the use of a stripping gas in which case conduit 12 and its associated equipment would be omitted a stream of liquid is withdrawn from the tank through a line 15 by a pump 16 and passed under a pressure for example in the range 5 psig to 15 psig to the upper end of a gas liquid contactor 17 oxygen enriched air for example having an oxygen content of at least 30 and preferably at least 90 is introduced into the liquid stream in line 15 through line 28 the oxygen may be introduced from a pressurised source or it may be drawn in by a vacuum action provided by inserting a venturi in the line 15 at the point where line 28 joins line 15 this location is adjacent pump 16 so that the great turbulence created in the stream of liquid causes relatively small gas bubbles to be formed in the stream it is also possible to inject the oxygen rich gas directly into the pump volute this gas is introduced at a rate to provide a quantity of gas in the liquid stream in the range 2 ppm to 10 ppm the contactor device 17 comprises an upper cylindrical portion 18 and a lower larger diameter cylindrical portion 19 the flow of liquid in the.