The invention relates to an apparatus and method for more effectively dissolving a gas into a liquid stream. A gas is introduced into a liquid stream, at atmospheric pressure, to create a mixed/dissolved stream. The mixed/dissolved stream is then pressurized to an elevated pressure and directed into a hydrocyclone where the gas is further dissolved and more thoroughly distributed in the liquid thereby creating a more intimately mixed/dissolved stream. The intimately mixed/dissolved stream is then directed to a pressure retention vessel where the liquid is held at a predetermined pressure for a predetermined time period for effectively treating, disinfecting and/or clarifying said intimately mixed/dissolved stream. The treated intimately mixed/dissolved stream can then be directed to further processing steps.