GB2298858A - Water treatment - Google Patents

Abstract

A method of killing microbial contaminants in water includes releasing metal ions, preferably silver, into the water, preferably by an electrochemical reaction and then removing them again, preferably by use of an ion exchange medium such as zeolites.

Description

WATER TREATMENT This invention relates to the hygienic treatment of water.

There are a number of applications where it is desirable to treat water so as to counteract any microbial contaminants therein. Frequently it is adequate to kill any such microbial contaminants.

A known technique for doing this is to liberate silver ions into the water, by an electrochemical reaction.

Silver ions are known to have an antimicrobial action.

This use of electrochemically generated silver ions has been discussed in a number of literature articles including Spadaro et al Antimicrobial Agents and Chemotherapy 6, 637 - 642, (1974).

Landeen et al Water Sci Tech 21, 267-70, (1989) Yahya et al Can. J. Microbiol 36 109-116, (1990) Pyle et al J. Applied Bacteriology 72, 71-9 (1992) UK Patents 1507324 and 1512146.

Suppliers of equipment for the electrolytic liberation of silver ions into water are Tarn-Pure Limited, High Wycombe, England and Tarn-Pure USA, Las Vegas, Nevada, USA. As mentioned in the Yahya et al and Pyle et al articles, copper and silver ions may be generated together, using an anode which contains both metals.

The silver is normally liberated into a solution at a concentration which does not exceed 0.1 parts per million (ppm). At such concentrations silver is rather slow in its cell killing action, which can be acceptable where the water can be stored for some while.

Thus this form of treatment has been applied to the water of swimming pools. It has also been used for water which is used for washing or cooling and then recycled or discarded. These are all applications where the concentration of metal ions can be kept low but slowness of cell killing can be accommodated, or where the water is not intended to be ingested so that the presence of heavy metal in the water is acceptable, at the concentrations which have been used.

We have now appreciated that the utility of metal ions for water treatment could be extended to other areas by arranging that the ions are subsequently removed from the water.

Therefore, according to the present invention, a method of treating water by introducing metal ions into the water to kill microbial contaminants therein, is followed by removal of these metal ions from the water.

By means of this two step procedure the water receives an antimicrobial treatment, but does not retain an enhanced concentration of heavy metal ions. This makes it possible to treat with metal ions at a concentration which will give faster cell killing but which would not be acceptable if the metal ions were retained in the water. This two step procedure can also make this form of water treatment acceptable in circumstances where the incorporation of a heavy metal might be regarded as undesirable regardless of concentration.

Water treated in this way may be used as potable water, which is incorporated into food or beverages or medicines.

Water which has been treated in this way may also be utilised in other kinds of industrial production, notably in the manufacture of aqueous liquid cleaning compositions such as products for cleaning windows and other fixed hard surfaces, liquid detergent compositions for hand dishwashing, aqueous liquid detergent compositions for fabric washing shampoos and other aqueous liquids for personal washing, and aqueous cosmetic products for application to skin or hair.

It is strongly preferred that the metal ions are removed by ion exchange, although other techniques may be employed, such as precipitation as an insoluble salt followed by filtration.

The preferred medium for ion exchange is zeolite, which is inexpensive and can be sterilised by heating, e.g. by exposure to steam.

Although the concentrations of metal ions in the water may be higher than customarily used hitherto, they can nevertheless be low concentrations, making it economic to discard the zeolite when it becomes loaded with the metal ions.

As an alternative to zeolite, a synthetic organic ion exchange resin could be employed. This might be regenerated when loaded with the metal ions, for instance by contact with nitric acid or sodium nitrate solution.

Zeolite could of course also be regenerated if desired, although this is unlikely to be worthwhile.

Silver ions can be introduced into the water in any convenient way.

The preferred method of introducing metal ions into the water is by electrochemical reaction, which is of course the technique that is already utilised when the metal ions are left in the water. The rate of liberation of the metal ions can readily be controlled by regulation of the electric current.

However, it would be feasible to introduce the metal ions into solution in some other way, such as by introducing a solution of a silver salt. A possibility would be the material dispensing of a silver nitrate solution using a peristaltic pump.

The metal ions which are liberated electrolytically are preferably selected from copper, silver and mixtures of the two, especially silver and mixtures of copper and silver. The electrolytic liberation of these ions requires apparatus which provides an electrolytic cell, with an anode containing the metal(s) to be released. This anode must of course be in contact with water to be treated.

This water may be in contact with the cathode, or the cathode may be immersed in a different aqueous solution, separated by a permeable membrane from the solution to be treated.

The anode may be silver, or a silver alloy from which both silver ions and other ions are liberated.

Various metals may be used as the cathode. Stainless steel (which is an alloy principally containing iron, chromium and nickel) is satisfactory. The cathode may be of the same composition as the anode. A known technique is to use anode and cathode of the same composition, place both in contact with the water to be treated, and periodically reverse the direction of the current so that both electrodes are consumed.

The water to be treated needs some conductivity. The concentration of ions which occur in normal tap water is usually sufficient for this, but if necessary a very small quantity of sodium sulphate or other innocuous electrolyte can be added.

The ions which are liberated into the water should of course be allowed to remain in solution for sufficient time to kill microbial contaminants. As mentioned above, the time required depends on the metal ion concentration.

We prefer that silver ions are liberated in sufficient quantity to yield a concentration of at least 1 ppm, better at least 3 ppm. The concentration is unlikely to exceed 500 ppm and the preferred range of silver ion concentration does not exceed 50 ppm, better still 25 ppm.

We have found that with 5 ppm of silver ions a contact time of 1 hour gives good cell killing. At 10 ppm a contact time of 5 minutes has proved sufficient.

When the silver ions are removed, the concentration of them is preferably reduced to 0.1 ppm or less, better below 0.08 ppm, even better below 0.05 ppm.

The invention may be carried out as a continuous process, or a semi-continuous process able to run for some hours at a time. If so, it may be arranged that water which has been treated electrolytically must pass through a run of pipework, or a sizable vessel, creating a period of delay before the ions are removed, e.g. when water contacts an ion exchange medium.

It will probably be desirable to sterilise the ion exchange medium after each regeneration or recharge of an ion exchange column or bed.

Another possibility would be to use the method of the invention from time to time when required, rather than regularly. For instance, the method could be employed to treat the water in a supply tank in the event that an outbreak of microbial contamination was detected.

Treatment in accordance with the invention could be continued for as long as necessary until the outbreak had been suppressed.

Embodiments of the invention, and apparatus for carrying it out will now be described with reference to the accompanying diagrammatic drawings in which Fig. 1 shows one form of apparatus and Fig. 2 shows a second for of apparatus.

Fig. 1 illustrates apparatus in which water from a supply 10 passes through a chamber 12 provided with a silver anode 14 and a stainless steel cathode 16. These are supplied with low voltage direct current causing liberation of a low level of silver ions into the water, at a concentration in the water of 10 ppm.

The water then flows into a holding vessel 20 which is of such a size that the residence time in the vessel 20 is approximately twenty minutes.

Next the water flows down a column 22 packed with zeolite particles. The silver ions are removed here, and the water leaves via outlet 24.

The apparatus can run for a lengthy period of time after which it must be stopped to change the zeolite. To do this the valves 26 and 28 are closed, the column is emptied via a bottom outlet 30, and refilled with fresh zeolite through a top inlet 32.

After replacing the zeolite, steam from supply 34 is admitted through valve 36 and passed through new zeolite in the column 22 to sterilise it. The steam outlet is at 38. Operation of the apparatus can then be resumed.

Fig. 2 illustrates a form of apparatus in which the invention is applied only when required. A number of parts of the apparatus resemble that of Fig. 1 and where the same reference and numerals are utilised, the parts are similar.

In normal operation of the apparatus water from a supply 10 passes through a chamber 12 in which silver ions are liberated into the water to provide a concentration in the water of less than 0.05 ppm. The water taken off from the holding vessel 20 for use via outlet 40 controlled by valve 42 and the very low concentration of silver ions introduced into the water is not removed.

From time to time it may be necessary to deal with an outbreak of contamination in the holding vessel 20 or it may simply be desired to sterilise the vessel after maintenance. To accomplish this the outlet valve 42 is closed and a quantity of silver ions, suitably in the form of silver nitrate solution, is introduced via the inlet valve 44. The quantity of solution introduced via the inlet 44 is sufficient to provide a silver concentration in the water of at least ppm, quite possibly much higher such as 50 ppm.

After a time the valves 26 and 46 are opened. Water from the vessel 20 is circulated through the vessel 22 removing silver ions from the water.

Eventually a concentration of silver ions in the holding vessel 20 falls below 0.05 ppm after which circulation through the column 22 is discontinued, the valves 26,46 are closed and normal operation with delivery of water through valve 42 can be commenced or as the case may be resumed.

It should be appreciated that this form of apparatus could be employed without the continuous introduction of a small concentration of silver ions, if it is considered that the incoming water is normally sufficiently pure not to require further purification. In this event the incoming supply 10 would delivery directly into the holding vessel 20.

After silver has been added to the holding vessel 20 and subsequently removed on the column 22, the packing of the column can be changed and the replacement zeolite in the column sterilised, all as described previously with reference to Fig. 1.

Claims (9)

1. A method of treating water by introducing metal ions into the water to kill microbial contaminants therein, characterised by subsequently removing these metal ions from the water.

2. A method according to claim 1 wherein the ions are liberated into the water by electrochemical reaction.

3. A method according to claim 1 wherein the metal ions comprise silver ions at a concentration in the range 1 to 50 ppm.

4. A method according to any one of the preceding claims wherein the metal ions are removed by ion exchange.

5. A method according to claim 4 wherein the ion exchange medium is zeolite.

6. A method according to claim 5 including sterilising the ion exchange medium with heat.

7. A method according to claim 5 including sterilising the ion exchange medium with steam.

8. A method of purifying potable water, comprising treating the water by the method of any one of the preceding claims.

9. A method according to any one of the preceding claims which is a method of counteracting microbial contamination of water, wherein metal ions are maintained in the water until the contamination has been removed, and the metal ions are then removed from the water.