A vast array of industrial operations require the separation of a solvent, predominantly water, from a liquid stream: Heat and Mass Transfer Report, NTU, Singapore

A vast array of industrial operations require the separation of a solvent, predominantly water, from a liquid stream. The ultimate objective of the separation may include the concentration of this liquid stream to reduce its volume before further processing, the retention of suspended or dissolved compounds that necessitate specific treatment, or the production of clean water.

Such a separation process implies negligible losses of the feed constituents, in order to maintain the quality of the concentrate or facilitate the processing of the permeate. The engineering application of osmosis, widely known as forward osmosis, is a membrane filtration technology that can provide this type of separation.

Forward osmosis has been intensively investigated over the last decades for a diverse range of potential applications, spanning from desalination, power generation, and treatment of various wastewater streams to the concentration of liquid foods. It has been suggested that forward osmosis can theoretically have a lower operating cost because lower amounts of energy are used for generating the driving force to run the process compared to its main counterpart, reverse osmosis.

The hydraulic pressure driving force applied in reverse osmosis imposes a high energy consumption on pumps, in order to deliver the fluid across the filtration system while generating a high transmembrane pressure, whereas forward osmosis simply utilizes an osmotic pressure driving force associated only with pumping for fluid circulation.

More importantly, it has been claimed that forward osmosis has a lower fouling tendency than reverse osmosis. The lower fouling tendency of forward osmosis has been attributed in many studies, to the fact that no hydraulic pressure is employed, hence no compression or compaction of the fouling layer occurs. This idea has been revisited more recently and the results have been contradictory.

Furthermore, since the first forward osmosis investigation, a tremendous improvement has been seen in the structure of the forward osmosis membranes in order to achieve a high water permeability and minimize other intrinsic material transport limitations. The fundamental material transport mechanisms governing the forward osmosis process have proven to be predominantly influenced by these membrane properties.