Nitrification of secondary effluent using a variant of the vertical flow bed system was investigated. In this vertical flow bed oxygen required for the nitrification process is supplied by a passive air pump. The passive air pump is based on a fill and draw cycle, where oxygen depleted air is removed from the system and fresh air is introduced. Each volume of effluent drained is displaced by an equal volume of fresh air. Experimental results of nitrification and the corresponding oxygen consumption show good agreement with theoretical calculations based on physico-chemical considerations.