Abstract: A replaceable filter cartridge is mounted in a housing body having an inlet and an outlet permanently attached to a line carrying the liquid to be filtered. The filter cartridge contains activated charcoal. An outlet in the filter cartridge can be closed by a buoyant poppet valve which is normally held out of engagement with its valve seat by a trigger wire. An impeller is rotated by the flow of the liquid through the cartridge and the impeller drives a toothed disc which abrades the trigger wire. The abrading eventually cuts through the wire and releases the poppet which then seats against the valve seat of the outlet and renders the cartridge inoperative. The amount of abrading of the trigger wire is proportional to the flow of liquid through the cartridge and the trigger wire is sized to fail after a quantity of liquid has passed through the filter sufficient to exhaust the filtering capabilities of the cartridge. The spent cartridge must be replaced with a new filter cartridge.