To solve the problem that water is rapidly increased in quantity after heavy rainfall and flows into a sewerage disposal plant, the excess of water over the capacity of a terminal biological treating plant is freed from sand and scum in a sand basin and then discharged into a river, etc., without being biologically treated, hence the rainwater in the initial stage has a high content of unsettled contaminant, and the water of the river, etc., having received the effluent is deteriorated.

The sand and scum contained in sewage are previously removed, and then the sewage is biologically treated in this method. Accordingly, when the quantity of sewage to be biologically treated exceeds a set capacity, the excess of sewage is filtered by a membrane filter and discharged into the river without being biologically treated.