GB2305439A - Water sterilisation wherein the electrical current applied to anode comprising biocidal metal ions is varied according to flow rate - Google Patents

Abstract

A water sterilisation system comprises a water flow conduit 1, an anode for introducing biocidal metallic ions e.g. silver and/or copper into the water flowing in the conduit, a power supply 3 for passing an electric current between the anode and a cathode, flow metering means e.g. insertion turbine 4 for monitoring the flow rate of water in the conduit, and control means 3 connected to the flow metering means and to the power supply for controlling the electrical current supplied to the anode as a function of the flow rate measured. A very low frequency magnetic field may also be induced in the water conduit to reduce build-up of scale on the electrodes.

Description

WATER STERILISATION SYSTEM AND METHOD Field of the Invention This invention relates to a water sterilisation system, and to a method of purifying water.

Background to the Invention The use of metallic ions such as silver and copper to kill bacteria in water is well-known, and different devices have been proposed to exploit this effect. Typi- cally, such devices are employed in systems in which water is re-circulated, for example cooling water for air conditioning systems, where control of the bacteria causing Legionnaires' disease, which can be carried by water droplets, is important.

Precise control of such systems is important, since too high a current on the silver anode which introduces ions into the water will waste silver and electricity, and risk the emission of potentially explosive gases, while too low a current will result in the silver ion concentrations being too low to kill off all the bacteria, allowing a build-up to occur. Control of the current has typically been a matter of calculating an appropriate average current level and maintaining this when the water is flowing. In an attempt to improve on this, it has been proposed to measure the conductivity of the water on a regular or continuous basis, and then to vary the current accordingly.Unfortunately, this method is not accurate, because it is not possible to ensure that the conductivity measured reflects with any precision the silver ion concentration; other ions may be present which affect the final result.

In GB-A-2 270 395 there is disclosed a water sterilisation system in which the silver ion concentrations are monitored by means of a platinum probe exposed to the water stream. The electrical charge induced in the probe gives a measure of the silver ion concentration in the water. The current applied to the silver anode in the water circulation system is then controlled according to the output from the probe. A problem with such an arrangement is that, at the concentrations of sil- ver ions used for bactericidal effect, the ion probes are operating at the lower limits of sensitivity, and the results cannot form the basis of accurate control; the silver ion concentration produced will still vary considerably in practice.

Summary of the Invention According to the invention, there is provided a water sterilisation system, comprising a water flow conduit, an anode for introducing biocidal metallic ions into the water flowing in the conduit, a power supply for passing an electric current between the anode and a cathode, flow metering means for determining the volumetric flow rate of water in the conduit, and control means connected to the flow metering means and to the power supply for controlling the electrical current supplied to the anode as a function of the flow rate measurement.

The invention also provides a method of sterilising water, comprising causing the water to flow through a conduit containing an anode comprising a biocidal metal, and passing an electric current between the anode and a cathode to intro- duce biocidal metallic ions from the anode into the water, characterised by monitoring the flow rate of the water in the conduit, and varying the electric current applied to the anode according to variations in the flow rate.

Preferably, the biocidal metallic ions are silver and/or copper ions, but other metals are also known to have biocidal effects, and may be used instead of, or as well as, these metals. The other known biocidal metals include zinc, zirconium, yttrium, iron, titanium, aluminium, ruthenium, iridium, platinum and tungsten.

The water flow in the conduit is preferably not a recirculating flow, being instead a supply of water for an end use, for example drinking water.

The flow metering means is suitably an electronic flow meter having a turbine which is located in the flow to be measured. Control means receive a signal from the meter and vary the electric current to the anode in proportion to the signal. The relation between the signal and the current applied to the anode may be determined by calibration during setting up of the device, or may be subject to calculation based on the dimensions of the conduit and of the electrode. It will be necessary to determine also the conductivity of the water in order to establish the correct size of the anode.

For use in hard water areas, the apparatus may be provided with a variable frequency magnetic field device, consisting of a wire cable wrapped around the con- duit for a predetermined number of turns and at a distance upstream of the anode, for example 1.2m. A variable frequency transmitter connected to the coil induces a very low frequency electromagnetic field in the conduit, for example in the range of 0.1 to 10 milli-Tesla, to reduce the build-up of scale on the electrodes. Although the mechanism by which such an arrangement reduces scaling is not clearly understood, such devices are commonly used to reduce the adverse effects of hard water.

The need to avoid scaling is important, as build up of scale on the electrodes would reduce the accuracy of control.

Brief Description of the Drawing The drawing is a diagrammatic representation of apparatus according to an exemplary embodiment of the invention.

Detailed Description of the Illustrated Embodiment The apparatus comprises a water conduit 1 which provides, for example, a drinking water supply, and which contains an electrode chamber 2. The chamber 2 houses a silver/copper anode and a cathode, electrically connected to a power supply/controller 3. Ahead of the electrode chamber, an insertion turbine 4 is installed to provide a signal to the controller 3 indicative of the flow rate of water through the conduit. A by-pass pipe 5 is provided around the electrode chamber, with valves 6, 7 and 8 permitting the electrode chamber to be isolated from the water flow, which is diverted through the by-pass pipe 5, when maintenance is required, for example for the replacement of the electrodes.Downstream of the by-pass 5, a small side-stream pipe 9 diverts a sample of the water to pass over an ion selective probe which tests for the silver ion concentration as a final check and provides a signal which permits the controller 3 to generate an audible and/or visible warning when the concentration measured falls below, or exceeds, predetermined levels.

The design of the apparatus includes measurement of the conductivity of the water to permit the correct power ratings and generating potential of the electrode chamber to be determined. To establish the resistance required, the formula: Q=LBx)(WGis used, where Q is the resistance, B=conductivity in ohm centimetres, G= required electrode Gap for B, L= electrode length, W= electrode width.

The voltage required to supply a fixed current can then be calculated from V=U x Q, where U is the current in amperes. Next the determinant of copper or silver must be established. This will relate to the typical analysis of a representative water sample and local requirements. If, for example, coliforms and E. coli are present in the water, a level of 700 to 800 yg/litre of copper will be required. Using the upper level as the requirement, the final power rating is established by the for- mula: Z = FMXK, where Z = current in amps, F= flow rate in litres/s, K= conversion factor, and M= migration rate of copper ions from a copper bar. The calculation enables accurate sizing of the apparatus to ensure that it is capable of generating metallic biocidal ions in solution at a concentration adequate to maintain potable drinking quality water.

Claims (8)

1. A water sterilisation system, comprising a water flow conduit, an anode for introducing biocidal metallic ions into the water flowing in the conduit, a power supply for passing an electric current between the anode and a cathode, flow metering means for monitoring the flow rate of water in the conduit, and control means connected to the flow metering means and to the power supply for controlling the electrical current supplied to the anode as a function of the flow rate measured.

2. A system according to Claim 1, wherein the biocidal metallic ions are silver and/or copper ions.

3. A system according to Claim 1 or 2, wherein the water passes over the anode only once.

4. A water sterilisation system, substantially as described with reference to, or as shown in, the drawing.

5. A method of sterilising water, comprising causing the water to flow through a conduit containing an anode comprising a biocidal metal, and passing an electric current between the anode and a cathode to introduce biocidal metallic ions from the anode into the water, characterised by monitoring the flow rate of the water in the conduit, and varying the electric current applied to the anode according to variations in the flow rate.

6. A method according to Claim 5, wherein the biocidal metallic ions are silver and/or copper ions.

7. A method according to Claim 5 or 6, which comprises passing the water through the conduit only once.

8. A method of sterilising water, substantially as described with refer- ence to the drawing.