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 .