ABSTRACT: The gravity selection pressure has been traditionally regarded as an ultimate driving force toward aerobic granulation. However, the essential role of feast and famine conditions in determining the success of aerobic granulation has been largely ignored. This study systematically analyzed 14 sets of data from sequential batch aerobic granulation studies and three sets of data from continuous flow aerobic granulation studies. The results suggest that sludge settleability is more sensitive to feast-to-famine duration ratio than to the gravity selection pressure applied. This observation explained why aerobic granulation has not been achieved in completely stirred tank reactors even with strong gravity selection pressure in place. In order to provide retrofitting guidance to existing wastewater treatment facilities, a theoretical equation was derived to correlate the feast-to-famine ratio to routine bioreactor operating parameters such as the Damköhler number used in chemical engineering and the F/M ratio used in wastewater treatment practice. The new understanding gained from this study warrants research attention toward economically providing feast and famine conditions sufficient to enable aerobic graduation in continuous flow wastewater treatment facilities already deployed throughout the world.