Green Desalination through Forward Osmosis

A new desalination process uses forward osmosis (FO) to produce potable water with just a fraction of the energy required for reverse osmosis. Developed by Yale’s Rob McGinnis and Prof. Menachem Elimelech, the idea behind the FO process was to “exploit the natural phenomenon of osmosis rather than treating it as a limiting constraint.”

The forward osmosis process is made possible by the solute ammonium (NH₄⁺), which is formed by dissolving ammonia (NH₃) and carbon dioxide in a water solvent. This special solute creates a high osmotic pressure, which helps to quickly diffuse the solution using FO.

As FO occurs, the ammonium salt in the draw solution and the sea salt in the feed solution are rejected by a semipermeable membrane. After the application of a low heat source (104 ͦF / 40 ͦC), the ammonium salts decompose into ammonia and carbon dioxide. At which point they are captured as gases and are ready to be introduced back into the draw solution to start the process all over again.

To instigate and maintain this forward osmosis, the distillation column must also be kept at a pressure lower than the surrounding atmosphere. A process which is actually self-sustaining, thanks to the condensation of gas into the draw solution and the rejection of FO heat, which together manage to maintain the gradient with ease.

The FO process is particularly impressive for how efficient it is with energy. While reverse osmosis processes are very energy intensive, forward osmosis uses phase change and a low-heat to remove solute at a fraction of the energy cost. It also recycles gases for repeated use, and operates with membranes that are much thinner, hydrophilic, and porous than traditional RO membranes.

What’s more, the only energy the FO system needs to outsource is the power needed to pump the clean water in the unpressurized system. Though if the system is enclosed and coupled with a hydro-turbine and pressure exchanger, it then becomes and osmotic heat engine (OHE) capable of producing electricity.

With 60% of the world’s people living within a few hundred kilometers of the coastline, green desalination processes such as McGinnis and Elimelech’s forward osmosis could have a profound impact on people’s everyday life. In fact, the two researchers figure that the only way to meet the demands of our growing population is through water reuse and desalination.

While only a part of the solution, the FO system is still a big step in the movement to manage our planet’s dwindling resources, and perhaps one of our best means of avoiding a Malthusian-style catastrophe.

Image Courtesy of Rob McGinnis and Prof. Menachem Elimelech