Abstract: The present invention relates to a waste water outlet for a sanitary plant, comprising a siphon forming a water seal to which is connected a waste pipe for connection with sewers and in which an automatic air valve device and a siphon are combined in one single apparatus so that their functions are complementary. A cylindrical body having a permanent water seal and into which extends an inlet pipe or equivalent connected to an apparatus such as a sink, a basin or waste outlet, is combined with an automatic air valve with peripheral air entry. Outside the inlet pipe extending into the water seal is an internal partition which separates the water seal and the automatic valve device situated at the upper part of the cylindrical body, thereby protecting the valve device against a raising or splashing of the liquid situated in the water seal.