Abstract: A gas diffuser is provided for water and wastewater treatment. The gas diffuser unit includes a vertical open-bottomed injection pipe extending downwardly and having a plurality of vertical slot-shaped ports adjacent the open bottom. A gas bubble distribution body including a lip portion is located above the open bottom of the injection pipe, and a conical shaped deflector may be mounted on the gas injection pipe above the distribution body. Gas released from the vertical slot-shaped ports forms relatively large diameter bubbles which rise due to buoyancy. The large diameter gas bubbles are broken into smaller diameter gas bubbles and are evenly distributed upon the bottom surface of the gas bubble distribution body above the open bottom of the injection pipe. The lip portion on the gas distribution body promotes turbulence in the rising bubbles, and produces smaller diameter gas bubbles.