A device for treating a flowing liquid comprises a chamber containing a treatment medium, having an inlet through which liquid enters the chamber and an outlet through which liquid leaves the chamber. A plate is provided in the outlet, having at least one opening extending through it positioned such that liquid passing out of the chamber passes through the opening for discharge from the device. The outlet includes a skirt located downstream of the plate so that the plate is recessed from the outlet end of the device defined by the edge of the skirt, and a peelable protective cover which is attached to the edge of the skirt at the outlet end of the device to prevent ingress of contaminants into the device at its outlet end.