Rapid gravity sand filters are normally employed in large raw water treatment plants wherein utilization of pressure sand filters become uneconomical from the point of view of the requirement of large plot areas due to limitation posed by sizes of sand filter vessels.

Rapid gravity sand filters are predominantly RCC constructions having multiple filter beds complete with internal piping, filter water collection channel, under drainage system, gravel media to support the sand media. Isolation of each filter bay is possible and a manual isolation gate is provided for this purpose.

The water flows from top to bottom through the filter and through the sand media with filtered water exiting from the bottom of the bed and collected in filtered box through a pipe. Filtered water (treated water) from filtered box flows to the filtered water reservoir by gravity.

Backwash water tank is normally provided above the filter house. The required quantity of filtered water will be taken for backwashing the filter bed unit from the backwash tank. The backwash storage tank is periodically filled using backwash water pumps. The operation of backwash water pumps is automated with the level in the back wash water tank.

Dirty Backwash water from the filter is routed by gravity to the Effluent collection sump. Dirty backwash water from the effluent collection sump is normally pumped back to the Stilling chamber/ Raw water reservoir by Effluent Disposal pumps for re-treatment.

Rapid gravity sand filters are provided with spare filter cell to facilitate back washing operations.

Air scouring blowers are provided to improve the cleaning of the filter bed during the filter back wash sequence.