To reduce the barrier sealant amount, the area of a throat (88), communicating an inlet compartment (78) to an outlet opening (64), is made small. Should sealant not be in the throat, wastewater in the throat would be correspondingly small. A snorkel (110) in a discharge section drain tube (98) extends downwards to above the level of the horizontal drain, and allows any air from the external drain to pass to the outlet compartment generally below the ceiling (90) so that, should wastewater cover the opening of the drain tube, such air is permitted to enter into the cartridge and to prevent any syphoning. A three rib channel guide mechanism (100) within the tube guides wastewater flow towards the drain tube center, and away from the drain tube walls to avoid deposit of any sediment contained in the wastewater.