Legionella - the need for vigilance
25 January 2013
An employer is responsible for identifying and assessing the risk of exposure to legionella bacteria from work activities and water systems, and for defining any precautionary measures.Neil Brown, technical director,Hygien
Legionnaire's disease is a potentially fatal infection, normally contracted by inhaling fine water droplets containing legionella pneumophila, causing a localised infection in the lungs. Legionella protects itself from the body's other natural defences by hiding within macrophages - white blood cells that normally attack invading bacteria.
This allows it to grow quickly, with severe untreated infections causing pneumonia.
Most people do not develop Legionnaire's disease, but around 12% of identified cases are fatal. However, if identified quickly enough, Legionnaire's disease can be treated effectively with antibiotics. Legionella needs food, warmth and somewhere to live; water systems can offer all of these. Any equipment producing a fine spray or aerosol adds an extra bonus, allowing the bacterium to travel.
It is a legal requirement for water system operators to keep them clean and demonstrate that they have been treated with suitable disinfectants.
This is usually required every six months, with an L8 certificate - referring to the HSE legionella control code of practice - issued by the cleaning contractor.
Ideally, a survey of all water systems should be undertaken, to include an asset register of associated plant, pumps, strainers and so on, with up-to-date drawings or diagrams. It can then be determined which components present a potential risk. Employers should consult employees on measures and actions taken.
If the assessment shows a foreseeable risk, the employer should appoint someone to take managerial responsibility and provide supervision for the implementation of precautions. This individual should know the potential sources of risk; immediate measures to be adopted, and measures to ensure controls remain effective.
Where it is impossible to avoid using systems posing the risk, a written scheme for controlling it should be implemented. This should include precautions to be taken, checks to ensure the control scheme's efficacy, and planned remedial action if the scheme is found to be ineffective.
The findings of the risk assessment, the written scheme, and results of any monitoring, inspection, tests or checks undertaken, with dates, should be included in records which should be retained and updated. Systems with an identified risk should be regularly drained, flushed out and disinfected.
Cooling towers The warm environment and high organic debris levels that can accumulate in cooling towers are ideal for rapid bacterial growth. A build-up of biofilm will also encourage growth of Legionella bacteria. The correct biocidal treatment regime will ensure bacterial populations remain under control.
The 'fill' - the trellis-type layers which permit air transfer but minimise the risk of water droplets escaping - must be removed and thoroughly cleaned and disinfected, while the tower's insides must be cleaned and disinfected. Treatment success depends on the compatibility of all chemical components used, and adherence to recommended application, monitoring and control procedures.However, it must be remembered that an operating cooling system is subject to unpredictable recontamination both by legionella and sources of nutrients. Regular microbiological testing will ensure the biocide programme remains effective. The tower's operations manual should include a detailed maintenance schedule, listing time intervals when system plant and water should be checked, inspected, overhauled or cleaned.
A typical cooling system requires weekly inspection for damage and soiling, and chemical checks of circulating water to monitor pH, hardness, temperature and the oxidising biocide level. It also requires weekly micro-checks to monitor general microbiological growth. A quarterly Legionella check is required, with a thorough clean of the tower interior, sump and fill twice yearly.
Each check must be meticulously logged, allowing observation of any trends to promote better tower and risk management, while proving the operator has discharged his legal duties.
For other water systems, temperature control, chlorine dioxide treatment, ionisation, and ozone and UV treatment can all contribute to reducing the risk of legionella build-up. All water services should be routinely checked for temperature, water demand, cleanliness and use. An annual check should comprise visual inspection of water storage tanks; recording water consumption to ensure water is not stagnating; checking hot and cold water circuit plans to ensure they are current; ensuring operation and maintenance schedules are readily available and updated; and checking connections to outside services.
Microbiological monitoring should also take place in the form of samples from both cold and hot water systems, carried out by a UKAS-accredited laboratory participating in the PHLS Water Microbiology External Quality Assessment Scheme for the Isolation of Legionella from Water. Remedial action is needed if legionella bacteria reach more than 100 cfu/L. However, this is unlikely if proper management systems exist.
Indeed, simple cleaning and maintenance regimes, and consultation with an expert on chemical use and certification, will ensure this never becomes a major issue.