Pack of 3x Granular Active Carbon Filters Granular Active Carbon is carbon that has been treated with oxygen to make it porous, but it has also been broken up into granules to further increase the surface area for chemical bondage to occur. The water passes through the GAC filter, allowing the organic materials and chlorine to effectively bond with the carbon molecules, thus removing these from the water. What's in the box 3x 10 inch Big Blue (10x4.5" size) Granular Activated Carbon (GAC) Filters