Abstract: A pump forms a pressurized stream of waste water, supplying it to a ring main which feeds the liquid to a plurality of conduits depending therefrom. Each conduit has a venturi whose throat has an inlet for gaseous oxygen formed therein. The throat of each venturi has a plurality of apertures through which gas is introduced from a chamber surrounding the throat. Each conduit ends in one or more nozzles immersed in a volume of liqud. Each venturi restricts the flow of water therethrough and creates turbulence so that a turbulent flow of liquid containing oxygen bubbles is emitted from the associated nozzle at relatively high velocity such that turbulence assisting the dissolution of oxyen is created in the volume of liquid.