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Water mains - why we need to flush them?

Technician flushing a water main

Delivering high quality drinking water to our customers, every day of the year, means that that we need to carry out a variety of maintenance type jobs designed to keep our system in tip-top condition.

One of these jobs is to periodically flush some of our water mains especially if we believe that natural sediments may have built up inside them.

The following information tries to explain what we do and why we do it.

Flushing water mains

What are you doing?

A specialist team of technicians create plans to systematically clean all of the water mains in the target area. We open hydrants on the water mains, one at a time, across the area we are working in to flush the water through the mains at high speed.

This process stirs up and removes any sediment in the mains and by opening and closing valves we ensure every length of water main is flushed at the best possible flow rate.

Why do you do it?

The water entering our mains after treatment is of a very high quality, however, over time sediments can build up in the pipes; particularly if the water is slow moving.

If left alone – this could result in customers occasionally receiving discoloured water, particularly if there has been a surge in the flow (perhaps because of fire-fighting or a burst water main). Too much sediment can also affect the taste, clarity and colour of water.

Where do you flush?

Every year, our scientists and engineers use information gathered from our customers, from water samples and from historical data to develop a programme making sure that we flush the areas that need it most.

To find out if we are planning to flush the mains in your area, take a look in our planned investment area.

How long will it last?

The flushes are targeted at areas which can be quite small and will only take a few days, or can be much larger and therefore take a few weeks.

Most individual customers are only directly affected for a few hours each as we flush past their property, although we may need to do this on more than one day when working in an area.

How will I know when you are flushing in my area?

A couple of weeks before we’re due to start the work you will receive a letter with dates and information about the programme in your area.

Please note: the vast majority of customers are only likely to be affected for part of the day.

How much water is used?

The flushing programme is designed to ensure that we flush the sediment out of the mains effectively, using the least amount of water possible.

The amount of water used to flush a particular section of pipe depends on a lot of factors, such as water main size, system pressure, the amount of sediment in the main and the ability to safely dispose of the water. As a rule of thumb – we use twice the volume of any pipe when we flush it.

Where does the water go?

Because of the large amounts of water we need to flush to clean a water main, we usually allow the water to either soak through verges (to boost ground water levels) or we allow it to drain back in the drains and sewers before flowing back into rivers through our network of Water Recycling Centres.

How does flushing water mains fit in with Water Conservation?

We are dedicated to water conservation. Water is a limited resource that must be carefully managed to ensure we have enough for our growing population and environmental needs.

However, because flushing is critical to maintaining high-quality drinking water, an efficient flushing program is also essential.

Other reasons to carry out flushing

Dead end mains - We also flush water mains where there aren’t enough customers to keep the water fresh all the time. This can occur towards the end of a water main and is often in Cul-De-Sacs, at the end of rural lanes or at the edge of villages. These flushes...

  • Won’t stir up the rest of the water main
  • Don’t affect pressures in the network and so
  • Aren’t noticeable to our customers.

Our modern plastic mains require less (if any) flushing than our older iron mains. Flushing is an effective way of keeping some older mains in use – so helping to keep customer bills as low as possible.

New water mains can also need a flush.
On many housing estates, we have to lay the water mains before the houses are completed and, once the first property is sold, the mains needs to be commissioned.
This means that customers aren’t there to use enough water to keep the water in the main at its freshest – and so we carry out short flushes ourselves until the houses are sold and their new residents arrive.

Large main-laying schemes (transferring water between our large reservoirs) also require flushing. This can be for a number of reasons – but is often when we have finished laying the water mains and have sterilised them ready for connection.
Our final job is usually to give them a flush, to ensure all the chlorinated water (used to sterilise the main) has been removed, before bringing them into use for the first time.

How will I be affected?
Your tap water will be safe to drink but on the dates between the start and end of the flushing in your area:

  • Your water may become discoloured or cloudy.
  • Your water supply may be interrupted for short periods of time.
  • You may experience lower water pressure.

You can find out more here.


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