A low-temperature evaporative water desalination system is set to slash the energy and maintenance costs of converting salt water to drinking water.
Many parts of the world have brackish water resources that could be tapped and purified to augment limited fresh-water supplies, but traditional desalination processes such as reverse osmosis and electrodialysis can consume vast amounts of energy.
Now, a new evaporative water desalination system developed by a New Mexico State University (NMSU) researcher not only converts salt water to pure drinking water on a round-the-clock basis, but its energy requirements are so low that it can be powered by solar energy.
The system, originally conceived by Dr Nirmala Khandan in the NMSU College of Engineering, is now being brought to market by Sterling Water, a privately held start-up company based in Doha, Qatar, after it acquired worldwide rights to the technology in January this year.
In the original system developed at the university, two 30ft vertical columns – one rising from a feed tank of saline water and the other from a tank of pure water – were connected at the top by a horizontal tube serving as a condenser. Due to the effects of gravity and atmospheric pressure, a near-perfect vacuum (approaching 0.02 bar) is created naturally inside the headspaces of the vertical columns and the condenser, allowing water to evaporate from the saline-water column and condense on the pure-water column at near-ambient temperatures.
At normal temperatures and pressures, water evaporates from the pure-water side of the system and flows to the saline-water side and condenses as the system seeks equilibrium. But by raising the temperature of the water in the space over the saline column by about 10-15˚ more than that of the freshwater column, flows are reversed so that pure, distilled water collects on the pure-water side and the brine concentrate is left behind in a separate container. In this manner, the system continues to run, drawing the saline water from the feed tank without any pumps.
The result is that the system uses so little energy that it can be powered by solar energy. The use of a static head of water to create a vacuum in the system also reduces the mechanical complexity of the system and eliminates the problems associated with the operation of vacuum pumps in conventional distillation equipment. It also eradicates the associated cost of the power needed to operate a vacuum pump.
The original work conducted by NMSU resulted in the construction and testing of an experimental treatment system that produced fresh water at the rate of five gallons per day. The researchers then took the idea and improved upon it, building a two-stage evaporator unit that produced fresh water at 200 gallons per day, which they went on to demonstrate using a salt-water feed from the Puget Sound in Washington state.
In the dual-stage system, the heated brine discharged from the condenser in the first stage of the system is used to preheat brine entering a second chamber, optimising the energy efficiency of the process and increasing the amount of pure water produced by the system.
As the new owner of the technology, Sterling Water intends to bring a commercial model of the dual-stage, 200-gallon-per-day unit into full production over the coming months. George Forbes, president of Sterling Water, said that the unit will be capable of operating from a photovoltaic system capable of generating as little as 2-5kW of power.
Forbes added that the company also plans to refine the technology further, as well as producing a triple-stage unit that he believes will be capable of producing 300-500 gallons of fresh water per day.
Forbes said that initially the Sterling Water system will serve the needs of people in rural areas, agri-business, oil and gas sectors, maritime, and in under-developed countries that do not have the capital or expertise to invest in and operate a more traditional desalination system.
The key facts to take away from this article
» An evaporative water desalination system was developed by NMSU
» The rights to the technology have been bought by Sterling Water
» The system can be powered by solar energy, reducing power costs
» It will initially serve the needs of rural areas and developing countries