Scientists have subsequently isolated and named no less than 46 species and 70 serogroups under the generic term Legionella. They know precisely what illnesses it causes and, to a great extent, the most effective ways of preventing their development. So, it is slightly puzzling that, thirty years after the first outbreak, people around the world are still dying from Legionnaires’ Disease.

Legionella bacteria are usually found in low numbers in natural environments such as rivers, lakes and reservoirs. They can survive temperatures as low as 6°C and as high as 50°C. While maximum growth occurs at temperatures in the range 20° degrees to 45°C, the bacteria seem to become most virulent at around 37°C. On the other hand they are killed by temperatures above 60°C.

Problems begin when Legionella bacteria migrate from their natural environments into man-made water systems. Enclosed, warm storage vessels, complex pipe-work with lots of dead spots and underused water systems with stagnant water provide the ideal habitat, particularly if sludge, sediment and scale are present to encourage the growth of algae, amoeba and other bacteria on which the Legionella bacteria can feed.

Legionnaire’s Disease - and the similar but less lethal respiratory illness known as Pontiac Fever- is generally contracted by deeply inhaling Legionella bacteria, either in tiny droplets of water (aerosol) or in droplet nuclei; the particles left after water has evaporated. Typical sources include cooling towers in industrial cooling water systems and in large central air conditioning systems, evaporative coolers, hot water systems, showers, whirlpool spas, architectural fountains, room-air humidifiers, ice making machines, misting equipment and similar disseminators that draw upon a public water supply. For the purposes of the Health & Safety legislation, this includes not just the main vessels but all associated pipe-work, pumps, valves and other ancillary equipment including heat exchangers and chillers.

In one research project 40 to 60% of all cooling towers tested contained Legionella. Other studies have demonstrated that Legionella pneumophila can be spread at least 6km from its original source. An outbreak of Legionnaires’ Disease that killed 18 people in Pas-de-Calais, northern France in 2003–2004 was traced back to a petro-chemical plant cooling tower 6–7km from where most of the victims lived.

Guidelines published by The European Working Group for Legionella Infections (EWGLI) on the actions to be taken to limit the number of colony forming units (i.e., the aerobic count) of micro-organisms per mL at 30°C (minimum 48 hours incubation) recommend:

Treatment & solutions

With the average mortality rate in any outbreak 12% it is obviously vital to defeat Legionella before it can get a grip; in other words, to stop it at source. A number of systems and techniques have been developed for this purpose, including:

• biocide treatments such as chlorine dioxide

• ionisation of the water using copper and silver ions

• thermal regimes designed to maintain water at optimum temperatures 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 or the accumulation of scale.

In cold water systems, the ideal is to maintain a temperature below 200C at all times while, in hot water systems, the optimum is storage at 60°C and higher reducing to 50°C or less at the point of use to avoid the risk of scalding.

Maintaining the right temperature

There are two keys to an effective thermal treatment regime. The first is a temperature programme that heats the water to the right temperature to kill the bugs. The second is to keep the water in movement to prevent scaling and dead spots developing. In an energy-conscious age, the system should either require low energy input or be designed to recycle the heat to minimise energy usage.

Conventional vessels such as calorifers can provide an ideal breeding ground for Legionella, particularly during periods of peak demand when hot water is drawn from the tank bottom. This leads to temperature stratification within the water which, in turn, can encourage bacterial growth.

By contrast, a domestic hot water heating system based on high efficiency, compact heat exchangers, meets these criteria, very well. One system that has just arrived on the UK market is the AquaProtect T. Designed for use in hot water systems in hospitals, hotels, schools, leisure centres and similar buildings, it disinfects the water by heating and maintaining it at 70°C for six minutes to ensure total sterilisation and a bug free system. After disinfection, the system cools the water to the optimum temperature for comfort and safety prior to use.

The AquaProtect system is designed with sufficient capacity to disinfect the entire hot water system at periodic intervals as well as continuously disinfecting all circulating water, to ensure maximum protection from bacterial growth. Since the heart of the system is a high efficiency heat exchanger, hot water used in the disinfection process is used to pre-heat incoming cold water, thus keeping energy losses to a minimum.

How it works

Water enters the system at ambient temperature and passes through the heat exchanger where it is heated to 70°C. It then flows through a reaction tank for six minutes, during which time all Legionella bacteria are killed. After passing through the reaction tank, the sterilised water flows into a storage tank in the normal manner.

When a tap is opened, this water, at 70°C, passes through another compact heat exchanger where it is used to pre-heat incoming cold water and, in the process, has its own temperature reduced ready for use. In this way, all of the energy used in the disinfection processed is recovered.

Water circulates continuously between the heat exchanger, reaction tank and storage tank to prevent the development of dead spots and avoid scaling and fouling. Once hot water has been drawn off the system, this circulation is reversed so that unused, stored water returns to the heat exchanger and the disinfection process starts all over again. When it has passed through the reaction tank, it enters the storage vessel at the top, thus avoiding stratification.

When an AquaProtect system is installed and prior to start-up, the entire hot water system, including all associated pipe-work, pumps and valves, is disinfected by circulating high temperature water for a specified time period. The same process can be used, periodically, throughout the life of the installation to ensure that it is kept bug-free.

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