A reverse osmosis system is usually a filtration unit that removes contaminants by forcing the water through a semi-permeable membrane with very small openings, usually fractions of a micron wide. Reverse osmosis is very effective at removing particulate matter and basically effective at filtering out biological contaminants.
For consumers worried about biological contamination, particularly E. coli, legionella or giardia, an ultraviolet filtration system should be installed. Unlike other filtration systems, a UV filter radiates the water with light in the ultraviolet spectrum. This destroys the simple bacterium and viruses but is too weak to cause the water to become irradiated -- a safe and environmentally friendly way to decontaminate water.