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Bacteriological Contamination of Drinking Water
Private water supplies can become contaminated by bacteria or viruses which can cause illness, such as gastroenteritis and dysentery. Hence, it is normal to test water for an indicator bacteria called Escherichia Coli (E.Coli).
This bacteria, from the coliform family, is found in the faeces of humans, warm blooded animals and birds. It is
rarely found from any other source. The water is also tested for total coliform bacteria. These bacteria may be
from an animal source, but can also be found in soil, rotting plant matter and on rusty surfaces, such as well lids.
Satisfactory results under the Private Water Suppliers Regulations 1991 is:
Total Coliforms
0 per 100 ml
0 per 100 ml
If coliform bacteria and especially E.Coli are found in a supply it indicates the presence of harmful organisms from
a human or animal source. As a short term measure it is advisable to boil all water before consuming it. A longer
term method is to provide permanent treatment to your private water supply.
Types of Treatment
There are two methods of treatment which can be adopted, namely, structural improvement works to the source
and disinfection of the water at the point of consumption.
Structural Improvement Works
Contamination may be the result of incorrect construction of a well, borehole or other source and you are advised
to make the following improvements where appropriate:-
The spring or well should be protected by a
fence at least 4 metres from the source to
keep out grazing animals and chickens.
The lid of well chambers and storage
chambers should be raised above ground
level and watertight to prevent the ingress of
ground water run-off.
All cracks and gaps in the chamber should
be repaired. Essential openings such as
overflows should be protected with a fine
mesh to keep out birds, animals and insects.
Spring Source - Schematic
access cover
Cut off ditch
To supply
Permeable wall
Coarse filter or
Plants, whose roots could cause structural
damage, should be removed and if they have
broken through brickwork, this should be
made good.
Overflow pipes should not be linked to other drainage pipes, to prevent back syphonage or contamination if
drains are blocked.
Fibreglass lids should not be used because they are transparent to light and lead to the growth of algae.
Rusty components such as lids and pipes may make and existing problem worse by encouraging the
growth and survival of bacteria. Where practical rusted ironworks should be replaced. In some cases
components can be treated with rust proof coatings.
Indoor storage tanks should also be protected from contamination by mice, bats and other animals through
the application of an airtight lid.
CM/CAB/April 2003
Disinfection of Water
It may not be possible to ensure a wholesome supply by structural improvements alone. In this case a disinfection
unit will be required. Special arrangements may be applicable in the case of dairy farms as detailed below.
Ultra Violet Treatment
Ultra violet light is a powerful disinfectant. The light passes through the water leaving no residue to affect the taste
or odour of the supply. The unit should be installed inside a building, as it is prone to frost damage. Hence this
method is more appropriate to smaller supplies. The unit should be installed such that it covers all the taps used
for drinking, food preparation and for cleaning teeth and washing hands. It is also preferable to site the unit after
any storage tanks which could be contaminated.
The efficiency of the unit will depend on the chemical quality of the supply. If the water contains suspended solids,
these could act as a shield to protect the bacteria from ultra violet light. All suspended solids should therefore be
removed prior to treatment using a 50 micron prefilter. Certain organic chemicals that give the water a yellow
colour can also absorb ultra violet light. This will reduce the effective dose of the unit. This problem may be dealt
with by either increasing the dose or removing the organic chemical’s using a graphite cartridge system. (a more
detailed fact sheet is available).
This is a chemical disinfectant which leaves a residue to continue to kill bacteria up to the tap. There are various
available methods of adding chlorine to a water supply.
Electric dosing systems are used to add chlorine in the form of sodium or calcium hypochlorite to water supply in a
controlled manner. In order to achieve effective disinfection, a concentration of 0.2 to 0.5 mg chlorine per litre of
water is recommended by the World Health Organisation. This is also the level of chlorine found in mains water. A
minimum contact time of 30 minutes is required for the chlorine to be effective, hence the dosing unit should be
installed as near to the source as possible in a purpose built housing. The dosing unit is ideal for shared supplies
as only one unit need be installed. Some pre-treatment may also be required, eg for removal of suspended
sediment or organic chemicals.
A solution of sodium hypochlorite, made up to a fixed concentration with well water, is often used by dairy farms to
wash down the dairy parlour and milking equipment.
Pellets of calcium hypochlorite placed in a hopper can be fixed inside a storage chamber such that the chemical
slowly dissolves. This method is not ideal as the tablets need to be replaced frequently, but is useful where a
source of electricity is available.
Bacteriological Filters
Filters made from porous ceramic can be used to remove bacteria and viruses. The filter cartridge needs to be
replaced every 18 months. As the units reduce water pressure they are normally fitted at a single tap. It is
therefore, preferable to install an ultra violet which can treat all the taps in the house. These filters are useful where
a source of electricity is not available or where pathogens have been identified are tolerant of chlorine and ultra
violet light.
Silver can act as a disinfectant and is added to some types of filter to discourage bacteria from growing in the unit.
As silver requires a long contact time to be effective it is not appropriate to treat bacteriological contaminated
drinking water. These systems should not be used in place of ultra violet or chlorine.
CM/CAB/April 2003