The activated sludge process consists of an aerated, mixed reactor (the aeration basin) followed by a clarifier-thickener (the secondary sedimentation basin). For domestic wastes, and some industrial wastes, 5-day biochemical oxygen demand (BOD5) and suspended solids (ss) removal are limited by secondary sedimentation basin performance before they become limited by kinetic removal rates in the aeration basin. The mode of operation of the aeration basin in terms of organic loading, dissolved oxygen (DO) levels, and mean cell residence time (qc)influences the physical properties of the activated sludge floc. In so doing it limits the ability of the secondary sedimentation basin to produce a well-clarified effluent and a concentrated return sludge.