Profile: Keystone Filter Division manufactures air, liquid, and gas filtration products for industrial, commercial & residential markets. Our industrial filter products include industrial liquid home drinking water filters, filter housings & replacement filter cartridges. Our standard, Giant® and bag housings are offered in a variety of materials including blue polypro, clear SAN, reinforced nylon and stainless steel. Keystone Filter is also one of the premier custom pleaters. We offer a complete line of standard filters, giant filters and whole house filters plus replacement cartridges, parts and accessories. Our cartridge types include pleated element, string wound, spun bonded, and activated, granular & carbon block in half, full, double & giant sizes.