of the world that need it.