granularfiltration

Granular filtration is a water treatment process that removes suspended solids from water by passing it through a bed of granular media. Common media include sand, anthracite, and garnet. The media bed acts as a physical barrier, trapping particles as water flows through. As the filter operates, the trapped solids accumulate in the media, increasing the head loss and eventually requiring cleaning.

The primary mechanism of removal in granular filtration is straining, where larger particles are physically caught by the pores between the media grains. Other mechanisms also contribute, including sedimentation, where particles settle onto the media surface due to gravity, and interception, where particles moving along the flow lines come into contact with the media and adhere. Adsorption, where particles stick to the media surface through chemical or physical forces, and inertial impaction, where larger, heavier particles cannot follow the tortuous path of the water flow and collide with the media, also play a role.

When the filter's efficiency decreases due to the buildup of solids, it undergoes a cleaning process called backwashing. During backwashing, the direction of water flow is reversed, and the flow rate is increased to expand the media bed. This expansion fluidizes the media, dislodging the trapped solids, which are then carried away with the backwash water. Granular filtration is widely used in municipal water treatment plants, industrial processes, and wastewater treatment for primary clarification and polishing.