Apparatus and method for multistage reverse osmosis separation

A method for two stage reverse osmosis separation comprising a first stage and a second stage, each comprising a plurality of modules, for collecting 50% or more of desalinated water which meets drinking water standard from seawater fed to the first stage, comprising:

• (a) a pretreatment for removing turbidity components from the seawater to produce pretreated seawater;
• (b) introducing an inlet stream of said pretreated seawater into a first stage;
• (c) collecting a first permeate stream of desalinated water from the first stage and removing a first concentrate stream from the first stage, pressurizing the first concentrate stream so as to raise the feed pressure and supplying the first concentrate stream to the second stage, wherein a total effective reverse osmosis membrane area of said second stage is in the range of 20-80% of that of said first stage, and the ratio of the operating pressure of the first stage, P1, and that of the second stage, P2, satisfies the following relationship:
• 1.15≦P2/P1≦1.8;
• and
• (d) collecting a second permeate stream of desalinated water from the second stage and returning a second concentrate stream to an energy recovering unit of said two stage system;
• and
• (e) removing the second concentrate stream from the energy recovering unit.