Reverse osmosis, or RO, is a water purification technology that uses a semipermeable membrane to remove ions, molecules and larger particles from drinking water. In other words, reverse osmosis is a form of filtering that removes contaminants such as lead and nitrates from drinking water. The process involves pushing water under pressure through a semi-permeable membrane that filters out most dissolved minerals and chemicals. This leaves you with clean but slightly demineralized (chemically neutral) water.