Hydrotech reverse osmosis systems are available in several diffrent configurations, but the most common are the 3-Stage System (pictured to the left) and 4-Stage System (pictured to the right).  The basic difference is that the 3-Stage System uses a "combination" sediment and carbon prefilter while the 4-Stage uses a  separate sediment prefilter and separate carbon prefilter.  US Water carries a full line of replacement filters that are GUARANTEED to be as good (actually better) than the factory replacements.  If you have any questions as to which filters or membranes you need, do not hesitate to call one of our water experts at 800-608-USWATER or e-mail us at  info@uswatersystems.com.