Mixing unit for mixing liquids treated in wastewater treatment apparatuses using large gas bubbles, which has a collection bell closed from above and delimiting a gas space, a medium guiding body cooperating with the collection bell delimiting a medium inlet opening, a medium outlet opening and a medium guiding passage located between these two, as well as a dispensing pipe end for introducing the gas into the gas space of the collection bell, where the medium guiding body has a base member and a deflecting mantle located along the external edge of the base member at an angle of inclination (α) to the plane of the base member of less than 180°, while the collection bell has a gas receipt member having a deflecting surface located at least partly above the deflecting mantle of the medium guiding body, a gas holding piece as a continuation of the gas receipt member at least partly running around the gas receipt member, and a discharge opening enclosed by the internal edge of the gas receipt member, the gas space of the collection bell is created by a trap space formed jointly by the gas receipt member and the gas holding piece and open from the direction of the dispensing pipe end but closed from the opposite direction, where the external edge of the deflecting mantle of the medium guiding body is positioned to extend into the trap space, and at least a part of the trap space is convex in shape viewed from the dispensing pipe end, the medium inlet opening of the medium guiding body is at least temporarily connected to the gas space of the collection bell, while the medium guiding passage of the medium guiding body leads to the discharge opening enclosed by the internal edge of the gas receipt member of the collection bell, and the gas holding piece is located to be in the path of the gas flowing out of the dispensing pipe end, wherein the base member of the medium guiding body is established to exclude the permeation of liquid, characterised in that a gas bubble forming element is inserted above the discharge opening separated by a gap from the meeting line of the gas receipt member and the gas holding piece of the collection bell, where the gas bubble forming element is created as an at least partially curved rotation body shell, the axis of rotation of the gas bubble forming element is coaxial with the main axis of the base member, and the gas bubble forming element is convex when viewed from the base member, furthermore, the deflecting mantle of the medium guiding body is a rotation body shell, and the axis of rotation of the deflecting mantle is coaxial with the main axis of the base member, and also the size of its cross-sections enclosed by the deflecting mantle, and perpendicular to its main axis decreases evenly from the external edge of the deflecting mantle in the direction of the base member, the deflecting surface of the gas receipt member of the collection bell is a rotation body shell, and the size of its cross-sections enclosed by the deflecting surface, and perpendicular to the main axis of the collection bell decreases evenly from the meeting line of the gas receipt member and the gas holding piece in the direction of the discharge opening at least at its part in the vicinity of the discharge opening.