The process of passing water through a thick layer of sand which act as a strainer is called filtration. By passing water through sand, suspended and colloidal matter is partially removed. The chemical characteristics of water are changed and the number of bacteria is reduced.
Mechanism of filtration:
1. Mechanical straining- It removes the particle of suspended matter which are too large to pass through the pores in sand.
2. Sedimentation and adsorption- This removes colloids, small suspended particles.
3. Direct and inertial impaction.
4. Electrostatic attraction.
Types of filters:-
1. Slow sand filter,
2, Rapid sand filter
3, Dual media filter
4. Multimedia filter