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Monday Morning Odor? Here’s Why Your Coolant Smells Like Rotten Eggs — and How to Fix It

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Monday Morning Odor? Here’s Why Your Coolant Smells Like Rotten Eggs — and How to Fix It

22 Oct 2025


Have you ever opened your factory on a Monday morning and smelled a rotten egg odor coming from your machines?

That’s not just spoiled coolant — it’s a sign of anaerobic bacteria growing inside your coolant sump.

Cause: Contaminated oil (such as hydraulic oil) leaks into the coolant and forms a thin film on the surface. This film blocks oxygen from entering the system, allowing anaerobic bacteria to produce foul-smelling gas (hydrogen sulfide) when the machines are idle.
To prevent and eliminate the bad odor caused by anaerobic bacteria, follow these steps:
  • ✅ Remove the source (contaminated oil):
The first step is to remove the oil film floating on the coolant surface, as it prevents air from entering the system. Use an Oil Skimmer or similar device to regularly remove this contamination.
  • ♻️ Maintain coolant condition:
After removing the oil, control the coolant quality by maintaining proper concentration and pH (should be above 9.0). A higher alkalinity effectively inhibits microbial growth.
  •  Increase oxygen to prevent recurrence:
The most important step is preventing oxygen deficiency, especially during long shutdowns (like weekends). Set a timer for periodic coolant circulation or use an air hose (aeration) to inject oxygen into the tank. This permanently disrupts the anaerobic bacteria’s growth cycle.

At Focus Mechanic, we offer a full range of coolant solutions — soluble oil, semi-synthetic, and full synthetic — all highly resistant to bacterial growth. We also provide coolant treatment systems that remove surface oil and bacteria, extending your coolant life.
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