Water quality testing and standards
The drinking water we supply to you must meet strict legal quality standards which are set out in European and UK legislation. These standards are known as Prescribed Concentrations or Values (PCVs for short). We have produced a list of the standards and an explanation of their significance.
We monitor the quality of our water as it leaves our water treatment works and service reservoirs (a large tank where treated water is stored) to make sure that these standards are being met. We also measure the quality of drinking water as it reaches our customers. Last year we collected over half a million samples to make sure that the quality of your drinking water met the legal standards. At consumers' taps in 2011 99.95% of water tests met these standards.
Our area is divided into water supply zones. The water supplied to a zone is usually from one particular water treatment works or service reservoir. Water samples are taken at addresses selected at random from customers' taps within a zone to monitor water quality in the zone as a whole.
The samples are tested at our laboratories and the results compared with the required quality standards. All the test results are recorded on the Drinking Water Register.
The Drinking Water Register – what does it mean?
The Drinking Water Register is split into a number of columns. The information below may help you understand the data for your area.
Parameter (PCV or indicator parameter)
A parameter is a substance or physical property that can be measured in water. In most cases, each parameter has a quality standard which must be met. Examples of parameters are lead, aluminium, iron and manganese.
Minimum, average and maximum
The next three columns show a summary of the results from the tests. The lowest, average and highest concentrations or values measured are shown. A '<' symbol shows that the value measured was below measurable limits. The average is calculated by adding up all the results and dividing by the total number of tests for that parameter.
The concentration or values are expressed in different units. Most are in milligrams per litre (mg/l) or micrograms per litre (µg/l). These are very small amounts. One mg/l is one part in 1,000,000 parts of water (or 0.0001%). One µg/l is one part in 1,000,000,000 parts of water (or 0.0000001%).
This column shows the percentage of tests which fail the standard for each parameter. This is shown as a percentage of the total number of tests for that parameter. For example, 2% shows that 1 sample in 50 failed. In most cases you will see a value of 0.00% which shows that the water has met the standard every time. We investigate all failures thoroughly and take action to sort out any problems.
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