Osmosis is a process which occurs when two solutions of different concentrations are separated by a semi-permeable membrane. Reverse osmosis water purification works by forcing the water under pressure against an ultrafine semi-permeable membrane designed to allow single water molecules to permeate through, while at the same time rejecting most contaminants known as waste water. The membrane acts as a mechanical filter, straining out particulate matter, micro-organisms, asbestos, even single molecules of heavier organic compounds.
A typical RO purifier consists of four filters in series plus a storage tank. The first is a sediment filter, the second a carbon block, the third a membrane and the fourth an activated carbon block to remove any remaining chlorine by-products. Such a system removes a wide spectrum of impurities from water; the only energy required is that of mains-water pressure.
Reverse osmosis effectively removes turbidity, sediment, colloidal matter, bacteria, chemicals, fluoride, totally dissolved solids, toxic metals, radioactive elements, pesticides, and herbicides. This can have significant health benefits.
A typical system produces water at a slow rate – almost drop by drop – so most under sink systems have a pressurised storage tank and a separate dedicated faucet or all in one three way mixer installed on the sink. Water drawn from the faucet or mixer comes from the storage tank.
A portable counter top reverse osmosis system works the same way except without a pressurised tank. Instead these systems attach directly to an existing faucet and used to fill a bottle or glass directly from the system. These systems are suited for use when renting, unable to plumb a system in, or for travel.
The average system produces about 300 litres per day if left to run 24 /7*, more than enough for an average family. (* Depending on inlet water pressure and membrane capacity.)
The average domestic RO will with modern technology achieve a waste water ratio of 1 – 1.
Unlike filters, RO membranes don’t accumulate pollutants but the membranes themselves gradually degrade with use. While the sediment and carbon filters will probably need replacement every 6 – 12 months, membranes should be changed every 4 – 5 years or as specified by the manufacturers or dependent on water conditions.