How Does a Three-Stage Reverse Osmosis System Work?

In a standard three-stage RO setup:

1. First-stage RO: Raw water passes through pretreatment and then enters the first RO unit. Most dissolved salts, heavy metals, colloids and microorganisms are removed here. The permeate water from the first stage still contains a small amount of ions.
2. Second-stage RO: The permeate produced by the first RO serves as the feed water for the second-stage RO. Further desalination is carried out to cut down most remaining dissolved solids. Sodium hydroxide dosing is commonly applied before the second stage to remove CO₂ and reduce conductivity.
3. Third-stage RO: The permeate from the second stage flows into the third RO membrane array for deep purification. This step removes residual trace ions to obtain high-purity product water.

The concentrate (reject water) of each stage can be partially recycled to improve water recovery rate. Compared with single-stage RO, three-stage reverse osmosis achieves far lower conductivity and higher salt rejection. It is widely used in electronics, pharmaceutical, laboratory and high-end industrial water treatment projects.