Reverse osmosis removes contaminants from or feed water, unfiltered water, when pressure forces it through a semipermeable membrane.  To provide clean drinking water，water goes from the more concentrated side of the RO membrane to the less side. The fresh water produced is called the permeated pure water. The concentrated water left over is called the waste water or brine.

A semipermeable membrane has small pores that could block contaminants but allow molecules to flow through. In osmosis, water becomes more concentrated as it passes through the membrane to obtain equilibrium on both sides. Reverse osmosis blocks contaminants from entering the less concentrated side of the membrane. For example, when pressure is applied to saltwater during reverse osmosis, the salt is left behind and only clean water flows through.