A critical review on draw solutes development for forward osmosis

• FO draw solutes have been comprehensively and critically reviewed.
• FO enabled by responsive draw solutes can have a significant energy cost saving.
• FO has advantages in niche applications where RO alone is unsuitable.
• Ideal draw solute development calls for interdisciplinary efforts.

Despite the impressive progress in forward osmosis (FO) membranes and pilot scale testing of FO process, lack of draw solutes that can be efficiently regenerated is still a limiting factor for more prolific commercial applications of FO technology. In the past decade, a large variety of draw solutes have been investigated. While many promising concepts were discussed, efforts are still needed to search for an ‘ideal’ draw solute which could enable the next breakthrough in FO technology. Besides giving a critical review on the development of FO draw solutes, we attempt to clarify some of the most important issues about draw solutes, to define the criterion for draw solutes, and to offer insights into challenges and opportunities concerning their future development. We intentionally avoid very detailed discussion on the issue of viability of FO, which was covered in several earlier reviews. Draw solutes are categorized into either non-responsive or responsive type according to their response toward external stimuli, such as heat and electromagnetic field. While the focus is on responsive draw solutes whose regeneration relies on their smart response to stimuli, non-responsive draw solutes are also discussed not only for a historic reason, but also for the valuable lessons learnt from these earlier systematic studies.

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