Abstract: An apparatus for mixing pressurized air with pressurized waste liquor at the inlet of a wet oxidation system heat exchanger device is disclosed. The apparatus is a T-shaped piping member with an inline first inlet and outlet and a second inlet oriented at about a right angle to said inline first inlet. The outlet is connected to an inlet tube of a tube and shell heat exchanger. A conduit supplies a pressurized waste liquor stream to the piping member through the second inlet. An oxygen-containing gas supply conduit, with circumferential sealing means, extends from exterior the T-shaped member, through the first inlet and terminates at a point beyond the intersection of the first and second inlets. The circumferential sealing means seals to the first inlet. The liquor entering the T-shaped member is heated to a selected elevated temperature prior to contacting a pressurized oxygen-containing gas supplied from the gas supply conduit.