Reverse osmosis purified drinking water systems
Unlike activated carbon filters and other filtration methods, reverse osmosis (RO) can remove a wide range of contaminants from the water supply including organics, inorganic, bacteria and other particulate matter. It cannot be claimed that RO water is totally pure - but it is the best you can get – and the taste is excellent.
The term reverse osmosis is the opposite of osmosis. Osmosis is the means by which water and nutrients are supplied to living cells. Cell walls are natural semi-permeable membranes. They are selective allowing only certain materials to pass through the membrane while rejecting others. The natural flow is from the dilute solution to the concentrated solution. Reverse Osmosis is the process of utilising pressure to reverse the natural flow through a semi-permeable membrane.
In an RO system water pressure is used to separate water into two streams. One part is forced through the membrane and emerges through the unit as product water; the other simply flows through the unit to drain. Unlike other filtration methods there is no accumulation of contaminants as these are continually washed away in the reject water.
The membrane in an RO is exceedingly fine. The pore sizes are about 5 Angstroms (five hundred millionths of a centimetre). As productivity of the membrane is proportional to size, the membrane is packaged into a spiral wound element. There are several types available but the thin film composite (TFC) membranes are the most effective.
The membrane module is the heart of an RO system. There is usually an activated carbon pre-filter and a post filter to ‘polish’ the water. As the filtered RO water is produced very slowly it is necessary to store the treated water in a small pressurised tank. This is linked to a separate drinking tap, which can supply the filtered RO water as and when it is required.
A good RO system can perform well over a long period of time and consistently removes 99% of bacteria. It will also remove giardia cysts and the majority of viruses. The system will also eliminate 90% of simple organics and 99% of large organic material.
When considering a reverse osmosis system it is important to evaluate the taste of the filtered RO water. In a good system the dissolved oxygen in the incoming water supply penetrates the membrane at an equivalent rate to the water. The filtered RO water should therefore have the same oxygen content. This is significant since it is the oxygen in the water, which gives good taste and not the mineral content. If oxygen is lacking the water can taste ‘flat’.
An in-home RO system can be installed under a kitchen sink. In some hard water areas it is advisable to use an RO unit in conjunction with a water softener. Calcium is known to clog membranes. Softened water can prolong the life of the system – take advice from your local supplier. The minimum water pressure required is 2.7 bar (40psi). Small Ro pumps are available to provide the correct incoming pressure if it is insufficient.
As the flow of water across the membrane constantly keeps it clean, an RO unit requires little maintenance compared with the other point of use (POE) systems. The unit should be disinfected or sanitised on installation and periodically to reduce the number of bacteria on the upstream side of the system. Servicing is normally required annually. Some pre and post filters may need to be replaced more often. Service scheduling will vary from manufacturer to manufacturer.
The reverse osmosis filtration process is an excellent way of removing most water contaminants. The taste of good RO water is first class and will improve beverage mixes, ice cubes, cooking and baking. RO water is known to enhance aromas and give rich flavours. As a drinking water it is comparable, if not better, than bottled water at a fraction of the cost. If you are concerned about drinking water quality then an RO system will give you peace of mind.
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