Abstract: In order to lower the cost of activated sludge wastewater treatment and reduce the energy of oxygen supply,energy saving approach via filamentous sludge bulking under low DO condition was studied by experiments.Real domestic wastewater was treated by A/O process.Limited filamentous sludge bulking stayed for a long time when ρ_DO was controlled as 0.5~0.7 mg·L^-1.The value of Sludge Volume Index was between 130 mL·g^-1 to 180 mL·g^-1.ouring this period,the system worked well and didn't lose sludge from clarifier.Compared with the condition of ρ_DO=2.5 mg·L^-1 and excellent sludge settleobility,the effluent was clearer.Suspended solids in effluent was less than 5 mg·L^-1,COD and TN removal rate was nearly as the same as usual.Areation cmsumption decreased,from 0.75 m³·h^-1 to 0.28 m³·h^-1,and about 60% of it was saved.