By arranging airflow holes with small diameters at multiple positions of the air diffuser, a uniform distribution of the diffused airflow can be achieved, and the efficiency of oxygen dissolution can be improved. Then with this air diffuser, it is difficult to completely avoid the blockage of the air flow holes and the diffuser tubes by the sludge that is not activated. When the air diffuser is clogged, it is difficult to biologically treat the activated sludge, and the filter membrane 53 of the membrane chuck 51 in the membrane separator next will be clogged with sludge adhering to the membrane surface. This impairs the function of the membrane separator.

Therefore, it is necessary to flush the air diffuser during periodic maintenance to avoid its clogging. This flushing job is usually bi-weekly, allowing the water in the diffuser tube of the air diffuser to flush the diffuser tube. However, in order to carry out this flushing, auxiliary equipment such as flushing tanks and flushing pumps must be installed.

In such membrane separators, the pull-suction pump is stopped when the flow of waste water decreases and the volume of waste water in the container falls below a predetermined level. In the type of gravity filtration that utilizes the head of water in the vessel as the driving pressure, it naturally stops working when the flow decreases and the head falls below a predetermined level.