> Based on thermodynamic principles and practical kinetic requirements, the theoretical minimal energy for desalination with FO is always higher than that without FO. In other words, using FO cannot reduce the minimum energy of separation.

https://www.researchgate.net/profile/Shihong_Lin/publication...

The idea behind forward osmosis is to replace the salt in water with some other solute, like sugar. A strict 1-particle-to-1-particle replacement can (I think) have arbitrarily low free energy cost, but then you're just stuck with sugar water instead of salt water. In certain special cases this is fine (like for emergency water generation at sea, when you don't mind drinking sugar water). Insofar as low levels of sugar can't be tasted, and insofar as you have access to free sugar, you can avoid the energetic cost of removing some of the salt by replacing it with sugar. But I don't think this is very much before it becomes noticeable, and sugar isn't free.

If you happen to have excess thermal energy from some source, there are also ways to harvest some of this using forward osmosis, but this is really just another way to smuggle in free energy. In principle, you could use the heat gradient to generate electricity and then use that to power normal reverse osmosis, although which is more efficient will depend on the details of your equipment losses.