Used as the second stage for the big blue iSpring whole house filtration system WGB21B or the first stage of iSpring whole house filtration systems WGB21BM and WGB21B-PB. The carbon block filter consists of high-quality activated carbon. It is effective in reducing free chlorine, bad taste, odor, THM, organics, hazardous chemicals, and more. Depending on source water quality or flow rate, it needs to be replaced every 6 months or about 50, 000 gallons of water. Each filter is individually sealed to keep freshness. Fits models using standard 10" x 4.5" big blue whole-house water filters.