The importance of knowing; ‘when’ to use supplementary control and ‘which’ control to use!
Part 3 - Supplementary Control: the use of Cu/Ag ionisation, UV & Filtration
Part 2 considered the merits and limitations of ‘traditional’ temperature control and supplementary control strategies, such as chlorine and chlorine dioxide. In Part 3, we expand this discussion to include supplementary control strategies to include copper-silver (Cu-Ag) ionisation, ultraviolet light [UV] and filtration and consider when these treatment methods may be indicated for use. Previously, in Parts 1 & 2, we discussed the importance of first understanding the water systems to be treated, to include but not be limited to:
- The ecology of water systems (microbiological concerns);
- The chemistry of water systems (water hardness, pH);
- The integrity of water system (materials of construction).
Developing an understanding of water systems in this regard may help with adopting a ‘proportionate approach’ to water risk management that is effective whilst minimising damage to the system components. Moreover, in Part 2 we discussed the benefits and limitations of using oxidising biocides such as chlorine and chlorine dioxide. The effectiveness of halogens in treating waterborne pathogens is well-documented, albeit water temperature dependent, however their use has also been associated with the premature dilapidation of water systems due to their corrosive nature.
This has given rise to alternative treatment strategies that offer biocidal control but are not associated with compromising the integrity of treated systems. Copper [Cu] and silver [Ag] ionisation may be considered such a strategy – demonstrating the effective treatment of Legionella species (spp) within both hot and cold-water systems. Cu/Ag ionisation entails the electrolytic generation of copper and silver ions by passing an electrical current between copper and silver electrodes. The ions generated have bactericidal properties in water and are reported to be particularly effective at treating Legionella bacteria and penetrating mature biofilms, although the ‘broad-spectrum’ effectiveness of this treatment is less well known.
System manufacturers typically indicate an optimum dosing range of 0.2-0.8 mg/l of copper and 0.02-0.08 mg/l silver respectively to control Legionella bacteria. The Water Supply [Water Quality] Regulations advise maximum permissible concentrations for both copper and silver in drinking water of 2mg/l and 0.1mg/l respectively; which necessitates an ongoing cost for regular testing within various parts of the treated system.
Guidelines indicate weekly testing of the supply water, monthly testing of sentinel outlets & annual testing of representative outlets [on rotation] to ensure that these metals remain within acceptable levels.
Cu/Ag ionisation is not however immune to criticism and the following limitations & problems have been noted:
- the efficacy of the system is contingent upon the adequate cleaning of electrodes and the pH of treated water;
- Cu/Ag systems that process ‘hard water’ are more susceptible to scale build-up and these systems have demonstrated to be less effective at treating waters of increased alkalinity (pH >7.6). Therefore, the additional cost of installing/maintaining water softeners is often indicated for use within these areas - whereby the Cu/Ag is positioned downstream of the softening plant.
- Water systems may be affected by unsightly staining – leading to the subsequent replacement of sanitaryware;
- the use of silver for water treatment has been criticised by microbiologists due to concerns surrounding the potential for increased antimicrobial resistance and the impact that this may have on the ‘therapeutic’ use of silver in medicine.
This leads nicely onto the remaining supplementary control options available to us; which include ultraviolet light [UV] irradiation and filtration. These methods are not chemistries but can still be used to augment the water quality of treated systems.
UV can be used to create ‘UV lesions’ on the bacterial cell wall until cell lysis [breaking open the bacterial cell] is achieved. Therefore, efficacy is dependent upon proximity to the point of application – to ensure that bacterial contaminants are exposed to an effective concentration of UV irradiation. This ‘modus operandi’ helps us to understand the main limitation of this treatment strategy – ‘Shadowing’ is the process of bacterial cells attaching to particles within an unfiltered water system and using these particles as a type of ‘sunscreen’ so that bacterial contaminants are only partially treated when passing through the UV system.
As such, partially treated bacterial cells maintain viability following photoreactivation (cell wall healing) as well as associated pathogenicity (in the case of P. aeruginosa and Legionella spp) and the ability to cause infection and subsequent disease. Therefore, for UV systems to remain effective then coarse (particulate removing) filtration is often indicated.
Filtration can be described as a ‘physical disinfection’ strategy whereby three types of installation are typically indicated:
- Point of entry (POE)
- Point of use (POU)
POE and process filtration systems may be indicated to improve upon microbiological quality whilst reducing particulate contamination within water entering buildings or specialist units, whereas POU or dead-end filtration is typically indicated to protect persons at the point of use/care.
POU filters often utilise a 0.2µm or 0.1µm sterilising-grade membrane thus ensuring that the ‘filtrate’ is free from waterborne bacterial contaminants. Finally, the limitations of using POU filtration centre on the implications of reducing or restricting water flow to filtered outlets and the associated increased potential for microbial growth within these systems.
In addition, whilst filtration may represent an effective means of achieving desired water quality, it’s prudent to consider that such filtration (unlike other supplementary control strategies discussed) does not ‘remediate’ the affected system. Therefore, whilst the sole use of filtration may not necessarily be the solution, the proportionate use of filtration may assist by helping to ensure that service users are protected at the outlet whilst remedial works are undertaken.
If you have questions regarding the issues raised above or you would like to speak with one of our consultants please click here to get in touch.
Editors Note: The information provided in this blog is correct at date of original publication - March 2020.
© Water Hygiene Centre 2020