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. 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, colloidial matter, 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 counter top 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 200 litres per day*, more than enough for an average family. (* Depending on inlet water pressure and membrane capacity.)
The average domestic RO will use about 40 litres per day to flush contaminants – average household consumption is around 1000 litres per day.
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 3 – 5 years or as specified by the manufacturers.