Something is still bugging us to death
Peter Rose reminds us of the very real, but avoidable, risks posed by legionella bacteria.Legionnaires’ Disease was first recorded in 1976. The disease — and the bacterium that causes it — took its name from its first victims, members of the American Legion who were attending an annual convention in Philadelphia. The previously-unidentified bacterium caused pneumonia in many delegates, a large number of whom subsequently died. Scientists have now isolated and named no fewer 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, 40 years after the first outbreak, people around the world still die 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 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 pipework with lots of dead spots and underused water systems with stagnant water provide ideal habitats for legionella — particularly if sludge, sediment and scale are present to encourage the growth of algae, amoeba and other bacteria on which the bacteria can feed. Legionnaires’ Disease and the similar but less lethal respiratory illness known as Pontiac Fever are 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-and-safety legislation, these sources include 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 6 km from its original source. An outbreak of Legionnaires’ Disease that killed 18 people in Pas-de-Calais, northern France, in 2003/04 was subsequently traced back to a petro-chemical plant cooling tower 6 to 7 km 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 millilitres at 30°C (minimum 48 hours incubation) made the recommendations summarised in Table 1.
Treatment and solutions
Table 1: Depending on the results of tests on a sample from a water system, various action may be required.
Bearing in mind that the average mortality rate in any outbreak of Legionnaires’ Disease is 12%, it is obvious that 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. • 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 20°C at all times. 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. More detail is given in Table 2.
Table 2: Danger temperatures for legionella in water systems are between 20 and 50°C.
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 yet will also cool it sufficiently prior to use to avoid scalding. The second is to keep the water moving 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 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 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 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 for disinfection is used to pre-heat incoming cold water, keeping energy losses to a minimum. Water enters theAquaProtect 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, 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 the energy used in disinfection 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. Peter Rose is marketing manager with Alfa Laval Ltd.