Perspective: A new future for desalination?
by Ronan K. McGovern
This decade may be remembered for a debate between two similarly named desalination technologies, perhaps to be known as the "reverse osmosis vs forward osmosis (RO vs FO) debacle". If reverse osmosis - a technology first developed in the 60s, and now dominant in desalination worldwide - would be Goliath, then forward osmosis - first promoted back in the 70s but only recently hitting the headlines - would be David. On David's side are 17 of the 20 most cited peer-reviewed journal articles dealing with forward osmosis, which promote the potential of forward osmosis for seawater desalination. On Goliath's side are reverse osmosis industry veteran's - the skeptics of the new forward osmosis technology.
In reverse osmosis the feed stream is pressurised - forcing pure water through a membrane. In forward osmosis a highly saline solution on the product (permeate) side of the membrane allows water to be drawn from the feed stream without significant pressure being applied (Image credit: htiwater.com)
In a "Perspective" article published this past weekend in the Journal of Membrane Science, Prof. John H. Lienhard V and I, at the Massachusetts Institute of Technology, provided a deeper look at the RO vs FO debate. Before giving the results, let me briefly describe how reverse osmosis (RO) and forward osmosis (FO) might be applied for seawater desalination. Reverse osmosis is a single step desalination process - a saline stream is pressurised such that pure water can permeate a membrane (as shown to the left of the adjacent image). Forward osmosis, by contrast, is a two-step process, of which only the first step is shown to the right hand side of the adjacent image, where water is first drawn from a feed solution, through a membrane, and into a solution of higher salinity. This higher salinity solution must then be desalinated to produce a final purified product (not shown) - either using reverse osmosis, which is pressure driven, or evaporative technologies, which are driven by heat. The crux of the RO vs FO debate is thus whether the advantages of the forward osmosis process - for example, reduced membrane blockage (fouling) in the first step - outweigh the fact that an addition second step is required for complete purification.
Our study illustrated two important considerations in the RO vs FO debate. Firstly, we showed that the FO process is most energy efficient when reverse osmosis is used (rather than evaporative methods) for the second step of the process. Secondly, assuming the use of reverse osmosis as a second step for the forward osmosis process we drew up a comparison, for seawater desalination, between a reverse osmosis and a forward osmosis process. For reverse osmosis we summed together the energy required for ultra-filtration (to remove large pollutants prior to filtration with RO) and for two passes of reverse osmosis - the second pass involving filtration of the low salinity product obtained from the first pass. For forward osmosis we summed together the pumping power required in the first step ("draw dilution" - shown on the right hand side of the figure above), where a highly saline solution draws water from the feed, and the second step, where reverse osmosis is used to purify that highly saline solution. The important result is that the FO process consumes more energy than RO.
There are a number of other factors that may be considered, such as optimisation of the process temperature, and allowance for the fact that membrane technology (both for forward osmosis and reverse osmosis) will improve with time. These factors are analysed within the article, but the conclusion that forward osmosis will struggle to compete with reverse osmosis for seawater desalination, now and in the future, holds firm. The fundamental fact that forward osmosis requires water to be moved into a higher salinity solution before final purification makes the problem harder, not easier. For this reason, we believe that forward osmosis research efforts should move away from seawater applications and focus instead on applications where energy consumption is not as crucial.
The author's version of this manuscript is available freely for download here.