What is the primary purpose of using poly-electrolytes in the pre-treatment stage of a desalination plant?

The primary purpose of using poly-electrolytes in the pre-treatment stage of a desalination plant is to enhance the coagulation and flocculation processes, which are crucial for removing suspended solids and colloidal particles from the feed water. Coagulation is the process of destabilizing these fine particles, allowing them to begin clumping together. These particles often have a negative surface charge, which prevents them from aggregating and settling out of the water. Poly-electrolytes are large, water-soluble polymers with multiple charged sites along their chain. These charged sites neutralize the surface charges of the suspended particles, reducing the repulsive forces between them and allowing them to come closer together. Flocculation is the subsequent process where the destabilized particles collide and aggregate to form larger, more easily settleable or filterable flocs. Poly-electrolytes can also act as bridging agents, where long polymer chains adsorb onto multiple particles, linking them together to form larger and stronger flocs. This enhances the settling velocity of the particles in sedimentation basins or improves their removal by downstream filtration systems like media filters or ultrafiltration membranes. By improving coagulation and flocculation, poly-electrolytes reduce the turbidity and Silt Density Index (SDI) of the feed water, which protects downstream RO membranes from fouling and scaling, leading to longer membrane life and more efficient desalination. The type and dosage of poly-electrolyte used depend on the specific characteristics of the feed water, such as pH, salinity, and the nature of the suspended solids.