A filter cartridge including a housing having a water inlet and outlet, a valve disposed in the housing and defining transversally aligned inlet and outlet passageways, and having a first key formation that vertically depends from the valve, a cartridge having transversely separated inlet and outlet ports transversely aligned relative to a longitudinal axis of the cartridge, each of the ports engageable in a corresponding one of the passageways, and a complementary key formation for engaging the first key formation for ensuring proper engagement of the ports in the passageways where the ports and the key formation project from a common plane. The valve is rotatable in the housing between an open position in which feed water passes through the inlet passageway and into the cartridge, and treated water flows to the outlet passageway, and a closed position in which water is prevented from flowing to and from the valve.