thumbnail|osmotic membrane processesforward osmosis is an osmotic process that, like reverse osmosis, uses a semi-permeable membrane to effect separation of water from dissolved solutes. the driving force for this separation is an osmotic pressure gradient, such that a "draw" solution of high concentration (relative to that of the feed solution), is used to induce a net flow of water through the membrane into the draw solution, thus effectively separating the feed water from its solutes. in contrast, the reverse osmosis process uses hydraulic pressure as the driving force for separation, which serves to counteract the osmotic pressure gradient that would otherwise favor water flux from the permeate to the feed. hence significantly more energy is required for reverse osmosis compared to forward osmosis. thumb|family of osmotic membrane processes, including reverse osmosis and forward osmosisthe simplest equation describing the relationship between osmotic and hydraulic pressures and water (solvent) flux is: j_w = a \left(\delta \pi - \delta p \right) where j_w is water flux, a is the hydraulic permeability of the membrane, ÎÏ is the difference in osmotic pressures on the two sides of the membrane, and Îp is the difference in hydrostatic pressure (negative values of j_w indicating reverse osmotic flow). the modeling of these relationships is in practice more complex than this equation indicates, with flux depending on the membrane, feed, and draw solution characteristics, as well as the fluid dynamics within the process itself. the solute flux (j_s) for each individual solute can be modelled by fickâs law j_s = b \delta c where b is the solute permeability coefficient and \delta c is the trans-membrane concentration differential for the solute. it is clear from this governing equation that a solute will diffuse from an area of high concentration to an area of low concentration. this is well known in reverse osmosis where solutes from the feedwater diffuse to the product water, however in the case of forward osmosis the situation can be far more complicated. in fo processes we may have solute diffusion in both directions depending on the composition of the draw solution and the feed water. this does two things; the draw solution solutes may diffuse to the feed solution and the feed solution solutes may diffuse to the draw solution. clearly this phenomena has consequences in terms of the selection of the draw solution for any particular fo process. for instance the loss of draw solution may have an impact on the feed solution perhaps d
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