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.
FO can also be combined with reverse osmosis (RO) or membrane distillation (MD) to produce clean water. Thus in this scenario the FO process is used to dilute a draw (e.g. single salt based) from an impaired feed (e.g. polluted water). Subsequently the diluted draw is re-concentrated in RO or MD to produce clean water. The advantage of this is that the RO (MD) process always will ‘see’ the same type of feed with FO as a pretreatment step. This will diminish the membrane fouling propensity and associated need for membrane cleaning.
Thus 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.