In England and Wales, two thirds of drinking water comes from surface water, including reservoirs, lakes and rivers, and the rest from ground waters.
Water treated for sewage gets dumped back into nearby natural water sources once it has been treated for sewage. They probably do not filter out radioactive particles. The cleaned water then goes back out into the natural water supply. So on one hand you have contamination of the natural water environment. Then England pulls water from the natural water supply, for purifying and then drinking and washing. Just guessing here. So water that is brought in for purification probably comes from the same natural sources. England is a big Island after all. More excerpts from the same web site describing treatment of natural water supply.
Clarification
Clarification is a complex process that removes silt, algae, colour, manganese and aluminium, and various other matter that may be present in the raw water. A chemical called a coagulant, which is usually an iron or aluminium salt, is added to the raw water and combines the material that has to be removed into larger particles. These are removed either by settling them out (sedimentation) or by using air to float them to the surface (flotation). Clarification also removes about 90 per cent of pathogens from the raw water.
Filtration
Any particles remaining after clarification are removed at the filtration stage. Filtration is also used to remove iron and manganese from ground water sources. There are a number of types of filter, the most common being rapid gravity filters. The water passes through a bed of sand or other suitable media, where the particles become trapped. After a given period of time the flow through the filter is reversed in order to clean the sand. It is very important to remove as many particles as possible at the filtration stage for the final disinfection stage to be effective.
Membrane filtration is used in special applications, such as the removal of Cryptosporidium oocysts (see question two in the frequently asked questions section at the back of this leaflet for more information on Cryptosporidium).
Disinfection
In this country the most common method of disinfection is the use of chlorine (occasionally you may notice a slight smell of chlorine in your tap water). It is a method that has been used extensively for over 70 years. The disinfecting process is essential to eliminate any bacteria in the water. Water companies have to ensure that enough chlorine remains in the water after it leaves the treatment works to help keep the water safe on its journey to the tap.
Other disinfectants include ozone and ultra violet light, but these do not remain in the water during distribution, so in both cases a small amount of chlorine is added before water goes into distribution.
Chlorine can react with some natural organic matter present in the water to produce trace amounts of other chemicals, some of which are potentially toxic. These are known as disinfection by-products (DBPs). However, the risks associated with DBPs are small, and far outweighed by the benefits of disinfection. Without it waterborne diseases such as cholera, dysentery and typhoid could rapidly re-establish themselves. DWI has a leaflet available free of charge called Chlorine, Smell, Taste.
Disinfection is not very effective against parasites such as Cryptosporidium and Giardia (which can cause illnesses with severe diarrhoea lasting a number of weeks). If there is a risk of these being present they must be removed during the filtration stage of treatment.
Some waters require more specialised treatment, such as:
Ion exchange
This process is used to remove nitrate from ground water, and also in some cases to soften water. Ion exchange is very similar to the process used in domestic water softeners, where water is passed through a bed of special resin particles.
Activated carbon and ozone
Activated carbon, often in association with ozone, is used to remove organic substances. Some of these occur naturally and others are contaminants that occur because of man’s activities, such as pesticides. The ozone breaks down the organics, which are then adsorbed on the surface of the carbon.