During all times except when decanting, air is trapped in the decanter forming a positive seal. After sludge has settled in the basin, an electric operated ball valve opens releasing air from the decanter. This procedure acts as a siphon and starts effluent withdrawal. The supernatant is withdrawn until bottom water level (BWL) is reached; a few inches above the bottom lip of the decanter. At BWL, the vent valve is opened and air is once again introduced, stopping the decant. By breaking siphon a few inches above BWL, no scum (if any is present) can enter the decanter.