The PT4000T36-GOLD reverse osmosis system is tested and licensed by the Water Quality Affiliation and has received a NSF/ANSI 58 Gold Seal.PuROTwist PT4000T36-GOLD reverse osmosis system utilizes a 4 stage filtration process to deliver the best quality drinking water. Stage 1 is a 5 micron sediment clear out that eliminates sand, dirt, sediment, and different particles. Stage 2 is a pre-carbon clear out that protects the RO membrane from sediment and chlorine. Stage 3 is the reverse osmosis membrane to take away dissolved inorganic fabrics. Stage 4 is a post carbon clear out to support style and odor of drinking water.