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Volume 77, 1948-49
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Silver, a Sterilising Agent.

It is well known that drinking water must conform to certain hygienic standards in order to be taken without danger to health. Whenever the water available does not fulfil these requirements or is of doubtful origin, it must be sterilised. There are many technically important methods for sterilising water, but none is so simple and neat as the process of employing silver, of which one form is known as the Catadyn process.

Although this method is not new, the first commercial apparatus having been developed more than 15 years ago, it is not as well known as it deserves to be. This is probably due to the fact that the older systems for sterilising water were already firmly established and backed by large financial interests and perhaps also to the fact that silver is a “precious” metal.

Before going into the details of this process, it may be interesting to note that the historian Herodotus mentions that the Persian King Cyrus used to have the water for his soldiers carried in silver containers because by this means it kept in good condition. This has probably been considered as an extravagant story about an Oriental king showing his wealth, but in the light of present-day knowledge it appears that this method of preserving water really had a scientific foundation.

In the nineties of the last century, the Swiss botanist Naegeli observed that some algae (Spirogyrae) died when put into a glass vessel which had previously contained solutions of copper salts. This was due to the fact that the walls of the vessel had absorbed some minute quantities of copper which in turn had been absorbed from the walls by the water. He observed the same phenomenon with silver salts and the remarkable fact was that the silver acted in extreme dilutions such as 1:100,000,000. As he did not think that these extraordinarily weak solutions could produce a chemical action on the cells of the algae, he expounded the theory of a special force which he called oligodynamic (from the Greek words oligos = small and dynamis = force).

Later other scientists investigated this phenomenon and studied the effects of various metals on different organisms. It was established that copper and silver had the strongest destructive action and this is particularly effective on some of the pathogenous bacteria which may find their way into the human body with contaminated drinking water. Another important result was that the so-called “activated” water maintained its sterile condition over long periods, and even if contaminated afterwards with bacteria became sterile again because some metal ions remained in solution. The activated water thus acquired a self-sterilising property. This effect is quite different from that of other sterilising agents which disappear after a short time and provide therefore no protection against further contamination.

Many scientists have tried to find out why silver or other metals act as sterilising means, although applied in a highly diluted state. Various theories have been put forward, but no satisfactory explanation of the phenomenon has been forthcoming. However, one assumption is that silver ions must come into contact with the surface of the living cells. They are then absorbed and so alter the protein constituents that the cells die.

It should be understood that not all bacteria are killed by oligodynamic action. Spore-forming bacteria are not affected by it, but luckily these are harmless to the human health and, as a matter of fact, they survive the effects of other sterilising agents also. Amongst those that are killed by the action of metal ions are: Bacteria causing typhoid and paratyphoid fevers, dysentery, cholera, coli, staphylococci and streptococci.

Bact. coli is generally used for tests because, although it is quite harmless, it has a similar or rather greater resistance than the above pathogenous bacteria. When the growth of Bact. coli has been inhibited, it is a sure sign that all the other related bacteria have been killed also.

For a long time these discoveries remained of purely theoretical interest because the time taken to obtain sterilisation was rather long and no one had yet found a simple way which would enable any inexperienced person to use this method.

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It was obvious that copper was not a suitable material as it is toxic even in small doses, but no such objection hold for silver. In fact, even excess doses of silver would not cause any ill effects since it is well known that for some diseases relatively large quantities of silver are injected into the blood stream of sick people. Moreover, experiments on animals which were given over-activated drinking water have proved conclusively that the silver has no harmful effects whatever. The quantity of silver taken by using silver forks and spoons is larger than that which one would receive by drinking activated water.

It was not until 1928 that Krause conceived a practical way of using the oligodynamic properties of silver for sterilising water. It was obvious that a large area of silver would have to be brought into contact with the water. To do this with solid silver would have meant prohibitive costs. He designed, therefore, a sterilising flask which was filled with a carrier material covered with a thin coating of silver. Thus a very large silver surface was created at a moderate cost.

These flasks had only a limited field of application for small water quantities up to about 3 gall. The time required to sterilise this quantity was four hours. The disadvantage was that the silver carrier was gradually covered with substances which precipitated from the water and therefore its activity had to be restored from time to time by cleaning.

It was Krause who coined the name Catadyn, combining thereby the words catalytic and oligodynamic, because, in addition to the oligodynamic theory, he believed that the silver had a catalytic action on the cells of the microbes.

This form of apparatus was quite successful, but its use was restricted to small groups of people on account of the small quantities of sterilised water produced.

Later Krause had the ingenious idea of applying the catadyn process to filter-candles. Filter-candles, such as the Chamberlain filter-candle, are usually cylinders of kieselguhr having very fine pores and giving a filtrate which is sterile as long as the candles are new, as the germs cannot pass through the cylinder. However, after a time, which varies according to the conditions of the raw water and its temperature, the microbes retained, but not killed, by the filter, multiply and thus gradually invade the filter until they grow right through it and appear in the filtered water. In this condition the water produced by the filter-candle is more dangerous than the contaminated raw water. It is necessary therefore to sterilise the filter-candles frequently, this being done by heating. It is a difficult and dangerous process as it may cause small invisible cracks so that afterwards the candle is no longer capable of retaining the bacteria. This danger is avoided by filling the cavity of the filter with silver. Since a filter-candle does not usually operate for 24 hours a day, there is always some time during which the filter-candle is idle but full of water. During the idle periods the silver goes into solution, permeates the filter-candle, and thereby kills the retained bacteria which it encounters. Such a filter-candle is therefore self-sterilising and does not require any attention or maintenance.

The only disadvantage of the self-sterilising filter-candles (and one which applies to all common filter-candles also) is that they need considerable pressure for their operation (about 14–28 lb. per sq. in.) if the output required is to be substantial, say 25 gall. per hour or more. Several candles can be put in parallel and thus the output can be increased. Ten candles in one container, producing about 250 gall. of water per hour may be considered a practical limit. Apparatus using these candles has been developed for mobile installations, and the necessary pressure in these cases is supplied by either hand- or motor-driven pumps.

It should be noted that the water from self-sterilising filter-candles is not “activated,” as the time of contact is far too small. The only purpose of the silver is to sterilise the candle.

As the two methods so far described are limited in their output, they are unsuitable for dealing with large quantities of water such as are required for communities or for swimming pools. The problem was to get relatively large and constant quantities of silver into the water in a short time. It had

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been found that with an activation of 50 gamma/1 (1 gamma = 10−6 grams), a time of about two hours was required to reach sterilisation (this is equivalent to 35 grains per 10,000 gall.). This could be cut down to one hour by an activation with 100 gamma. It will be remembered that with the contact method a considerably longer time was necessary, which was due to the time taken for putting the silver into solution.

Krause's next step was to dissolve the silver not by contact, but by electrolysis. It is known that an electric direct current passing between two electrodes will carry metal ions from one electrode, the anode, to the other, the cathode. According to Faraday's law 1 amp.-hour causes 4.023 grams of silver to be dissolved. If the silver is prevented from reaching the cathode by removing the electrolyte continuously it will remain in the water. Thus by making the water flow between two electrodes and by regulating the current to suit the flow velocity, any desired degree of activation can be imparted to the water. This is the method used in the so-called electro-catadyn process. As water is not a perfect electrolyte it has been found that in practice Faraday's law is followed only to about 50 per cent., i.e., for every 1 amp.-hour 2 grams of silver are dissolved.

In order to avoid electrolysis and with it alteration of the water itself, the voltage for activation is kept in practice at between 1 and 1.5 volts. It should be stressed that here as well as in the contact process, the quantity of silver used is so small that the natural qualities of the water are not affected in any way whatever, which is more than can be said of most other methods of sterilisation.

The device containing the electrodes is called the activator, and its size can be designed to suit the quantity of water which it is desired to treat.

As no two waters are alike, it is always necessary to determine the current density which the electrodes can tolerate within the voltage limits for a particular water. The current density referred to the electrode area may range with different waters from 0.25 to 2 amp./m2, with an average of about 1 amp./m2. When designing mobile installations, which may be used with waters of varying resistivity, allowance is made in the size of the electrodes so that even those with high resistivity can be treated.

Depending on the size of the installation, the electric power may be supplied by dry batteries, accumulators, or from a.c. mains after suitable transformation and rectification. A rheostat in conjunction with an ammeter in the d.c. circuit is all that is required for regulation. The actual activation may be varied to suit different conditions of water flow. Apart from the usual complement of electrical equipment, comprising switches and fuses, a polarity-changing time-switch is used. As both electrodes are of the same size and material while only one is consumed, the polarity is changed at regular intervals of about 15 minutes. Thus both electrodes are used up uniformly, and in addition difficulties with polarisation of the electrodes are avoided. The installation is quite simple. Once the water flow and the d.c. (in accordance with the activation desired) have been regulated no further attendance is required.

For individual use a small activator has been designed which can be carried easily in a pocket. In this case a 3-volt torch-type dry battery is used as power supply. The electrodes are so dimensioned that a rather high activation is achieved so as to cut down the time of sterilisation and to make sure that enough silver is available even if the water is not very clear.

This brings up the important point that the water to be activated must be reasonably clear, as all impurities absorb some silver which is thus not available for sterilisation. The water must be filtered if this fundamental condition is not fulfilled.

Where iron and manganese salts or sulphuretted hydrogen are present in the raw water, steps must be taken to eliminate those substances as they would inhibit the action of the silver. The former precipitate as colloids and absorb silver, while the latter causes the formation of silver sulphide, which is rather ineffective as far as sterilisation is concerned.

Two interesting designs in mobile plants were developed for the Italian Army in 1938. The first apparatus can be carried on the back like a soldier's pack, its weight is about 40 lb., and its output is about 25 gall. per hour. The second is a plant delivering about 125 gall. per hour. It is divided into two parts,

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which are to be carried by a mule. In both types self-sterilising filter-candles are used, followed by an electro-activator. The pressure is supplied by a hand pump. This combination has been made purposely because there is always the possibility that the purified water may be carried over long distances before it is actually consumed. During the time which elapses between purification and consumption, the water may be contaminated again through dust, flics, etc., and may therefore not be safe. However, if the water has been activated with the silver ions, the bacteria are destroyed as fast as they develop and the water is always safe and drinkable.

The self-sterilising property of activated water makes it ideal for swimming pools where contamination occurs, so to speak, without interruption. In modern swimming pools the same water is generally used for quite a period and in order to keep it in good condition it is circulated continuously through a purification plant having a filter and some means for sterilisation. It is usual to inject chlorine into the water in order to effect sterilisation. Chlorine is very prone to effect bathers' eyes and ears and is therefore not altogether harmless. Activation with silver ions avoids any such difficulties, and since the silver accumulates in the water as it passes again and again through the activator, only comparatively low activation need be given each time. The activation thus does not cost any more than chlorination. Yet the protection is better, as the silver stays longer in the water than chlorine.

Another application of sterilisation with silver ions is in the preservation of vinegar. Even in the best-conducted vinegar factories there is always the possibility of the vinegar being infected with vinegar eels and film or mother formation.

If the vingear before bottling is activated with 1,000–4,000 gamma/1, those two enemies of the vinegar cannot develop. The quality of the vinegar is not changed at all by the activation.

Activation can also be used with great advantage in order to interrupt the fermentation of wines. An additional advantage here is that the silver ions precipitate some of the colloidal matter contained in wines and thereby effect a certain amount of clarification.

In soft-drink manufacture activated water permits eliminating other preservatives yet it does not alter the taste of these drinks at all.

Where artificial ice is used in contact with food, the use of activated water for the production of the ice is also beneficial. Although the temperature of the ice inhibits the growth of germs, they are not killed. Activated ice is not only sterile, but sterilises the food with which it comes into contact. In this connection it is interesting to relate a case where activated ice had a rather undesired effect. A cheese factory manufacturing a special type of fermented cheese was using activated ice. It was found that the moulds necessary for this particular type of cheese died without any apparent reason. In this instance it was necessary to return to the use of ordinary ice.

In breweries strongly activated water provides a cheap disinfectant for the cleaning of pipes, barrels, etc.

Another field worth considering is that of artificially maturing alcohol. An important use of alcohol is in the manufacture of perfumes and liqueurs. In both cases the fresh product has to mature for a considerable time before it is ready for consumption. The effect of maturing is that of increasing the esters and decreasing the fusel-oils and aldehydes. The maturing time can be cut down to almost nothing by activating the alcohol with silver ions. To manufacturers this is a very important point, as storage space required normally for maturing is eliminated to a considerable extent.

It is thus seen that silver in highly diluted solutions can be put to various uses not generally appreciated. Before concluding it may be interesting to mention that while it is true that the concentrations required are very small, e.g., 50 gamma/1 for drinking water, 1 gamma (= 10−6 grams) contains 5 × 1015 silver ions. Therefore in terms of ions the quantity of silver is by no means small.


Mr. O. H. Keys: Are any swimming pools in New Zealand equipped with silver sterilisation? Comparing silver sterilisation with the residual chlorine the order of magnitude of concentration is not so much different. If chlorine concentration is properly regulated in accordance with the pH value of the

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water, there should be no ill effects on bathers' eyes, ears, etc. Are the costs of silver sterilisation the same as for chlorine?

Have the experiments with silver sterilisation been checked as to their effects? It had been found that minute quantities of some substances may be quite harmful to health.

Dr. Rothmann: There are no swimming pools in New Zealand equipped with silver sterilisation.

Operating costs for swimming pools depend largely on the operating cycle. They are roughly the same as for chlorine or a few per cent. higher.

As to effects on eyes, etc.: Chlorine must be very carefully regulated if these are to be avoided. However, even so some people are very sensitive to chlorine, and get inflammations, etc.

Even if the residual chlorine concentration is low and similar to silver it must be remembered that weight for weight one obtains a higher concentration for chlorine, which is a gas, as compared with silver, which is a metal.

Concerning the effects on health: My information goes only up to 1939. Until then all the facts reported in this paper had been checked frequently and in various countries and had all been confirmed.

Experiments on rats with very strongly activated water given for long periods showed conclusively after dissection, that no alteration in their tissues took place, nor did the general appearance of the rats indicate any different development from that of the control animals.

Mr. McLennan: Has silver been used for the sterilisation of city water supplies?

Dr. Rothmann: No, only for small communities. The cost of silver sterilisation for drinking water is higher than that for chlorine. On present-day prices it costs about 0.4d per 10,000 gallons of water with chlorine, but approximately 4d per 10,000 gallons with silver. This, however, is of minor importance, as the cost of sterilisation is only a relatively small fraction of the total cost of a water purification plant which includes pumps, filters, etc. (A diagram of comparative costs for three different projects for the city of Rome was shown on the screen.)

Mr. Korrick: Has silver been used for sterilising other liquids, such as, e.g., milk?

Dr. Rothmann: It is not suitable for milk as colloids would precipitate the silver ions. It can be used for washing cans and pipes, as in breweries.