Any STP which deals with ...

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Any STP which deals with combined sewage (foul and rainwater) should have an overflow tank. This assumes that you are not able to separate out the inflow to the sewer network during rainfall. The main process should be designed to deal with 3x average DWF. Inflow in excess of this should be diverted, via a screen to a balancing tank and returned to the main process when rainfall ceases. Any flow in excess of 6xDWF may be diverted direct to the environment after screening and may be designed as an overflow from the balancing tank or at the inlet.