When the temperature is used to

control pathogens in water systems, TMVs need to be fitted to blend hot and cold water to avoid scalding. Blended water, however, promotes microbial growth not only because the temperature is lowered to a temperature at which microbes grow rapidly, but also because biofilms can form on the surfaces of the internal components of the valves. In the HSE HSG274 Part 2, it is stated

that ‘although using temperature is the traditional and most common approach to control, sometimes there can be technical difficulties in maintaining the required temperatures, particularly in older buildings with complex water systems’. Therefore it is not recommended as the primary control regime, and the HTM 04-01 now recognises that temperature control is a traditional, but no longer preferred, strategy. In the HSE HSG274 Part 2, it goes on

to recommend that, if another control programme is applied, which is proven to be effective against Legionella, and there is no requirement to store hot water at 60˚C (or distribute at 50˚C), then hot water temperatures can be lowered. It is, of course, essential that reducing hot water temperatures is done safely ensuring that there is no risk of infection. There are other control methods

described in the original and revised versions of HTM 04-01, including copper and silver ionisation. Copper and silver ionisation has been

shown to be effective against Legionella from studies done in vitro and with model plumbing systems (BSRIA TN6/96). In-vitro studies and studies with

model plumbing systems have also demonstrated that biofilm formation was controlled with copper and silver ionisation (BSRIA TN6/96, Shih and Lin 2010). Copper and silver ionisation is the only

control method for which many field studies of efficacy have been published. In hospitals worldwide, its efficacy against Legionella has been thoroughly studied (Lin et al 2011, Bedford 2012). Studies have also demonstrated that

copper and silver ionisation is effective against other waterborne pathogens, including P. aeruginosa, S. maltophilia, Mycobacteria and Acinetobacter baumannii (Shih and Lin 2010). It has also been demonstrated that

copper and silver ionisation is effective against Legionella, P. aeruginosa, S. maltiphilia, and Mycobacteria regardless of water temperatures (Lin et al 2011, Bedford 2012). Therefore, hot water temperatures can be reduced when copper and silver ionisation is applied and adequately managed, without the risk of infection being adversely affected. One NHS Trust deliberately and safely


lowered the hot water temperature with copper and silver ionisation, demonstrating that where a biocide regime is correctly applied temperature control can be relaxed, reducing energy costs. Copper and silver ionisation is

explained in detail in the HSE HSG274 Part 2 technical guidance document: “Ionisation is the term given to the

electrolytic generation of copper and silver ions providing a continuous release of ions in water. These are generated by passing a small electrical current between two copper and silver electrodes; copper and silver alloy electrodes may also be used. When correctly used, copper and silver ionisation is shown to be effective at controlling Legionella and can penetrate and control established biofilms.” It recommends values of ‘more than

0.2 mg/L copper and more than 0.02 mg/L silver at outlets to ensure effective control of Legionella.’ The HSE HSG274 Part 2 and the new

HTM 04-01 also emphasise the maximum allowed value of 2 mg/L for copper in drinking water in the Water Supply (Water Quality) Regulations, 2001, but that no maximum allowed value for silver is given in these regulations. Both documents, therefore, refer to the maximum allowed value of 0.1 mg/L set by the World Health Organisation (WHO) for silver in drinking water. The WHO guidance published in 2014 explains where the maximum level of silver, of 0.1 mg/L, derives from: “Present epidemiological and

pharmacokinetic knowledge states that an entire lifetime oral intake of about 10g of silver can be considered as the human No-Observed-Adverse-Effect-Level (NOAEL), and since the contribution of drinking water to this NOAEL will normally be negligible, the establishment of a

health-based guideline value is not deemed necessary. On the other hand, some situations may exist where silver salts are used to maintain the bacteriological quality of drinking water. Higher levels of silver of up to 0.1 mg/L (a concentration that gives a total dose over 70 years of half the human NOAEL of 10 g), could then be tolerated without risk to health.” The HSE HSG274 Part 2 highlights

that “maintaining adequate silver ion concentrations in hard water systems can be difficult, due to the build-up of scale on the silver electrodes potentially obstructing the release of copper and silver ions. It also highlights the pH sensitivity of the ionisation process and that dosing levels may need increasing for pH levels greater than 7.6.” Formation of scale on the electrodes is

prevented in the Orca system from ProEconomy Ltd by using pure copper and pure silver electrodes, instead of copper/silver alloys. Being able to increase or reduce the electrical current output controls separately, controls the release of the copper and silver ions. The polarity of the electrodes is also automatically switched from anode to cathode and vice- versa, which results in scale formed by cations, such as calcium and magnesium ions that may have adhered to the surface of the cathode, falling off its surface when the polarity switches to the anode.

References l Link to all four Health Technical Memorandum 04-01 documents [ publications/hot-and-cold-water-supply- storage-and-distribution-systems-for- healthcare-premises].

l Bedford B. 2012. Legionella control in water systems using copper and silver ion generation systems. PhD thesis. Cranfield University, UK. Available from: https:// 7983/1/Birgitta_Bedford_Thesis_2012.pdf

l Building Services Research and Information Services 1996 Application Guide AG 2/93. Water treatment for building services systems. Building Services Research and Information Services (BSRIA) Technical Notes TN6/96 1996. Ionisation water treatment for hot and cold water services.

l Lin Y, Stout JE, Yu VL. Controlling Legionella in hospital drinking water: an evidence-based review of disinfection methods. Infect Cont Hosp Ep 2011; 32: 166-73.

l Shih H-Y, Lin YE. Efficacy of copper-silver ionisation in controlling biofilm- and plankton-associated waterborne pathogens. Appl Environ Microb 2010: 76 (6): 2032-5.

l WHO 2014. Silver in Drinking-water: Background document for development of WHO Guidelines for Drinking-water Quality. WHO/SDE/WSH/03.04/14. Available from: dwq/chemicals/silver.pdf



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