an improved apparatus and method for operating a granular medium filter is disclosed . the apparatus comprises an improved underdrain structure made up of a plurality of rectangular compartments which allows the filter bed to be &# 34 ; air pulsed &# 34 ; as well as backwashed for thorough cleaning of the bed . nozzles are positioned within the rectangular compartments and directed vertically into the filter bed . fluid from the nozzles helps to break up the bed during backwashing as well as countereact size classification of the medium during this process . the method of operating a filter with the improved underdrain structure is also described .