The feed water, depending on its source, may contain various concentrations of suspended solids and dissolved matter. Suspended solids may consist of inorganic particles, colloidal particles and biological impurities such as microorganisms and algae. Dissolved matter may consist of highly soluble salts, such as chlorides, and sparingly soluble salts, such as carbonates, sulfates, and silica.

During the Reverse Osmosis process, the volume of feed water decreases, and the concentration of suspended particles and dissolved ions increases. Suspended particles may settle on the membrane surface, thus blocking feed channels and increasing pressure drop across the system. Sparingly soluble salts may precipitate from the concentrate stream, create scale on the membrane surface, and result in lower water permeability through the RO membranes. This process of formation of a deposited layer on a membrane surface is called membrane fouling and results in performance decline of the RO system. The objective of the feed water pretreatment process is to improve the quality of the feed water to the level which would result in reliable operation of the RO membranes.

Pretreatment of the Raw Water may include following Steps based on the Raw Water Quality:

• Clarification followed by Sand Filtration for Turbidity removal
• Water disinfection with chlorine
• Hardness reduction by Softening
• Addition of scale inhibitor
• Reduction of free chlorine using sodium bisulfite/ Activated carbon filters
• Final removal of suspended particles using cartridge filters / Ultra Filtration