ZIP Countertop Water Filter Replacement Filters Bundle FIRST STAGE FILTRATION - 5-micron Prefilter removes Sediment, Rust, and other large particles. SECOND STAGE FILTRATION - Carbon Block filter removes Chlorine and other Disinfection by-products. FOURTH STAGE ALKALINE POST FILTER - Polishing Alkaline Post Filter adds beneficial minerals  Calcium and Magnesium to purified water for the best taste. It also increases the alkalinity of the water to a healthy level of 7-8 pH.