Technology-based effluent requirements based on federal law have mandated a general upgrading in treatment levels for industries and municipalities. In many instances, effluent requirements have been established that are more stringent than biological secondary treatment has traditionally been considered capable of producing. In these instances, the usual practice of the environmental engineering profession has been to add effluent filtration after biological secondary treatment. However, in many cases, costly effluent filtration can be avoided by the careful design of secondary sedimentation tanks and optimization of the biological treatment process to enhance sedimentation efficiency. In this paper, the factors influencing sedimentation efficiency in the activated sludge process are discussed.