Microbial contamination of diesel fuel

Diesel bug is contamination of diesel fuel by microbes such as bacteria and fungi.

Water can get into diesel fuel as a result of condensation, rainwater penetration or adsorption from the air — modern biodiesel is especially hygroscopic. The presence of water then encourages microbial growth which either occurs at the interface between the oil and water or on the tank walls, depending on whether the microbes need oxygen. Species which may grow in this way include:[1]

• bacteria — clostridium; desulfotomaculum; desulfovibrio; flavobacterium; hydrogenomonas; pseudomonas; sarcina
• fungi — aspergillus; candida; fusarium; hormoconis resinae

Fuel companies agree that if left untreated fuel will remain reliable for just 6–12 months. After which fuel contamination (such as the diesel bug) begins to appear. Most industrial engine manufacturers now recommend a fuel conditioning programme to ensure the reliability of fuel.

How to minimise the risk of fuel contamination occurring[2]

• Where possible purchase your diesel fuel from reliable sources to known specifications
• Ensure all tanks are well maintained, in good condition and leak-free
• Avoid contact between fuel and copper or zinc as they promote oxidation
• Always follow recommended methods when refilling tanks to avoid the accidental introduction of contamination
• Where possible, keep fuel cool (below 20°C) by storing it indoors or under shade to minimise water absorption
• Keep fuel tanks as full as possible to reduce possible water condensation from moisture-laden air
• Use tank drains to periodically remove tank bottom water and sediment
• If storing fuel in multiple tanks, employ a rotation system to use the oldest fuel first
• Filter fuel every time it is moved – this is considered to be ‘best practice’ as fuel is at most risk from contamination when it is being transferred
• When you are unable to guarantee the quality of the fuel being used in your plant, generators, etc. appropriate fuel conditioning equipment should be fitted onto the engine or appropriate filtering measures should be employed when re-fuelling
• Ensure all appropriate staff are trained in good fuel housekeeping. Processes and procedures should be in place to minimise the possibility of accidental fuel contamination.
• Introduce a fuel conditioning programme that includes:
• A regular test program to check fuel for water, particulate and microbial contamination
• If contamination is found, thoroughly clean both fuel and tanks
• Regularly run an on-tank fuel polishing system to remove water and other contamination
• Support the on-tank polishing unit with a fuel additive and biocide to maintain fuel stability and minimise microbial growth