Reverse osmosis is a multi-stage water treatment process that removes contaminants from unfiltered water, or feed water, when pressure forces it through a semipermeable membrane. Residential reverse osmosis systems utilize at least three stages of treatment to reduce levels of virtually all contaminant types. In the final stage, the RO membrane, water flows from the more concentrated side (more contaminants) of the membrane to the less concentrated side (fewer contaminants) to provide clean drinking water. The fresh water produced is called the permeate. The concentrated water left over is called the waste or brine.

​