In natural water contamination depend on water source like Bore well & surface. Two type of contamination you can see like CHEMICAL & MICROBIOLOGICAL FINDINGS. Surface water contain mostly Microbiological findings, but Bore well water contains both Chemical & Microbiological findings, not only that their limit to much high than Human permissible limit (as per IS 14543)
Formally, reverse osmosis is the process of forcing a solvent from a region of high solute concentration through a semi permeable membrane to a region of low solute concentration by applying a pressure in excess of the osmotic pressure. The largest and most important application of reverse osmosis is the separation of pure water from seawater and brackish waters; seawater or brackish water is pressurized against one surface of the membrane, causing transport of salt-depleted water across the membrane and emergence of potable drinking water from the low-pressure side. Reverse osmosis (RO) is a water purification technology that uses a semi permeable membrane to remove ions, molecules and larger particles from drinking water. In reverse osmosis, an applied pressure is used to overcome osmotic pressure, a colligative property, that is driven by chemical potential differences of the solvent, a thermo dynamic parameter. Reverse osmosis can remove many types of dissolved and suspended species from water, including bacteria, and is used in both industrial processes and the production of potable water. The result is that the solute is retained on the pressurized side of the membrane and the pure solvent is allowed to pass to the other side. To be "selective", this membrane should not allow large molecules or ions through the pores (holes), but should allow smaller components of the solution like essential Mineral, Nominal quantity salt etc. So, Reverse osmosis is the Ultimate solution of water treatment for new generation.