Reverse Osmosis is a fantastic method for producing very pure water. It will remove virtually all contaminants, good and bad! Nowadays it is a practical and affordable technology for the home. There are many systems out there, and if you know exactly what you need then you might wish to go straight to our shop to look. On the other hand, here is some further information on RO that might help.

RO is a process whereby a liquid flows along a pipe, and at the same time, seeps across the pipe wall to the other side. The “pipe” is in fact a semi-permeable membrane that only allows the H₂O and molecules of similar dimensions across. The membrane has pores in it, or extremely small holes if you like. The water molecules are small enough to get across but the other molecules are too big and get left behind. The result is very pure water. In order to maintain a flow on the “impure” side, it's necessary to flush away the impurities left behind, so the system has to be designed so as to leave a significant proportion of the water behind too. For this reason, a typical RO system will use say 5 litres of water for every 1 litre of pure water produced, generally considered OK if it is only drinking water that is being produced, and one of the reasons why you wouldn’t wish to process all of your domestic water this way!

In the UK, a 4-stage or 5-stage system would be typical for mains water. The stages are generally as follows: 1) sediment removal; 2) sub 5-micron filtration; 3) chlorine removal; 4) reverse osmosis membrane; 5) “water polishing” 6) mineral replenishment. Not all of these stages are always present, and sometimes stages 1 and 2 are sub-divided so that for example there might be 2 sediment removal stages.

Don’t worry about the order of the stages for the time being, but here are some brief explanations of what the stages accomplish:

Occasionally people find brown water coming out of their taps, or very fine sediment gathering in the bottom of saucepans after filling with tap water. This is more common in rural areas, where excessive amounts of soil might be washed off the land and into the treatment plants during periods of heavy rainfall. Yes it sounds pretty horrible, but it this is one of the reasons why the public water supply is protected by chlorine in the first place. Besides being unpleasant, sediment can damage taps and appliances. It is easily removable using polypropylene filter elements housed in separate in-line filter bowls. The whole water supply for a house may be treated this way. The proof of the pudding is in the eating, as they say, and over a 3 or 4 month period, most polypropylene filters will become visibly discoloured, proof of the sediment they have removed.

If a whole house sediment removal system is present, then the sediment removal stage in an RO system intended to treat only the house’s drinking water is not so important. However in the context of most systems, it is a small part of the cost and you might still opt for it as part of your RO system to stay on the safe side. If for any reason the household system is put into bypass mode, for example if the filter becomes clogged and a replacement filter hasn’t yet been supplied, then the extra sediment protection in an RO system might give extra assurance to the householder.

Typically, sediment removal in an RO system will be in one or two stages - a 25 micron filter followed by a 5 micron filter is common, the 25 micron filter protecting the finer 5 micron filter from becoming clogged in the instance of a higher than usual burst of coarse sediment. Alternatively there might be a single 25 micron filter or else a single 5 micron filter.

Chlorine Removal and “Polishing”

The UK’s water is generally of excellent quality, though occasionally things do go wrong. That is why chlorine is added to mains water. Chlorine is the principal reason why the UK no longer suffers outbreaks of cholera and typhoid. The amount of chlorine in the water varies from place to place. If you can’t taste it then perhaps you don’t need to worry about it. On the other hand, if it bothers you then it’s easy to get rid of. Chlorine is not removed by traditional filters which work along the same lines as a sieve, but by the process of adsorbtion.

As the name suggests, Granular Activated Carbon consists of carbon granules. If you shake a GAC filter you can often hear the granules rattling. If these become unevenly distributed within the filter body, then channelling of the water can occur, reducing the effectiveness of the filter. “Carbon block” is made by pulverising the GAC, heating it and then extruding it in block form. The result is a finer, more efficient material, with even greater surface area and with even greater adsorbtion properties than GAC, and without the possibility of channelling.

These chlorine removal filters also remove the by-products known as chloramines, produced when chlorine reacts with organic materials - dead bacteria. Besides chlorine and chloramines, the activated carbon used in the filters also removes other volatile or gaseous contaminants, but there is an efficiency factor to consider in this context. The first activated carbon filter the water encounters will usually remove 90% or more of the dissolved carbon, which will make the water far more attractive to taste. It will remove a similar percentage of other volatile components too. The problem is that as the water becomes purer and purer as it passes through the system, any residual tastes become more noticeable as they are no longer being masked by other impurities. Human nose and taste buds are extremely sensitive, and one of the unfortunate effects of the whole RO process is that in removing some impurities it unmasks relatively harmless contaminants with molecular weights small enough to escape the RO membrane itself! The simple remedy for this is the addition of a final “polishing” stage after the membrane, which is successful in removing any such “after tastes”. Thus in many RO systems you will find that two of the stages are carbon filters, one either side of the membrane.

The GE membrane used in our premium RO systems are spirally wound membranes - tightly packed filter material sandwiched between mesh spacers and wrapped in a small diameter tube. The membrane's operating conditions are fine-tuned to balance the flux, or the amount of water which passes through the membrane, with specific rejection rates of contaminants achieved of up to 99.8% salt rejection at low pressures and high flux rates. In the RO process, pure water moves from the concentrated stream to the pure stream on the other side of the membrane. It sounds somewhat counter-intuitive, but this movement is driven by the system’s operating pressure, which must be greater than the osmotic pressure of the dissolved contaminants to allow flow across the membrane. If the prevailing water pressure is inadequate, then the system simply will not work at all. In such cases, the solution is to use a mechanical booster pump to enhance the entry pressure of the mains water.

In a properly designed domestic system, the membrane should be capable of lasting up to three years if the correct pre-filters are in place. It is the key part of the system, and the most expensive too.

An integral part of the RO process is the de-mineralisation of the water. Water that has been through RO is often insipid to taste to the point of being objectionable. Whilst RO water is indeed pure, it also has no minerals. In order to make the RO water more palatable, many RO systems also include a porous ceramic element through which the treated water passes. The effect is to reinstate some mineral content into the water, with the result that the water becomes far more palatable than it would otherwise.

We hope this brief guide will help you understand the different parts of an RO system. To find out more, please refer to each individual listing.

Note - 19th December 2008 - RO listings will appear in our shop shortly.