Sold as a single filter element Super Sterasyl™Ceramic Filtration Elements have been in continual use since 1827 Silver impregnated to inhibit bacterial growth Contains granulated activated carbon to reduce chemical and organic compounds 7" element installs in the upper chamber of all Berkey systems or British Berkefeld systems 9" element installs in the upper chamber of most Berkey systems or British Berkefeld systems; Not compatible with Travel Berkey system 9" Imperial element installs in the upper chamber of the Imperial Berkey systems and Crown Berkey systems; Not compatible with Travel Berkey system Durability: Recommended manufacturer's replacement for each Ceramic Filter Element is six months or 535 gallons after use Re-cleanable and self-sterilizing for the life of the filter Meets NSF Standard No. 42 for materials