But if cheap energy isn't on its way any time soon, energy efficient water purification is a good interim step. Engineered osmosis could be a key to addressing the global need for affordable clean water, according to two Yale researchers.
Doctoral student Robert McGinnis and his advisor Menachem Elimelech, Chair of Chemical and Environmental Engineering, say they have designed systems that harnesses the power of osmosis to harvest freshwater from non-potable sources like seawater and also generate electricity from low-temperature heat sources, such as waste heat from conventional power plants. Naturally, this is not the glory days of research for the public good so they have created a company called Oasys to get rich making sure poor people don't die of dehydration. Since this research was funded by the National Science Foundation Graduate Research Fellowship Program, the Office of Naval Research and the NSF Science and Technology Center, you can take satisfaction knowing that they will try to patent something your tax dollars paid to create.
Back to the science, they say their approach requires only one-tenth the electric energy used with conventional desalination systems, and it was featured in Environmental Science&Technology.
According to the authors, desalination and reuse are the only options for increasing water supply beyond that which is available through the hydrologic cycle — the continuous movement of water on, above, and below the surface of the Earth. However, conventional desalination and reuse technologies use substantial energy.
Using a new twist on an old technology, the engineers are employing “forward osmosis,” which exploits the natural diffusion of water through a semi-permeable membrane. Their process “draws” pure water from its contaminants to a solution of concentrated salts, which can easily be removed with low heat treatment — effectively desalinating or removing contaminants from water with little energy input.
Another application of engineered osmosis the Yale researchers are pioneering is an 'osmotic heat engine' to generate electrical energy and Elimelech and McGinnis say that it is possible to produce electricity economically from lower-temperature heat sources, including industrial waste heat, using a related method — pressure-retarded osmosis. In this closed loop process, the “draw” solution is held under high hydraulic pressure. As water moves into the pressurized draw solution, the pressure of the expanded volume is released through a turbine to generate electrical energy. The applied hydraulic pressure can be recovered by a pressure exchanger like those used in modern reverse osmosis desalination plants.
“The cost of producing electricity by this method could be competitive with existing means of power production” says Elimelech.
The research was also funded by WaterCAMPWS (Center for Advanced Materials for the Purification of Water with Systems).
Citation: Environmental Science & Technology 42: 8625-29 (2008) doi:10.1021/es800812m