PURPOSE: To reduce the press. applied to the concn. side of a reverse osmosis module, in desalination of seawater or concn. of harmful matter by a reverse osmosis method, by making the soln. temp. of the concn. side higher and the soln. temp. of the permeation side lower to give an osmotic press. due to the temp. difference.

CONSTITUTION: A soln. such as seawater to be treated is charged into tank 1, heated to 30°C with heater 5, pressurized with high press. pump 2, and fed into reverse osmosis module 3, where water and the solute in the soln. are separated through semipermeable membranes 3'. The water permeated through the membranes is stored in permeated water tank 6, fed to cooler 8 with pump 7, cooled to about 15°C, and recycled to the permeated water side of module 3. The soln. to be treated is supplied to tank 1 in a vol. corresponding to the vol. of permeated water discharged from the system through (e). A conc. soln. leaving module 3 is passed through reduction valve 4 to reduce the press. and returned to tank 1. After finishing concn. conc. soln. (g) is drawn out of the bottom of tank 1.