Bacteria and Fungi in Coolants

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Nothing kills the morale of the CNC machine operator than the dreaded Monday morning coolant smell. It smells like a combination of dirty socks and towels and rotten eggs, and the stench typically will permeate into the whole shop floor. Bacterial growth is usually the cause, and while somehow unavoidable, it can be controlled and prevented with the right practices, as enumerated below:

  1. ) Always use clean water and containers (pails and drums) to prevent contamination.


  1. ) Have dedicated containers, hand-pumps, dippers, and other paraphernalia for EACH type of metalworking fluid, and lubricant. Do not inter-change usage.


  1. ) Use the water-soluble cutting fluid at the recommended manufacturer’s concentrations, at all times. At the correct mix-ratio, there would normally be sufficient biocide content to combat any attacks.


  1. ) Add a ph booster, or alkalinity improver once pH level hits 8.7 or below. The target is to boost pH to 8.8-8.9, an alkaline scenario that is not conducive to strong biological activity.


  1. ) Skim all tramp oil (leaked slideways and hydraulic oils) from the surface of the coolant, either manually by foam or dipper, or via an automated belt skimmer. Tramp oil prevents the entry of oxygen into the coolant, enabling the rapid propagation of anaerobic batercia.

cleaning bacteria in coolant

  1. ) During long-breaks, or long week-ends, keep the fluid system aerated by circulating the fluid at selected intervals. Alternatively, one may inject compressed air into fluid.


  1. ) Remove accumulated chips and metal fines from the bottom of the sump regularly, as bacteria hides and thrives in that location.

cleaning bacteria in coolant

In situations where the above actions do not prove to be of effective, a dose of biocide directly to the fluid, at 0.01-0.02% concentration should be considered. Highly acidic, the biocide should only be used by trained personnel, while using the apt personal protective equipment (PPE), such as nitrile gloves and goggles.

On the other hand, Fungi, while preventable by recommended actions above, are more persistent once it becomes present.  Aside from odors, the presence of fungi is manifested by the development of slimes and scum that can line machine walls, envelope the coolant surface, and worse, cause coolant pipe blockage.   Biocides can prove to be effective in removing fungi, though the slime and scum must be removed physically.

While expensive, some shops periodically check the bacterial and fungal population by use of agar slides, as illustrated below.  The yellow side with red dotted growth indicates bacteria, while the red side with white splotches indicate fungi.

bacteria level

Looking for a cutting coolant supplier in the Philippines? Choose Fluid Solutions! We offer various lubrication solutions for your business. Our product range also includes hydraulic oils, slideways oils, gear oils, synthetic lubricants, high-temperature greases, and anti-rust oils. Contact our Lubrication Consultants now!

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