a filter cartridge has a shell defining an upper portion with an upper interior volume and a lower portion with a lower interior volume . the cartridge is for use in gravity - fed water treatment systems have an upper untreated supply and a lower filtered container . a hydrophilic porous particulate carbon powder block filter in a polymeric binder is disposed in the cartridge . the filter has a central opening and is open on both ends . the top end of the block is fitted with an air vent tube . the lower end of the block is fitted into a rubber gasket to direct water flow into the lower container . as water flow through the filter cartridge , air is vented through the block filter central opening and out through the vent tube .