What standards does 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'.

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.


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.


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.


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.

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.

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