his 3.11 in x 3.54 in Filter Cartridge (Type I) features ultra fine heavy-duty paper with more pleats for better filtration. Patented built-in chlorination to suspend chlorine tablets in the middle to maximize chlorine effectiveness. Replace filter cartridge every 2 weeks. Features Ultra fine heavy-duty paper with more pleats for better filtration Patented built-in chlorinator to suspend chlorine tablets in the middle to maximize chlorine effectiveness Replace filter cartridge every two weeks