Thermal Inactivation of Legionellae
In the environment, legionellae will survive at temperatures below 20oC but are unlikely to multiply. As the temperature rises above 50oC legionellae are less able to survive, and at 60oC the majority will be killed within 5 minutes.
Maintaining water temperature below 20oC and above 50oC is therefore currently the main method of controlling legionellae in domestic water systems.
Calorifiers in a recirculating hot water system should distribute hot water at 60oC and receive the return hot water supply at not less than 50oC. This will eliminate most legionellae which may enter the system. However, should legionellae survive this process, either by entering the system when the temperature of the hot water supply has fallen (eg at peak demand), or by evading the effect of high temperature as intracellular parasites of amoebae, then the legionellae may enter pipework serving outlets from the recirculating supply.
If outlets are infrequently used, the warm stagnant water trapped between the recirculating supply and the outlet may encourage legionellae to colonise the lumen of the pipe and tap or shower components. When the outlet is eventually opened, legionellae may be subsequently disseminated as respirable aerosols.
Some alternative methods for controlling legionellae in potable water systems have been the subject of research performed at the Royal Liverpool University Hospital (RLUH). These include regular flushing of outlets, ultraviolet light, ozone, ionisation and continuous dosing with chlorine dioxide.
Regular Flushing of Showers
One of the most effective and simplest methods of controlling legionella in dead-legs in our studies was regular usage of the associated outlet (Makin 5 Hart 1990). Two adjacent showers heavily colonised with legionellae were flushed on their maximum hot setting for two minutes each day for two weeks. This was sufficient to remove and further dilute residual water within the 9 ft of feed-pipe (dead-leg) connecting each shower unit to the main circulating hot water supply and the main cold water supply. Samples of shower water were collected daily prior to each flushing and cultured for legionellae.
Legionellae were not detected in shower water immediately after flushing commenced, and remained undetectable for approximately one week after daily flushing had stopped. However, approximately two weeks after flushing had stopped, legionella counts in both showers had risen to levels far higher than those in the pre-flushing period.
The findings of this limited study imply that the use of regular flushing of outlets as a means of controlling legionellae can be effective, but once initiated should be continued. To facilitate this, the latest hands-free electronically controlled shower units may be programmed to automatically flush outlets at a set time and for a defined period, and would remove the potential for failure associated with human error.
Ultraviolet Light
At 254nm, ultraviolet light is an effective bactericidal agent to which L. pneumophila is particularly susceptible. Researchers in this field have shown that UV light was effective in controlling legionellae in real and model plumbing systems.
In the UV study at the RLUH (Makin 5 Hart 1992), showers known to be heavily colonized with micro-organisms including L. Pneumophila and amoebae, were disinfected using high temperatures and chlorine.
The hot and cold water systems were connected to in-line 10μ pre-filters and low pressure mercury lamps, which delivered a UV dose of 30mW/s/cm2 to water passing through the shower. The filters would allow legionellae to pass, but removed large particulate matter that may shield micro-organisms from the UV light.
During the 8 month period that the UV lamps were lit, no legionellae or amoebae were detected in shower water, whereas legionellae and amoebae were detected continuously in the control shower not irradiated with UV light. Two weeks after the UV lamps were turned off, L. pneumophila and amoebae were isolated simultaneously from both showers. Therefore in this study UV light at 254nm was effective in preventing recolonisation of a shower by L. pneumophila. Furthermore a significant reduction in legionellae in shower water was observed after the mixer valve had been re-colonised and the UV lamp was reactivated. The cause of this disinfection approximately 1 metre downstream of the UV lamp may have been associated with the production of superoxide radicals in water induced by the 186nm wave band also emitted by low pressure mercury vapour lamps.
Ultra Violet is as effective as any biocide. However, it has similar disadvantages as ozone, in that it is not a dispersive process. In addition its effectiveness can be erratic in water, as any bacteria shielded from the rays at the point of treatment are not eliminated. Even a small stain on a UV bulb is sufficient to interfere with the efficiency of the treatment.
Ozone
This is a very powerful and effective oxidising biocide, even more powerful than chlorine dioxide. Its’ major disadvantage is that it is not a dispersive process, therefore a water system is quickly reinfected after treatment; it is also expensive to install. Biofilms, other than those close to the point of injection, are not controlled. Capital equipment for Ozone generation is expensive, with a number of operational and health and safety considerations to be taken account of.
Ionisation
Copper & Silver Ion manufacturers have vigorously promoted their systems; however, there are a number of questions as yet unanswered. There is very little toxicity data on the use of silver ions; the DWI recently concluded that products containing silver salts could not at this time be recommended for approval for continuous use as disinfectants for water for public supply.
High levels of use, up to 80ug/L, are likely to be required to kill Legionella; this level may be used for water treatment, but greatly exceeds the level of 10ug/L recommended for potable water. There is no data on corrosion, and recent suggestions are that pitting corrosion may be a feature of these systems. Recent test work shows that the systems are inefficient in hard water.
Five commercially available copper/silver ionisation systems were tested as part of a recent study to investigate the use of ionisation to control Legionella in operational hot water systems. The study showed that all the ionisation systems, both in soft and hard water, had difficulty maintaining a stable level of silver over a long time period. However, during the 24-hour testing period the silver levels remained fairly consistent for each of the sites. Careful control and maintenance would appear to be required to maintain stable silver levels. In general the average silver concentration was found to be below the value of 30 to 40 μg/l, thought to be necessary to inhibit the growth of bacteria.
For hard water systems, silver ion concentrations can be difficult to maintain due to:
•The build-up of scale on electrodes
•The high concentration of dissolved solids precipitating the silver ions out of solution
•For both hard and soft water, the ionisation process was pH sensitive and it became increasingly difficult to maintain silver ion concentrations above pH 7.6
The build-up of scale and concentration of dissolved solids need to be carefully controlled so that suitable ion levels are consistently maintained throughout the system.
Additional water treatments such as scale control, filtration, pH control and water softening may therefore be necessary, particularly in hard water areas, to overcome these problems.
Chlorine Dioxide
The necessity of using powerful biocides for biofilm removal is illustrated in recent work which demonstrated that it took four times as much bromide to destroy bacteria in biofilm as it took to destroy bacteria in water flow. Chlorine dioxide can bring down bacteria levels to single figures, and more particularly is very effective against Legionella which can exist in systems even when the Total Viable Count in the water is well within accepted standards.
Chlorine Dioxide’s effectiveness against Legionella in both hot and cold systems has enabled building services managers to provide safe water, even in systems which have difficulty maintaining the 60oC level recommended. Dosing the system with chlorine dioxide can also prevent the pipe corrosion caused by the co-existence of two quite different types of bacteria in the biofilm. The upper layers of biofilm are populated by aerobic bacteria, and the lower layers (lacking in oxygen) by anaerobic bacteria. This co-existence creates a corrosion potential between the oxidising and reducing bacteria, resulting in the removal of pipe metal by natural electrolysis. Biofilm develops faster in plastic pipes than in metal pipes, as the biofilm will find nutrients in the organic content of the pipe.
Apart from its effectiveness in penetrating biofilm and killing the actual bacteria, Chlorine Dioxide solutions have a long residual level of disinfection. It is less reactive than other biocides such as Chlorine or Ozone which stay in the water for a significantly shorter period of time. By using solutions based on Stabilised Chlorine Dioxide, protective levels of biocide can be maintained in water systems for 14 days or more and so provide protection during "Shut downs". The need to flush out systems or carry out chlorinations prior to reopening can be eliminated.