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.