Abstract: An aerobic treatment of waste water is accomplished in such a manner as to effectively reduce the emission of volatile organic compounds contained therein by first re-introducing oxygen containing gas previously used in the process as supplemented with additional oxygen containing gas to fulfill make-up requirements during the operation. The reuse is effected in an enclosed vessel having a lower sparging compartment within which both a suitable quantity of mixed reused and make-up gas is thoroughly mixed with contaminated waste water being introduced therein for treatment. The thus humidified gas and preliminarily treated waste water are then moved upwardly within and through a central biological growth area, and the treated waste water is formed as a liquid level in an upper compartment and removed through a trap therefrom to maintain a water level therein and collect the resultant treated oxygen containing gas above the water level.