Reverse Osmosis, as its name implies - is the reversal of the natural flow of osmosis.
By applying pressure to the water solution of higher concentration (the incoming water), the flow of liquid is reversed. Under these conditions, the membranes still rejects the contaminants, but allows the fresh cleansed water to pass through.
Reverse Osmosis (RO) is a water purification technology that utilizes normal household water pressure to force water through a selective semi-permeable membrane that separates contaminants from the water. The purified water is collected in a holding tank and the contaminants are flushed away.
The process, originally designed to make seawater drinkable for the Navy, removes sodium, as well as lead, arsenic, nitrates, asbestos and a range of many other contaminants from household drinking water.