Reverse Osmosis
Reverse Osmosis (RO) is a technology that that was originally developed to desalinate sea water, but is now widely used to provide pure water for the food, mining and pharmaceutical industries, as well as for the home consumer.
Osmosis is a natural process which occurs when two solutions of different concentrations are separated by a semi-permeable membrane. The water passes through the membrane in the direction of the more concentrated solution.
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 also acts as a mechanical filter, straining out virtually all particulate matter, turbididty, micro-organisms, asbestos, even single molecules of the heavier organic compounds. The size of the pores in a reverse osmosis membrane are in the order of 0.0005 microns, which cannot even be seen by the best optical microscope!
However, the RO membrane allows oxygen to pass through so that the pure water doesn’t have the “flat” taste, as can sometimes be found with distilled water. Reverse Osmosis units typically consist of three filters in series with a storage tank for collecting the product water. The first cartridge is a sediment and chlorine removal filter (chlorine can damage PA membranes), the second is the housing containing the RO membrane and the third is an activated carbon cartridge to remove any remaining chlorine by-products such as chloroform or THM’s.
The contaminants that are rejected by the membrane are washed down the drain, keeping the membrane surface clean. Unlike filters, RO membranes don’t accumulate pollutants, but the membrane gradually degrades with use and is usually replaced after about three years. In the past, RO membranes used to reject around 10 litres of water for every 1 litre of pure water produced, but now advances in technology has reduced that to 3 litres per 1 litre produced.
Reverse osmosis can remove and reject a wide spectrum of impurities from water, the only energy required is that of mains water pressure. With the exception of distillation, reverse osmosis is the only process which will effectively remove or reduce turbidity, sediment, colloidal matter, total dissolved solids, toxic chemicals, fluoride, radioactive elements, pesticides and herbicides.
Reverse osmosis is a slow process, therefore most underbench units have a pressurised storage tank, whereby the pure water is always available in large volumes under pressure, through a separate faucet. When the storage tank volume drops, the unit automatically refills the tank. The typical residential RO unit will produce around 140 litres per day.