Microbial contamination of fuel oil
The diesel bug: the mixed community at the fuel water interface, why distillate is vulnerable, how it is detected under IP 385 and IP 613, and how it is treated.
Microbial contamination of fuel oil is the growth of a mixed community of bacteria, yeasts and moulds at the interface between fuel and the free water that collects in a tank bottom. It is commonly called the diesel bug and commonly attributed to a single fungus, which is a simplification: the accepted name of the organism most often named is Amorphotheca resinae, but the contamination is a community rather than one species.
The full article will explain why marine distillate is vulnerable where residual fuel largely is not, why fatty acid methyl ester blending raises the risk further, and what the growth actually does: filter blocking by biomass, emulsification of water into the fuel by microbial surfactants, corrosion of tank internals by organic acid metabolites, and injector and pump problems downstream.
The article will cover detection and control. ISO 8217:2024 sets no microbial limit, so the governing documents are the test methods and the industry guidance: the filtration and culture method in IP 385, the thixotropic gel culture method in IP 613 and ASTM D7978 that reports colony forming units as absent, light, moderate or heavy, immunoassay screening, the ASTM guide to microbial contamination in fuels and fuel systems, and the Energy Institute guidelines. Prevention is water control, tank drainage and housekeeping before it is biocide. It links to marine gas oil .