F orward osmosis is a natural process in which concentration differences between two aqueous solutions are equalized by solute or water transport from the side with a low solute concentration (Feed) to the side with a high solute concentration (Draw). Usually, the two differently concentrated aqueous solutions would mix, and solute would diffuse equally in the system. By introducing a semi-permeable membrane that does not allow solute transport the only way for the system to equilibrate is by water transport from the side with higher solute concentration to the side with lower solute concentration. This osmotic pressure gradient is used to induce a flow of water from the feed through the membrane into the draw, thus effectively concentrating the feed while at the same time diluting the draw.
The draw solution can consist of any type of molecules capable of generating an osmotic pressure. Simple salts such as sodium chloride and magnesium chloride can be used but also more compounds (e.g., polymers) tailored for specific applications.
Most of the applications of FO fall into two broad categories: product concentration (i.e., concentration of the feed) and product dilution (i.e., dilution of the draw). Amongst examples of the first category are wastewater concentration, fruit juice concentration, and the second category encompass applications such as production liquid fertilized using an impaired irrigation water source as feed (e.g., brackish water or polluted surface water) and concentrated fertilizer solution a draw. In most industrial cases FO has to be combined with a recovery step such as reverse osmosis (RO) or membrane distillation (MD) to produce clean water.
In short, FO can concentrate waste, turning waste into a product and/or produce clean water. FO has applications in many different industries, including but not limited to: Wastewater treatment, Water Reuse and Desalination; Food and Beverage; Mining; Oil and Gas; and the Power Industry.