double-stage reverse osmosis

Separate pure water from a solution containing solutes with a semi-permeable membrane that only allows water to pass through. At this time, water on the pure water side will spontaneously pass through the semi-permeable membrane into the solution side, and the liquid level on the solution side will rise. This phenomenon is called osmosis. When the liquid level rises to a certain height, the pressure on both sides of the membrane reaches equilibrium, and the liquid level on the solution side no longer rises. At this time, there is a pressure difference between the two sides of the membrane, which is called osmotic pressure. 

If a pressure greater than the osmotic pressure is applied to the solution side, water molecules in the solution will be squeezed to the pure water side. This process is just the opposite of osmosis, which we call reverse osmosis. It can be seen from the reverse osmosis process that due to the action of pressure, water molecules in the solution enter the pure water, the amount of pure water increases, and the solution itself is concentrated.

The reverse osmosis membrane used in reverse osmosis equipment is a semi-permeable spiral wound membrane. When raw water is sent to the reverse osmosis membrane at a certain pressure, water passes through the tiny pores on the membrane and is collected to obtain pure water, while impurities in the water such as soluble solids, organic matter, colloidal substances and bacteria are retained by the reverse osmosis membrane, concentrated in the retentate and removed. The double-stage reverse osmosis equipment uses the pure water produced by the first stage as raw water for the second stage, which can make the produced pure water reach higher quality. It is suitable for the needs of high-purity water production and can also be used in occasions where the conductivity of raw water is high.