Fresh approach to water desalination

Canada, 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 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 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 with 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 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. 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 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 groundbreaking research of Philip Jessop, an organic chemistry professor at Queen’s University. When in its salt form, the additive is used to create a superconcentrated brine solution that draws pure water out of the much lowerconcentrated seawater on the other side of a membrane. Sea salts and contaminants are left behind. At this point the magic of green chemistry takes over. To end up with pure water, the additive in the superconcentrated solution must be removed. “We simply switch off the salt and turn it into a gaseous byproduct that just leaves the water,” Resendes said. Huh? How does one “switch off” salt? That’s the Cadbury secret, but it has to do with bubbling air through the solution at about 50 C. 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 superconcentrated 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,” Resendes said. “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 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 focused on building a larger demonstration unit.

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

Forward Water has a multinational water company 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 $13billion 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 take a bite out of the world’s major issues.”