Premium Fluoride water filter element compatible with all Berkey water purification systems. Compared to the original part PF-2. Filter is made with fluoride particles. It is designed to work with black elements BB9 in the water system to adsorb fluoride, arsenic and pre-oxidized arsenic and other residual typical heavy metal ions. Dimension: 6.25" x 2.5". 4 elements included. Recommend to replace each pair of fluoride water filter after 1,000 gallons of use, under normal conditions. Premium Fluoride water filter element compatible with all Berkey water purification systems. Compared to the original part PF-2. Filter is made with fluoride particles. It is designed to work with black elements BB9 in the water system to adsorb fluoride, arsenic and pre-oxidized arsenic and other residual typical heavy metal ions. Dimension: 6.25" x 2.5". A pair of 4 elements included.