Cleantechnica has a post on Forward Osmosis. It is better explained here. The authors description in a gist:

…conventional desalination and reuse technologies use substantial energy. “forward osmosis,” exploits the natural diffusion of water through a semi-permeable membrane. Their process “draws” pure water from its contaminants to a solution of concentrated salts, which can easily be removed with low heat treatment — effectively desalinating or removing contaminants from water with little energy input.

The concentrated salt solution is probably a mixture of ammonia and/or carbon dioxide dissolved in water. The ammonia and CO2 are later easily boiled off and reused. The energy required for this quite less even low-cost solar heaters may be sufficient.