The filter is fitted with a system which uniformly distributes the raw feed water at the base of the sandy filter bed via ring-type distributor.
The water passes through the bed of sand flowing from the bottom to the top and exits filtered via an open-sided orifice at the surface. At the same time the dirty sand is removed from the filter bed by means of an air pump, which draws it off from the bottom and rises to the top of the filter.
Here it is introduced into a washing chamber (washer) where the impurities are separated from the sand. In the washer the sand is cleaned by a flow of raw water moving counter-current with respect to the motion under gravity of the sand itself. The washed sand falls back onto the top of the filter bed.
The dirty wash water leaves the filter in a continuous flux via a separate drain.
