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Recovering Unusable Diesel with Fuel Polishing
Dennis Vastergaard
8 min
18 June 2026
At a hyperscaler data centre in Europe, 8,000 litres of B7 diesel had been classified as unusable waste. One week of fuel polishing later, it was cleaner than new fuel. Here is how it was done.
The Problem with Condemned Diesel
At a hyperscaler data centre in Europe, the standard procedure for heavily contaminated diesel fuel is disposal. The fuel is pumped out, collected by a waste contractor, and replaced with new fuel.
When 8,000 litres of B7 diesel in a storage tank was classified as unusable, the facility operator faced exactly this decision. Instead, a test was initiated to determine whether the fuel could be recovered.
Why Diesel Degrades in Storage
- Water ingress: Tank breathing draws in humid air. Moisture condenses on tank walls and settles at the bottom of the fuel. FAME biofuels (including B7) are particularly hygroscopic and absorb water readily.
- Particle contamination: Water at the bottom of the tank accelerates corrosion of tank walls and fuel system components. The corrosion products - iron oxide particles - enter the fuel and provide catalytic reactions. Dust and debris enter through vents and during fuel deliveries - particles traps water and oxygen that accelerate the oxidation processes.
- Microbial growth: Bacteria and fungi thrive at the water-fuel interface. They feed on hydrocarbons and produce acidic metabolic by-products that raise the fuel's acid number, generate biomass that plugs filters, and accelerate corrosion.
- Oxidation and degradation: Diesel oxidises over time, particularly in the presence of water and metal catalysts. Oxidation produces gums, resins, and sediment that increase the fuel's colour, raise the acid number, and reduce combustion quality.
The European Hyperscaler Case: Starting Conditions
The 8,000-litre tank of B7 diesel had been in storage for an extended period. A CJC FuelPolish Separator with an Oil Contamination Monitor (OCM) to continuously track and monitor the cleaning process:
Parameter | Starting Value | OEM specification |
|---|---|---|
Particle cleanliness (ISO 4406) | ISO 21/20/18 | ISO 18/16/13 |
Microbial contamination | Positive - active growth | Negative |
The fuel was classified as unusable on the basis of particle contamination, and microbial activity - both parameters were outside the OEM specification for diesel fuel.
The Fuel Polishing Process
A CJC FuelPolish Seperator (FPS) was connected to the tank in a recirculation loop. The FPS combines coalescing technology - which causes water droplets to merge and settle - with cellulose depth filtration, which removes particles, and captures bacteria through polar attraction. The system ran continuously for one week, recirculating the full tank volume multiple times.
Results After One Week
Parameter | Before | After | OEM specification |
|---|---|---|---|
Particle cleanliness (ISO 4406) | ISO 21/20/18 | ISO 7/7/0 | ISO 18/16/13 |
Microbial contamination | Active growth | Negative | Negative |
The recovered fuel at ISO 7/7/0 was significantly cleaner than the OEM specification and cleaner than typical new diesel fuel delivered from a refinery. The 8,000 litres of fuel that had been classified as waste was recovered to a condition exceeding the standard for new fuel.
The Financial and Environmental Case
The cost of disposing of 8,000 litres of contaminated diesel - including waste contractor fees, transport, and the cost of replacement fuel is substantial. The cost of one week of fuel polishing with a CJC FuelPolisher is a fraction of that figure. Beyond the direct financial saving, fuel recovery eliminates the environmental impact of disposing of 8,000 litres of hazardous waste and producing an equivalent volume of new diesel.
For data centre operators managing multiple large diesel tanks, the implication is significant: contaminated fuel that would otherwise be condemned can be recovered to a standard exceeding new fuel specification, at a fraction of the replacement cost.
Conclusion
Diesel classified as unusable is not necessarily waste. The European hyperscaler case demonstrates that even heavily contaminated B7 diesel - with active microbial growth, free water, and particle contamination well outside specification - can be recovered to a condition exceeding new fuel standards in one week of continuous fuel polishing.
For data centre facility managers, fuel polishing is both a cost-saving and a sustainability measure that should be evaluated before any decision to condemn and replace stored diesel.
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