HSG 274 Part 2 is clear on the role of a cleanliness in a hot and cold water system, as it states in paragraph 2.80 “….For control measures to be effective, it is essential to keep the whole system clean, as biofilms or inorganic matter such as scale can reduce the efficacy of any type of control measure significantly…..”
This highlights the need to keep systems clear from any deposits within a system so that the chosen control method has a chance to work. In the UK the preferred method of control is temperature with a biocide addition as a secondary measure if required.
However, it is important to understand that each of these methods has its limitations, for example Allegra et al. (Allegra S, Grattard F, Girardot F, et al. Longitudinal evaluation of the efficacy of heat treatment procedures against Legionella spp. in hospital water systems by using a flow cytometric assay. Appl Environ Microbiol. 2011;77:1268–1275) proposed that Legionella bacteria can become heat resistant, as some Legionella submitted to superheating in the environment for a long time develop resistance to high temperatures as demonstrated by the high proportion of culturable cells and viable but non-culturable (VBNC) cells still present after a 30-min treatment at 70°C.
In another study (Farhat M, Moletta-Denat M, Frère J, et al. Effects of disinfection on Legionella spp., eukarya, and biofilms in a hot watersystem. Appl Environ Microbiol. 2012;78:6850–6858), a 30-min heat-shock treatment at 70°C performed twice in a test loop was unable to remove the biofilm. The results showed that although Legionella diversity was reduced, pathogenic Legionella species (Legionella pneumophila and Legionella anisa) remained after the heat shock and after chemical treatments.
Therefore hospitals, in particular, often choose a secondary biocide such as chlorine, chlorine dioxide or copper/silver ions. But even these can have limitations, in 1983 and 1985 two separate reports by Kutcha et al (Enhanced Chlorine Resistance of Tap Water-Adapted Legionella pneumophila as Compared with Agar Medium-Passaged Strains John M. Kuchta, Stanley J. States, Jane E. Mcglaughlin, Jean H. Overmeyer, Robert M. Wadowsky, Ann M. Mcnamara, Randy S. Wolford, and Robert B. Yee. Applied and Environmental Microbiology, July 1985, p. 21-26) showed L. pneumophila to be much more resistant to chlorine disinfection than several organisms, especially members of the coliform group commonly used as indicators of potability of public water supplies. The reports concluded evaluations of the effectiveness of chlorine in disinfecting hospitals, and other buildings with recirculating hot-water systems should be determined empirically. It is likely that in actual practice higher dosages of chlorine for longer contact times may be required to achieve disinfection.
The above shows that Legionella bacteria are difficult to remove from a hot or cold-water system and many factors must be considered, not just temperature and biocide addition. These other factors include, for example, materials of construction, physical layout of the pipework, are secondary or tertiary loops present and are they functioning correctly, are TMV’s present, the type of taps and presence of aerators or flow straighteners.
This complex subject therefore needs good quality risk assessors who are aware of these points and actively look for them in the risk assessment along with good quality staff who look after the systems on a day to day basis. So therefore, staff training is a key component in ensuring your staff understand these issues and can keep your site safe.
Further a comprehensive Written Scheme (or in the case of a hospital a Water Safety Plan) will then bring all of the above points together specifically for your site to enable you to manage the site safely protecting staff and public alike.
Collaton Consultancy Limited are expert Legionella consultants working for both water treatment companies and end users alike, Expert Witness services are also offered should a legal case arise. If you have any specific issues relating to the above you would like help with then contact Collaton Consultancy Limited via firstname.lastname@example.org.