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| The
Inlet Pipes |
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| Under much
of Three Valleys Water's area there is a layer
of porous chalk called an aquifer.
As the rainfall filters through the ground,
it reaches this layer. The aquifer
traps the rainwater in its many air pockets,
which gradually fill up to saturation, like
a sponge in a bath. |
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| To get the
water out of this aquifer, water companies
drill down about 100 metres, until they have
reached some depth into the waterlogged chalk.
This hole is called a borehole and
is very much like a straw in a glass sucking
out the underground water using large pumps.
The pump that pushes the water up and out
is under the ground, in the borehole. |
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| Although
the water has already filtered through the
ground to get to the aquifer, it may not be
safe to drink. The water must be cleaned and
disinfected to remove any herbicides,
pesticides, bacteria, viruses and dirt. |
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| At the top
of the borehole is a pumping station. The
water gets pumped from here to a water treatment
works through large inlet pipes. As water
enters the water treatment works, it is injected
with a gas called ozone. |
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| Ozone |
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| Ozone is
added to the water as it kills 99% of the
bacteria and breaks up any herbicides and
pesticides that can be found in raw water. |
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| It is made
by machines called ozonators. These create
ozone gas from another gas - oxygen. |
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| The oxygen
is put into glass tubes in the ozonator and
electricity (around 11,000 volts) is passed
through. The electricity changes the oxygen
into ozone. |
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| The water
has ozone pumped into it as it passes through
the two inlet pipes and then is kept in large
contact tanks for seven minutes. This
gives the ozone time to be absorbed into the
water where it kills 99% of the bacteria and
breaks up any herbicides and pesticides. |
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| Any excess
ozone not absorbed into the water is destroyed
by a machine called a destructor. |
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| The ozone
that has been absorbed into the water is neutralised
using a chemical called sodium bisulphate.
This makes the water safe again and ready
for the next stage of cleaning, called filtration. |
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| Filtration |
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| Although
the ozone has broken down the impurities,
they are still in the water and need to be
removed. The quickest and most efficient way
to do this is to put the water in a large
tank and filter it through Granular
Activated Carbon (GAC). |
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| GAC is like
a very rough, black charcoal sand and has
a very large surface area (½ teaspoon of GAC
has about the equivalent surface area
of a football pitch!). This large surface
area makes GAC a very effective filter. The
GAC is put in large filter beds, each
of which has ten cubic metres of GAC at the
bottom. As the water filters down through
the GAC, the large surface area picks up all
the tiny bits such as any dirt or pesticides
that are suspended or dissolved
in the water. |
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| This means
the GAC needs to be cleaned regularly - it
is washed approximately once every 12 days.
Every five years it is sent to be regenerated
where any impurities are burnt off. |
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| As the water
filters down through the filter beds it collects
in contact basins where chlorine is added. |
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| Super
Chlorination |
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| Excess chlorine
is added deliberately (this is called super
chlorination) and is left in the water
for 20 minutes to kill any remaining bacteria.
After this time, chlorine levels are reduced
by adding a chemical called sodium bisulphate
(the same chemical that is used in ozonation). |
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| A little
bit of chlorine is left in to make absolutely
sure that even after the water has gone all
the way from the reservoir to your home, it
is still absolutely clean and safe to drink.
The water then goes to the final treatment. |
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| Ultrafiltration
Membrane Plant |
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| We are leading
the way in membrane treatment and by March
2005, two-thirds of our groundwater supplies
will be covered by this form of treatment. |
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| The Ultrafiltration
Membrane Plant is state of the art technology.
The plant at Clay Lane in Bushey was officially
opened in June 2001. It works by filtering
the water through thousands of porous straws.
The pores in the filter are smaller than the
pores in your skin. |
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| Storage/Delivery |
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| After disinfection,
the treated water, which is amongst the cleanest
drinking water in the world, goes direct to
the taps in our customers' homes in the surrounding
area. It is also stored in water reservoirs
on the site. |
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| The quality
of our drinking water is carefully checked
and assessed by thorough monitoring at every
stage from the source, right through to your
drinking water tap. Every week we conduct
about 6,000 tests on water from treatment
works, storage reservoirs and customers own
taps, to ensure it is of a very high standard
and meets the rigorous standards set. |
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