Theory of filtration

THEORY OF FILTRATION

Since sedimentation alone is insufficient for complete removal of suspended matter and bacteria from water, it is filtered in order to remove very fine suspended particles and bacteria. This process involves passing water through beds of fine granular material such as sand. This process is known as filtration. The four basic filtration mechanisms involved are:

• Sedimentation: This mechanism is due to force of gravity and the associated settling velocity of the particle. This causes it to cross the streamlines and reach the bottom of the tank or the collector.
• Interception: Interception is common occurrence for large particles. When a large enough particle follows the streamline, that lies very close to the media surface it will hit the media grain and be captured.
• Brownian motion: Diffusion towards the media granules occurs for very small particles such as viruses. Particles move randomly within the fluid due to thermal gradients. This mechanism is important only to particles with diameter less than 1 micron
• Inertia: Attachment by inertia occurs when larger particles move fast enough to travel off their streamlines and bump into media grains