Microbes and food 1. Menu - you are what you eat
Reservoir photo
Picture 1.13a A reservoir in the Brecon Beacons.
1.13 Mains water
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Water is essential for all life. Its natural normal flora of micro-organisms, mainly bacteria, is usually harmless, but drinking water that has become contaminated with harmful microbes can cause illness. Dirty water often poses a big risk to health in developing countries.

Mains water or bottled water, what's the difference?

Mains water, in developed countries at least, is treated to make it safe to drink. It can be taken from many sources, including lakes and rivers, and has to travel miles through the pipes from the treatment works. When it comes out of the tap it should contain very few microbes, and certainly no pathogens.

Bottled water is very popular. Some people do not like the taste of mains water because it has been treated with chemicals, others believe that some types of water, containing high levels of certain minerals, are actually good for you.

Most bottled water comes from springs or deep boreholes, is bottled at source under hygienic conditions and is not treated with chemicals. As a result it can contain harmless, natural water bacteria. These gradually die off in the bottle during storage as they use up the small amount of nutrients.

Water treatment
Picture 1.13b A filter bed at a water treatment plant.

How is it produced?

The treatment depends on where the water comes from. In England and Wales surface waters, such as lakes and rivers, account for two thirds of our mains water. The rest is ground water from deep aquifers, formed when rain soaks through porous layers of rock. It is extracted through boreholes. Surface water is stored in reservoirs for several months where particles settle out and sunlight kills a high proportion of the viruses and pathogens.

Water is next clarified to remove silt, algae, colour and other material. It is coagulated into larger particles with aluminium or iron salts, and the lumps are then removed. This process also gets rid of about 90% of pathogens.

Any remaining particles are filtered out either through man-made beds or natural systems such as sand dunes, and sometimes activated carbon. Membrane filtration may be used at this stage to remove the cysts of the parasite Cryptosporidium as these are not killed during disinfection (see below).

Finally the water is disinfected before it is distributed to eliminate any microbes, including pathogens. Disinfection should persist in the water so that it remains safe between treatment and when it comes out of the tap. During distribution the water may become contaminated from the pipes, soil entering the system through breaks or from untreated water, which can get into some sources after heavy rainfall.

Disinfection methods used are:

Chlorine – the most widely used disinfectant. It is effective against bacteria and viruses, but some parasites and protozoa, e.g. harmful ones such as Cryptosporidium and Giardia, which cause severe and prolonged diarrhoea, can survive chlorination. It persists well in distribution systems, but the disadvantage is that the water sometimes still tastes of chlorine.

Ozone – due to its strong oxidising capacity it reacts quickly with organic matter, including microbes. It is very effective against bacteria and viruses but has little residual activity; there are concerns that toxic bi-products may also formed during treatment. A small amount of chlorine may be added to the water after ozonation to protect it in the pipes.

Ultra-violet light – at certain wavelengths it inhibits and kills microbes, although higher lethal doses are needed for viruses and UV is ineffective against fungi and protozoa. Suspended particles can also protect the microbes from this treatment. As with ozone, UV has no residual activity.

Can it be harmful?
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Pathogens can get into water from animals through run-off from agricultural land and from humans by many routes including sewage. In most countries the microbiological quality of water is protected by law. Regular tests have to be carried out and the levels of indicators of faecal pollution, such as coliforms and E. coli, are checked against stringent standards. In some parts of the world water quality is not controlled in this way and to avoid diseases like cholera, typhoid fever or virus infections, advice should be sought before drinking it.

In the UK all bottled waters have to meet the requirements of the Natural Mineral Water, Spring Water and Bottled Drinking Water Regulations (Amendment) Regulations 2003 which ensures that they are safe to drink. Mains water quality is governed by the Water Supply (Water Quality) Regulations 2000.