Copper / Silver Ionisation : burns, boats, and buildings

A Short History

Hippocrates (460-370 B.C.) the ‘father of modern medicine’ used silver to treat wounds. Silver was used clinically early in the last century for sutures (stitches). Subsequently Silver is in use for sutures and dressings and 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. The British Navy used Copper for sheathing the hulls of wooden ships to prevent rot in the 18th century. 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.

So that explains the title for you - there is historical evidence of the effects of copper and silver on burns, boats and buildings.

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 the most susceptible.

Applying copper and silver to external surfaces or topically is one thing, ingesting them is another. The concentrations of copper and silver in solutions normally required to control microorganisms would probably be harmful to us too if ingested. So copper or silver solutions cannot be used in drinking water. However, activated forms of copper and silver can still retain their antimicrobial effects on microroganisms at much lower and safe-to-drink concentrations. Cu/Ag treatments are effective in low concentrations 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.

Copper / Silver Ionisation

Copper / Silver (Cu/Ag) ionisation is a relatively well-established technology for disinfection of potable water systems. This technology has published reports of notable successes and notable failures.

An electric current passes between electrodes containing the metals and this ionises the Cu/Ag. The electric current ionises the metals and releases them as chemicals into the water supply. Regulating the voltage across the electrodes and water chemistry regulates the concentrations of the ions in the water.

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) for human exposure. WHO Drinking Water Quality guidelines suggest that silver concentrations in drinking water stay below 1 ppm maximum.

A simple diagram of copper /silver ionisation

Normally in a static solution the ions move from the anode to the cathode and deposit there. However, in a potable water system they wash away with the flow of water to chemically dose the system. Free ions are then circulated 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, or other positively charged ions. So, cleaning the cathode regularly is necessary to remove this build up or the system fails because the cathodes 'cakes up'. Obviously as time goes by the anode loses all its ions and needs replacing.

Critical Review

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 the systems need frequent and regular maintenance by competent water treatment / plumbing professionals to be effective (enHealth Guidelines for Legionella control 2015 below).

As the table below points out water chemistry is a major issue. In hard water the cathode will attract positive ions like Calcium and Magnesium and electrodes can scale up causing the system to fail very rapidly and need constant attention. Zinc (in hot water sacrificial anodes) also interferes with the copper / silver ionisation process. If there are any sacrificial anodes within the system (storage hot water cylinders have these for instance to reduce corrosion) this will compromise the disinfection process.

Higher doses are necessary in water with pH above 7.6. As a result, 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.

Just like other chemical treatments 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 to give microbial control. 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 (softeners) 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.

Operational Considerations

As discussed Cu/AG systems cannot maintain effective disinfection residuals when they are fitted to a warm or hot water system. In these circumstances a reliable residual concentration after the tempering device is impossible (see Table above).

The UK HSE recommends taking monthly Copper / Silver water samples for testing in an accredited laboratory. It is not possible to test for Silver at the time of sampling. 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 may be necessary, especially in hard water. Monitoring of the pH is also important. Electrodes may 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 elevating operational costs.

As with all disinfection systems, Cu/Ag should be part of a comprehensive Water Safety Risk Management Plan. the technology will only perform if it is part of a comprehensive maintenance and monitoring program built around the unique characteristics of the system design, operation, and water quality.

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Our business delivers industry leading processes, systems, training and support as well as market leading disinfection products that will enable you to manage and reduce risk of waterborne infection. We are an ISO accredited business and hold Systems, Safety and Environmental certification, please feel free to contact us if you would like to talk more.