Pressure retarded osmosis (PRO) is a technique to separate a solvent (for example, fresh water) from a solution that is more concentrated (e.g. sea water) and also pressurized. A semipermeable membrane allows the solvent to pass to the concentrated solution side by osmosis. The technique can be used to generate power from the salinity gradient energy resulting from the difference in the salt concentration between sea and river water. In PRO, the water potential between fresh water and sea water corresponds to a pressure of 26 bars. This pressure is equivalent to a column of water (hydraulic head) 270 meters high.  However, the optimal working pressure is only half of this, 11 to 15 bar. 
In 2014 researchers verified that 95% of a PRO system's theoretical power output can be produced with a membrane that is half (or less) the size needed for achieving 100%. Output is proportional to the salinity. Desalination yields very salty brine, while treated municipal wastewater has relatively little salt. Combining those streams could produce energy to power both facilities. However, powering an existing wastewater treatment plant by mixing treated wastewater with seawater could require a membrane area of 2.5 million square meters.
To deal with these membrane requirements, scientists are working on rolled membranes  that will take up much less space.
- Electrodialysis reversal (EDR)
- Forward osmosis
- Green energy
- Osmotic power
- Osmotic pressure
- Renewable energy
- Reverse electrodialysis (RED)
- Reverse osmosis
- Semipermeable membrane
- Van 't Hoff factor
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