Keep Legionella bugs at bay

In sensitive areas such as hospitals, senior residences and nursing homes, bacteria in the hot-water circulation system represent a potential hazard. Large buildings equipped with older, frequently “over-dimensioned” heating systems are particularly susceptible to Legionella growth in the water. Drinking this contaminated water will not cause any problems, but if the small, rod-shaped bacteria are inhaled together with very fine droplets of water – as can occur under a shower – they may cause Legionnaire’s disease.

The disease got its name following an outbreak of the illness in 1976 in a hotel in Philadelphia during a congress of veterans of the American Legion. Some 200 of the approximately 4,000 participants fell ill and about 30 of them died. This infection, which is often fatal, is caused by bacteria called Legionella, of which there are more than 40 types. The most important is Legionella pneumophila.

The disease (Legionellosis) begins two to 10 days after infection, with symptoms including headaches and pains in the limbs and thorax, accompanied by fever. The progress of the disease is similar to that of a simple pneumonia and is often diagnosed as such. This is why relatively few cases of the disease become known.

In most cases the general public hears only about spectacular outbreaks, such as one in the summer of 2002 in a public facility in the UK which led to a county official being charged with manslaughter in the spring of 2004. In Germany, which has a population of about 80 million, the authorities estimate that approximately 10,000 people become infected with Legionella each year – and that most of these infections remain undiscovered.

The people most seriously at risk from Legionellosis are those with weakened immune systems, such as the elderly, small children, people with chronic illnesses and smokers. The infection is transferred via the lungs when a person inhales the bacteria in a mixture of air and water in the form of an aerosol. Typical potential sources of infections in hot-water systems are showers or whirlpools.

Legionella, which can occur in water of any kind, multiply particularly quickly in stagnant or standing water at temperatures between 20°C and 45°C. At temperatures above 50°C a pasteurisation process begins and the number of bacteria drops.

In the technical discussion on suitable preventative measures in large domestic hot-water systems, the circulation line is rapidly becoming the focus of attention. Traditionally, the circulation line is intended to provide more comfort and, above all, to save energy and water: hot water should be available as quickly as possible at every outlet to avoid energy and water losses from waiting for the water to run warm.

However, to keep such domestic systems free of germs it is important that the water circulates sufficiently through all parts of the system and that a certain minimum temperature is maintained. This requires modern methods of hydraulic adjustment, centred around a “thermal balance”. In many old systems, this has never been attempted, with the result that certain sections of the system are not flushed by the circulating water. This is a potential hazard to general health that must be rectified.

Regulations

In many countries regulations define the design and dimensioning of circulation lines and deal with technical measures for the reduction of the growth of Legionella. They also provide an overview of suitable disinfection methods. Conventional methods include the addition of chemicals such as chlorine dioxide, calcium hypo-chlorite or hydrogen peroxide. Disinfection can also be achieved with anodic oxidation or UV irradiation of the water. UV irradiation is frequently used with ultrasonic treatment to kill the Legionella, which are hosting in amoeba. Frequently, thermal disinfection appears to be the easiest solution, as the necessary technical prerequisites, such as a water heater, already exist. A pasteurisation process begins at a temperature of 50°C and the number of bacteria is reduced; increasing the temperature reduces the necessary disinfection time. In general it is recommended that the entire system be heated to more than 70°C to achieve satisfactory thermal disinfection.

Temperature issues

One method of ensuring permanent germ-free operation of the heating system is by maintaining a circulation temperature of 60°C throughout the entire system. In order to ensure a uniform temperature thermal adjustment is necessary.

In the past this was achieved by the time-consuming adjustment of manual balancing-valves, but today it can be achieved far more easily and automatically with thermostatic circulation valves that, even under varying operating conditions, regulate the temperature effectively over a period of many years. A thermostatic valve installed in the circulation line is simply set to the desired temperature; it then automatically controls the flow of water in order to maintain this temperature.

If the user wishes to carry out thermal disinfection it is possible to use valves that permit disinfection at temperatures of more than 70°C. To maintain the thermal balance of the system during disinfection the valves used should actually reduce the flow rate at temperatures above 70°C, rather than simply open a bypass line. Particularly flexible valves for this purpose are valves that can be upgraded while the system is running, since thermal disinfection is often not necessary under normal operating conditions but will be needed at other times. In addition, such valves have separate thermostats for thermal adjustment and thermal disinfection, thus ensuring greater accuracy for each of the two functions.

If a circulation system is operated at a high temperature of 60°C or more, the accompanying risks must be taken into account. The first of these is the risk of scale precipitation within the system, since this depends not only on the hardness of the water but also, to a considerable degree, on the water temperature.

If the system contains galvanised iron pipes or fittings there is also a significant risk of corrosion. The risk of scalds to users should not be ignored either: at a water temperature of about 60°C, first degree burns occur within a few seconds; at a water temperature of about 50°C, they occur after approximately five minutes. Elderly people and small children who may not be able to react quickly enough are particularly endangered.

An alternative to these temperatures could be the operation of the circulation system at a lower temperature and the use of one of the above-mentioned disinfection methods to ensure freedom from Legionella bacteria. If the water is treated centrally with UV radiation, or with electrolytic or thermal processes, a lower circulation temperature is quite feasible, provided that the system is thermally balanced to prevent the accumulation of slow-moving water at lower temperatures in sections of the system.

Permanent disinfection

A method often used, in the case of public swimming baths for example, is an intelligent process with a circulation temperature of about 40°C and involving permanent thermal disinfection of the water in the storage tank. The entirety of the water is disinfected by keeping it at a temperature of more than 60°C in the tank for a sufficiently long period. The disinfected water is then cooled in a heat exchanger to about 40°C before it is distributed. Disinfection of the entire circulation system is also possible by bypassing the heat exchanger.

In addition to the above-mentioned methods, the pipe system can also be subjected to a periodic thermal treatment, either as an independent or additional safety measure. With the option for automatic thermal disinfection mentioned above, the thermostatic circulation valves only permit disinfection at temperatures above 70°C. Depending on the hydraulic design of the system and the principle of operation of the valves used in it, all sections of the system will reach the flow rate necessary for disinfection after a shorter or longer time. Further control and optimisation of the disinfection process is not possible.

In such cases, the new thermostatic valves, which can be upgraded in a modular manner, such as the MTCV from Danfoss, offer better capabilities. With an actuator, a temperature sensor and the electronic controller CCR it is possible to control the disinfection temperature and duration precisely for each section of the circulation system. Once set to the correct temperature the MTCVs automatically maintain the thermal balance of the circulation during normal operation. When the boiler outlet temperature reaches the pre-defined disinfection temperature the CCR starts the disinfection process.

The actuators open bypass lines on the MTCV valves and the system sections with the best flow are first disinfected at the appropriate temperature. When the disinfection time programmed in the CCR has elapsed the bypass is closed. The entire process or periodic thermal disinfection can be controlled in this manner and, more importantly, the times can be optimised.

After completion of the disinfection process the CCR sends a signal to the boiler controller to reduce the outlet temperature to its normal operating value. This not only saves energy, but also reduces the risks associated with water at high temperatures. The status of the disinfection process and any problems which occur can be displayed on the controller.

In addition the CCR can be easily integrated into existing or planned building management systems (bms) via its internal RS 485 interface, permitting central control and monitoring of the process in large buildings such as hotels or senior citizen residences.

A base for the safe operation of heating systems is a thermally balanced circulation system that prevents stagnation of the water and the renewed growth of germs. When selecting the circulation temperature, various other aspects, such as the risk of scale deposits and injuries to users, must be taken into account, in addition to the desired reduction in the number of Legionella bacteria. The thermal balance between the various sections of the system is achieved with thermostatic circulation valves. Modern designs permit “modular upgrading” of these sections while running the system, as well as automatic thermal disinfection, at temperatures of at least 70°C along with electronically regulated thermal treatment of the water as a standalone measure.