Purifying the Dirtiest Waters

In the break room of a South Boston research center, a gleaming fish tank stands next to shelves of Cheez-It crackers. Inside angelfish, catfish, and minnows swim around brightly colored plastic seaweed. The only sign of the water’s polluted past is a small jar of tar-black liquid—wastewater from hydraulic fracturing—sitting on top of the tank.

“When oil and gas water comes to the surface, they take the hydrocarbons off and we bring it all the way to potable water,” says Jim Matheson, CEO of Oasys Water.

Oasys Water is one of a number of companies that are taking decades old water purification technology known as reverse osmosis and turning it on its head. The new approach, called forward osmosis, can treat far dirtier water, often using significantly less energy than existing purification methods. The new technique now treats some of the most challenging industrial wastewaters, provides emergency hydration, and could soon bring relief to many of the world’s most water scarce regions.

water pollution
Where traditional water filtration may choke on polluted waters like this, forward osmosis can make it potable.

When reverse osmosis was first developed in the 1960s, it dramatically reduced the amount of energy and cost required to extract freshwater from the sea. Previously, seawater had to be boiled, turned to steam and then re-condensed to separate freshwater from the salts that made it undrinkable. Reverse osmosis cut the cost of desalination by forcing seawater through a semi-permeable membrane that allows freshwater to pass but blocks salt and other impurities.

Today, 21 billions gallons of fresh water—enough to meet the needs of 300 million people—is desalinated worldwide each day. The vast majority of that water comes from reverse osmosis. Yet to coax freshwater through membranes in reverse osmosis, the seawater has to be pressurized, a process that still requires a significant amount of energy.

This is where forward osmosis comes in. Instead of pressurizing water and pushing it through a membrane, forward osmosis relies on the natural tendency of water to flow through a semi-permeable membrane from concentrated to less concentrated solutions, eventually equilibrating the two. “Instead of using pressure, we are letting nature do it’s job,” Matheson says. “It’s the way your body uses osmosis to move water for cellular processes.”

The Draw

Suppose you want to use forward osmosis to purify water. You’d start with two mixtures of water, one on each side of a semi-permeable membrane. The first solution, for example the hydraulic fracturing wastewater that Oasys Water wanted to clean, is called the feed solution. The second, on the other side of the membrane, is the draw solution. It has a higher salinity than the feed solution and pulls or draws water across the membrane through osmosis. At this point you’ve drawn freshwater out of dirty wastewater but have moved it into another solution that is even saltier than what you started with.

The key to Oasys’s approach is the salt they use in their draw solution, ammonium bicarbonate, turns into carbon dioxide and ammonia gas when heated. “If you warm it up a little bit, to 80˚ C, you vaporize it.” Matheson says.

Oasys treated tens of thousands of gallons of hydraulic fracturing wastewater from the Marcellus and Permian basins of Pennsylvania and Texas in pilot tests starting in 2011. Hydraulic fracturing or fracking involves injecting water, sand, and a cocktail of often undisclosed chemicals deep underground to release oil and natural gas trapped in shale formations. Water that surfaces with the gas is typically several times more saline than seawater and often includes heavy metals and radioactive material.

Membranes in this Oasys Water system will clean 4,000 barrels of fracking wastewater per day.

Using forward osmosis, Oasys was able to turn roughly 60% of the wastewater into freshwater that met or exceeded drinking water standards set by the EPA. The water is too saline to treat with conventional reverse osmosis—the water pressure required to overcome such high salinity would cause its membranes to shred. The only option previously available to purify such wastewater was thermal evaporation, a process that uses 25–50% more energy than forward osmosis, according to Matheson.

In 2013, Oasys partnered with oil and gas equipment supplier National Oilwell Varco to sell a forward osmosis system that can turn up to 4,000 barrels of wastewater into fresh water each day.

The bigger prize for developers of forward osmosis, however, is the multi-billion dollar global market for seawater desalination. In 2011, Oasys’s founders, executives who are no longer with the company, claimed they could produce fresh water from the sea using one-tenth the fuel or electricity required by reverse osmosis. Their claims, however, rested on the presumption that they could harness “waste heat” from power plants at little or no cost.

Power plants and other industrial facilities often release low-grade heat in the form of steam or hot water. Oasys needs heat to “regenerate” their draw solution or remove the ammonium bicarbonate salt to unlock the freshwater that remains. But harnessing this heat proved difficult.

“Waste heat is very seldom free,” says Brent Giles, a water analyst with Lux Research. “The barrier they ran into was you had to build a cathedral to tap enough waste heat out of typical low grade source.”

To reduce energy requirements of forward osmosis, researchers are now pursuing a number of novel techniques, including nanoparticles that can be filtered out, metallic salts that can be drawn out with a magnetic field, and oil based salts that can be skimmed off the top of the water column, to remove salt from their draw solution. “A ridiculous number of solutions have been tried,” Giles says. “It’s really hard to find a good one.”

Finding a Focus

A 2014 study by researchers at the Massachusetts Institute of Technology concluded that the energy requirements of forward osmosis will always be greater than those of reverse osmosis for seawater desalination. “In forward osmosis you are moving water into something more concentrated,” says study co-author Ronan McGovern, a postdoc at MIT. “It will always require more energy than reverse osmosis.”

At the same time, the scale up of reverse osmosis desalination has made it a very inexpensive method of water purification. “You can get 1,000 liters of water for less than $1 using reverse osmosis,” says Menachem Elimelech, a professor of environmental engineering at Yale University and one of the researchers behind the initial development of Oasys Water’s technology. “It’s very difficult at this stage to beat that.”

The MIT study recommended that developers of forward osmosis focus on the treatment of exceptionally dirty or salty water where reverse osmosis can’t compete or where the draw solution doesn’t need to be regenerated.

One consumer application that meets both of these requirements is a line of hydration packs developed by Hydration Technology Innovations, a startup company based in Albany, Oregon. Plastic pouches filled with a sugary syrup that acts like a highly saline solution draws freshwater from any source across a forward osmosis membrane to provide a single use, emergency hydration drink. The technology was developed through a U.S. Department of Defense Advanced Research Projects Agency (DARPA) grant for soldier hydration. It is now also marketed for use by hikers, boaters, and wilderness first responders.

A forward osmosis plant built by Modern Water in Al Khaluf, Oman

Another application that avoids the need to regenerate the draw solution is fertilizer-driven forward osmosis. A 2011 study by researchers at the University of Technology in Sydney, Australia showed that 2.2 pounds of highly concentrated chemical fertilizers can draw can draw up to eight gallons of freshwater that can then be applied on agricultural fields. The concept is currently being developed by Danish startup Aquaporin.

Other companies are pairing forward osmosis with other technologies to extend its capacity for desalination. Trevi Systems, a company based in Petaluma, California, is coupling forward osmosis with solar power to desalinate seawater. The company is building a pilot plant in the United Arab Emirates that will use solar thermal energy to heat and regenerate its draw solution. The system will be completed in May and will have an output of 1,760 cubic feet of freshwater per day, according to Trevi CEO John Webley. “It will be the first renewable-powered forward osmosis system in the world,” he says.

One of the earliest adopters of forward osmosis, U.K.-based Modern Water, completed its first commercial seawater desalination plant in Oman in 2009 and a second in 2012. Seawater in the region has a higher salinity as well as higher concentrations of algae and other suspended solids that make it more prone to fouling in reverse osmosis than seawater from other regions. Modern Water uses forward osmosis to pretreat the water before using reverse osmosis, a combination that reduces fouling.

“In reverse osmosis, all the stuff in seawater is being pushed toward the membrane,” Giles says. “With forward osmosis, water has an attraction to the other side of the membrane that other materials don’t have. You aren’t pushing material against the membrane, you are letting it draw through.”

Zero Waste

Oasys meanwhile is pursuing something called “zero liquid discharge” a process that that could reduce the environmental impact of desalination. Desalination as it is most commonly done today takes seawater and turns it into two products, freshwater that we drink and brine that is roughly twice as saline as seawater. The later is typically dumped back into the sea in a process that increases ocean salinity in the immediate vicinity. As desalination increases worldwide the environmental impact on ocean water is becoming more of a concern.

“If you have a glass of water and keep adding a tablespoon of salt every day, you are eventually gonna get pretty damn salty,” Matheson says. Oasys is currently running a pilot desalination plant near the Bohai Sea in northern China. Their system takes brine wastewater from an existing desalination facility and extracts more freshwater from the brine. Oasys’s goal is to draw all of the water from the brine leaving only dry salt.

“There is significant dialogue in China, especially in the Bohai Sea, because there is a lot of industrial discharge and very little water movement,” Matheson says. “There is pending regulation that discharge in the Bohai Sea, or potentially all of China, may go to zero liquid discharge.”

Oasys’s demonstration plant currently draws 80% of the water from the brine. The company is now completing a commercial-scale zero-liquid-discharge wastewater treatment system at a coal-fired plant elsewhere in the country. The facility will use forward osmosis to draw 90% of the freshwater from the salt. The remaining brine will be heated until only dry salt remains.

At the same time Oasys is pursuing inland desalination, treating brackish water from underground aquifers far from the sea. Such aquifers offer a vast water resource that is difficult for reverse osmosis to treat. Though the water is less saline than seawater, it often contains other minerals including silica, calcium, and magnesium that would foul the membranes used in reverse osmosis.

Room 2
Engineers complete the installation of an Oasys Water system at the Hua Neng Changxing coal-fired power plant near Hangzhou, where it will clean wastewater produced by flue gas scrubbers.

“It would open up an entirely new capability,” Matheson say. “If you look at California today, it could be a significant game changer for the Central Valley. India, China, the Middle East all have significant sources of brackish water. If you could clean that up, you could bring a significant localized freshwater resource.”

Giles agrees saying the capacity to treat brackish water could have immediate applications in California and elsewhere, but cautions that Oasys Water still has some challenges to work out. “They need waste heat or at least heat, and it’s not immediately obvious where they will find that in the olive fields.”

New developments in conventional reverse osmosis could also provide a competing solution, Giles says. Desalitech, a Newton, Massachusetts, startup is already treating brackish water in California and Israel by running conventional reverse osmosis in a small batch process that allows the company to flush salt and minerals from its membranes as it accumulates.

In South Boston, the ground floor of Oasys’s research facility is littered with plastic totes containing brackish water from the American Southwest, Europe, China, the Middle East, and Australia. Lab benches overflowing with a dizzying array of plastic tubing, custom-built membranes, and novel draw solutions test the mineral rich waters.

As Matheson describes these brackish waters, he rubs his fingers together suggesting a grittiness you can feel in your hand. Standing amongst the totes you get the feeling that major changes in desalination are close at hand, so close that you can almost feel it.