Every chemical we use meets strict British Standards, and we continuously monitor the raw water quality and the process to make sure only the minimum amount needed is added. Any chemical that is dosed during treatment is removed or reduced to safe levels through later stages of the process, so it does not remain in the final drinking water.
Because the quality of raw water varies, the exact chemicals and quantities used can differ from site to site. We adapt and optimise our treatment processes based on the type and condition of the local water source.
Step 1: Water monitoring and screening
We continuously monitor the quality of the water that comes from the lake, river or reservoir. This helps us understand how much chemical we need to add to treat the water at the water treatment works.
We will then use water screening to remove large debris like sticks and leaves from the water.
Step 2: Coagulation and flocculation
A chemical (coagulant) is added to the water to bind small particles into larger clumps (flocs), making them easier to remove. This process uses either aluminium sulphate or iron (ferric) sulphate. We also add a polymer chemical which makes all the sediments in the water stick together and form a floc.
Step 3: Clarification
The coagulated water sits in large tanks, so flocs settle to the bottom, or tiny air bubbles float particles to the surface to be removed.
Step 4: Disinfection
Chlorine or UV light is used to kill harmful bacteria, viruses, and pathogens. Sulphur dioxide and sodium bisulphite can be used if there is a need to reduce the levels of chlorine in the drinking water before the water is sent into our mains network.
Step 5: pH adjustment
Chemicals, such as sodium hydroxide, calcium hydroxide, sulphuric acid may be added to stabilise pH and phosphate is added to prevent corrosion of lead pipes.
Step 6: Storage and distribution
Treated water is stored in reservoirs or tanks before being distributed to consumers.
At the request of the Secretary of State we also add Hexafluorosilicic acid to raise the concentration of fluoride in the drinking water in parts of Cheshire and parts of West Cumbria. The Water Industry Act 1991 states that the Secretary of State can instruct a water company to fluoridate the water supply in a particular area if it is technically feasible to do so. If it is technically feasible, we cannot refuse to add fluoride. We do not make or influence these decisions.
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