iSpring F4-CUA4 4-Piece Filter Pack for 4-Stage Ultra-Filtration Under Sink Water Filter System CUA4 1st Stage: 5 Micron sediment filter that can effectively remove rust, sand, and solid impurities in the water to protect the delicate UF membrane. 2nd Stage: State-of-the-art UF Hollow Fiber membrane that uses standard home water pressure to push water through a semi-permeable membrane. 3rd Stage: KDF media (or Kinetic Degradation Fluxion Media) with activated carbon. 4th Stage: Post Activated Carbon Filter. Adopting fine coconut shell activated carbon with ultra-micron aperture, this carbon filter absorbs residual chlorine, organics, humus, and odor.