1. Microbial Contamination

• Mechanism:
  • Aerobic and anaerobic bacteria (especially Sulfate Reducing Bacteria – SRB) decompose emulsifiers, esters, and lubricating additives in the fluid.
  • Generate H₂S (rotten egg odor), NH₃, and organic acids (butyric, propionic, lactic).
  • Molds and algae grow when exposed to light and organic matter.
• Consequences:
  • pH decreases (< 8.5), fluid turns dark brown or greenish.
  • Biofilm forms on sump walls and piping.
  • Fluid degrades quickly, causing foul odor, corrosion, and skin irritation to operators.

2. Tramp Oil Contamination

• Sources: way lube oil, hydraulic oil, gear oil, machine lubricants.
• Mechanism:
  • Forms a free oil layer on the surface → blocks oxygen dissolution → anaerobic conditions → microbial growth.
  • Esters and fatty acids in way lube oil serve as nutrients for microbes.
  • Causes errors in refractometer readings (false high Brix).
• Consequences:
  • Fluid changes color from pale yellow → light brown → dark brown.
  • Stable foam, oil scum, and foul odor appear.
  • Biocide consumption accelerates, emulsion destabilizes.

3. Poor Maintenance

• No sump or piping cleaning before refilling → residual microbes and dirt remain.
• No filtration of chips, sludge, or dust → accumulate as nutrients.
• Long intervals between changes or topping up with water only → concentration too low, additives insufficient.
• Dead zones in the sump without circulation → breeding ground for anaerobes.
• Consequence: fluids turn foul quickly even after replacement, unpleasant odor spreads across the shop.

4. Adverse Operating & Environmental Conditions

• High temperature: fluid heats up to 35–40 °C → microbial growth rate multiplies.
• Poor ventilation: dissolved oxygen low → anaerobic growth.
• Light exposure in the sump: promotes algae growth → fluid turns greenish, fishy odor.

5. Poor Water Quality

• Excessive hardness (>150 ppm CaCO₃): Ca²⁺, Mg²⁺ react with emulsifiers → soap scum, emulsion break.
• Too soft water (<50 ppm CaCO₃): foam formation, unstable pH.
• High chloride (Cl⁻) or sulfate (SO₄²⁻): corrosion, microbial growth.
• Water already contaminated with microbes: introduces bacteria during mixing.

6. Improper Concentration & pH

• Too low concentration (<3–4%): insufficient corrosion inhibitors and biocides → fluid deteriorates, foul odor.
• Too high concentration (>12%) with tramp oil → unstable emulsion, still foul odor.
• Low pH (<8.5): microbial acids → favorable for bacterial growth → rapid spoilage.
• Excessively high pH (>9.8): causes skin irritation, still prone to microbial issues if tramp oil present.

7. Insufficient Biocide/Inhibitor

• Fluids with little or no biocide.
• Biocide consumed during use but not replenished → microbial outbreak.
• No preventive dosing program → fluids foul faster.

Technical Summary

Main causes of foul odor:

1. Excessive microbial growth (due to low pH, abundant nutrients).
2. Tramp oil contamination → anaerobic conditions.
3. Poor sump and fluid management (residues, chips, dead zones).
4. Environmental factors (heat, light, low oxygen).
5. Poor water quality.
6. Uncontrolled concentration and pH.
7. Lack or depletion of biocides.