Scales are formed when dissolved substances in the feed water reach their maximum solubility limit somewhere in the RO unit. When further concentrated, they precipitate out of the solution and onto a surface. The surface may be a suspended particle in the feed/concentrate stream, the surface of the membrane, the feed water spacer, or any other surface.
The most common scales are:
• Calcium Carbonate (CaCO₃)
• Calcium Sulfate (CaSO₄)
• Barium Sulfate (BaSO₄)
• Strontium Sulfate (SrSO₄)
• Silica (SiO₂)


Other materials can cause scaling, such as metal hydroxides, calcium fluoride, and magnesium salts, but the above five are the most common. If these substances reach their maximum solubility in an RO unit, they precipitate onto a surface as a relatively hard, crystalline layer.