WATER TREATMENT METHODS: REVERSE OSMOSIS
Reverse Osmosis (RO) systems produce the purest water practicably available. They are purification systems, not filtering systems.
Independent tests have shown them to remove over 99 per cent of pesticide residues and chlorine, over 99 per cent of metals and 90 per cent of nitrates. They also remove most bacteria, parasites and viruses. Distillation systems are the only other water treatment systems which produce comparably pure water, but they have greater disadvantages.
RO systems work by forcing part of the water flow through a semipermeable membrane, combined usually with a sediment filter and an activated carbon filter (see below), which also absorbs contaminants. The water is stored in a small reservoir tank until needed.
The main benefit of RO systems is the purity of the water resulting. In addition, contaminants are washed away. They are not held in any treatment medium and cannot wash back into the system.
They have drawbacks, however. They have a very low flow-rate and need a high water pressure to work well. They are extremely wasteful of water. More than 75 per cent of the water used goes down the drain and this could be an important cost factor if water metering is introduced. Mineral salts are removed and the water has a bland, insipid taste which some people dislike.
Most of the reverse osmosis systems available are for plumbing in under a sink or basin with one tap outlet. A skilled DIYer could instal them, but many people will need a plumber’s help. These undersink systems are also bulky and need a lot of space. They cost between £300 and £500, excluding any plumbing costs. The membranes need replacing every 18 months to three years and cost between £50 and £90. Carbon filter cartridges need replacing every six to twelve months and cost between £15 and £30.
A portable end-of-tap version is available at £90 (1992 prices), which is very convenient, and has the advantage that you can take it with you outside the home – to work, or on visits.
Firms will quote specially for installing reverse osmosis equipment to cover a whole water system. This will probably cost well over £1000 before plumbing costs, and the low flow-rate and water wastage would be very inconvenient in practice.
It has been known for some people – extremely isolated cases to react to RO water. However, it is the purest available and if you are unlucky enough to be that sensitive, it is probably still the best you can find. Manage your expectations if you are thinking of trying it though. Do not pin your hopes on it, just in case, if you have a history of exceptional sensitivity. Test it systematically before installing an expensive system. System suppliers can provide samples.
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