An orbital wastewater treatment system generally including a tank assembly and at least one impeller in the tank assembly for moving mixed liquor under process about the tank assembly is provided with an wastewater inflow sensor, an impeller power meter, and a control unit for periodically and incrementally varying impeller speed and weir height in order to reduce energy consumption. Operating such a wastewater treatment system comprises flowing mixed liquor or wastewater into a treatment pool in the tank assembly, rotating the impeller within the pool to move the mixed liquor or wastewater, and adjusting, in concert, a rate of rotation of the impeller and a level or depth of the pool to reduce a rate of energy usage per unit inflow volume while maintaining at least one process parameter such as dissolved oxygen content substantially constant at a given level in the pool.