Diesel Bug

Diesel bug 101: how to deal with increasing fuel contamination risks

Are you a user or provider of middle distillate fuels, such as diesel?

If so, you need to be aware of the growing danger of diesel bug. Recent changes to fuel such as the addition of biodiesel mean that, although it has clear environmental advantages, it is increasingly likely to affect your business.

Left unchecked, it can lead to operational issues and even serious corrosion. That means a big hole in your diesel tanks—and an even bigger one in your wallet.

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What is diesel bug?

Diesel bug is a single microorganism or a consortium (group) of microorganisms that grow on fuels.

It includes bacteria, filamentous moulds and yeasts. They feed on the hydrocarbons in the fuel and create biomass, biofilms and damaging by-products.

These microorganisms will live very happily in even the smallest amounts of water, often from condensation, and at the interface of fuel and water, where they can create thick mats known as biomass. They can also coat surfaces and ‘climb’ up storage tank walls by creating what we know as biofilm.

Diesel bug is often called diesel fungus or algae—though in reality, it has nothing to do with algae.

Why are diesel bug microorganisms found in diesel fuel?

Microbes, which include bacteria, yeasts and fungi, are all around us.

Needing tiny amounts of water and food to proliferate, they live almost everywhere, including in fuels such as diesel.

As soon as fuel leaves the sterile environment of a refinery, it is exposed to these microbes. Feeding off even the tiniest amounts of water, they go on to form what is known as diesel bug.

Diesel bug microbes prefer to live at the fuel/water interface, living in the water and feeding on the fuel. They have a tendency to ‘hide’ in the lower flow areas of a fuel system, where the lack of turbulence allows them to thrive in relative safety.

What are the most obvious diesel bug symptoms and effects?

If you’re not testing your fuel regularly, and you have a dangerous diesel bug infection, you may not notice it at first.

However, it is inevitable that you will see clear symptoms as the problem worsens. Diesel bug symptoms and effects include:

Poor engine performance (caused by insufficient fuel reaching the engine)

Increased fuel consumption

Altered fuel composition/ degradation of fuel components

Higher fuel usage

Engine failure, in severe cases

Blocked filters

Exhaust emissions

Leaking fuel tanks (caused by corrosion)

If you’re seeing these symptoms, it means significant damage has already occurred. Diesel bugs, through their metabolism, can induce corrosion over time that can damage the structural integrity of the tank and injectors. Dealing with this requires expensive mechanical intervention and significant downtime to clean the tank and fuel system.

How diesel bug affects filters and fuel systems

Diesel fuel contamination can have extremely serious effects on fuel systems and tanks.

The contamination will block filters and clog up injectors, thereby causing problems with fuel gauging systems. When allowed to proliferate over a long period of time, these microbes begin to corrode the fuel system.

Diesel bug in fuel can also seriously affect filter operations. The issues are usually related to pump and injector wear, or metering and gauging problems resulting from diesel bug build-up.

Even at moderate levels of contamination filter blockage, this can mean costly operational issues.

Why can’t we just prevent diesel bug from getting into fuel and growing?

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 practicing good housekeeping of diesel fuel is the key to limiting the problem.

Diesel bugs in fuel will develop very quickly, particularly in hot and humid conditions where 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 (around $19,000 or £15,600). Diesel bug was passed on to boat owners from a single supplier with contaminated tanks.

The changing nature of diesel means the risk of diesel fuel bug contamination is increasing

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 practicing good housekeeping of diesel fuel is the key to limiting the problem.

Diesel bugs in fuel will develop very quickly, particularly in hot and humid conditions where 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 (around $19,000 or £15,600). Diesel bug was passed on to boat owners from a single supplier with contaminated tanks.

Water – FAME – Diesel Bug

Water is likely to be present in all diesel fuel systems, including storage and operational. It may exist as free water, visible as a separate layer below the fuel, or as a hazy layer between the water and fuel layers. The hazy layer consists of microscopic droplets of water suspended in fuel. Both are excellent conditions for microbial growth.

FAME, also known as bio-fuel, is more hygroscopic than traditional fuels—in other words, it attracts and holds water. This means that it’s more prone to produce condensation issues in fuel systems. And increased condensation means better conditions for the growth of diesel bug, also known as diesel fuel fungus.

The changing nature of diesel means the risk of diesel fuel bug contamination is increasing

Between 1999 and 2006, the EU and US introduced legislation to reduce the amount of sulphur in diesel. The intention was to reduce the amount of harmful emissions, and it led to the introduction of ULSD (Ultra Low Sulphur Diesel) into fuels. These days, USLD is often combined with Fatty Acid Methyl Esters (FAME) in fuel (biofuel).

While the removal of sulphur is good for the environment, it potentially poses risks for fuel integrity. There are some indications that sulphur may have a bio-static effect, so may suppress the growth of diesel bug. This remains to be scientifically proven, but what is clear is that the combination of lower sulphur and increased levels of FAME has enhanced the potential for fuel contamination problems.

FUELSTAT® 15-minute diesel bug test kit

Industry-approved tests used in 130 countries worldwide

How to treat diesel bug

In general, diesel bug is best dealt with through good fuel husbandry, such as regular removal of water.

There will always be some presence of diesel bug, but the real problems arise when and when it reaches moderate or severe levels of contamination.

Once the contamination reaches these stages, you may need to use biocides to control it. In more severe cases, complete cleaning of the tanks may be required as is fuel remediation known as fuel polishing or total fuel write-off or downgrading of the diesel fuel. All of these solutions are extremely costly.

Regular testing is the safest, most cost-effective way to prevent diesel bug problems

Due to the prevalence of FAME, and the challenges in removing water from fuel, the risk of diesel bug is increasing. Yet, many companies only realise they have a problem after they put contaminated fuel in their assets—by which point the damage is done.

The fact is, identifying contamination early and treating it with biocide is far cheaper than dealing with engine failure, or corrosion of the tank structure.

The majority of operators have concluded that detecting and dealing with the problem early is the most cost-effective answer. This is because the costs of testing are minimal compared with the potential costs of serious contamination. In other words, you need to test your fuel regularly.

What you need to know when choosing a fuel test

There are three main types of fuel tests available, and each varies in terms of time, and cost, along with equipment and expertise required.

It’s worth noting that some of the most popular tests can lead to misleading results, i.e false alarms. This is where results appear to show dangerous contamination, but are actually indicating microbes that may not be damaging to fuel.

Misleading results can be costly if they lead you to take assets out of operation or treat fuel when it isn’t necessary.

What options are available for diesel bug testing?

Immunoassay-based (on-site) tests

Immunoassay based tests are the quickest and easiest available today, reliably detecting the specific microbes that threaten fuel.

Equipment: Requires minimal equipment and zero investment beyond simple test kits.

Location: Can be conducted on-site, so fuel does not need to be sent anywhere.

Time: Gives test results within minutes.

Level of testing: Only searches for microbes that are threatening to fuels. This means that the user doesn’t need to take any special sterility measures on-site.

Other considerations: results are given in a ‘traffic light’ green/amber/red system. This means that you don’t have to spend time calculating levels of contamination. You get an instant readout of negligible/moderate/heavy contamination according to industry standards.

Immunoassay tests like FUELSTAT® also let operators test diesel fuel at the point of uploading. You can then ensure that you aren’t accepting moderately or heavily-contaminated fuel.

ATP (Adenosine Triphosphate) testing

ATP testing detects any living microorganism present, but also tests for microorganisms that aren’t dangerous to fuel. This can lead to misleading results that result in assets being taken out of operation for days unnecessarily.

Equipment: Requires a ‘reader’, and this equipment costs thousands of dollars/pounds.

Location: Testing is not conducted on-site, so fuel samples have to be transported as a hazardous good, at high cost.

Time: The tests themselves take minutes, but as they usually take place off-site in a laboratory, the process of transporting samples adds days to the process.

Level of testing: ATP is ‘catch-all testing’ that finds all living microbes, including those that aren’t directly threatening to fuel at that time, such as E.coli and the flu. This means results can sometimes be overstated or misleading.

Risk: Overstated results can mean positive contamination alerts. This typically leads to re-testing of fuel, or expensive biocide treatment when it isn’t actually needed.

Colony counting methodologies (CFUs)

CFUs are laboratory-based tests that are based on 19th century Pasteur technology. While the tests themselves take 4-10 days on average and it’s very difficult to stop background contamination entering the sample, so misleading results can occur.

Equipment: Requires a laboratory-quality incubator.

Location: Testing is not conducted on-site, so fuel samples have to be transported as a hazardous good, at high cost.

Level of testing: CFU is ‘catch-all testing’ that finds all culturable microbes, including those that aren’t directly threatening to fuel, such as E.coli and the flu.

Other considerations: Tests are subject to cross-contamination, so require sterility measures e.g. transporting fuel in refrigerated containers.

Risk: Because CFU results can lead to misleading results, you may end up applying expensive biocide treatment when it isn’t actually needed. This means taking the asset out of operation for several days, potentially costing millions.

Additionally, CFU tests miss all microorganisms that do not grow in culture, thus there is also a risk of false negative results.

FUELSTAT® Diesel Plus – FMD8 fuel test kit

The 15-minute fuel test used in over 130 countries

FUELSTAT®: The on-site fuel testing kit that detects diesel bug in just 10 minutes

FUELSTAT® is an immunoassay test that provides rapid screening of fuel samples.

It gives a quick and accurate assessment of the presence of Hormoconis resinae (formerly known as Cladosporium resinae), other fungi, bacteria, and yeasts that make up diesel bug.

The test measures the severity of microbial contamination in the sample and provides actions and alert levels within minutes. This is in contrast to current CFU tests, which require a minimum of 72 hours to provide results.

How FUELSTAT® works

Each FUELSTAT® test kit comes as a heat-sealed foil test pouch. This contains a FUELSTAT® test with desiccant sachet and pipette in one section, and a Sample Extraction Bottle with flat cap, dropper cap and instructions in the other section.

Users simply put 4 drops of fuel sample onto the test wells to get results, and only minimal training is required.

The FUELSTAT® Diesel Plus (FMD8) test measures the amount of microbial contamination in the sample. It provides results based on a ‘traffic light’ scenario:

Negligible (green)

negligible contamination

Low Positive (amber)

moderate contamination

High Positive (red)

heavy contamination

The fast response means engineers no longer have to send diesel fuel samples to the lab for analysis, so there’s no more downtime or shipping costs. And because FUELSTAT® only tests for diesel bug, there’s no risk of misleading results.

Learn more about FUELSTAT®

See how FUELSTAT® can save you time and money by testing for diesel bug early