factsheet 3
water treatment
explained
Your mains water supply is treated
different sources of raw water
using various treatment processes to
The treatment process that we use depends upon the
remove any impurities from the raw
type of water source and the quality of the raw water
water. The water that we supply is
available in a particular area.
required to meet the standards laid
down under the Water Supply (Water
Water from springs and boreholes is generally of a
Quality) (Scotland) Regulations 2001.
higher quality and may only need simple filtering
followed by disinfection.
This factsheet provides you with
information on:
Water from upland sources such as lochs, reservoirs
• different sources of raw water
and streams, will normally require some sort of
• where we sample
treatment process.
• how we treat your water
Water from lowland sources such as rivers, may require
• how to contact us
more extensive treatment as this is more likely to
contain some pollutants.
Scottish Water will select the appropriate form of
treatment based on the raw water quality data at
each source.
We monitor the quality of our raw water sources to
ensure their integrity and security. For more information
on this process please see Scottish Water
Factsheet 1
your water explained.
where we sample
Water is sampled regularly at our treatment works,
service reservoirs and at our customers’ taps to monitor
the quality of the drinking water. In addition to this,
some water quality parameters are continuously
monitored at major treatment works. Across Scotland
over 350,000 laboratory tests are carried out on water
samples each year for regulatory purposes. Many more
samples are taken by staff for operational reasons
(e.g. bursts, new mains, complaints). The percentage
of all regulatory samples complying with the relevant
standards in Scotland is over 99%.
water treatment explained
factsheet 3
how we treat your water
Rapid Gravity Filtration – This process removes any
remaining impurities through direct filtration using
As the treatment of your water depends upon its
either sand or a mixture consisting of anthracite (coal)
source, our treatment works may use one or more of
and sand.
the following processes:
1. storage
Please note that in the absence of coagulation, raw
water will pass through the sand filters untreated and
Surface water is stored in reservoirs to provide a
impurities will not be removed.
continuous supply to meet demand throughout
the year.
Slow Sand Filtration – This process is used as an
alternative to coagulation, clarification and rapid gravity
2. screening
filtration. With the help of gravity, the water passes
Water is passed through mesh screens on leaving the
slowly through a large sand filter. Tiny natural organisms
reservoir or borehole. These screens remove debris, such
in the filters break down any impurities by forming a
as leaves, weeds and sticks which come in the raw water
layer at the top of the filter surface. The sand needs
from the catchment area.
periodic skimming and washing. This type of process is
3. particle removal
not effective in removing colour in the water.
Coagulation – Colloids and other impurities in the raw
Membranes – Membrane filtration involves the filtering
water need to be removed. Coagulants like alum
of raw water under very high pressure through
(Aluminium Sulphate) are used to help bind the
prefabricated membranes. This process is more expensive,
impurities together to form particles. This process is
but the results give a better quality of water in situations
called solid-liquid separation, and requires precise
where the raw water source is of a poorer quality.
dosing (measurement in milligrams per litre) and pH
control along with good mixing to be effective.
Sludge – This is the material left over from the
clarification and filtration processes.This is disposed of
Flocculation – By adding chemicals called polymers,
either to landfill, or by passing it directly to the sewer
the particles formed during the process of coagulation
where it can be treated in a waste water treatment works.
are further increased in size for effective removal. The
choice of the polymers used may differ with the quality
4. treating taste and odour
of the raw water.
Granular Activated Carbon, commonly called GAC, is
used to absorb and remove many tastes and odours
Sedimentation (also called Clarification) – The
particles created from the coagulation and flocculation
present in water supplies. This is an expensive process to
processes then form themselves into a suspended mass
maintain, and is more often used during the summer
of material called a sludge blanket. This blanket is
months when taste and odour problems are more likely
removed and water is then allowed to settle in tanks.
to occur.
The water is then passed onto filters for further
5. pH adjustment
purification.
pH is a scientific term used to describe the relative
Dissolved Air Flotation (DAF) – This is a different
acidity or alkalinity of a substance. It is necessary for us
type of clarification in which the impurities are floated
to control the pH level of drinking water. If the water is
to the surface, instead of being allowed to settle at the
too acidic it may corrode metal pipes, and if it is too
bottom. A mass of fine bubbles is created by saturating
alkaline it may cause deposits to form inside the pipes.
the water with air under high pressure, and these are
then released and mixed with the flocculated raw water
to help float the impurities to the surface. These
impurities gradually collect to form a layer on the top
which is skimmed at regular intervals into a separate
channel, whilst the remaining water underneath, is
allowed to pass onto filters.
water treatment explained
factsheet 3
6. disinfection
Disinfection is vital to ensure that water-borne diseases
are eliminated, and that the drinking water that we
supply to you meets the Water Supply (Water Quality)
(Scotland) Regulations 2001. Chlorine is predominantly
used for disinfection either in gas or liquid form.
However, chloramination (the addition of small
quantities of ammonia mixed with chlorine) is also now
being used in some areas of Scotland as it lasts longer in
the supply system with less obvious taste and odour.
Other less frequently used processes include the use of
UV (ultra violet light) and Ozone for disinfection.
For more information please see Scottish Water
Factsheet 5 chlorine explained, and Scottish Water
Factsheet 6 chloramination explained.
7. lead in water
Phosphate may be added at our water treatment works
to help prevent lead being absorbed from lead service
pipes at customers’ properties.
For more information please see Scottish Water
Factsheet 7 lead explained.
8. fluoride in water
Scottish Water does not add fluoride to any water
supply in Scotland. Low levels of fluoride are found
naturally in some raw water supplies.
water treatment explained
factsheet 3
how to contact us
Details of water quality testing in your area can be
Alternative formats of this leaflet can be made
obtained by contacting us and asking to speak to one
available free of charge. Textphone users please call
of our specialists in the Public Health team.
0845 603 8855. For information on Braille, large print,
If you require more information on our services, please
audio tapes and a variety of languages, please call
contact us:
0845 606 8855.
By phone on our
Customer Helpline 0845 601 8855
Please quote this reference code when contacting us:
On the web at
www.scottishwater.co.uk
SWFact WT2 08/08
Or in writing at
Scottish Water, PO Box 8855,
We record all calls for quality and training purposes.
Edinburgh, EH10 6YQ
