The top and bottom distributors are fixed on the riser and this assembly is fixed to the valve. The top distributor prevents the media from exiting the tank whilst backwashing and the bottom distributor prevents the media from exiting during normal filtration (these processes are discussed further below).

The flow rate through the filter depends on the size and number of bottom distributors and the size of the valve (if the flow rate is increased, it is also necessary to increase the size of the top distributor and riser).