UK municipal water supplies have to meet strict quality standards set out in the Water Supply (Water Quality) Regulations. These regulations limit the concentration of harmful substances such as pesticides and nitrates in domestic supplies, but permit other substances such as naturally occurring minerals to be present in solution. In fact the theoretical idea of pure water implies the absence of everything except for H2O. In practice if you drank a glass of 'pure' water you would find it to be very flat and uninteresting - it's the presence of natural minerals that give drinking water its taste, although these minerals can also produce scale.
Another taste found in UK drinking water is due to chlorine added to ensure that the water is free from bacterial contamination. Only 150 years ago contaminated water caused many outbreaks of waterborne disease such as typhoid, cholera and dysentery which have now been virtually eliminated with modern treatment methods and in particular the introduction of chlorine as a disinfection agent.
The extent to which chlorine impairs the taste of drinking water is a subjective matter, but it is certainly true that some people find it to be unpleasant. (Note -- if you fill a jug or bottle with tap water, cover it, and leave it in the fridge for an hour or so, this will help reduce the chlorine taste although we recommend that this water should not be kept too long as water may deteriorate if saved for more than 24 hours).
In summary, whilst the standards of tap water in the UK are high there are still potential benefits in installing additional filtration, depending on the application. Water filters are generally designed to remove one or more of the following:
Fine particulates (such as sediment or rust)
Volatile Organic Compounds (trace herbicides/pesticides, etc.)
Particulate filters are simple mechanical filters designed to trap small particles of dirt and debris in much the same way as a very fine sieve. Sediment filters typically remove particles in the 5 micron to 20 micron range. Some are designed to remove particles as small as 1 micron.
Organics and chemicals such as chlorine and chloramine are typically filtered out through inexpensive activated carbon filters. Four types of carbon filters are commonly used. From least to most expensive, they are granulated activated carbon (GAC), carbon-coated fiber, extruded carbon block and moulded carbon block.
Prevention or control of scale is more complex. Scale forms much the same way a raindrop or a snowflake does.
The phenomenon of scale (limescale) is due to the presence of naturally occurring calcium and magnesium salts (bicarbonates, carbonates and sulphates). These salts are readily soluble in cold water, but as hard water is heated to boiling point the dissolved carbon dioxide is driven off, which reverts the bicarbonate salt to the insoluble carbonate state which then precipitates out of solution on cooling. This is the origin of the scaling, which occurs on all water boiling products in hard water areas where the concentration of these salts is highest. It is most commonly seen as a light grey 'fur' on kettles. Boiling water products are particularly susceptible to scaling especially in hard water areas, with scale formation being greatest for products that are in constant use. The following map shows the areas in the UK with hard water.
Polyphosphates additives can be incorporated into filters to produce inexpensive scale inhibitors, but they become less effective at higher temperatures (above 30 degs C) and so are not well suited to protecting boiling water units, for example, from scale buildup. The addition of polyphosphate into water can also to make tea cloudy.
More sophisticated ion exchange resins filters exchange a hydrogen ion for a calcium ion offer a better solution for hot water beverage maker machines. In these filters the hydrogen ion binds with spare oxygen molecules and helps to slightly increase the acidity, making it more difficult for scale to form. Although more expensive than polyphosphate based filters, they are much more effective and can virtually eliminate the need to de-scale boiling water units between filter changes (typically every 6 months).
Water Filter Applications:
Water Chillers/Drinking Water Fountains
The goal is here to improve the water's taste by removing chlorine and chloramine and volatile organic compounds (VOCs). A good cost effective solution is a granular activated carbon filter.
Boiling Water Units The goal is to remove VOCs, chlorine and chloramines and also to inhibit or reduce scale without compromising the quality of hot drinks. A good solution here is a a carbon filter and hydrogen ion exchange resin.
Dish Washing. Scale buildup is a big problem in dishwashers as it can clog spray nozzles and make heating elements less efficient. A good solution is using heat-resistant polyphosphate in a point-of-use sediment filter which would be less expensive than a traditional water softener. An even better option would be an ion exchange filter based on high-efficiency hydrogen resin.
Filter Sizing and Replacement
Filters have a load capacity measured in litres of water throughput before replacement, and are typically specified to last about six months or more before they are exhausted and have to be changed. A simple water flow meter can be used to measure remaining filter life. A filters may limit the maximum flow through a water supply system, and so must be sized to ensure that the filter does not have a detrimental effect on the overall system operation.