Method for treating outlet air in a pneumatic waste conveying system, in which the outlet air is at least partly the conveying air to be used in waste conveying. In the method in a first phase additional air is brought into the body of conveying air in the partial-vacuum generator of the pneumatic waste conveying system before the output aperture of the partial-vacuum generator, and that in the method in a second phase additional air is brought into the body of outlet air as a consequence of the suction effect brought about by the outlet air in such a way that the outlet air and the additional air mix with each other at least partly. The invention also relates to an apparatus.