Concentration of solution by the reverse osmosis process

A multistage method of concentrating a solution by reverse osmosis, comprising the steps of:

• providing means for maximizing the concentration of a solute in a solution in a multistage apparatus having only standard capacity pumps, including steps of providing first concentrating means for concentrating a solution to a first concentration, said first concentrating means comprising at least one concentrating unit which is positioned upstream with respect to a direction in which a solution to be concentrated flows, and providing second concentrating means for concentrating the solution that has been concentrated by said first concentrating means to a second concentration which is higher than said first concentration, said second concentrating means comprising at least one concentrating unit which is positioned upstream with respect to said direction;
• said concentrating units consisting essentially of respective membrane modules and respective standard capacity pumps, the membrane module of the concentrating unit of said first concentrating means comprising a tight reverse osmosis membrane with a high rejection percentage sufficient to achieve a high concentration of said solution with a high flux density of solvent flowing through the membrane, and the membrane module of the concentrating unit of said second concentrating means comprises a loose reverse osmosis membrane with a lower rejection percentage sufficient to easily achieve a further concentration of said solution which has been concentrated by said first concentrating means with a high flux density of solution flowing through the membrane;
• said lower rejection percentage of said loose reverse osmosis membrane ranges from 40-80 percent;
• and
• a total number of the concentrating units of said first and second concentrating means ranges from 3-11.