Industrial applications, Research

Efficiency of forward osmosis desalination by McGovern et. al. 2014

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Efficiency of forward osmosis desalination by McGovern et. al. 2014
15
Oct
ForwardOsmosisTech
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Efficiency of forward osmosis desalination by McGovern et. al. 2014Article suggesting that forward osmosis is not energy efficient compared to reverse osmosis gains social media attention

On June 21st 2014, McGovern, a post-doc at MIT’s  Department of Mechanical engineering, and co-workers published the article “On the potential of forward osmosis to energetically outperform reverse osmosis desalination” in the Journal of Membrane Science. The main take-home message of the article should come as no surprise for scholars and experts in the field of membrane based water treatment, namely that:

A hybrid forward osmosis (FO)/ reverse osmosis (RO) seawater desalination system using a sodium chloride (NaCl) based draw solution will always use more energy than a direct desalination system based on RO alone.

What came as a surprise to us, however, was the amount of social media attention the article received in the months following its publication. Numerous tweets and Google+ posts started appearing during July 2014 – all singing to the same tune of “Forward osmosis is not energy efficient”:

 

 

McGovern and co-workers actually did forward osmosis desalination (and forward osmosis commercialization in general) a huge favour – here’s why

At a first glance, McGovern et. al.’s article could be viewed as a show-stopper for forward osmosis desalination activities and perhaps even limiting for forward osmosis commercialization in general, given the article’s overall take-home message:

It appears best for forward osmosis research to focus fully on high salinity applications and applications that do not require draw regeneration, where reverse osmosis cannot compete. McGovern et. al. 2014.

We, however, are of a different opinion. For too long, forward osmosis desalination proponents have been claiming energy savings of 30-80% without presenting operational data to back their claims. This has, without a doubt, resulted in a natural scepticism towards the economical viability of forward osmosis desalination (and as a consequence also forward osmosis in general) among industrial end users and water treatment experts. McGovern et. al’s article has become a scientific vessel for sceptics to latch onto, which explains the degree of attention it has received so far.

Moving forward, forward osmosis desalination proponents will have to beef up their efforts in producing real operational data to be held up against McGovern et. al’s. scientific benchmark.

It is ForwardOsmosTech’s opinion that this is a very healthy – and much needed – development in the forward osmosis field, which will undoubtedly benefit the industry in the long run. Because, at the end of the day, any real market traction in the water treatment industry is critically dependent on real life operational data showing economical viability.

ForwardOsmosisTech’s take on McGovern’s article and forward osmosis based desalination in general

As mentioned earlier in this piece, the main take-home message in McGovern and co-workers research is that a hybrid forward osmosis (FO)/ reverse osmosis (RO) seawater desalination system using a sodium chloride (NaCl) based draw solution will always use more energy than a direct desalination system based on RO alone. If you think about it, this is really just common sense. In order to extract water from seawater through a forward osmosis membrane, the draw solution must have a higher osmotic pressure than seawater and hence the subsequent RO system for draw solution regeneration and fresh water production must operate at a higher hydraulic pressure. No operational advantages from coupling a forward osmosis with a reverse osmosis system will be substantial enough to compensate for the energy penalty of operating at higher pressure. Dr. Nathan Hancock from Oasys Water recently touched upon this subject in an excellent IDA webinar on forward osmosis.

So how can forward osmosis based desalination ever become more energy efficient that reverse osmosis desalination, you may ask? The solution is to use alternative – and lower energy – draw solution regeneration systems, which in the opinion of ForwardOsmosisTech – and contrary to the comments made by McGovern et. al. – are still very much in play:

  • Exchanging NaCl with larger ionic species as draw solute enables the draw solution regeneration step to be performed with nanofiltration membranes, which are capable of sufficiently high rejection to the draw solute while at the same time lowering the overall energy requirements of the draw solution regeneration step. Although real life transparent operational data is still lacking on forward osmosis / nanofiltration hybrid system, the potential for energy savings compared to RO systems is still very much present. Modern Water have seemingly had some success already in this area.
  • Exchanging ionic draw solutes with cloud-point polymers, which can be separated from bulk water by applying waste heat. Using waste heat has the potential to significantly reduce the energy costs of draw solution regeneration, but according to McGovern et. al. these energy cost reductions are offset by the large capital investments needed for heat exchangers. Indeed, heat exchangers are needed to transfer waste heat to the draw solution in order to separate out the polymeric draw solutes, but referring to the same study cited by McGovern et. al., the cost of heat exchangers can be as low as 200USD/kW, which should be sufficiently low to render temperature based draw solution regeneration economically viable at heat source temperatures of 60°C. Again, real life operational data are needed to prove the viability.

We greatly appreciate our reader’s thoughts on this subject!

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