GB2094775A - Bactericidal method and apparatus - Google Patents

Abstract

To kill bacteria in an aqueous liquid, especially an oil-water emulsion which is used as a coolant and lubricant for machine tools, an apparatus is set up comprising means for inflicting damage to the cell walls of the bacteria in association with a source providing heavy metal ions arranged to expose the bacteria to such ions substantially immediately after the damage is inflicted. The preferred form of the invention is a gear pump in which at least the gear wheels are silver-plated.

Description

SPECIFICATION Bacterial method and apparatus This invention relates to a method and apparatus for killing bacteria present in aqueous liquids. It is of particular importance in relation to reducing the population of live bacteria in oil-water mixtures, especially the so-called "soluble oil"~water emulsions used as a coolant and lubricant.

"Soluble oil" is so called because the neat oil readily forms an emulsion when it is mixed with water, the oil:water ratio being typically about 1:24 to 1:32 by volume, i.e. about 3% to 4% of oil in the emulsion. (All oil:water ratios mentioned in this specification are by volume.) Such emulsions are used widely in the metal cutting and forming industries to cool and lubricate the workpiece. It has been estimated that some 20 million British gallons (90 million litres) per year of soluble oil are used for this purpose in the UK.

Aqueous emulsions of soluble oil become contaminated very easily by bacteria and the bacteria grow very rapidly. Many bacteria obtain sufficient nutrients for this purpose from the hydrocarbons present in the oil, others from additives in the oil or contaminants, e.g.

pieces of food, cigarettes or urine, which are introduced when the oil is circulating. Within a few weeks, a bacterial population of the order of millions per ml. of emulsion is often obtained.

The bacteria chiefly found in oil are of the genus Pseudomonas, especially the species Pseudomonas oleovorans. Other Gram-negative bacteria commonly found are those of the Escherichia, Aerobacter, Proteus and Klebsiella generaa, including E. coli. The Grampositive genus Streptococcus is also found.

(See Wort, Lloyd and Schofield in Tribology International, February 1976, pages 35-37.) The effects of contamination of the oil are serious. They include.

1. Degradation of hydrocarbons and decomposition of some additives.

2. Acidic intermediate products in the degradation or decomposition of constituents of the oil cause corrosion problems.

3. The emulsion is often de-stabilised so that an oily phase separates out.

4. The size of the oil droplets increases, thus interfering with lubricating action.

5. Sludge and slime form in sumps or containers and rancid smells are produced, particularly after a shut-down of the machinery. This causes the oil to be replaced, at great expense.

6. Infectious diseases are easily transmitted from oil containing these high counts of bacteria. Dermatitis, enteritis and bronchial complaints are believed to arise in this way. This is perhaps not surprising, considering that a bacterial count of 107 per ml. is about the equivalent to that found in untreated sewage.

Various attempts have been made to overcome the problem of bacterial contamination of soluble oil. Bactericides have generally not been successful, because strains resistant to them are developed quickly. It has been proposed periodically to clean out the whole system in which the oil circulates and add a bactericide, see "Metalworking Production", April 1980, pages 78-80. However, to clean the whole system is a laborious operation.

It has now been found that bacteria in an aqueous liquid can be killed by inflicting physical damage to the cell walls of bacteria and substantially immediately exposing the damaged bacteria to a source providing bactericidal heavy metal ions. Accordingly, the present invention provides such a method of treating an aqueous liquid, especially an oil-water mixture and most especially an emulsion of soluble oil in water. The invention also includes apparatus comprising means for inflicting physical damage to the cell walls of bacteria in association with a source providing heavy metal ions arranged to expose the bacteria to such ions substantially immediately after the damage is inflicted.

A preferred way of carrying out the invention is to circulate the bacteria-contaminated liquid through a gear pump, the gear wheels of which have been silver-plated. By this means it has been found possible to reduce the number of bacteria per ml. from 107 to 102 or 103 within 12 hours.

The invention is most surprising, bearing in mind that merely inflicting physical damage on the cells and merely exposing the cells to a silver surface are individually ineffective to reduce the number of bacteria significantly.

Indeed, even the combination is ineffective unless the bacteria are exposed to the silver in such a way that silver ions penetrate into the bacterial cell before the damage done to the.

cel wall can be repaired by the bacterium. The biochemical reasons for the success of the invention are not yet fully understood. Seen at a simple level, it appears that the energy of the cell is consumed in attempting to produce materials for the repair of the damaged wall and that during this period it is more vulnerable than usual to attack by silver ions. It seems that this effect is peculiar to use of metal ions as the bactericidal agent. Use of a conventional gear pump (unplated) in combination with a conventional biocide is ineffective.

In principle, the silver ions could be provided in other ways than by bringing the liquid into contact with a silver surface. For example introduction of a silver salt into the liquid could be contemplated. However, it is a specific advantage of the invention that the silver or other bacericidal heavy metal is provided in an economical and convenient manner by plating. In this way, there is no need to rely on an operative carrying out an instruction to add chemicals to the system, and no risk of waste or pilferage of supplies of such chemicals. Further, the use of silver-plating provides a slow release of silver and therefore a relatively long-lasting bactericidal action.

The exact mechanism by which silver exerts a bactericidal action is not known. It appears, however, that it acts to provide silver ions within the bacterial cell.

Other bactericidal heavy metals, e.g. copper or a copper alloy, could be used in place of silver. However copper metal itself is unsuitable on account of its softness. Copper-containing metals are also liable to attack by sulphur present in oils. Brass, cast iron and steel are ineffective.

Desirably, physical or mechanical damage to the bacteria is inflicted by a device which is plated by silver or other bacericidal heavy metal. Such an arrangement ensures that the heavy metal will strike at the bacteria when they are at their weakest, in accordance with the principles explained above. Instead of a gear pump other means of inflicting the damage could be used. Thus the liquid could be subjected to violent pressure changes, turbulence, intensive multi-directional agitation or any combination of two or more such effects sufficient to cause the damage, and preferably to burst a significant proportion of the cells.

Ultrasonic energy has been found to be particularly effective, but the high cost of the probe and asociated equipment makes this an unrealistic choice.

The primary function of the gear pump, preferred as the damage-inflicting means, is to cause the damage. The main pumping action could be provided by another pump in the circulatory system, if desired.

The invention is applicable to any aqueous liquid which is circulated. The principal application of current interest is to circulating oilwater mixtures, but it is contemplated that the invention could be used to reduce bacteria in ordinary domestic or commercial water systems. In many of these systems some of the bacteria are killed by the action of heat, but this does not entirely dispose of the problem and there is a build-up of bacteria in cold water tanks or header tanks.

In the application of the invention to the circulating soluble oil-water mixtures the oil to water ratio is preferably maintained at about 1:30, especially 1:24 to 1:36. If the ratio is allowed to fall too far, e.g. to below 1:10, the bactericidal effect does not persist for as long as normal. 1:36 is about as dilute an emulsion as is normally likely to be useful.

The bactericidal effect can be enhanced by adding a formaldhyde-based biocide and even such an unusually small proportion as 1 Og./litre is useful for this purpose.

It might be necessary to clean the gear pump periodically to remove slime and sludge, but this can be done with various solvents and is an easier operation than cleaning tanks and other parts of the system.

Preferred features of the invention will now be described with reference to the accompanying drawing. The drawing is a plan view of a gear pump, partly cut away to show its interior. Within the housing 1 is a gear chamber 2 of approximately figure-of-eight configuration, containing a driving gear wheel 3 meshing with a driven gear wheel 4 of the same diameter (1 inch; 2.5 cm.) and having the same number of teeth, namely sixteen.

Gear wheel 3 has a shaft 5 for connection to an electric motor (not shown). A pipe 6 leads into an inlet 7 in the gear chamber. An outlet 8 from the gear chamber leads back into an outlet pipe 9. Oil from inlet 7 is impelled around the gear chamber 2 by the gear wheels and leaves via outlet 8. Bacteria in the oil are "chewed" between the meshing gear teeth.

At least the gear wheels and teeth should be silver-plated; as a matter of convenience it is preferred to plate the whole surface of the chamber shown in the drawing.

Any kind of gear pump could be substituted for the particular pump shown, but preferably it should be one having a small tolerance. The pump shown has a meshing clearance of a few thou (1 thou = 0.001 inch = 0.025 mm.). Generally, the smaller the meshing clearance, the greater the damage that can be inflicted on the cell walls. It is usually sufficient merely to abrade the wall, without necessarily piercing it. It has been observed however that many of the bacteria emerge from the outlet of the pump in a burst condition, i.e. with severe rupture of the wall.

The silver-plate need have a thickness of only 1 thou (0.025 mm.). A greater thickness is preferably in order to prolong the useful life of the pump before it has to be re-plated and it is expected that about 3 thou (0.075 mm.) will be very suitable. Provided that there is sufficient clearance between the gear teeth greater thicknesses than 3 thou, for example up to 5 thou (0.125 mm.) or even greater would be usable.

The following Example further illustrates the invention.

EXAMPLE A gear pump as specifically described above with reference to the drawing having silver-plating of thickness 1 thou (0.025 mm.) on its interior gear-carrying surface was installed in place of a conventional gear pump in the cooling and lubricating system of a machine tool. The lubricant/coolant was an emulsion of the soluble oil "Dromus B" (made by Shell) in water in the ratio 1:30.

The gear pump was run at a speed of 2,000 rev./min. and with a throughput of 2 gallons (9 litres) per minute. Within 4 to 12 hours the concentration of bacteria was reduced from 107/ml. to 102 or 103/ml.

When 10g./litre of a biocide "Grotan" was added to the oil, the concentration of bacteria was around the lower end of the range, i.e. of the order of 1 02/ml. "Grotan" is a formaldehyde-based biocide.

The effectiveness of this method diminished after about 6 or 7 weeks, owing to a build-up of sludge on the silver-plated surfaces of the gear pump. However, when the pump was backwashed with a caustic alkali or detergent, its efficiency was restored.

Claims (14)

1. Apparatus suitable for use in killing bacteria present in an aqueous liquid, comprising means for inflicting physical damage to the cell walls of the bacteria in association with a source providing heavy metal ions arranged to expose the bacteria to such ions substantially immediately after the damage is inflicted.

2. Apparatus according to Claim 1 wherein the damage-inflicting means and the heavy metal ion source are provided by a device which is plated on damage-inflicting surfaces thereof with the heavy metal.

3. Apparatus according to Claim 1 wherein the damage-inflicting means is a gear pump.

4. Apparatus according to Claim 2 wherein the damage-inflicting means is a gear pump of which at least the gear wheels are plated.

5. A gear pump of which at least the gear wheels are silver-plated.

6. A gear pump substantially as hereinbefore described with reference to the accompanying drawing.

7. A method of treating a bacterially contaminated aqueous liquid which comprises inflicting physical damage to the cell walls of bacteria in said liquid and substantially immediately exposing the damaged bacteria to a source providing bactericidal heavy metal ions.

8. A method according to Claim 7 wherein the damage is inflicted by circulating the liquid through a gear pump.

9. A method according to Claim 7 or 8 wherein the heavy metal ions are silver ions.

10. A method of treating a bacterially contaminated aqueous liquid which comprises circulating it through a gear pump of which at least the gear wheels are silver-plated.

11. A method according to any one of Claims 7 to 10 wherein the aqueous liquid is a mixture of oil and water.

12. A method according to Claim 11 wherein the mixture is an emulsion having an oil:water volume ratio of from 1:10 to 1:36.

13. A method according to Claim 1 or 12 wherein the liquid contains bacteria of the genus Pseudomonas.

14. A method of killing bacteria in an aqueous liquid with the aid of a bactericidal heavy metal and bacteria-damaging means, substantially as hereinbefore described.