factsheet 2
water quality
standards explained
At Scottish Water, it is our duty to
where the water quality standards
ensure that there is a safe and clean
come from
supply of high quality water for
In Scotland, The Water Supply (Water Quality) (Scotland)
everyone to use. Essentially, all of our
Regulations 2001 ensures the production of an
drinking water comes from rain.
adequate and high quality supply of water. These
When it rains, water flows into
regulations detail the acceptable levels of certain
streams, rivers, lochs and reservoirs.
characteristics, elements and substances allowed
The vast majority of water supplied in
in drinking water. Usually this is a maximum allowed
Scotland comes from these sources.
level but occasionally a minimum is also set (e.g. pH).
This permissible level is known as the Prescribed
Water that seeps through the ground
Concentration or Value (PCV).
until it reaches rocks which it cannot
pass through, and then forms pools,
where we sample
is called ground water. This is often
Water is sampled regularly at our treatment works,
very pure: as the rain seeps through
service reservoirs and at our customers’ taps to monitor
the ground, pollutants are often
the quality of the drinking water. In addition to this,
filtered out naturally.
some water quality parameters are continuously
monitored at major treatment works. Across Scotland
Water supplied for drinking
over 350,000 laboratory tests are carried out on water
may contain some of the substances
listed in this factsheet.
samples each year for regulatory purposes. Many more
samples are taken by staff for operational reasons (e.g.
These substances can:
bursts, new mains, complaints). The percentage of all
• occur naturally in raw water
regulatory samples complying with the relevant
• result from treatment processes
standards in Scotland is over 99%.
• be introduced to safeguard public health
This factsheet provides you with
what happens if a test fails
information on:
If a sample fails a test, this does not necessarily mean
• where the water quality standards come from
the water is unfit to drink. Sometimes the water in our
• where we sample
mains or pipes and in the neighbouring properties is
• what happens if a test fails
good, but the failure is caused by the householder’s
• units of measurement
own plumbing system. However, we take all failures of
• individual parameters/substances
these standards very seriously and these are dealt with
• how to contact us
by a team of specialists. All failures are recorded,
investigated and action is taken to resolve the problem.
All PCV failures are also reported externally to the
Drinking Water Quality Regulator (DWQR), respective
health boards, environmental health departments and
Health Protection Scotland (HPS).
water quality standards explained
factsheet 2
units of measurement
turbidity
The units of measurement used in this factsheet are
Turbidity is caused by very fine insoluble materials that
as follows:
may be present in water. Levels are closely monitored
during the treatment processes.
• 1 milligram per litre (mg/l) is one part per million
• 1 microgram per litre (ug/l) is 1 part per billion or
• PCV at the customer’s tap is 4.0 NTU
thousand million
• 1 nanogram per litre (ng/l) is 1 part per million million
odour and taste
• NTU – Nephelometric turbidity units (for turbidity
Customer complaints quite often relate to taste and
measurement)
odour. Quality control tests are carried out to measure
• mg/l Pt/Co - Milligram per litre (mg/l), Pt/Co -
the level of taste and odour and are performed by a
Platinum-cobalt units Standard (for colour
specialist testing panel.
measurement)
• uS/cm – micro siemens/centimetre (for conductivity
For more information please see Scottish Water
measurement)
Factsheet 4 Colour, taste and odour.
• E-Coli or Enterococci - 0 per 100 ml
• PCV for each = Dilution Number 3 at 25˚C
individual parameters/substances
conductivity
pH
Conductivity is proportional to the dissolved solids
This is a scientific term used to describe the acidity or
content of the water and is often used as an indication
alkalinity of a fluid. We need to control the pH of water
of the presence of dissolved minerals, such as calcium,
because:
magnesium and sodium.
• if water is too acidic then it may corrode metal pipes
• PCV is 2500 uS/cm at 20˚C
in the distribution system
• if water is too alkaline it may cause deposits to form
hardness
in the pipes
Hardness is normally caused by rocks through which the
The standard is to keep water pH levels in the
water has passed. In Scotland most of our water is soft.
6.5-9.5 range.
Hardness is a measure of the calcium and magnesium
concentrations in water. Hardness means you may have
colour
to use more soap when washing as hard water lathers
The colour of drinking water is usually dependent on the
less than soft water. It has no adverse effects on health.
presence of naturally occurring dissolved organic matter.
There is no standard specified in the regulations.
For example, the higher the peat content of a
catchment, the higher the colour. However, colour may
chlorine
also be due to the presence of iron contributed by old
Chlorine is added to water to ensure water is free from
cast iron mains. For more information please see
bacteria. When chlorine is added not all of it is used up
Scottish Water
Factsheet 4 colour, taste and odour
in the process. Some remains as ‘free chlorine’ to make
explained.
sure the water remains safe as it passes through the
• PCV for colour is 20 mg/l Pt/Co.
distribution system. For more information please see
Scottish Water
Factsheet 5 chlorine explained, and
Sometimes the water coming out of the tap has a milky
Scottish Water
Factsheet 6 chloramination
or cloudy appearance, which is usually caused by excess
explained.
air dissolved in the water. This is not harmful and if the
No PCV is prescribed for chlorine in the Regulations and
water is left to stand for a few minutes it will clear from
these levels are set to ensure that a small concentration
the bottom upwards.
remains at the end of the distribution system.
water quality standards explained
factsheet 2
E-coli and Enterococci
copper
If present, these indicate a possible breach in the
Copper can occur naturally in some water sources, and is
integrity of the water supply system. An effective
normally found in low concentrations in drinking water.
treatment process will kill any organisms present.
• PCV = 2 mg/l
As bacteria can flourish in contaminated taps, be careful
not to contaminate your drinking water tap e.g. when
iron
washing food or by hanging a dishcloth over the tap.
This is one of the most abundant metals found naturally in
PCV standards are:
surface and ground waters. After treatment it is normally
• 0 per 100 ml for E-Coli
reduced to trace concentrations in drinking water.
• 0 per 100 ml for Enterococci
Increased levels can occur due to the corrosion of old cast
iron water mains. There is no known health risk associated
coliforms
with high iron concentrations, but staining can occur and
the water may be unacceptable to customers as it may
These are naturally present in the environment. Their
appear coloured.
presence may indicate a possible breach in the integrity
of the supply system or contamination from the kitchen
• PCV = 200 ug Fe/l
sink or taps.
manganese
nitrite and nitrate
Manganese occurs naturally in water. High concentrations
Normally only trace amounts of these compounds are
of manganese are unacceptable to customers as the
found in water.
water may appear coloured.
• PCV for nitrite = 0.5 mg NO2/l
• PCV = 50 ug Mn/l
• PCV for nitrate = 50 mg NO3/l
chloride
Chloride in water originates from natural sources such
as mineral deposits. It can contribute to taste which
may be unacceptable to customers if the standard is
exceeded.
• PCV = 250 mg Cl/l
fluoride
Scottish Water does not add fluoride to any water
supply in Scotland. Fluoride can be found naturally
in some raw water supplies at low levels.
• PCV = 1.5 mg F/l
sulphate
Sulphate occurs naturally in water and originates
from mineral deposits. High concentrations may give
rise to taste problems and in the long term, damage
pipe work.
• PCV = 250 mg SO4/l
water quality standards explained
factsheet 2
aluminium
trihalomethanes (THMs)
Aluminium can occur naturally in water within certain
THMs occur in drinking water as by-products of the
catchments. However, aluminium compounds are used
reaction of chlorine with naturally occurring dissolved
in the treatment process to help remove impurities.
organic materials. In drinking water only four compounds
Any aluminium compounds added during the
out of the group of THMs have health significance, the
treatment process are removed before the final
most common of which is chloroform. The PCV is based
treated water leaves the treatment works.
on the sum of the concentrations of all four
constituents.
• PCV = 200 ug Al/l
• PCV = 100 ug/l
sodium
other substances
Sodium occurs naturally in trace amounts in water.
High concentrations may impart a level of taste that
In addition to those listed and explained above, we also
is unacceptable to customers.
test for substances such as hydrocarbons, pesticides and
herbicides, phenols and organic carbon. We also carry out
• PCV = 200 mg Na/l
extensive monitoring of our supplies for cryptosporidium
through sampling of raw and final treated water.
lead
Lead is not normally present in water sources, but
significant concentrations may be present at customers’
taps if lead or copper pipes with lead joints have been
used in the plumbing system. For more information
please see Scottish Water
Factsheet 7 lead
explained.
• PCV = 25 ug/l
This will reduce to 10 ug/l from 2013.
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 WQ2 08/08
Or in writing at
Scottish Water, PO Box 8855,
We record all calls for quality and training purposes.
Edinburgh, EH10 6YQ
