The invention relates to an installation for the mechanical purification of liquids, said installation comprising a container for separating suspended matter, and a transporting device for transporting the suspended matter to a discharge point. Said container comprising an inlet and an outlet for the liquid. A collecting chamber for suspended matter is formed in the bottom part of the container. The container for separating the suspended matter comprises at least two other chambers, at least one being embodied as a ventilated chamber and at least one as a non-ventilated chamber. The invention also relates to a method for separating suspended matter from liquids in a container comprising an inlet and an outlet for the liquid, and a collecting chamber for suspended matter in the bottom part thereof. According to said method, the suspended matter is separated in a multi-stage manner. At least one stage is a ventilated stage and at least one other stage a non-ventilated stage.