Limited filamentous bulking caused by low dissolved oxygen (DO) was proposed to establish a low energy consumption wastewater treatment system. This method for energy saving was derived from two full-scale field observations, which showed pollutants removal would be enhanced and energy consumption could be reduced by atleast 10% using limited filamentous bulking. Furthermore, preliminary investigation including the abundance evaluation and the identification of filamentous bacteria demonstrated that the limited filamentous bulking could be repeated steadily in a lab-scale anoxic-oxic reactor fed with domestic wastewater. The sludge loss did not occur in the secondary clarifier, while COD and total nitrogen removal efficiencies were improved by controlling DO for optimal filamentous bacterial population. Suspended solids in effluent were negligible and turbidity was lower than 2 NTU, which were distinctly lower than those under no bulking. Theoretical and experimental results indicated the aeration consumption could be saved by the application of limited filamentous bulking. (C) 2009 Published by Elsevier Ltd.