The Omnipure Q-Series Filters are one of the most common RO filters on the market.  The Q-Series Fiters are found on reverse Osmosis systems from IWW, GWW, PuROtwist, TGI, EZ Pure, Farris, Purely EZ and numerous other brands.  The Q-Series filter are a simple "screw on or off design" and are encapsulated so that all filter changes are sanitary. The main difference between the 3 and 4 stage systems is that the 3-Stage system (pictured on left) uses a "combination" sediment and carbon prefilter, while the 4-Stage uses a seperate sediment and separate carbon filter.  US Water carries only the original replacement filters manufactured by Omnipure.  If you have questions or need help in deciding which filter to use, plase call one of our water experts at 800-608-USWATER or e-mail us at info@uswatersystems.com.