The system makes it possible to control and treat gas emanations inside a wastewater access shaft. The system includes a cover with at least one pivoting part, which is normally closed, having a substantially horizontal rotation axis and able to allow solid and/or liquid matter falling onto the main upper surface of the cover to pass toward the bottom of the access shaft. The cover also includes a raised part that has at least one lateral wall and includes a plurality of orifices situated vertically above the main upper surface of the cover. The orifices define an outlet of a gas-emanations circuit formed between the underside and the top of the cover. An air filter is arranged across the circuit. The filter is moistened by a liquid that comes from a reservoir in order to treat the emanations.