The Gravity Filter system has four ceramic filters and one media-tray. This page allows you to purchase single filters or the whole set.
The 4 ceramic filters of the Gravity Filter System remove all microorganisms in the water. They should last indefinitely. However be careful, because they can break or crack. If one does you will need to replace it.
The Media Tray removes heavy metals and toxic chemicals from the source water. It is rated for an average of 2,000 gallons (depending on the quantity of toxins in your water). Make sure to change it minimally every 2 years of use.
See more at the Gravity Filter main product page.