WATER TREATMENT: GRAVITY FILTERS
Gravity filters are open vessels that depend on system gravity head for operation. Apart from the filter media, the essential components of a gravity filter include the following:
· The filter shell, which is either concrete or steel and can be square, rectangular, or circular. Rectangular reinforced concrete units are most widely used.
· The support bed, which prevents loss of fine sand or anthracite through the underdrain system. The support bed, usually 1-2 ft deep, also distributes backwash water.
· An underdrain system, which ensures uniform collection of filtered water and uniform distribution of backwash water. The system may consist of a header and laterals, with perforations or strainers spaced suitably. False tank bottoms with appropriately spaced strainers are also used for underdrain systems.
· Wash water troughs, large enough to collect backwash water without flooding. The troughs are spaced so that the horizontal travel of backwash water does not exceed 3-3 ft. In conventional sand bed units, wash troughs are placed approximately 2 ft above the filter surface. Sufficient freeboard must be provided to prevent loss of a portion of the filter media during operation at maximum backwash rates.
· Control devices that maximize filter operation efficiency. Flow rate controllers, operated by venturi tubes in the effluent line, automatically maintain uniform delivery of filtered water. Backwash flow rate controllers are also used. Flow rate and head loss gauges are essential for efficient operation.