Treatment plants at which operators can switch wastewater in-flow from one bay of an aeration tank to one or more other bays (step-feeding) have additional ways to meet the process demands of the activated sludge system. Recognition of the process demands that call for such control changes and knowledge of what happens when step-feeding is employed provide the foundation for successful operation of such plants. The curves on Figure 1 show how shifting wastewater in-flow locations exerts forces on mixed liquor sludge oxidation that are opposite to those exerted on wastewater treatment. Knowledge of these facts alone permits operators to shift step control in the proper direction to correct sludge or final effluent deficiencies and to restore best process balance.