Cutting fluids and coolants are complex mixtures of many components. When bacteria, yeast or mould multiply, they can destabilize the fluid, shorten its service life and cause odours, corrosion, foaming and health risks. Effective control combines monitoring, tramp-oil removal, filtration, oxygen exchange and disciplined maintenance.
A putrid or “rotten egg” smell often indicates increasing anaerobic decomposition.
Grey-black, brown or unusually cloudy fluid may be linked to microbial growth, sludge and decomposition products.
A sudden increase in foam can indicate that microbial metabolites are disrupting the emulsion.
Trends in pH, nitrite, hardness and nitrate can reveal abnormal conditions early.
Early Detection
Putrid or sour odours, obvious discolouration, increased sediment or emulsion separation all require further investigation. A normal smell does not necessarily mean that microbial counts are acceptable.
Abnormal foaming, reduced cooling performance, shorter tool life, workpiece corrosion or fluctuating surface quality may result from a combination of fluid chemistry and microbial activity.
Regularly record concentration, pH, nitrite, hardness and temperature. If nitrate rises above the initial value, include microbial activity in the investigation. Use laboratory analysis when necessary to identify organisms and contamination levels.
A More Reliable Assessment
A single measurement only reflects one point in time. Consistent sampling locations, test intervals and record formats are needed to show whether the fluid is becoming unstable. For broader treatment options, see coolant purification and KSS filtration systems.
Causes of Contamination
Microorganisms can enter through make-up water, workpieces, personnel, air or equipment surfaces. They use tramp oil, cleaner residues, fine particles and organic components of the emulsion as nutrients, multiplying quickly in warm, low-flow or oxygen-poor areas.
Key Point
Disinfection can only reduce microbial counts temporarily. If tramp oil, particles, dead zones and concentration deviations remain, contamination is likely to return.
Prevention First
Mix the fluid according to the supplier’s instructions, use suitable water quality and regularly verify concentration, pH and temperature. Avoid incorrect ratios and frequent unmeasured additions.
Slideway and hydraulic oils form a floating layer and supply nutrients. Use FRIESS oil skimmers or FRIESS oil-water separators for continuous oil removal.
Use magnetic filters and coolant purification systems to reduce the particle load, while eliminating stagnant zones and maintaining effective mixing and oxygen exchange.
Establish a Consistent Routine
Inspect the liquid surface, odour and foam daily; measure concentration and pH on schedule; and regularly clean filters, tank bottoms and return-flow dead zones. Preventive maintenance usually costs less than treating an entire contaminated system.
Reducing Existing Microbial Counts
Controlled heating can reduce microbial counts without adding chemical biocides. However, energy use, equipment suitability, temperature uniformity and effects on the emulsion formulation must all be assessed. Thermal treatment does not replace tramp-oil removal, filtration or system cleaning.
No added biocideAssess energy useVerify fluid compatibility
Biocides can reduce microbial counts quickly, but the product must be compatible with the coolant formulation and used in strict accordance with dosage, occupational-safety and regulatory requirements. Under the CLP system, some products may be classified as hazardous; improper use can irritate skin, eyes and mucous membranes or cause allergic reactions.
Use only on professional adviceAvoid overdosingUse appropriate PPE
Treatment Principle
The coolant supplier, occupational-safety personnel or a qualified service provider should decide whether thermal treatment or a biocide is appropriate, based on microbial counts, fluid condition and the cause of contamination. Never dose a product based on odour alone.
If the nitrate concentration in coolant that remains in service is higher than the initial value of freshly mixed fluid, this may indicate microbial contamination and an increasing organic contaminant load.
During the nitrogen cycle, nitrate can form through microbial decomposition of nitrogen-containing organic compounds. A nitrate level above the initial value should therefore be treated as one possible indicator of microbial contamination.
Maintenance Conclusions
Extending coolant service life does not depend on one disinfection step. It requires continuous control of the conditions that support microbial growth: reducing tramp oil and particles, eliminating stagnant zones, keeping concentration and pH stable, and using scheduled tests to identify trends early.
For systems with large oil volumes, complex contamination sources or unusual tank layouts, select a purification solution based on treatment flow, tank volume, target cleanliness and maintenance conditions. Explore coolant purification and KSS filtration systems for mechanical cleaning, mobile treatment and equipment combinations.
