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Guide to Legionnaires’ Disease and its prevention

Best practice for eliminating the risk of Legionella bacteria in hot water systems



1 The origins of Legionnaires’ Disease



In July 1976, members of the American Legion attending a Bi-Centennial Conference in Philadelphia, Pennsylvania contracted a mystery respiratory illness. 34 of the 221 struck down subsequently died. The origins of the infection were traced to bacteria discharged from the conference centre’s air conditioning system. Doctors named the guilty bacterium Legionella pneumophila to acknowledge the source and circumstances of its discovery. Since that time, scientists have isolated and named no fewer than 46 species and 70 serogroups under the generic term Legionellosis, the cause of a number of diseases, of which Legionnaires’ Disease is the best-known.



Potential sources of infection


Health & safety obligations


UK legislation


How the disease is contracted




Legionella in Domestic Hot Water (DHW) Systems


Control measures


Alfa Laval equipment for DHW systems


Despite increased awareness of the disease and its potentially lethal impact, particularly on the frail and the old, outbreaks of Legionnaires’ Disease continue to occur on a regular basis. There are, on average, 350-400 cases a year reported in the UK (source: HSE). The purpose of this Pocket Guide is to examine the origins of the disease, the threat it poses and the different methods available to prevent its development in hot water systems.

Potential sources of infection 3


Legionella bacteria occur in low numbers in natural environments such as rivers, lakes and reservoirs where they can survive temperatures as low as 60C and as high as 500 C. Generally speaking they thrive at temperatures between 20 to 450 C and are at their most virulent at 370 C. The problem is that the bacteria can migrate into and flourish in man-made water systems. Enclosed, warm storage vessels, pipe-work with lots of blind spots and underutilised water systems containing stagnant water provide an ideal environment in which bacteria can prosper, particularly if there is sludge, sediment and scale for them to feed on.

Interestingly enough, researchers isolated the bacterium as early as 1947 but regarded it as a threat only to animals. It is thought that previous epidemics of Legionnaires’ Disease had occurred many times before it was finally isolated and named in 1976. They were generally diagnosed as atypical pneumonia because the cause of the infection was unknown prior to 1976.

In the case of the original Legionnaires’ outbreak, the cooling tower feeding the hotel’s AC system was identified as the main source of the bacteria. Cooling towers still represent a problem as do other typical sources, including: • • • • • • • •

Evaporative coolers Hot and cold water systems Showers Hot water spas Decorative fountains Ice makers Room humidifiers Machinery & plant coolant systems

(For Health & Safety purposes, this can include not only the main vessels but all of the pipe-work, pumps, valves and other ancillaries including heat exchangers and chillers associated with them.)

Health & safety obligations

UK legislation



Any person, either individually or as an officer of a company or group, that owns or operates water systems on any premises, is legally responsible for the control of Legionella and its attendant risks. Under general health and safety law, it is necessary to assess the risks that may affect staff or members of the public and take suitable precautions. An employer or person in control of premises such as a landlord must: • • •

identify and assess potential sources of risk; prepare a course of action to prevent or control the risk; implement and manage the scheme – appointing a ‘responsible person’ to be managerially responsible. keep records and check that what has been done is effective

The key is to design, maintain and operate hot water services under conditions which prevent or control the growth and multiplication of Legionella bacteria. Other people and organisations also have obligations including those involved in the supply of water systems and their components, such as designers, manufacturers, water treatment companies and suppliers. By law, such equipment must be designed and manufactured to be safe to use and easy to clean and maintain.

UK Legislation relating to the control of Legionella in the work place • • • • • • • • •

The Health and Safety at Work Act 1974 Management of Health and Safety at Work Regulations 1999 Control of Substances Hazardous to Health Regulations 1999 (COSHH) Notification of Cooling Towers and Evaporative Condensers Regulations 1992 Reporting of injuries diseases and Dangerous Occurrences Regulations 1995 (RIDDOR) Safety Representatives and Safety Committee Regulations 1977 Health & Safety (Consultation with Employees) Regulations 1996 ACOP (L8) Legionnaires’ disease Guidance (HS(G) 70 ) The control of Legionellosis

How the disease is contracted




Legionnaires’ Disease is generally contracted by deeply inhaling Legionella bacteria, entrained in tiny droplets of water (aerosol) or in droplet nuclei; the particles left after water has evaporated.

Legionnaires’ Disease usually develops between 2 to 14 days after exposure to the Legionella bacteria. It frequently begins with the following signs and symptoms:

The ideal conditions for Legionella growth are:

• • • •

a suitable temperature for growth, 20 to 45oC; a source of nutrients for the organism, e.g. sludge, scale, rust, algae, and other organic matter stagnant water, where Legionella can accumulate amongst biofilm without being flushed by out flowing water the presence of amoebae, single cell animals prevalent in most water systems that encourage Legionella proliferation

Legionella bacteria start to die at temperatures above 500 C but die rapidly (within minutes) at temperatures above 600 C

headache muscle pain chills fever that may be 1040F (400C) or higher

By the second or third day other signs and symptoms may include: • • • • • • •

cough, which may bring up mucus and sometimes blood shortness of breath chest pain fatigue loss of appetite gastrointestinal symptoms, such as nausea, vomiting and diarrhoea confusion or other mental changes

Legionella in Domestic Hot Water (DHW) Systems 9

If Legionella is not correctly identified and no effort is made to pinpoint the source, the risk to public health remains.

The symptoms of Legionnaires’ Disease can be diverse and may not be the same as those seen in classical pneumonia. Consequently, cases of Legionnaires’ Disease frequently go undetected and are generally classified as atypical pneumonia with no recognised cause. Special laboratory tests are required to diagnose Legionnaires’ Disease accurately. According to the National Health Service (NHS), there were 551 cases of Legionnaires’ Disease reported during 2006 in England and Wales, roughly 10% of which (52) resulted in death. However, the actual number of cases could be much higher due to people with mild symptoms of the condition being misdiagnosed with more common forms of pneumonia. This bears out similar findings in the United States where many cases go unrecognized because the disease cannot be diagnosed by its symptoms alone and the special tests needed to confirm the diagnosis are not routinely used in hospitals. As many as 90% of Legionnaires’ Disease cases may go undetected.


Domestic as used here applies to all non-process water used for showers, hot water, heating etc. Legionnaires’ Disease may be contracted from any of these sources by inhaling aerosol droplets. Any water heaters that maintain water at below 600C represent a potential risk, especially if poorly-maintained. The most common sources for reported outbreaks are hot and cold water systems in buildings. DHW systems should be designed to avoid potential areas of stagnation such as dead-legs and to re-circulate water frequently. Dead-legs are areas of pipe work that have been by-passed or capped. Similarly, under-utilised sinks or showers can harbour stagnant water and provide ideal breeding conditions for many bacteria, including Legionella. Large scale calorifiers and similar conventional storage vessels can also provide a breeding ground for Legionella because they are prone to temperature stratification and water at the base of these vessels is commonly maintained at between 20- 450C which encourages bacterial growth. If the calorifier cannot maintain an output temperature (>600C) required to kill Legionella continuously – especially during periods of peak demand – then the bacteria can migrate from the calorifier into the distribution system and colonise dead-legs and under-utilised taps and showers.

Control measures 11


The only sure way to defeat Legionella is to stop it at source. A number of systems and techniques have been developed for this purpose, including: • • • •

biocide treatments such as chlorine dioxide treatment of the water with copper and silver ions UltraViolet (UV) radiation thermal regimes designed to maintain water at optimum temperatures (600C+) to prevent bacteria developing or thriving

Of these techniques, the one most frequently used is the thermal option because it tends to be the least expensive, involves no chemicals and is easier to maintain. As with any system, strict hygiene regimes are absolutely vital to deter the growth of organic slimes (biofilms) or the accumulation of scale and corrosion deposits where bacteria can reside.

Thermal Disinfection: There are three keys to an effective thermal treatment regime. 1. a temperature programme that heats the water to the right temperature (at least 60 0C) to kill the bacteria yet will also cool it sufficiently prior to use to avoid scalding. 2. a method to keep the water in movement to prevent dead spots developing. 3. a system as free as possible of scale and corrosion deposits and biofilm.

Alfa Laval equipment for DHW systems 13


Alfa Laval provides compact heat exchangers and modularized systems for Domestic Hot Water heating in industrial, residential and commercial buildings. All are designed to work with a wide range of heat sources, including conventional boilers, district heating networks and solar heating systems. Most importantly, they all have the ability to provide full security against impurities, including Legionella bacteria from the primary side.

AquaCompact: A compact, semi-instantaneous system for use with District Heating or individual boilers, consisting of a stainless steel or enamelled tank and a charging kit which can employ gasketted, brazed or all steel heat exchangers.

AquaStore: provides semiinstantaneous access to large volumes of hot water in large buildings such as apartment blocks, hotels, hospitals, nursing homes and the like.

AquaProtect: Developed specifically to prevent bacterial growth in DHW systems, the AquaProtect range uses recovered heat to provide low cost energy-efficient and effective thermal disinfection. Using the latest heat exchanger technology and electronic controls, AquaProtect provides guaranteed protection against Legionellosis.

AquaFlow: For instantaneous and controllable supplies of domestic hot water to schools, hospitals, hotels and similar large buildings, AquaFlow provides the simple answer. Prewired, tested and approved at the factory, AquaFlow modules are designed for plug-and-play simplicity.

AquaMicro: Developed specifically for use with District heating systems, the AquaMicro provides a hot water sub-station for individual flats and houses.

For more detailed information on all of our systems for DHW production visit our website at Alternatively, call 01276 63383 and ask to speak to one of our specialists.

Alfa Laval Limited Doman Road, Camberley, Surrey GU15 3DN e-mail: Tel: 01276 63383 Fax: 01276 413632

Guide to Legionnaires' Disease  

A guide to Legionnaires' Disease, its origins, how it develops and methods to prevent it.

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