Studor 20297 Maxi-Filtra Septic Tank Drain Vent Two-Way Active Carbon Filter, 3- or 4-Inch ConnectionActive carbon filter designed to eliminate bad odors produced by the drainage system Designed for use on septic tanks and lift stations; fits 3 or 4 inch piping Eliminates sewer gases from entering buildings and polluting the surrounding areas (when used as part of the complete Studor System); includes replaceable carbon filter (maximum life of 2 years) to remove bad odors and a rubber connecter Install externally anywhere the odor from existing vents