How we treat water

To provide water that's good for you, we have to meet the drinking water quality standards set by the Water Supply (Water Quality) Regulations.

The type of treatment process required depends on the type of raw water source and its quality in a particular area.

Water from springs and boreholes is generally higher quality and may only need basic treatment followed by disinfection. Water from sources such as rivers, lakes and reservoirs, will normally require more complex treatment as it may contain more impurities (for example, pesticides) that have to be removed.

We monitor the quality of our raw water sources to make sure they haven’t become contaminated – and also to decide which type of treatment to use for a particular source of raw water. Follow the link to find out more about how we monitor, test and maintain water quality.

The treatment process

Treatment of water depends on the source, so any particular water supply works may use one (or more) of the processes below. We don't add fluoride to water – follow the link to find out more about fluoride in water.

Surface water is stored in reservoirs to provide an adequate supply to meet demand throughout the year. This also enables particles to settle. Ultraviolet light from the sun helps to reduce bacteria levels and break down organic compounds in the raw water.

Water is passed through mesh screens which remove larger-sized material in raw water such as leaves, weeds and sticks.

Particles are removed through the processes below:

  • Coagulation: Coagulants are substances that cause particles in liquid to stick together. In water treatment, coagulants are added to cause so solids and particles to be removed more easily. The process is called solid-liquid separation. The amount and type of coagulants added to water are precisely measured.
  • Flocculation: When particles have been increased in size and strengthened by adding polyelectrolytes (coagulant aids that strengthen the particles formed during coagulation) they can be removed by filtration. The selection of polyelectrolyte may differ with the quality of raw water.
  • Sedimentation (clarification): The material produced by the coagulation and flocculation process forms itself into a suspended mass called a sludge blanket in tanks called clarifiers, where the solids settle out as sediment. The clarified water that is left above the sediment is then passed to filters to remove fine particles.
  • Dissolved Air Flotation (clarification): This is a type of clarification process in which impurities are floated to the surface instead of being allowed to settle at the bottom of a tank. Micro-bubbles, created by saturating water with air under high pressure, are released into the flocculated water, attaching to floating the impurities to the surface. The impurities gradually form a sludge layer on the top which is skimmed off at regular intervals into a separate channel, while the clarified water underneath is allowed to pass on to filters.
  • Rapid Gravity Filtration: This process removes very small particles not removed by clarification using either sand alone or mixed layers consisting of anthracite (coal) and sand. It’s important to note that in the absence of coagulation, raw water passes through the filters untreated and no impurities are removed.
  • Membrane Filtration: This involves filtering raw water under very high pressure through prefabricated membranes. The process is more expensive, but gives better quality of water where the raw water source is of poor quality.
  • Sludge: is the material left over from clarification and filtration processes.  This is disposed by either putting it to a landfill site or directly to the sewer where it is treated at a waste water treatment works.

Granular activated carbon (GAC) is used to remove many tastes and odours. The process causes the materials responsible for taste and odours to be absorbed by the particles of GAC. It's often used during the summer when taste and odour problems are more likely to occur, as well as being used to remove pesticides from drinking water supplies.

The scientific term pH is used to describe the degree of acidity or alkalinity. The pH levels of water need to be controlled because if water is too acidic it may corrode distribution and domestic pipes, while if it’s too alkaline, it may cause deposits to form inside the pipes or cause taste or odour problems.

Satisfactory disinfection is vital to prevent waterborne diseases and to ensure we supply water that meets the Water Supply (Water Quality) Regulations. Chlorine is used for disinfection either in gas or liquid form. Other disinfection processes include use ultraviolet light (UV) or ozone.

Lead can be present in domestic pipe work and the pipes connecting to the water mains. Phosphates may be added at water supply works to help prevent lead being leached into the water from the pipes.