Abstract: A mixing device for mixing air and water in a water purifier includes a water inlet pipe (2) and an air inlet pipe (3). The air inlet pipe (3) extends coaxially within the water inlet pipe (2) and defines therewith an annular gap (4) for providing an annular water jet. Downstream of the annular gap (4) there is provided a mixing member (6) for mixing water and air. The mixing member (6) includes a water flow disturbing device (7) which is provided to be hit by the annular water jet. At least those parts (2a) of the water inlet pipe (2) and/or those parts (3a) of the air inlet pipe (3) which define the annular gap (4), are made of plastic material. The water flow disturbing device (7) includes at least one helical member (8) which extends along the inner side of the mixing member (6) around through-flow portions (6a) thereof within the helical member (8) such that helical movements are imparted to the annular jet of water when it flows downwards through the through-flow portions (6a).