Diesel Bug- the basics
It’s been some years now since the introduction of ULSD and legislation which increased biofuel content of diesels. The predicted ‘end of the world as we know it’ didn’t quite happen, but the combination of ULSD and FAME (Fatty acid methyl esters) has certainly had an effect on the number of heavy contamination incidents by diesel bug that we see in our labs. These amendments to environmental legislation have changed the nature of diesel fuel and increased the risk of microbial contamination.
As a result, those involved in its production, distribution and use are finding that they need to change, or at least review, the way they manage their operation to maintain fuel quality.
What is the diesel bug and how can it be prevented?
Diesel bug is one of the many names for a consortium of microorganisms able to grow on fuels, utilising the alkanes and additives, creating biomass, biofilms and damaging by products. Also known as the Jet fuel fungus and on some occasions Algae (although it has nothing to do with algae at all!!!), it is a mixture of bacteria, filamentous moulds and yeasts.
This consortium of organisms will live very happily in any water bottom and particularly at the fuel water interface where it can create thick mats known as biomass. This consortium can also coat surfaces and ‘climb’ up storage tank walls by creating what we know as biofilm.
How does the bug form?
If conditions suit, the bug will develop very quickly, particularly in hot and humid conditions when condensation is rife. One case in New Zealand struck down more than 600 boats and cost on average NZ$27-30,000 per vessel to fix. The bug was passed on to boat owners from one single supplier with contaminated tanks.
Fungal spores and bacteria can be carried in both air and water, and cannot be kept out of fuel. Restricting the infiltration of water and good housekeeping is the key to limiting the problem.

Why have the changes had an impact on the bug?
These changes, intended to be positive for the environment have had less than desirable impacts on fuel integrity. The unintended consequences include lubricity challenges and the threat of microbiological contamination.
It has proved relatively easy to find solutions to the lubricity issues but dealing with diesel bug is proving to be a much more complex nut to crack.
The recommendation to remove the water to minimise the diesel bug problem is as true today as it is ever was. However, it is much more difficult to effectively remove water from diesel containing biofuel. FAME is extremely hygroscopic which means biofuels absorb and hold on to water to a greater degree than traditional mineral diesel. In fuel containing FAME, free water can usually be found, as expected, at the bottom of a tank, but above this free water there is often a hazy layer of suspended water. Whilst this hazy fuel can still be drained away, more difficult to deal with are water droplets which can be entrained throughout the fuel. The problem is that potentially at each and every water-fuel interface bugs can live in the water phase whilst they feed on the fuel.
Completely removing the water is becoming more of a challenge thereby increasing the risk of diesel bug. Prolonged heavy contamination is an expensive affair. It can produce corrosion and require expensive mechanical intervention and significant downtime to clean the tank and fuel system. The fuel will require both polishing and biocide treatment if it is to be retrieved for use. Even at moderate levels of contamination filter blockage, pump and injector wear plus metering and gauging problems cause costly operational issues. Here just biocide and or fuel polishing should be sufficient. The majority of operators have concluded that detecting and dealing with the problem early is the efficient, cost effective answer. In other words; a proactive approach.
A fuel maintenance regime, designed according to risk, should arguably begin with fuel testing to gauge the levels of diesel-bug in the system and there are several different types of test available. There are pros and cons with each method with regards to the time involved to obtain results, the cost, the equipment and expertise required to conduct the test. The choice of test will depend on the nature of the operation and the requirements of the engineer, but on-site testing puts the operational team in control and delivers immediate actionable information.
Immunoassay based tests are the quickest, easiest, zero investment, on-site choice. But ATP testing is popular where the investment required for a reader may be justified by high volumes of testing. Colony counting methodologies are ideal where time is not of the essence and quantification of culturable organisms may be important.
On site testing also means that operators can test fuel at the point of uploading to ensure that they are not accepting moderately or heavily contaminated fuel. Unfortunately, bugs are everywhere no fuel is bug free once it leaves the sterile refinery environment.