Thermal Disinfection of Coolant Lubricants

Whoever works with coolant lubricants knows the problem: You enter the production hall in the morning and it stinks to high heaven, because the coolant in one of the tooling machines has turned.

As is well known the cause of this are microorganisms, namely fungi and bacteria which find a per­fect breeding ground in the coolant. These microorganisms mainly occur in water-soluble coolants and become more frequent in summer and in southern countries because of higher temperatures. The odour nuisance comes from the excretions of anaerobic bacteria.

Apart from the smell bacteria and fungi also have other unwanted side effects: They reduce the effectiveness of the coolant and so impair its performance. Occasionally pipes and tubes are blocked by fungus threads and/or mucus bacteria, exceptionally even fungus growths the size of a cabbage occur.

Maintenance of the coolant in most cases includes filtering the coolant with the aid of specific coolant filters as well as collecting surface oils with the help of skimmers and oil separators. Such mechanical metho­ds, however, are powerless against existing microorganisms. Even monitoring the coolant with respect to nitrite, ni­tra­te and the pH-value only allows to determine that resp. when new measure have to be taken.

For fighting bacteria and fungi in the cool­ant only very few methods are available. Widespread is exposure to ultra­vio­let light. This works relatively well with transpa­ren­t fluids but with coolant the UV rays can only disinfect at the surface because of the milky quality and impurities. As a result the coolant has to be reduced to a very thin layer so that UV rays can still penetrate the coolant. In practice this is a significant challenge and results are often rather modest: Often only 60-80%, and sometimes less than 50% of the existing bacteria and fungi can be eliminated.

Certainly most frequently applied are biocides, i.e. bactericides and fungicides. Without a doubt these have the advantage of being highly cost effective and normally also of being very efficient. On the other hand, biocides are hazardous substances under the CLP ordinance of the EU, which require a professional (and expensive) application.

So, in many respects one evil is replaced with another because biocides can result in considerable health complaints. These span from easy infection of everyday wounds to eczema and allergies all the way to irritations of the skin, the eyes, and mucous membranes. Concentrates in particular must not come into contact with the skin as even smallest droplets can lead to significant health issues.

Avoiding biocides would therefore reduce the health risks for the employees and contribute to reducing sick days.

Little known but all the more effective is the thermal disinfection, known as pasteurisation in hospitals and in the food industry. For this purpose the coolant is heated up to 65°C. As proteins and therefore bacteria and fungi are destroyed at ca. 62-63°C this method is close to 100% effective.

In a test series conducted by ph-cleantec in Fell¬bach thermal disinfection was analysed at 107 customers with 75 different coolants. 88 customers showed bacteria, at 29 of the 107 also fungi were found in the coolant.

Note: The quantity of fungi and bacteria prior to a ther¬mal disinfection of a coolant only shows that at a specific customer and at a specific point in time a specific amount of germs was found in the respec¬tive coolant. This may be a function of the tem¬pe¬rature, the specific application, or the age of the coolant, and does not say anything about the quality of the respective coolant or whether a par¬ticular coolant is particularly prone to developing germs.

Graphs 1a, 1b, and 2 show the results of the ther­mal disinfection for several coolants in practice. The graphs are on a logarith­mic scale, so 10^3 means 10 x 10 x 10 = 1000. Measured were colony-forming units per milliliter (cfu/ ml).

In 90% of cases – 79 out of 88 customers – bacteria were reduced to zero (graphs 1a and 1b). Only in 9 cases bacteria remained at all, but could be reduced by a factor 10^3 on aver­age. Even in cases of severe infestation – 10^6 or 10^7 bacterial germs per milli­liter – after the disinfection no bacteria were found in the coolant in 23 out of 26 cases, and in the remain­ing cases bacteria were re­duced by a factor 1.000 or 10.000.

Even more successful was the thermal disinfec­tion of fungi (graph 2): In all 29 cases these were reduced to zero.

From the users’ perspective particularly rele¬vant is that the thermal disinfection works in real life and in the long run. Companies using the system were able to completely avoid biocides and to pro¬lon¬gate the life span of their coolants signi¬fi¬cantly once they started to dis¬infect their coolant regularly. This is also acknow¬ledged by coolant producers.

As a result thermal disinfection is certainly one of the most efficient methods to disinfect coolants. It is, as no chemicals and no bio¬cides are needed, also health and environ¬men¬tally friendly. Finally, the thermal disin¬fec¬tion is par¬ticularly cost efficient as a rela¬ti¬vely low invest-ment for purchasing a machine for ther¬mal dis¬infection – e.g. the Ther¬mo-EK or the 1000 SR with a built-in option for ther¬mal disinfect¬ion from ph-cleantec in Fellbach – as wells as minimal operating costs, namely for elec¬tri¬city, are re¬quired. In any event, the dis¬infection should be run during full produc¬tion so that the content of tubes and pipes can be disinfec¬ted also. As a result there are also no stand still costs during disinfection.

This can be illustrated with a simple example: Consider a company with 10 tooling machines (TM) of 500 liters each where the coolant is changed twice a year the annual consumption amounts to 10,000 liters of coolant. At costs of €5/l of coolant concen­trate and a con­centration of 6%-that costs 10,000 l x 6% x €5 = €3.000. Say the disposal costs another €0.12/l, that is 10,000 l x €0.12 = €1,200. Add the cost of stand still times: Assuming the TM stands still for eight hours to change the coolant, and an hour of stand still time costs €100, then this costs 10 TM x 2 changes per year x 8 hours x €100 = €16,000. Total costs therefore amount to €20,200.

If the life span of the coolant can be prolon­ga­ted by 50% due to thermal disinfection the company saves €6,733 p.a., and if the life span of the coolant can be doubled the company even saves €10,100 per year. On top the costs for bactericides und fungicides can be saved. Last not least some benefits which cannot easily be quantified in terms of money need to be cited: No unpleasant smells, a clean production environment, and, importantly, less health problems.

In practice the machines used for thermal disinfection are mobile and can easily be driven to the respective tooling machine. The coolant is sucked into the disinfection device from the tank of the tooling machine, disinfected, and then re-injected into the tank of the tooling machine. With a dis¬infection capacity of 5-6 li¬ters per minute a tank of for example 500 liters can be disinfected once in less than two hours. Also an incorporation into a central cooling system is feasible.

Graph 2 Ideally, the sterilised coolant should be transferred to a sterile container and the machine tool should be thoroughly cleaned and sterilised before the coolant is reintroduced into the machine. However, this is hardly practical. In practice, the disinfected coolant is directly reintroduced into the bath and mixed with the non-disinfected coolant. This means that non-disinfected coolant remains in the tank of the machine tool, in particular the biofilm on the floor and walls of the machine tool. However, this is no different with all other methods, even with biocides, so the situation is no worse here.

In practice, it is recommended to disinfect the cooling lubricant three times directly one after the other and, if possible, to mix it up a little so that as much as possible of the germ-contaminated residues in the corners, pipes and on the floor are swirled up and disinfected also. Depending on the specific application, it will take weeks, sometimes months, until the KSS is so heavily contaminated again that a new disinfection is necessary.

Nevertheless, thermal disinfection should be repeated regularly, e.g. once a month, in order to prevent excessive germ formation from the outset. As this can and should be done in bypass mode, the ongoing operation is not affected.

To conclude, a significant prolongation of the life span of the coolant is possible – with corres­ponding savings potential for purchasing and disposing of the coolant, as well as reduced health complaints by employees and therefore potentially lower sick times.