A wastewater treatment system comprising a membrane bioreactor, the membrane bioreactor comprising: a) a reaction vessel (7, 10); b) a membrane separation system (1, 8, 12, 16); c) a water production system (9, 13, 17); and d) an aeration system (2, 3, 4, 5, 6); wherein: the membrane separation system is disposed in the reaction vessel; the water production system communicates with the membrane separation system to pump filtrate out of the membrane separation system; the aeration system is employed to aerate the reaction vessel and the membrane separation system; the aeration system is adapted to enable a dissolved oxygen concentration in the reaction vessel to be larger than 0 and smaller than 1.5 mg/L, so that the reaction vessel is maintained at a facultative-organism-adapted environment; a dissolved oxygen concentration in the membrane separation system is larger than 0 and smaller than 1.5 mg/L, a dissolved oxygen concentration in the reaction vessel excluding the membrane separation system is larger than 0 and smaller than 0.5 mg/L, and the dissolved oxygen concentration in the membrane separation system is higher than the dissolved oxygen concentration in the reaction vessel excluding the membrane separation system; the membrane separation system employs a microfiltration membrane or an ultrafiltration membrane, and the aeration system employs microporous aeration, perforated aeration, or a combination thereof; the membrane separation system is flushed by concentrating an aeration rate at the membrane separation system by the aeration system; when the aeration system is a perforated aeration system, the amount of holes per unit area or the bore size of a perforated aeration pipe of the perforated aeration system is larger beneath the membrane separation system than in the reaction vessel excluding the membrane separation system, so as to increase the aeration below the membrane separation system; and when the aeration system is a microporous aeration system, the amount of microporous aeration disks per unit area of the microporous aeration system is larger beneath the membrane separation system than in the reaction vessel excluding the membrane separation system, so as to increase the aeration below the membrane separation system. a) a reaction vessel (7, 10); b) a membrane separation system (1, 8, 12, 16); c) a water production system (9, 13, 17); and d) an aeration system (2, 3, 4, 5, 6); the membrane separation system is disposed in the reaction vessel; the water production system communicates with the membrane separation system to pump filtrate out of the membrane separation system; the aeration system is employed to aerate the reaction vessel and the membrane separation system; the aeration system is adapted to enable a dissolved oxygen concentration in the reaction vessel to be larger than 0 and smaller than 1.5 mg/L, so that the reaction vessel is maintained at a facultative-organism-adapted environment; a dissolved oxygen concentration in the membrane separation system is larger than 0 and smaller than 1.5 mg/L, a dissolved oxygen concentration in the reaction vessel excluding the membrane separation system is larger than 0 and smaller than 0.5 mg/L, and the dissolved oxygen concentration in the membrane separation system is higher than the dissolved oxygen concentration in the reaction vessel excluding the membrane separation system; the membrane separation system employs a microfiltration membrane or an ultrafiltration membrane, and the aeration system employs microporous aeration, perforated aeration, or a combination thereof;