Automatic chemical cleaning of granular medium filters

Claim 1

A process for automatically initiating a chemical cleaning cycle for a granular medium bed filter with upper and lower bed surfaces, said granular medium bed supported above an underdrain cavity, comprising the steps; (a) filtering a liquid through said granular medium bed by introducing liquid above said bed for down-flow passage to said underdrain cavity to produce a filtrate, and further introducing liquid above said bed to raise the level of liquid above said bed to a backwash cycle initiating level; (b) backwashing said granular medium bed whereby said filtrate flows upwardly through said bed and out of said filter through a backwash outlet therein; (c) selecting a predetermined time interval required for the level of liquid above said filter bed to drain from said backwash outlet level to said upper bed surface level following a backwash cycle; (d) measuring by measuring means the time interval required for the level of liquid above said filter bed to drain from said backwash outlet level to said upper bed surface level following a backwash cycle; and (e) initiating a chemical cleaning cycle for said filter upon said measured time interval of step (d) exceeding said predetermined time interval of step (c).