Water treatment process

Drinking water in the South East mainly comes from rainwater. When it rains, water flows into streams, rivers, lakes and reservoirs – this is called 'surface water'.

What standards do drinking water have to meet?

Drinking water in the South East mainly comes from rainwater. When it rains, water flows into streams, rivers, lakes and reservoirs – this is called 'surface water'. You can find out more about your water and where your water comes from in the region. 

If water seeps through the ground until it reaches rock that it can’t penetrate, it forms underground aquifers. The water in these aquifers is called ‘groundwater’ and is often very high quality, because as it seeps through the ground, many contaminants are naturally filtered out. It also tends to be hard water.

Water supplied for drinking may contain some of the substances listed below. These substances can:

  • occur naturally in raw (untreated) water
  • enter water during the treatment process
  • be deliberately added to safeguard public health.


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 – find out about the natural levels that occur.

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.


Why we test water quality

It's our duty to ensure a wholesome supply of drinking water for our customers. 

The Water Supply (Water Quality) Regulations set the standards required to produce this. They explain, in detail, the levels of certain characteristics, elements and substances that are allowed in drinking water to protect public health, and how much of each substance should be in the water supply.

Usually the limit is a maximum level but occasionally a minimum value is also set (for example, the pH, or acidity levels) – this permissible level is known as the Prescribed Concentration or Value (PCV).


How we test water quality

We regularly sample drinking water to monitor its quality at our water supply works, service reservoirs and customer taps.

We also continuously monitor some water quality measures at our water supply works. Hundreds of thousands of samples are analysed across our region each year to comply with the water quality standards. We also carry out checks every time we do something that might affect water quality, for example when we’ve responded to a burst supply pipe or installed new water mains.

When we receive complaints about water quality we take samples – more than 99% of all samples we check comply with the water quality standards.


What we test for

We test for the parameters and substances listed below. The common units of measurement for the Prescribed Concentration or Value (PCV) of a substance are:

  • one milligram per litre (mg/l) is one part per million
  • one microgram per litre (ug/l) is one part per billion or thousand million
  • one nanogram per litre (ng/l) is one part in a million million
  • NTU = Nephelometric Turbidity Units (to measure turbidity – the level of cloudiness caused by particles in water)
  • °H = Degree Hazen (for colour measurement)
  • uS/cm – microSiemens/centimetre (to measure conductivity)
  • E. coli or enterococci = number per 100 ml.

If water seeps through the ground until it reaches rock that it can’t penetrate, it forms underground aquifers. The water in these aquifers is called ‘groundwater’ and is often very high quality, because as it seeps through the ground, many contaminants are naturally filtered out. It also tends to be hard water.

Water supplied for drinking may contain some of the substances listed below. These substances can:

  • occur naturally in raw (untreated) water
  • enter water during the treatment process
  • be deliberately added to safeguard public health.


If a test fails

If a sample fails a test, it doesn’t necessarily mean the water is unfit to drink. Sometimes the water in mains, pipes and neighbouring properties meets all the required standards, but there's an issue with the householder’s plumbing system. In this case, we'll let the customer know and give them advice on what action to take next.

We have specialist teams that deal with all samples that fail, and we quickly record, investigate and act upon any water quality failures to make sure any problems are resolved as soon as possible. We also report failures to the Drinking Water Inspectorate, health professionals (Public Health England) and the local environmental health services.


Frequently asked questions

pH (Hydrogen ion concentration)

This is a scientific term used to describe the degree of acidity or alkalinity. We need to control the pH of water because:

  • if it is too acidic then it may corrode metal pipes in the distribution system
  • if it is too alkaline it may form deposits in pipes and cause taste problems.

Regulations require the pH of drinking water to be in the range of 6.5-9.5.


The colour of drinking water is usually due to the presence of naturally occurring dissolved organic matter. However, colour may also be due to the presence of iron sediment caused by old cast iron mains in the water distribution network. High colour may be unacceptable to consumers on aesthetic grounds. See Fact Sheet 4 – Common issues with taste, odour and colour for more information.

The PCV for colour is 20 degrees Hazen (°H).


Turbidity is caused by very fine particles suspended in water. Turbidity is closely monitored during the treatment process.

Sometimes, water coming out of the tap has a milky / white appearance. This is usually caused by excess air dissolving in the water (similar to opening a bottle of fizzy drink). It isn’t harmful and if water is left to stand for a few minutes, it will clear from the bottom up.

The PCV for turbidity at customers’ taps is 4.0 NTU.

Taste and smell

Occasionally customers complain to us about the taste and smell of their water. Quality control tests to measure the level of taste and odour are performed by a specialist panel.

The PCV for Taste and for Odour is dilution number 1 at 25oC.


Conductivity is a measure of the dissolved solids content of water and is often used as an indication of the presence of trace levels of dissolved mineral salts of calcium, magnesium and sodium.

The PCV for conductivity is 2500 uS/cm at 20oC.


Chlorine is added during treatment to disinfect water and make sure it’s free from harmful bacteria. When we add chlorine in strictly controlled amounts, not all of it is used up in the process. Some remains as ‘free chlorine’ to maintain microbiological quality as it passes through the distribution pipes.

No PCV is set for chlorine. We carefully calculate how much to add to make sure that once cleaned, water contains sufficient chlorine to keep it clean throughout the entirely of its journey through the network and retains small concentrations when it comes out of your taps for drinking.

E.coli and Enterococci

If present, these suggest a possible breach in the water supply system. An efficient treatment process will remove and kill any organisms present.

As bacteria can flourish in taps, be careful not to contaminate your drinking water tap by, for example, hanging a dishcloth over it. Be especially careful when washing food as bacteria can easily splash back on to the tap.

The PCV standards are 0 per 100 ml for E.coli and 0 per 100 ml for Enterococci.

Nitrate and nitrite

Nitrites and nitrates are two different molecules that are made up of both nitrogen and oxygen. The chemical difference between nitrites and nitrates is how many oxygen atoms each compound contains.

Normally only traces of these compounds are found in drinking water. However, mainly because of agricultural activity, nitrate levels can increase in raw untreated water. When this happens, we treat the water to remove nitrates.

Nitrate is used mainly in inorganic fertilisers but is also used in the production of explosives and glass-making. Nitrate occurs naturally in plants and is a key nutrient that helps them grow.

Sodium nitrite is used as a food preservative, especially in processed meats. Nitrite can form in drinking water distribution pipes due to the action of bacteria, where stagnation of nitrate-containing and oxygen-poor drinking water occurs.

  • The PCV for nitrite is 0.5 mg NO2/l
  • The PCV for nitrate is 50 mg NO3/l.


Chloride in drinking water originates from natural sources such as mineral deposits, and from tidal mixing of river and groundwater. It contributes to taste which may be unacceptable to consumers if the standard is exceeded.

The PCV for chloride is 250mgCl/l.


We don't add fluoride to water, but fluoride can be found naturally in raw water supplies at low levels.

The PCV for fluoride is 1.5mgF/l.


Copper occurs naturally and is normally found in low concentrations in drinking water. Issues with high levels of copper can be caused by domestic plumbing and fittings.

The PCV for copper is 2mgCu/l.


Iron is one of the most abundant metals on earth and is found naturally in surface and groundwater. After treatment it’s normally reduced to trace concentrations in drinking water. Increased levels can occur due to corrosion in old cast iron water mains sediment. Normally there’s no health risk associated with higher levels of iron except in the case of some specific medical conditions. However, iron can cause staining on domestic fittings and the water may look unacceptable to some people.

The PCV for iron is 200ugFe/l.


Manganese occurs naturally in water. High concentrations of manganese are unacceptable in drinking water, as they affect how the water looks and can be seen as purple or black spots on laundry.

The PCV for manganese is 50ugMn/l.


Aluminium can occur naturally in water, and aluminium compounds are used in the water treatment process to help remove impurities. Any aluminium salts added in controlled amounts during the treatment process are removed through the clarification and filtration processes. 

The PCV for aluminium is 200ugAl/l.


Lead isn’t normally present in water sources but significant concentrations can be found in drinking water if lead pipes, or copper pipes with lead joints, have been used in the domestic plumbing system.

The PCV for lead is 10ugPb/l.

Phosphate dosing (treatment to prevent lead dissolving in pipework) is installed at our water supply works. We sample and analyse water regularly at our water supply works, service reservoirs and at our customers’ taps – and when sampling specifically for lead, we take a first draw sample (first thing in the morning) and a flushed sample.

Find out more information about lead in water.

Trihalomethanes (THMs)

Trihalomethanes (THMs) principally occur in drinking water as trace by-products of the reaction chlorine has with naturally occurring, dissolved organic materials. In drinking water the significant measure is the sum of a number of compounds classed as THMs – the most common is chloroform.

The PCV for THMs is 100ug/l (sum of all compounds measured).

Other substances and compounds

In addition to the substances listed, we also test water for a wide range of compounds that include:

  • hydrocarbons
  • pesticides
  • herbicides
  • phenols
  • organic carbon.

We also carry out extensive monitoring of our supplies for cryptosporidium (a parasite that can cause diarrhoea) through sampling of raw and treated 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.

Particle removal

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.

Treating taste and odour

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.

Adjusting pH levels

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.

Disinfection (chlorine, UV and ozone)

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.

Preventing lead

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.