Reverse Osmosis (RO) is a method of filtering water that uses semipermeable membrane. The membrane will pass some atoms or molecules through but not others.
Reverse osmosis will filter up to 99% of contaminants from water - including bacteria, most viruses, heavy metals, minerals, etc.
A typical reverse osmosis system consists of a pressurized stream of water passing through pre-filters to reduce contaminants, semipermiable membranes and a by-pass flow to carry the concentrated contaminants away from the membrane.
RO systems are used in large water treatment plants, on boats and ships and in homes. A small RO unit can be purchased for as low as $150. Units suitable for municipal water treatment plants can cost millions.