Forward osmosis (FO) is an osmotic process that uses a membrane permeable to water only in order to separate water from dissolved solutes. The driving force for this separation is an osmotic pressure gradient between a solution of high concentration, often referred to as a “draw” and a solution of lower concentration, referred to as the “feed”.
The 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), 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.