Forward osmosis desalination of seawater is “significantly less energy efficient compared to reverse osmosis”, according to a study by the Massachusetts Institute of Technology (MIT).
Published in the Journal of Membrane Science, the study performed an energetic comparison of reverse osmosis and forward osmosis membranes to “identify their respective energy consumptions”.
It was performed by MIT professor John Lienhard and postdoc Ronan McGovern, both of the Department of Mechanical Engineering.
In forward osmosis, water is drawn from the seawater into a concentrated salt solution, known 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.
McGovern said that for forward osmosis, 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, he said, 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 McGovern, forward osmosis is better suited to alternate applications, such as the production of hydration drinks. In such applications, he said, 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.
Modern Water, a pioneer of forward osmosis technology with two facilities operating in Oman, said it did not wish to comment on the study.
The company did highlight another technical paper it published here for the International Desalination Association (IDA) World Congress.
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