Microbial Fuel Contamination: Symptoms, Causes & How To Fix

Microbial fuel contamination is unfortunately common when storing red diesel (gas oil), white diesel (DERV), biodiesel and heating oil (kerosene).

More often than not, when our customers purchase fuel, they keep it in storage and then forget about it until they need to use it – out of sight out of mind one might say. So it can be a huge surprise when it fails to do its job when it’s needed the most.

Even the best quality fuels will degrade over time when stored, and it’s important to be able to spot the warning signs and take action as soon as possible. One of the most common problems with diesel engines is microbial fuel contamination. Fuel owners and users must regularly be on the lookout for signs of contamination or degradation – both which can cause significant problems.

Contamination spreads quickly in industrial machinery, grain dryers, generators, milk bottling machines or anywhere else where diesel is used and can render your fuel useless if you don’t take steps to prevent it. Fortunately, these problems can be avoided through the implementation of planned preventative maintenance solutions and regular fuel testing.

What is microbial fuel contamination?

Fuel contamination is caused by microorganisms that manage to enter the fuel. Certain types of microorganisms are strong enough to thrive within your fuel and if the conditions allow it they can cause serious damage to fuel systems, tanks and bunkers.

The cause of microbial fuel contamination

microbial fuel contamination

Changes to fuel specifications brought about by rigorous emission regulations for engine designs to meet a higher level of fuel cleanliness have led to a greater need for regular fuel testing to prevent and manage fuel contamination.

In 2004, the British Standard for off-road diesel (BS2869) was amended to mandate up to 7% fatty acid methyl ester (FAME) to be included in white diesel (DERV) to allow renewable fuels to be introduced under the Renewable Fuel Transport Obligation (RFTO).

Biodiesel is a diesel replacement manufactured from renewable materials such as animal fats and plant oils. The production process converts the oils and fats into long-chain molecules called fatty acid methyl esters (FAME).

FAME offers several benefits over traditional fuels such as reduced emissions, low toxicity, low sulphur and biodegradability. However; unlike diesel, FAME is hygroscopic and the methyl esters absorb significantly more water than petroleum-derived diesel and hold it in suspension in the fuel.

FAME is very different from diesel chemically and can cause problems with engine and fuel systems if used in high concentrations. It’s the perfect source of nutrients for microbes to grow, leading to foaming, reduced stability and gelling at lower temperatures.

Common fuel contamination causes


microbial fuel contamination

The build-up of water is almost inevitable in stored fuels. With the modern blends of ULSD fuels, almost any stored diesel fuel left for a period of time will end up with water at the bottom of the tank.

Water can be found within fuel as three forms: dissolved, emulsified or free water.

  • Dissolved water – spreads throughout fuel molecule by molecule – most fuel can hold up to 200 ppm of dissolved water content
  • Emulsified water – once the water content surpasses 200 ppm, water separates from the fuel and creates a fuel-water emulsion, with several water droplets suspended in the fuel
  • Free water – as the water content grows past this point, the water will turn into free water and form a separate layer on the bottom of the tank

How does water get in a fuel tank?

  • Rainwater can get in through the seals
  • Moist outside air can enter through the vents
  • Fluctuations in the outside temperature and humidity can cause water vapour to travel into and out of the tank, where it condenses on the cooler surfaces within the tank

Water droplets will travel down the tank walls when the air cools down at night and then settle on the bottom of the tank.

The problems of water in fuel:

  • Water increases the chances of microbial contamination
  • Water accelerates the oxidation and breakdown of fuel
  • Water causes tank corrosion
  • Water increases fuel gelling in colder temperatures
  • Water in your fuel lines can cause machinery failure

Water is a fertile ground for diesel bug to grow. If you suspect water contamination, the issue needs to be resolved as soon as possible to prevent further damage to the engine.

Diesel bug and microbes

Microbial contamination or “diesel bug” are the main terms used to describe the main cause of degraded fuel quality problems. As the fungal and bacterial microbes responsible for diesel bug both need a combination of diesel and water to thrive, once water gets inside a fuel tank, diesel bug won’t be far behind.

Water provides the perfect environment for microbes to grow and reproduce, increasing the rate of fuel degradation over time. These microbes reside at the interface with the water and fuel, pulling in the required elements and nutrients from both areas.

Over time, the microbes grow, plugging filters, affecting the pH of the fuel and causing a tank to corrode. This increases the likelihood of system failure and engine shutdown which can lead to:

  • Engine deposits
  • Black smoke
  • Inefficient engine performance

Common microbial fuel contamination symptoms

First and foremost, to avoid expensive issues it’s important to be aware of the common diesel contamination symptoms so you can identify them before the problem becomes irreparable.

Cloudy appearance

microbial fuel contamination

Fuel should ideally be a clear and bright substance. It’s important to look out for the following signs which will indicate a significant level of contamination:

  • A dark or cloudy appearance means it’s likely that the fuel is severely contaminated
  • A dark appearance can indicate signs of asphaltenes which are hard particulates that can clog engine filters once amalgamated
  • A murky appearance can be a sign of water within which is the perfect breeding condition for diesel bug to develop

Eroded fuel tanks

Microbial bacteria are strongly linked to corrosion in fuel storage facilities. Biological acid production from diesel bug attacks and damages metal surfaces, corroding the tank exterior and causing extensive and expensive damage. Any signs of tank corrosion need to be repaired as soon as possible to avoid fuel leaking through degraded tank walls and causing environmental damage.

Sludge build-up

Water and diesel bug can cause sludge build-up on the bottom of a tank and clog filters which restricts the flow, placing additional strain on pumps and fuel infrastructure. Diesel forms heavy polymers that drop out and build up on the bottom of a fuel tank, leaving sludge deposits within. If drawn into engines, these deposits prevent efficient combustion and can cause lasting damage.

Engine problems

microbial fuel contamination

While many fuel users inspect their supplies on a regular basis, it’s almost impossible to notice problems on the bottom of your tank by simply looking in the lid. This can mean that you don’t notice an issue until it starts to affect the engine’s performance, such as when engines may experience spluttering and have trouble starting. By then the damage has been done, and it’s vital that you repair the damage before it spreads any further.

It’s important to get regular checks by a professional company who will be able to take fuel samples from the top, middle and bottom of your tank to save you the cost and hassle of expensive equipment repair and replacement.

Clogged filters

microbial fuel contamination

Fuel filters are designed to prevent diesel bug and other particulates from passing into the engine. Over time, microbes build up in fuel tanks and produce biomass and slime growths which eventually plug these filters and lead to other issues in the fuel system. Once the problem spreads to vehicles and equipment, clogged filters can break them, causing unnecessary costs.

If you’ve noticed an increase in needing to change filters, it’s likely you need to take a look at what’s happening inside your fuel.

Symptoms of fuel contamination in vehicle fuel tanks

  • Poor engine performance
  • Loss of acceleration
  • Loss of RPM
  • Spluttering
  • Black or white smoke
  • Vehicle not starting

If you believe that your vehicle has been filled with dirty fuel, get it to a garage as quickly as possible. For advice on what to do visit Resolver. It’s important to note down the date and time of when you bought the fuel and contact your insurance company immediately.

Tips on how to avoid diesel fuel contamination

microbial fuel contamination

It’s now more important than ever for businesses to have emergency backup and fuel storage facilities to implement planned preventative maintenance strategies into their operations. To avoid the huge costs and inconvenience associated with fuel contamination it’s important to follow these steps:

  • Have professional inspections annually (at least) – investing in regular fuel tests by a professional company will keep your fuels and oils in good condition and help tackle the problems of microbial fuel contamination. We actually suggest a fuel test twice a year; once before the winter months and again before the spring, as these seasons are the most problematic for your fuel system.
  • Buy from a reputable supplier – while oil is a commodity, it’s important to shop around and make sure that you’re purchasing from a trusted company. If you see unusually low prices, it’s likely because it’s poor quality and will do more damage to your fuel system than good.
  • Maintain your fuel tanks – while regular fuel tests are crucial, it’s also important to invest in tank cleans at the same time. If you put clean fuel into a corroded or damaged tank, it will simply contaminate the fuel again straight away – a waste of time and money!
  • Rotate your fuel usage – if you have several bowsers or tanks that you use to store fuel, ensure that you rotate their use regularly so you’re using the oldest fuel first.
  • Keep your tanks topped up – by minimising the amount of empty space within your tank, you’re also reducing the chances of water condensation forming. This, in turn, reduces your risk of microbial contamination.
  • Switch to a FAME-free renewable diesel – HVO is 100% bio and FAME-free, so it doesn’t face the issues of stability faced by many existing biodiesel and gas oil products. This makes HVO a much more stable product than mineral diesel with a reduced risk of degradation
    when in storage due to zero FAME and sulphur content. The production process affords a shelf life of up to ten years, providing good storage and maintenance, as opposed to the comparatively short 1-year shelf life of mineral diesel.

What to do if your fuel is contaminated

microbial fuel contamination

If you suspect that your fuel is contaminated, get in touch with Beesley Fuels today on 0330 123 1144. The longer you ignore contaminated fuel, the worse the problem will get.

We’ll collect a fuel sample

Our engineers will come to your site and take samples from the top, middle and bottom of your tank so we can get a full picture of the condition of your fuel. They’ll also perform a visual inspection of your tank, checking for the tell-tale signs of corrosion or damage.

We’ll test it

Our industry-leading onsite technicians can test for fuel contamination in a large diesel fuel tank, bowser, generator, boiler or any other storage facility you have. They will analyse and test it in our state of the art onsite laboratory to look for signs of water, bacteria, bugs, particulates and provide you with an easy-to-interpret analysis in a detailed report.

If possible, we’ll polish it

If possible, we can then come back to your site with a fuel polishing unit and clean it and return it to its former quality, so it’s as good as new. This is a more cost-effective option than replacing your entire fuel stock. Learn more about our fuel polishing service here.

If not, we’ll uplift and replace it

If your fuel is severely contaminated, it may be that it cannot be polished and instead needs replacing. We can uplift the dirty fuel and take it away and supply you with a fresh, clean fuel delivery. It’s that simple!

We’ll clean your tank no matter what

Whether you need your fuel polishing or replacing, we will always carry out an industrial tank clean to avoid the fuel becoming re-contaminated.

To avoid the troublesome consequences of fuel contamination, get in touch with the fuel experts on 0330 123 1144 today. 

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