Rain water treating device in combined sewage, backwashing method therefor

A sewer-treating system in a combined sewer-system, said system comprising a first sewer-treating section and a second sewer-treating section, said first sewer-treating section comprising a sand-settling basin for receiving sewer, a first sedimentation basin provided downstream of the sand-settling basin and a reacting vessel provided downstream of the sedimentation basin, said second sewer-treating section being provided downstream of the sand-settling basin and comprising a plurality of upward flow type high-speed filter vessels each having a layer of a floating filter medium, a common treated water vessel which is located at upper faces of said high-speed filter vessels and is adapted to collect treated water from the high-speed filter vessels, a used wash water vessel which collects used wash water from said high-speed filter vessels, a rainwater inflow channel for distributing the entering rainwater into upper portions of the high speed filter vessels, and rainwater inflow portions for flowing downwardly the rainwater distributed from the rainwater inflow channel under the respective floating filter media of the filter vessels, wherein when an amount of sewer to be treated is not more than a preset design water amount, the sewer is treated in the first sewer-treating section, whereas when the amount of the sewer to be treated is more than the preset design water amount, the sewer treatment is effected in the first sewer-treating section and in addition a portion of the sewer exceeding the preset design water amount is treated in the second sewer-treating section.