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Extract relating to Silver
Most metals are “oligodynamic;” this means that “with only a small amount, they can produce an effect.” Metals like silver, copper, mercury, manganese and iron, among others, are all potential water disinfectants. However, of all of these metals, only silver, for several reasons, has been used to some extent to disinfect water for human consumption, and this use dates back to ancient times.
Silver is not particularly toxic to human beings and, on being ingested, the body absorbs only a very small fraction. Large doses of this metal used for certain medical treatments have been found to cause discoloration of the skin, hair and nails (argirosis), but no problem has been noted with the small concentrations needed to disinfect water. The WHO has not proposed any guideline value for silver in drinking water, precisely because of its relative safety. Treatment of drinking water with silver produces no abnormal taste, smell or color. Nor are any DBP generated.
Silver has disinfecting properties only in its colloidal state —when it is in the form of extremely small particles in suspension which, because of their size, are easily charged electrically. In this state, it is also known as silver protein, silver salts, weak silver protein and strong silver protein. The salts that are used are silver chloride and silver iodide.
In its colloidal form, silver does not eliminate viruses, but is considered highly effective in destroying different types of bacteria. Silver’s disinfection mechanism acts by inactivating bacteria and mold cell enzymes that need oxygen for their metabolism; its causes their cellular disruption, although over periods that vary widely according to the water temperature. Very long periods are required at temperatures of 10 °C or less, making it difficult to determine silver’s precise germicidal power. Colloidal silver can remain in the water for a long period of time, but is not considered to have good residual power because of the slowness of its reactions in eliminating organic matter. The recommended dose for high germicidal efficacy is in the range of 25 to 75 micrograms of silver per liter (0.025 – 0.075 mg/l).
Three methods are used for disinfection with silver. The first or “contact” method requires the passage of the water through silver saturated devices, such as tanks with walls and screens coated with special silver-containing paint. The second method consists of dosing low concentration silver solutions in the same way used for chlorine solutions, with similar equipment and feeders. The third, electrolytic, method appears to be the most practical. A number of silver electrodes are connected to the positive pole (anode) of a low power electric source.
An inert Water disinfection electrode is used as a negative pole, where hydrogen
is produced and freed. Through electrolysis, the silver ions are freed by the
electrodes inside the water current to be treated in proportion to the supply
current. The method is appropriate, for the dose can be varied by changing the
The electrolytic method is used only for small water supply systems. From a
practical and safety viewpoint, a certain degree of automation and complexity are needed in the control system, which should have sensors to check the properness of the disinfection. This simply cannot be done manually. A connection should be made to a solenoid valve that can cut off the water flow automatically at any moment if the system is unable to produce the proper dose.
There is no simple test for measuring the silver content of the water. The measurements taken with the existing test show a considerable degree of error The most effective method is to dose the water with controllable amounts of silver —in other words, the control is carried out basically through the dosing and not analytically following it.
Silver paint is not very expensive, but this is the least appropriate method. The dosing of a silver solution for a small population requires the equipment already cited in the case of chlorination; a wide range of diaphragm metering pumps can be used and their cost is not high. The solutions, on the other hand, are extremely costly, especially compared with the equivalent chlorine solutions that provide the same bactericidal capacity.
The electrolytic devices are expensive for small systems and can cost anything from $1,000 on up. The final cost depends on the size of the flow to be disinfected and the ancillary equipment needed. Insofar as the operating cost of this equipment is concerned, not only must the cost of the silver solution be considered, but also that of the electric power. In the case of its maintenance, it is necessary to consider the cost of electrodes, which must be replaced frequently because they are the only source of the silver ions and they wear out fairly rapidly.
Silver’s apparent advantages for water treatment are that it does not produce any taste, odor or color in the treated water and that no by-products are formed as a result of its use.
The methodology is very simple and easy to handle in rural areas of the developing world. For that reason, it is appropriate for household water disinfection. One of its disadvantages is the difficulty in controlling dosing for lack of a simple laboratory analysis. The second disadvantage —and this has proven to be an insurmountable barrier throughout history— is the high production cost. Both the electrolytic method, in which the electrodes that are needed to produce the silver ions wear out fairly rapidly, and the dosing of colloidal silver are expensive.
Disinfection with silver has been estimated to cost between 200 and 300 times more than chlorination.