How to Eliminate Bad Odours from Your Cutting Fluid

Bad-smelling cutting fluid is one of the most common and unpleasant problems in machining workshops. That rotten or sulphurous smell is not just a nuisance: it is a sign that your coolant has a serious problem that, if left untreated, will completely degrade the emulsion.

Why does cutting fluid smell bad?

Bad odour is almost always caused by one thing: bacterial proliferation. Anaerobic bacteria (that live without oxygen) multiply in the cutting fluid and produce hydrogen sulphide (H₂S), the gas responsible for the rotten egg smell.

These bacteria appear when several conditions combine:

• Tramp oil on the sump surface blocking oxygen
• Extended stops without agitation (weekends, holidays)
• High temperatures in the workshop (summer is worst)
• Low concentration (less biocide available)
• Accumulated dirt: chips, food waste, etc.

Consequences of not acting

• Corrosion inhibitors degraded → parts rust
• pH drops → emulsion destabilises
• Contact dermatitis in operators
• Drastically shortened fluid service life

Immediate solution: emergency protocol

1. Measure pH: if below 8.0, the sump is heavily degraded
2. Remove tramp oil from the surface
3. Raise concentration to the manufacturer's maximum (usually 8-10%)
4. Aerate the sump: run an aerator for 24-48 hours
5. Filter to remove particles and organic matter

Prevention: how to stop it happening again

1. Aeration during stops

The most effective weapon against anaerobic bacteria is simply moving the sump and injecting air. An aerator on a timer running over weekends prevents most odour problems.

2. Tramp oil removal

Install a belt or tube skimmer to continuously remove oils from guideways and hydraulic systems.

3. Concentration control

Keep concentration in the correct range. A diluted cutting fluid has less biocide and is more vulnerable to bacterial attack.

4. Filtration

A filtration unit removes metal particles and organic matter that feed bacteria.

When to change the cutting fluid

• pH drops below 7.5 and does not recover after adjusting concentration
• Odour persists after 48h of aeration and adjustment
• Visible fungi appear (floating colonies or on walls)
• The emulsion breaks (separates into water and oil)