Copper / Silver ionisation is a relatively well established technology for disinfection of potable water systems. This technology has published reports of notable successes and notable failures in these applications.
Copper / silver ionisation (Cu/Ag) first hit the disinfection market in Australia in the late 1980’s. Although at this time there was little field or laboratory data validating how well it works. Since then a number of studies have demonstrated its effectiveness. It’s value as a disinfectant is now available in a number of reference documents. Also a number of studies have shown it’s weaknesses. Let’s dig a little deeper into the history and value of this treatment for potable water.
A Brief History of Silver and Copper as Disinfectants
Some suggest that Silver was used as a disinfectant in water jars in Ancient Egypt. However it is more likely it was there for cosmetic purposes. Silver was believed to be good for the skin and complexion. Placing silver rods in water jars is more likely to be about beauty than bugs, especially as bugs were unknown at that time! Hippocrates (460-370 B.C.) the ‘father of modern medicine’ also used silver to treat wounds. Silver was used early in the last century for sutures (stitches). Subsequently Silver is in use as a topical (‘skin surface’) disinfectant for burns and wounds and does appear to speed up the healing process.
Copper has well known antimicrobial properties. It is in widespread use to control fungal pests in agriculture and algae in water bodies. Recent studies show microorganisms are less likely to colonise copper pipework and surfaces than polymer and copper free metallic surfaces. UK HSE guidelines recommend that copper fittings should replace flexi-hoses in health care premises to reduce potential for Legionella multiplication.
Copper / Silver (Cu/Ag) as a Disinfectant
Copper and Silver are only effective as disinfectants when they are ions (have a ‘charge’). When charged they will attach to oppositely charged molecules like proteins and stop them from functioning. An electric current passes between electrodes containing the metals and this ionises the Copper / Silver. So the electric current ionises the metals and releases them into the water supply. Regulating the voltage across the electrodes and water chemistry regulates the concentrations of the ions in the water. Typically target values of between 0.2-0.8 mg/L and 0.02-0.08 mg/L are set for Cu and Ag respectively. This is well within the recommendations of the UK Health and Safety Executive (less than 2 ppm for Copper). Obviously this is much less than concentrations used in the examples above. WHO guidelines suggest that Silver concentrations in drinking water stay below 1 ppm maximum.
Normally the ions move from the anode to the cathode and deposit there. However in a potable water system they wash away into the system. Free ions are then circulating through the water system. Of course to balance the loss of the ions landing on the cathode from the anode other positive ions deposit. These can be Calcium and Magnesium which will form a scale coating on the cathode. So the cathode may need regularly to remove this build up or the system will fail. Obviously as time goes by the anode loses all its ions and needs replacing.
Effects on Microorganisms
Both metals are ‘heavy metals’ on the lower end of the toxicity scale. Together they have a synergistic (complementary) disinfectant effect. Many higher organisms such as fungi, protozoa, algae and humans have reasonable resistance to the toxic effects. This is often by producing proteins that bind up the metal ions. However most bacteria are unable to make these proteins and this makes them more susceptible. There is very little evidence to confirm bacterial resistance to copper or silver. Legionella and other bacteria with outer membranes and thin cell walls are most susceptible. Other bacteria, like Mycobacteria, are less susceptible as the thick cell wall blocks entry of the metals ions and protects the cell membranes. Some published independent data shows Copper / Silver is much less effective (100 times) against Mycobacteria and their relatives in building water systems. Protozoa that are the hosts for Legionella and other bacteria are less susceptible to Copper /Silver ions than bacteria.
A number of reports show successful treatment of Legionella contamination using Copper / Silver technology in health care facilities. Some reports show rapid return of Legionella once ion levels fall below desired levels. The general consensus is that most systems need frequent and regular maintenance by competent water treatment / plumbing professionals to be effective (see table extracted from EnHealth Guidelines for Legionella control 2015 below).
As the table points out water chemistry is a major issue. As we state above the cathode will attract positive ions like Calcium and Magnesium. In hard water electrodes can scale up and the system fail very rapidly and need constant attention. The system works best below pH 7.6. Over this pH the kill time takes longer so higher doses are necessary. Consequently for these reasons Cu/Ag is very difficult to operate in cooling water and process water systems where hard water and pH well above 7.5 are the rule rather than the exception.
It is essential to install the system on the mains supply to the building. Past reports show that installation on warm or hot supplies will fail. This is because the ions dilute by mixing with the cold water supply at thermostatic mixing valves, tempering valves and sensor taps and lose their effect. Other in-line equipment can remove the ions so it is essential to install dosing units after them. Newly installed systems usually need an intense period of system adjustment and water testing to optimise the process. Copper leaching can occur if the systems are installed on new copper pipe work. In this case a program of water testing needs to ensure that copper levels stay below the 5 mg/L set by the Australian Drinking Water Guidelines.
The UK HSE advises that Zinc sacrificial anodes installed on hot water services to reduce corrosion can cause problems. The free Zinc ions (Zn2+) may interact with the ionisation process and compromise the system.
Cu/AG systems cannot maintain effective disinfection residuals when they are fitted to a warm or hot water system. Tempering devices will mix unknown amounts of untreated cold water (depending on it’s temperature) with the hot / warm supply. The volume flowing through these devices also changes during operation. In these circumstances a reliable residual concentration after the tempering device is impossible (see the enHealth Table above). Cu/Ag systems that can proportionally dose ions based on flow rates to the cold water supply can overcome these problems.
The UK HSE recommends taking monthly Copper / Silver water samples for testing in an accredited laboratory. It is not possible to test at the time of sampling. Test results will indicate the condition of the system at sampling not in the present. This is less convenient and more costly than many other more popular chemical disinfectant alternatives that you can measure on-site.
Weekly cleaning of the electrodes is necessary, especially in hard water. Monitoring of the pH is also important. Proportional dosing of the ions in relation to water usage is recommended. Electrodes will need replacing regularly depending on water quality and system demand.
Installation of the units is relatively quick and easy and comparable in cost to other disinfection methods. However the systems require ongoing maintenance from skilled providers on a weekly and monthly basis which is likely to elevate operational costs.
enHealth Guidelines for Legionella control in the operation and maintenance of drinking water distribution systems in health and aged care facilities (2015).
Lin et al (1988) Inactivation of Mycobacterium avium by copper and silver ions.Water Res. 32 (7)1997-2000.
US EPA (2016) Technologies for Legionella Control in Premise Plumbing Systems: Scientific Literature Review. Office of Water EPA 810-R-16-001