Abstract: Apparatus and methods are disclosed for facilitating dissolution of one or more gases into a liquid. Preferred gases for use with the apparatus are oxygen, air, and ozone. An apparatus of the present invention comprises a dissolution tank that includes a pressure vessel, at least one liquid spray nozzle, and a fluid outlet. The apparatus also comprises a gas source, means for passing fluid into the pressure vessel, and a discharge device connected to the fluid outlet, which discharge device is provided with at least one orifice. Preferred applications include wastewater treatment, treatment of drinking water, fermentation, and bioremediation.