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Why Achilli et. al. 2010 is a must read within the field of forward osmosis R&D
In their article “Selection of inorganic-based draw solutions for forward osmosis applications” from 2010, Achilli and co workers screen more than 500 different inorganic compounds for their potential as draw solutes for forward osmosis applications.
An initial desktop screening removed all but 14 candidates through the following criteria:
- Must be water soluble
- Must be a solid at room temperature
- Must be non-hazardous
- Must have an osmotic pressure of more than 100 bar at saturation concentration
- Must cost less than 10 USD/L based on chemical unit prices from Fisher Scientific
The fact that Achilli et. al. base one of their selection criteria on unit prices from Fisher Scientific – and not bulk chemical prices from bulk supplier web portals such as Alibaba.com – is our only criticism of their work. It would be very interesting to see how the final draw solute selections change when bulk chemical prices are considered instead of costly R&D products.
Nevertheless, with their short list of 14 draw solute candidates in place, the researchers then proceed to rate each candidate by the following parameters:
- Forward osmosis water flux (based on a CTA forward osmosis membrane from Hydration Technology Innovations) – should be as high as possible
- The loss of draw solute from reverse salt flux during forward osmosis operation – should be as low as possible to minimize FO OPEX
- The loss of draw solute when re concentrating the draw solution after it has extracted feed water – should be as low as possible to minimize FO OPEX
- The propensity of the cation/anion component of the draw solute to cause mineral salt scaling inside the membrane and on the membrane surface – thus irreversibly reducing membrane performance – when diffusing into real-life feed streams containing mineral salt scaling counterparts – mineral salt scaling should be avoided in as wide a range of feed streams as possible
The final conclusion in the article is that there is no preferred draw solute that is universally applicable to all water treatment applications. However, for water and waste water applications (where many mineral salt scaling counterparts are present), MgCl2 is the best choice due to its low scaling propensity and good FO performance potential. And for applications with pure feed streams (e.g. food processing), KHCO3 and NaHCO3 are good choices because of their ability to deliver high water fluxes in combination with low reverse salt fluxes.
We regard the work of Achili and co-workers as a must read for anyone wanting solid background material on which to base their choice of draw solute. Additionally, Achilli et. al. 2010 has been cited 105 times since publishing – according to Google Scholar, February 2014 – which confirms the article’s place among the most influential recently published forward osmosis literature.