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Controlling Legionella in water systems – which method is effective and safe?
Fri, 22 May 2015 16:32:09 GMT
Legionella pneumophila is a disease-causing microorganism, which is ubiquitous in water systems and can infect people, being particularly dangerous for hospital patients that have a compromised immune system. L. pneumophila is well known for causing Legionellosis (Legionnaire’s disease and Pontiac fever). L. pneumophila thrives in temperatures between 20 and 45°C and multiples rapidly in untreated or ineffectively treated water systems.
Legionella makes its way into a human system when it is caught by inhaling particles of aerosol emitted by showers and taps. These microorganisms have been responsible for many deaths and, therefore, need to be controlled, especially in hospitals and care homes water systems. In the UK Legionnaires’ disease requires to be reported to the Health Protection Agency (HPA) and the total confirmed cases of Legionnaires’ disease in England and Wales reported to the HPA from 1980 to 2009 was 6750, of which 837 were fatal. Cases of deaths and resulting legal cases have been reported in the press from time to time. L. pneumophila was responsible f or four Legionnaire’s disease deaths in Scotland during an outbreak in 2012, when 92 cases were registered (BBC News website http://www.bbc.co.uk/news/uk-scotland-edinburgh-east-fife-32216664).
The most commonly control measures for Legionella currently in use are intermittent heat and flush, temperature control, chlorine dioxide and copper-silver ionisation, but which method is the safest, as well as effective?
Heat and flush (thermal disinfection) is a ‘one-off’ approach and can be temporarily effective. However, it is expensive, energy inefficient and labour-intensive and, if not managed correctly, can present a major risk of scalding to both staff and patients. Research has shown that bacterial levels can soon return to pre-disinfection levels a few months after treatment resulting in further disinfection being necessary, which means further risk of scalding too.
Temperature control has been the traditionally applied method for the control of Legionella in water distribution systems. This control method entailed obtaining 50ºC, and above, after running any hot water tap for 1 minute, and 20ºC, and below, after running any cold water tap for 2 minutes (HTM04-01, 2006). However, only 13% of the results of tests carried out by the UK Building Services Research and Information Association (BSRIA), in 1996, using temperature control were free of Legionella. In summary, it is not effective!
Intermittent shock injection of chlorine into the water system, to achieve 20 to 50 mg/L of chlorine throughout the system, can be effective in the short term, however, bacteria re-colonisation often occurs after the disinfectant levels decrease (Lin et al. 1998). Protozoan cysts, such as amoeba, that harbour Legionella survive free chlorine levels of 50mg/L (Kilvington and Price, 1990). Chlorine also reacts with organic materials and accelerates the production of trihalomethanes (THMs), which are the only regulated disinfection by-product in the UK, and it is required by law that the sum of four THMs does not exceed 100 μg/L (Bougeard et al., 2010).
Chlorine Dioxide: the UK Drinking Water Inspectorate prescribed that 0.5mg/l of chlorine dioxide should not be exceeded. Maintaining between 0.3 and 0.5mg/l chlorine dioxide at outlets, which some studies with model systems demonstrated could control Legionella, is difficult as it decomposes to chlorite and chlorate and decays over distance and at elevated temperatures (Sidari et al., 2004). Chlorite and chlorate are not only toxic but also less active, compared to chlorine dioxide, against Legionella and inactive against protozoa and biofilm that harbour Legionella.
There are health hazards to humans and environmental concerns associated with chlorine dioxide and to this effect, the UK Health Protection Agency has produced a guide on how to deal with chlorine dioxide incidents (https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/338711/Chlorine_Dioxide_Incident_management.pdf). Some of the key points of the document are, chlorine dioxide:
• Reacts violently with organics, phosphorus, potassium hydroxide and sulphur, causing fire and explosion hazard.
• Emits toxic fumes of chlorine when heated to decomposition.
• Highly irritating via inhalation or ocular exposure.
• CHIP: Very toxic, corrosive and oxidising
• Inhalation causes irritation of eyes and nose with sore throat, cough, chest tightness, headache, ataxia and confusion. Dyspnoea and stridor due to laryngeal oedema may follow. Pulmonary oedema with increasing breathlessness, wheeze, hypoxia and cyanosis may take up to 36h to develop.
• Dangerous to the environment and the Environment Agency must be informed of substantial incidents.
Furthermore, chlorine and chlorine dioxide, being oxidising chemicals, create problems in older distribution systems by causing a pitting type of corrosion.
Copper and silver ionisation involves the continuous release of copper and silver ions in water. The ions are generated by passing a low electrical current between two pure copper and two pure silver electrodes or between two copper and silver alloys (normally 70:30 ratio). Copper and silver ions are positively charged ions (cations) and thus seek opposite polarity and find this in Legionella bacteria as well as in biofilm; the copper and silver ions attach, through electrostatic bonds, to negatively charged sites on bacterial cell walls; this distorts and weakens the cell wall allowing penetration of the silver ions; the silver ions attack the cell by binding at specific sites to DNA, RNA, cellular protein and respiratory enzymes denying all life support systems to the cell, causing death; the copper and the silver ions need to work together, since without the copper ions the silver ions cannot penetrate the cell wall of the target organisms.
The UK Regulations do not specify a maximum concentration value for silver, but the US Environmental Protection Agency (EPA) has set maximum containment levels for drinking water of 1.3mg/l for copper and 0.1mg/l for silver (Lin et al., 2011). The levels of copper and silver needed to eradicate Legionella in water are much lower than that (<0.2 mg/L copper and 0.02 mg/L silver). A long-term study (5 – 11 years) on the efficacy of copper and silver ionisation to control Legionella in 16 hospital water systems, to reduce incidence of hospital-acquired Legionnaires’ disease concluded that copper and silver ionisation was the only disinfection modality to have fulfilled all criteria recommended to be applied to disinfection approaches (Stout and Yu, 2003).
Looking through the key points regarding dangers related to the use of chlorine dioxide and chlorine, it is quite surprising that chlorine dioxide is still being used as a Legionella control measure, when there is a much safer method available, i.e. copper-silver ionisation.
In conclusion, the only Legionella control method which ticks both boxes, safety and effectiveness, seems to be copper-silver ionisation.
Bougeard CMM, Goslan EH, Jefferson B, Parsons SA. Comparison of the disinfection by-product formation potential of treated waters exposed to chlorine and monochloramine. Water Research 2010;44(3):729-740.
Kilvington S, Price J. Survival of Legionella pneumophila within cysts of Acanthamoeba polyphaga. Journal of Applied Bacteriology 1990;68:519-525.
Sidari FP, Stout JE, van Briesen JM. Keeping Legionella out of water systems. J. Am. Water Works Assoc. 2004;96:111-119.
Stout JE, Yu VL. Experiences of the first 16 hospitals using copper silver ionisation for Legionella control: Implications for the valuation of other disinfection modalities. Infection Control and Hospital Epidemiology 2003;24(8):563-568. [Legionella photo](//muut.com/u/leapt/s1/:leapt:b0uX:legionellaphoto.jpg.jpg)