Filmtec (TW30-4021) Tap Water Membrane 900 GPD @ 4"X21"Filmtec Commercial RO Membrane TW30 4021 Tap Water Membrane 900 GPD High rejection 4"21" commercial membrane for tap water applications delivering 99. 5% stabilized salt rejection 900 GPD permeate flow 99. 5% salt rejection 36 ft active surface area 4"21" form factor The Filmtec TW30 4021 is engineered for small commercial reverse osmosis systems purifying less than one gallon per minute. With 36 ft of active membrane surface area and 99. 5%