Metals such as copper and silver can be used for water disinfection, if they are ionized.
When was the disinfection mechanism of copper and silver discovered?
Archeological excavations show, that people have been using copper for more than 11.000 years and have been using silver for more than 5000 years. Copper can be easily extracted and processed. More dan 7000 years ago people developed a copper extraction mechanism for copper ores. The Roman empire gained most of its copper from Cyprus, the isle that gave copper its name.
Nowadays copper is mainly extracted form ores, such as cuprite (CuO2), tenorite (CuO), malachite (CuO3·Cu(OH)2), chalcocite (Cu2S), covelite (CuS) and bornite (Cu6FeS4). Large deposits of copper ores have been found throughout the US, Chili, Zambia, Zaïre, peru and Canada.
Silver can be obtained from pure deposits, from silver ores such as argenite (Ag2S) and horn silver (AgCl) and combined with ore deposites that contain lead, gold or copper.
Both copper and silver have been applied for centuries because of their biocidal mechanism. The Vickings used copper strings on their ships to prevent the growth of algae and shells. Modern ships still use the same technology.
Most anti-fouling paints contain copper, reducing the number of marine species growing on the walls of ships. Because of this measure, ships can reach their destination faster.
Nomads used silver coins to improve drinking water quality. Well water containing copper and silver coins is very bright, due to the biocidal affect of these metals.
Since 1869 various publications have appeared on disinfection properties of silver. Some European and Russian villages have been using silver for drinking water treatment for many years.
Copper-silver ionization was developed in both Europe and the United States in the 1950’s.
Figure 1: the Vickings used copper strings to prevent algal growth on ships
How are copper-silver ions produced?
Copper-silver ionization is brought about by electrolysis. An electric current is created through copper-silver, causing positively charged copper and silver ions to form.
Copper-silver ionization brings us back to basic chemistry: an ion; an electrically charged atom, has a positive charge when it gives up an electron and a negative charge when it takes up an electron. A positively charged ion in called a kation and a negatively charged ion is calles an anion. During ionization, atoms turn into kations or anions. When copper-silver ionization is applied, positively charged copper (Cu+ and Cu2+) and silver (Ag+) ions are formed.
The electrodes are placed close together. The water that is disinfected flows past the electrodes. An electric current is created, causing the outer atoms of the electrodes to lose an electron and become positively charged. The larger part of the ions flows away through the water, before reaching the opposite electrode. Generally the amount of silver ions at a copper ion rate of 0,15 to 0,40 ppm lies between 5 and 50 ppb.
The ion concentration is determined by the water flow. The number of ions that is released increases, when electric charges are higher.
When copper ions (Cu+) dissolve in water, they are oxidized immediately to form Cu2+ ions. Copper can be found in the water in free form. It is commonly bond to water particles. Copper (Cu+) ions are unstable in water, unless a stabilizing ligand is present.
What are the applications of copper-silver ionization?
Copper-silver ionization is suitable for a large number of applications. It became of interest when NASA used copper-silver ionization for drinking water prodcution aboard Apollo space ships in 1960. The ion generator that was used, was the size of a matchbox.
Because of copper-silver ionization, drinking water could be produced safely in space without the use of chlorine.
Figure 2: NASA used Apollo flights for one of the first copper-silver ionizations
In England, copper-silver ionization is applied in about 120 hospitals successfully for the deactivation of Legionella bacteria.
In the United States, copper-silver ionization is mainly used for swimming pool water disinfection. Copper-silver is often used to limit disinfection byproducts formation during chlorine disinfection.
Because of its specific properties, copper-silver ionization is very suitable for fishpond disinfection. Copper-silver ionization is not dependent on temperatures. It is active in the entire water system.
Copper-silver ionization is used by water bottling companies and companies that recycle water throughout the United States.
What is the disinfection mechanism of copper-silver ionization?
Electrically charged copper ions (Cu2+) in the water search for particles of opposite polarity, such as bacteria, viruses and fungi. Positively charged copper ions form electrostatic compounds with negatively charged cell walls of microorganisms. These compounds disturb cell wall permeability and cause nutrient uptake to fail. Copper ions penetrate the cell wall and as a result they will create an entrance for silver ions (Ag+). These penetrate the core of the microorganism. Silver ions bond to various parts of the cell, such as the DNA and RNA, celular proteins and respiratory enzymes, causing all life support systems in the cell to be immobilized. As a result, there is no more celular growth or cell division, causing bacteria to no longer multiply and eventually die out. The ions remain active until they are absorbed by a microorganism.
What are the disinfection applications of copper-silver ionization?
Swimming pools and copper-silver ionization
In the United States, copper-silver ionization is applied as an alternative for chlorine disinfection. Chlorine use can be reduced by 80%. However, another disinfectant should be added in addition to copper-silver. This is because copper-silver cannot remove organic matter, such as skin tissue, hairs, urine and skin flakes, from swimming pool water.
Cooling towers and copper-silver ionization
Cooling tower water requires disinfection, to prevent the growth of microorganisms. This can be brought about by a combination of copper-silver ionization and chlorine disinfection. Chlorine concentrations that are required are much lower.
Copper-silver ionization can also be used to kill Legionella bacteria in cooling towers.
Legionella in hospitals and nursing homes and copper-silver ionization
Copper-silver ionization is applied in hospitals and nursing homes to prevent the distribution of Legionella bacteria. The main source of Legionella distribution is the warm water system. Circumstances in warm water systems are ideal for Legionella bacteria to grow and multiply. Contagion mainly takes place through shower steam. Copper-silver ionization can sufficiently kill Legionella bacteria. Copper-silver can actively deactivate Legionella, as well.
Drinking water and copper-silver ionization
In the United States, several drinking water production companies use copper-silver ionization as an alternative for chlorine disinfection and to prevent the formation of disinfection byproducts. The standard for trihalomethanes was decreased by EPA from 100 to 80 µg/L.
When copper-silver ionization is combined with chlorine disinfection, it is an excellent disinfection mechanism to deactivate viruses and bacteria.
What are the terms of copper-silver ionization?
The affectivity of copper-silver disinfection depends on a number of factors:
Firstly, the concentration of copper and silver ions in the water should be sufficient. The required concentration is determined by the water flow, the volume of water in the system, the conductivity of the water and the present concentration of microorganisms.
Secondly, the electrodes should be in good condition. When the water is hard or fouling takes place as a consequence of water hardness and quality, there will be a decrease in electrode release and the additional effect will decrease. By using pure silver and pure copper, the supply of copper and silver ions can be regulated separately. These electrodes suffer from less limestone formation and fouling.
Thirdly, the affectivity of copper-silver ionization depends on the pH value of the water. When pH values are high, copper ions are less effective. When the pH value exceeds 6, insoluble copper complexes will precipitate. When the pH value is 5, copper ions mainly exist as Cu(HCO3)+; when the pH value is 7 as Cu(CO3) and when the pH values is 9 as Cu(CO3)22-.
Fourthly, copper-silver ionization affectivity is determined by the presence of chlorine. Chlorine causes silverchlorine complex formation. When this occurs, silver ions are no longer availbale for disinfection.
How effective is copper-silver ionization?
Copper-silver ionization can deactivate Legionella bacteria and other microorganisms in slow-running water and still water. Legionella bacteria are very susceptive to copper-silver ionization. Copper-silver ionization also takes care of bio film. Copper remains within the bio film, causing a residual effect.
It appears that copper-silver ionization additon causes the number of Legionella bacteria to diminish. After a short period of time, however, the number of Legionella bacteria will rise again because they can also be found in the bio film. Copper that stays behind in the bio film takes care of these bacteria. When copper and silver ions are added to water constantly, the concentration of Legionella bacteria remains low.
The deactivation rate of copper-silver ionization is lower than that of ozone or UV. A benefit of copper-silver ionization is that ions remain in the water for a long period of time. This causes long-term disinfection and protection from recomtamination. Copper and silver ions remain in the water untill they precipitate or absorb to bacteria or algae, and are removed from water by filtration after that.
What are the benefits and drawbacks of copper-silver ionization?
Copper-silver ionization affectively deactivates Legionella bacteria and bio film and it improves water quality. Copper-silver ionization has a larger residual effect than most other disinfectants. Copper and silver ions remain in the water for a long period of time. Because of its local affectivity, the effect is larger than that of UV. Copper-silver is effective throughout the entire water system, even in dead-end points and parts of the system that contain slow-running water. Copper-silver use affectivity does not depend on water temperature. When copper-silver is used, less maintenance to the water system is required. Copper-silver is non-corrosive; it causes less strain on the distribution system. Because of a decrease in the use of chemicals, the lids and pumps are not affected. Furthermore, shower heads, tanks and taps are not contaminated. When copper-silver ionization is applied, there are no transport and storage difficulties.
Copper-silver affectivity depends on the pH value of the water. At a pH value of 9, only one tenth of all Legionella bacteria are removed. When dissolved solid concentrations are high, silver will precipitate. This means silver ions are no longer available for disinfection.
Silver ions easily react with chlorines and nitrates that are present in the water, causing them to no longer be effective.
Some species of microorganisms can become resistant to silver ions. They can remove metal from their systems or convert it to a less toxic product. These microorganisms can become resistant to copper-silver ionization.
Allthough it is suggested that Legionella bacteria can develop resistance to copper-silver ionization, this disinfectant still appears to be effective for Legionella deactivation.
To affectively kill pathogenic microorganisms, copper and silver ions should be present in the entire water system. When the system is used little and the water flow is quite slow, or when there are dead-end points in the system, this can causes problems for disinfection.
What are the health effects of copper-silver ionization?
Unsufficient eveidence has been found on the possible health effects of long-term exposure to copper-silver ionization. Little is known on the general health effects of copper-silver ionization.
Legislation for copper-silver ionization
The European Union does not dictate any standards considering silver concentrations in the water. Copper, however, has a maximum value of 20 μg/L, because it corrodes waterworks. Copper concentrations should be measured in taps. (EU Drinking water directive 98/83/EC, 1998)
The WHO does not disctate any standards considering the concentration of silver as a drinking water disinfectant, because the organization found the available data to be insufficient to recommend a health standard. (WHO, Guidelines drinking water quality, 3e editie)
The United States dictate a maximum value of 1 mg/L of copper and a maximum value of 0,1 mg/L of silver. (EPA, National Secundary Drinking Water regulations, 2002)
How is copper-silver ionization controlled?
When copper-silver ionization is applied, a log of the entire system must be kept. Water analysis and tests must be conducted to prove system affectivity, because this concerns an alternative disinfectant. The first analysis round takes place before the application of copper-silver ionization. Copper and silver concentrations in the water are measured and the amount of Legionella bacteria and the aerobic growth number at 22 ˚C and at 37 ˚C are determined. When the system is placed, the outcome of water analysis should be checked and reported monthly.
More information on water disinfection?:
What is water disinfection? Necessity of drinking water disinfection History of water disinfection Waterborne diseases Factors that influence disinfection Conditions of water disinfection Regulation drinking water disinfection EU USA