Legionnaires Disease - The control of Legionella
Bacteria in Water Systems
Identify the risk : Risk Assessment.
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Is there a reasonably foreseeable
potential for harm to health from exposure to legionella
containing aerosols?
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What measures are required to prevent,
or adequately, the risk?
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Are legionella bacteria present?
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Are conditions suitable to growth
of legionella (temperature of 20 - 45C Source of
nutrients)
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Can an aerosol be generated?
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Can people be exposed to the aerosol?
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Review the risk assessment regularly
(at least every 2 years)
Manage the risk: Management responsibilities
/ training / competence.
- a person must be appointed to take managerial responsibility
and to provide supervision for the implementation
of the control measures.
- Persons carrying out the risk assessment, and drawing
up and implementing the control measures, must be
competent to do so.
- Staff responsibilities and lines of communication
must be properly defined and clearly documented.
Preventing or controlling the risk : Implementing
control measures.
There must be a written scheme for controlling the
risk from exposure.
The scheme should include:
- Up-to-date schematic of the plant
- Description of the correct and safe operation of
the plant.
- The precautions to be taken
- Checks to ensure the efficiency of the scheme.
- Remedial action to be taken if the scheme is show
to be ineffective.
Typical precautions include:
- Controlling the generation and dissemination of
aerosols.
- Avoiding water temperature and conditions favourable
for legionella
- Avoiding water stagnation
- Avoiding system materials which support microbial
growth
- Maintaining the cleanliness of the system and the
water in it.
- Using appropriate water treatment
Monitoring and routine of the system should include:
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Checking the condition and performance of the plant.
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Inspecting accessible parts of the system for
damage and signs of contamination.
- Monitoring to ensure the treatment regime continues
to control to the required standard.
Record Keeping
Good record keeping is a vital part of the legionella
control process. The responsible person must ensure
that adequate records are kept which detail:
- The person(s) responsible for conducting the Risk
Assessment and managing the written scheme.
- The significant findings of the risk assessment
- The written scheme, and details of its implementation
- The results and dates of inspections, monitoring,
tests and checks.
Guidance on controlling the risk from exposure
to Legionella Bacteria.
Cooling systems (incorporating cooling towers
or evaporative condensers)
These systems must be registered with the local authority
under the notification of cooling towers and evaporative
condensers regulations 1999
Cooling systems must be:
- Properly commissioned to ensure that they operate
correctly.
- Kept in regular use wherever possible.
- Kept filled with biocide and inhibitor treated water
if out of use for longer than 1 month.
- Properly maintained according to a comprehensive
schedule.
Typical Industrial Cooling System
Treatment programmes for cooling systems
must encompass control of:
- Microbiological activity - by using biocides.
- Corrosion - by using anodic and cathodic inhibitors.
- Scaling - by using preventatives or pre-treatment.
- Fouling - by using dispersants or side stream filtration.
None of these four processes can be neglected
- each impacts upon the other.
Monitoring of the cooling system should
encompass routine tests of:
- The composition of the make-up and system water.
- The reserves of water treatment chemicals
- Microbiological tests (general fouling and legionella
bacteria)
Microbiological monitoring action levels
for cooling systems are:
Testing for legionella should be conducted
at least quarterly during the year.
Cleaning and disinfection of the cooling
systems should be done at least twice a year, or more
frequently if the system is particularly prone to fouling.
This important maintenance activity incorporates
pre-cleaning deienfection, manual cleaning and post-cleaning
disenfection stages.
Free chlorine is the disinfection of choice
- though other oxidizing biocides such as bromine or
chlorine dioxide can be used.
Hot and Cold Water Services.
Design and construction considerations
should ensure that:
- Systems are not over-sized
- Systems comply with the water supply (water fittings)
regulations
- Materials which support microbial growth are not
used.
- Low-corrosion materials are used where possible.
- Water tanks are fitted with adequate lids.
- Multiple linked tanks are avoided (unequal flow
rates)
- Showers which would be used infrequently are not
fitted.
- Calorifiers are sized to meet fluctuating demands
adequately.
- Time-controlled shunt pumps are fitted to large
calorifiers.
- Hot water distribution pipes are insulated.
- Trace heating is considered for non-circulatory
hot water pipework.
- Low use cold outlets are installed upstream of high
use outlets
- The volume of cold water stored is minimised.
Typical gravity system with recirculation
Management of hot and cold water services
should ensure that:
- New systems are commissioned correctly
- Existing systems are re-commissioned correctly after
being out of use.
- Water undertakers are warned of excessive incoming
mains temperatures.
- Temperature stratification in calorifiers is avoided.
- Infrequently used outlets are regularly flushed.
- Treatment and control programmes should ensure that:
- Hot and cold water systems are kept clean
- Suitable temperature regimes are used to control
Legionella growth
- Store hot water at 60C
- Hot water not <50C at outlets
- Cold water stored at not >20C
- Biocides, if used, are dosed under strictly controlled
conditions (chlorine dioxide or ionisation can be
used as on-line biocides)
Monitoring of hot and cold water services
should include:
- Checks for temperature, water demand and cleanliness
- Details of who carried out the checks and when
- Schematics of the hot and cold water systems.
- Careful record keeping
- Annual checks of systems and maintenance schedules.
Testing hot and cold water systems for
the presence of general fouling bacteria is unnecessary
because the water supplied is of potable quality.
Legionella testing of these systems should
be carried out when:
- Temperatures are reduced because biocides are in
use.
- Temperature or biocide regimes are shown to be inadequate
- Legionellosis is suspected or has been identified.
- "At risk" communities are involved, e.g
hospital wards.
Cleaning and disinfection of hot and cold
water services should be carried out:
- When routine inspection shows it to be necessary
- When the system has been substantially altered
- Following a suspected or actual case of legionellosis.
Disinfection of hot systems involves either:
Thermal disinfection
Raise the temperature of the water in
the calorifier to at least 60C and curculate it through
the system for not less than 1 hour.
Chemical disinfection
Chlorinate the water in the cold storage
tank to 20 50 mg/L free residual chlorine, then allow
it to flow to all parts of the system for several hours
Other risk systems
There are other types of systems which
may harbour legionella bacteria and disseminate them
in the form of aerosols, for example, spa and whirlpool
baths and humidifiers.
The general principle of legionella control
as applied to cooling systems and hot and cold water
services are also relivent to these other risk systems.
A risk assessment of these systems will reveal the potential
to harm health and indicate the control measures which
should be implemented.
For spa and whirlpool baths, careful attention to design,
maintenance and cleaning is essential. Regular water
treatment (disinfection) is required to prevent or control
the risk of legionella.
Atomising humidifiers can become heavily
contaminated. Unless they can be regularly and regorously
cleaned and disenfected, it may be more prudent to replace
them with humidifiers which do not create a spray, i.e
steam humidifiers..
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