Series PWRO4 Four-Stage Reverse Osmosis System is a tried and true reverse osmosis system! The 4-Stage RO System begins with Stage 1, a 5-micron Sediment filter that traps particulate matter like dirt, rust, sand, silt and sediment that will affect the taste and appearance of your water. Stage 2 is a 5-micron Carbon Block filter that provides a reduction of chlorine, chloramine and other materials that cause bad taste and odors (these are called PRE-filters because they filter in front of the reverse osmosis membrane). Stage 3 is the heart of the reverse osmosis system, the RO Membrane. This semi-permeable membrane is 1/10,000 of a micron and will effectively reduce TDS (total dissolved solids), sodium and a wide range of contaminants such as arsenic, lead, perchlorate, chromium, copper and radium. It will also remove over 99.95% cysts such as giardia and cryptosporidium. Stage 4 is a high quality GAC final filter (this is called a final filter because it filters after the membrane). The filtered RO water passes through the membrane and enters the storage tank. When you're ready for a taste, it will leave the storage tank and pass through this fourth stage on its way up to the faucet. The end result — crystal clear, high quality, and great tasting water!