Water treatment cartridges and processes related thereto

The invention relates to a process for adding a second treatment material to an existing water treatment cartridge, said process comprising the steps of: (a) providing a water treatment cartridge comprising a housing, an inlet for introducing water into said water treatment cartridge, an outlet port for egress of water from said water treatment cartridge, and a first treatment material, said first treatment material having a core region; (b) providing a second treatment material; (c) providing a liquid-pervious retainer; (d) placing said second treatment material into said core region of said first treatment material through said outlet port; and (e) capping said outlet port with said retainer.