Forward osmosis

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.

One example of an application of this type may be found in "hydration bags", which use an ingestible draw solute and are intended for separation of water from dilute feeds.

With sufficient contact time, such water will permeate the membrane bag into the draw solution, leaving the undesirable feed constituents behind.

Typically, the draw solutes are sugars such as glucose or fructose, which provide the additional benefit of nutrition to the user of the FO device.

A point of additional interest with such bags is that they may be readily used to recycle urine, greatly extending the ability of a backpacker or soldier to survive in arid environments.

[2] This process may also, in principle, be employed with highly concentrated saline feedwater sources such as seawater, as one of the first intended uses of FO with ingestible solutes was for survival in life rafts at sea.

[4][5][6] In March 2010, National Geographic[7] magazine cited forward osmosis as one of three technologies that promised to reduce the energy requirements of desalination.

The first separation step of FO, driven by an osmotic pressure gradient, does not require a significant energy input (only unpressurized stirring or pumping of the solutions involved).

[12] Brine concentration using forward osmosis may be achieved using a high osmotic pressure draw solution with a means to recover and regenerate it.

[15][16] Because ammonia and carbon dioxide readily dissociate into gases using heat, the draw solutes can effectively be recovered and reused in a closed loop system, achieving separation through the conversion between thermal energy and osmotic pressure.

NH3/CO2 FO brine concentration was initially demonstrated in the oil and gas industry to treat produced water in the Permian Basin area of Texas, and is currently being used in power and manufacturing plants in China.

[17][18] One unexploited application[19] is to 'soften' or pre-treat the feedwater to multi stage flash (MSF) or multiple effect distillation (MED) plants by osmotically diluting the recirculating brine with the cooling water.

[27] In 1954 Pattle[28] suggested that there was an untapped source of power when a river mixes with the sea, in terms of the lost osmotic pressure, however it was not until the mid ‘70s where a practical method of exploiting it using selectively permeable membranes by Loeb [29] and independently by Jellinek[30] was outlined.

This process was referred by Loeb as pressure retarded osmosis (PRO) and one simplistic implementation is shown opposite.

Some situations that may be envisaged to exploit it are using the differential osmotic pressure between a low brackish river flowing into the sea, or brine and seawater.

[31] In more recent times a significant amount of research and development work has been undertaken and funded by Statkraft, the Norwegian state energy company.

This occurs when treating a high saturated draw effluent, resulting in the membrane getting obtruded and no longer making its function.

An area of current research in FO involves direct removal of draw solutes, in this case by means of a magnetic field.

Small (nanoscale) magnetic particles are suspended in solution creating osmotic pressures sufficient for the separation of water from a dilute feed.