In forward osmosis, water is drawn from the seawater into a concentrated salt solution as a draw solution. Then, a second step is required to regenerate the concentrated draw solution and produce purified water. With reverse osmosis, the seawater is directly desalinated by being pressurized and driven through a membrane that only allows water to pass through.
MIT scientists performed an energetic comparison of reverse osmosis and forward osmosis to identify their respective energy consumptions. They found that even if the second step of draw regeneration — in which the concentrated salt solution is dewatered, producing fresh water — can achieve the same level of efficiency as the reverse osmosis process, the actual energy consumption of forward osmosis will consistently surpass that of reverse osmosis. This is because the salt solution that results from the first step of forward osmosis is necessarily more highly concentrated than standard seawater, meaning it always requires a higher level of energy for regeneration.
According to postdoc Ronan McGovern, who won an IDA scholarship in 2012, forward osmosis is better suited to alternate applications, such as the production of hydration drinks. In such applications, only the first step of the forward osmosis process is required — where a concentrated sugar syrup is diluted to a desirable level — placing forward osmosis at an advantage to reverse osmosis.
The study was published in the Journal of Membrane Science.