The heart of a reverse osmosis system is a semi-permeable membrane with very fine pores. Water is forced through this membrane, and any particles or impurities that are too big to fit through the pores are flushed away. The pores are approximately 0.0005 microns in size - for comparison, bacteria range from 0.2 to 1 micron and viruses are 0.02 to 0.4 microns. A typical human hair is about 100 microns wide.
Reverse osmosis units also incorporate their own pre-filter to remove sediment that can clog the membrane, and activated carbon filters to help improve taste.
Although the membrane is fine enough to filter out bacteria, it is still possible for bacteria to pass through leaking seals or possible imperfections in the membrane, so reverse osmosis systems should not be used as a means of disinfection. An ultraviolet light or chlorination system should be used to provide disinfection, if required.
The principle drawback of a reverse osmosis system is its limited capacity, and the amount of water used. For every 5 gallons (or 20 liters) of water produced, a typical residential system will send 10 to 20 gallons (or 40 to 80 liters) of water down the drain.