New desalination approaches using forward osmosis claim significant reduction in desalination energy costs
Two un-related announcement have put forward osmosis desalination technology in the limelight. Of the two, one has come from a university spin-off while the other is a private entity the selling point in both cases is reduced energy costs. The Yale University spinoff, called Oasys, claims that its desalination device reduces the energy needed to purify water to one-tenth of that required by conventional systems, while QuantumSphere claims that its method reduces desalination energy cost by 70% compared to conventional methods like thermal desalination and RO.
The most common approach to desalination is currently RO, and the market for this technology is expected to grow at a rate of 10% per year. RO involves forcing a solution through a semipermeable membrane using hydraulic pressure or thermal evaporation. The energy required to do this has spawned new thinking and innovation on lower-energy purification technologies. "The primary driver behind this technology is to get at the heart of the problem of energy cost," says Aaron Mandell, CEO of Oasys. Unlike conventional desalination systems, the Oasys system establishes an osmotic pressure gradient instead of using pressure or heat to force water through a purifying membrane. The approach exploits the fact that water naturally flows from a dilute region to one that's more concentrated when the two solutions are separated by a semipermeable material, thereby saving the energy normally needed to drive the process.
In Oasys's system, a 'draw solution' is added on one side of the membrane to extract clean water from dirty water. The solution used by Oasys is designed to have a high osmotic pressure and be easy to remove through heating.
"Forward osmosis is not a new technology, but trying to find the optimal draw solution to make it efficient and create the proper balance of ammonia and chlorine is critical," says Michael LoCascio, senior analyst with Lux Research.
Oasys's draw solution uses ammonia and carbon-dioxide gases dissolved in water in specific proportions. Crucially, the solution can be reused after being removed from clean water and the membrane required is also nearly identical to those already used in reverse osmosis. While other companies are doing forward osmosis, Oasys claims that its draw solution makes its technology much more efficient.
RO currently produces water at a cost of about $0.68 to $0.90 per cubic metre. Oasys estimates that engineered osmosis will cost just $0.37 to $0.44 per cubic metre once fully scaled up. The startup has so far established a pilot-scale plant to test the technology by producing one cubic metre of water per day.
Oasys says that the first market it will focus on will be wastewater reuse. The second will be reprocessing wastewater produced by the oil and gas industries. Instead of having to pay to haul this water away, companies would treat it on-site using the Oasys system.
QuantumSphere, too, has constructed a prototype system that can purify several cubic metres a day for demonstration purposes and intends to scale the process with large development partners to serve large-scale municipal drinking supplies. Subra Iyer, principal technologist for QuantumSphere, said: "The RO process also requires tremendous amounts of energy for desalination. The forward osmosis process we've developed can purify water at less than 3,000 kWh per acre foot, which is approximately 70% cheaper than the energy cost of traditional RO processes."
Forward osmosis using QuantumSphere's proprietary process can also purify brackish and polluted water. The process uses a semi-permeable membrane to separate water from salt water into a special organic solution across the membrane.
The diluted organic solution is then warmed to cause the specially formulated organic solute to drop out, leaving only fresh drinking water after a final purification step through activated charcoal.
QuantumSphere which develops advanced catalyst materials, high-performance electrode systems, and related process chemistries for portable power and clean-tech applications, has filed for a US patent for its water purification process.
QuantumSphere plans to build larger prototypes to accelerate the water purification rate of forward osmosis to 1,000 gallons a day (3.8m3/day) and then 10,000 gallons a day (37.85m3/day), the latter being sufficient to supply a hotel or a seaside resort. Within three years, the company intends to bring the rate up to a million gallons a day (3785 m3/day), or enough to supply a small municipality.
© H2O 2009
