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Design to Minimize Microbial Multiplication

July 01, 2012
Dr. Alan C. Eachus
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The growth of bacteria and fungi in machine-tool coolant sumps is counter-productive for a number of reasons.  Microbial metabolism can cause irreversible chemical changes to the fluid itself, impede the flow of fluid through the system, damage surface finish of the worked parts and induce health problems among the workers.  The ultimate result of such uncontrolled growth is loss of productivity and increased costs.

A recirculating metalworking-fluid system can be compared to the blood-circulation system of the human body; in both cases fluid is pumped to its job site, performs its function and then is recirculated back to a reconditioning location to be renewed.  Reconditioning for a metalworking fluid involves removal of swarf, metal chips and fines, and tramp oil.  Chemical rebalance of the fluid may also be necessary.  The capacity of the fluid return system must be sufficient to permit all the contaminants to be transported to the separation point. 

Additionally, the fluid-system piping design must permit consistent laminar flow throughout.  In locations where smooth fluid flow is disrupted, such as dead-legs and excessive pipe bends, microbes can establish colonies attached to interior pipewalls.  Such attached microbial populations will result in biofilms, providing constant sources for fluid contamination and whose inhabitants are physically protected from contact with antimicrobial pesticides by a matrix of cellular matter. 

The layout of the machine-shop floor should permit easy machine access for maintenance, housekeeping and fluid sampling. Moreover, ventilation must be designed so that aerosol particles of contaminated fluid are not transported to other locations within the shop where they could inoculate uncontaminated fluid or serve as an inhalation-health hazard to workers.  Open grates which cover fluid return lines in concrete floors should be replaced with removable plates, so as not to allow dirt or floor sweepings to easily enter the fluid system.  Contamination such as this can serve as a major source of microbial introduction.

For more information on this topic, look to STLE's educational programming, including the Metalworking Fluid Management Certificate Course and the 2013 Annual Meeting Education Course, Metalworking Fluids 115: Basic Metal Removal Fluids.

Dr. Alan C. Eachus is an STLE member and member of the Metalworking Fluids Education & Training Subcommittee. He is a consultant and subject matter expert on metalworking and metalworking fluids. You can find his contact information in the membership database.


  • Laminar fluid flow is smooth, non-turbulent.
  • Biofilm is a mass of cells adhering to each other and attached to a surface. It’s generally slimy. (Free-floating individual microbial cells are called “planktonic.”)  Dental plaque is a type of biofilm. For more on biofilms, see the webinar, Emerging Issues in MWFs: Biofilm Control, given by Fred Passman.
  • Swarf is the very fine metal particles removed during a metalworking operation such as grinding.

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