Alternating osmotic pressure from a metal salt osmolyte to enhance forward osmosis processes

CO2 absorption and desorption affords differing osmotic pressure metal salt osmolyte draw solutions from a common solution. These draw solutions serve a staged forward osmosis membrane process. First stage draw solution is the lowest osmotic pressure osmolyte. First stage concentrate is fed to the second stage and fresh water is externally extracted from the first stage diluted osmolyte. Concentrated first stage osmolyte returns from fresh water extraction, blends and is heated with solid precipitates of the lower osmotic pressure solute. CO2 desorbs from the lower osmotic pressure osmolyte converting to a higher osmotic pressure osmolyte. The higher osmotic pressure osmolyte serves as second stage draw solution to further dewatering the first stage concentrate. Second stage concentrate conveys to external processing or discharge. CO2 absorption converts the dilute high osmotic pressure osmolyte from the second stage to the lower osmotic pressure osmolyte serving as draw solution in the first stage.