Abstract: Apparatus and method for periodic backwashing and inoculating with anaerobic denitrifying bacteria a columnar filter bed in a waste water treatment system. A minor portion of backwash water containing a substantial quantity of denitrifying bacteria is retained for return to the filter bed media by gravity flow after completion of the backwash cycle. The apparatus and method have particular utility in an activated sludge sewage treatment system the effluent from which contains nitrates (converted from organic nitrogen and ammonia) and suspended solids which are removed in a filter bed comprising several discrete layers of filter sand of different particle sizes supported on coarse gravel.