Guide to Managing Fuel Contamination in All Weathers

8 out of 10 diesel engine failures are due to diesel bug

Stored diesel fuel can deteriorate over time as it reacts with oxygen from the air. Diesel usually has a shelf life of around 1-year, so good maintenance and correct storage conditions are vital to reduce its risk from fuel contamination. Diesel bug, water and sludge can degrade your fuel’s quality and eventually make their way into equipment and machinery, blocking fuel injectors and damaging other parts of your engine or boiler.

Degraded fuel can also cause internal tank corrosion which can damage dispensers, block filters and eventually lead to a halt in operations. Once a tank’s integrity has become damaged, this can also increase the danger of fuel leaking into the nearby environment, risking large fines and prosecution.

What is a diesel bug?

There are a wide range of types of bacteria that live in fuel; the most prevalent is diesel bug, which is also known as microbial contamination. Diesel bug can multiply extremely quickly and double in size every 20 minutes. Therefore, it doesn’t take long until a biomass of diesel bug is several centimetres thick, causing chaos in your engine, tank or machinery.

If left untreated, microbial contamination can clog filters, corrode fuel tanks, destroy engines and bring your operations to a standstill.

Diesel Bug

Microbial contamination appearance

Microbial contamination can often look like slime, but its colour depends on the type of diesel bug. It varies from a milky/cloudy appearance to a red deposit on the bottom of your tank or on the surface of the fuel filter. As it lives in the layer between water and the fuel, it is often associated with a rotten egg-like smell.

Biodiesel FAME is to blame for contamination

The legislated rise of FAME (fatty acid methyl ester) in fuels has led to an increase in performance issues that decrease their shelf life when in storage. FAME biodiesel is hydroscopic so absorbs atmospheric water, including moisture from the air, which increases the risk of microbial attack. The attraction of water encourages the growth of diesel bug, which can cause gums and resins to attach to the sides of your tank and cause it to build up over time.

FAME also has varying degrees of unsaturation which are vulnerable to oxidation, which causes sediment to form in the fuel. This means diesel must be checked regularly to ensure that it’s reliable and functional.

Top tip: Why not use a FAME-free synthetic diesel fuel?

HVO is an advanced renewable fuel that can be used as a direct, drop-in replacement for mineral diesel that reduces net greenhouse gas CO2 emissions by as much as 90%. HVO meets legislated biological content requirements with zero FAME content and therefore doesn’t face the instability and operability issues that many mineral diesels and biofuels face. These qualities give it a stored lifespan of up to 10 years compared to one year for regular diesel.

How to manage diesel contamination throughout the year

The British weather is known for its unpredictability and, unfortunately, this can put your fuel system at greater risk from diesel bug. As a UK leading fuel and environmental services provider, we have the expertise and breadth of knowledge to help protect your vital resources. Read on to learn how to protect your fuel supply in all weathers.

Fuel contamination issues in winter

Top tip: During autumn, we encourage all of our customers to plan ahead and prepare for any periods of bad weather. Being proactive will help protect your stored fuel and keep costs low.

Freezing temperatures

The winter period is more likely to cause contaminated fuel, particularly from freezing temperatures. The fuel contains wax paraffins which are an important component as they give the fuel its high cetane number to allow efficient combustion. These paraffins are usually in liquid form and are dissolved in the fuel, but when the temperature drops low enough, the molecules start to form crystals and solidify.

If fuel storage systems are exposed to the elements or have been badly designed, they can have insufficient thermal protection and the wax crystals can easily plug filters and pipework. In some instances, the narrow bore pipework can bend and unions can become fully blocked, causing the engine to fail or stop completely due to fuel starvation.

Top tip: Before the winter, you must ensure that you’ve switched any summer grade diesel over to winter grade fuel – summer grade diesel is only certified for use in summer as it has a lower viscosity and is more susceptible to gelling in cold temperatures.

Winter grade diesel contains an additive which prevents wax crystals from forming at low temperatures and helps it resist gelling in cold temperatures. This allows it to be used all year round as it’s more resistant to the cold.

Most fuel suppliers automatically change over to the correct grade diesel in early October so it’s always best to get a fresh delivery of winter grade fuel in plenty of time.


The freezing temperatures that come with snowfall aren’t the only danger; you should also be aware of the damage heavy snow can cause to plastic fuel tanks. Most plastic tanks aren’t designed to be load bearing, which is why it’s important to shelter your tank if you can. The weight of the snow can cause stress to the tank’s walls, leading to weak points, cracks or even splits which compromise your tank’s integrity.

Top tip: Be sure to remove any large deposits of snow off the top of your tank as it could impair its integrity. Lagging or insulating any exposed pipework is also recommended to avoid further damage from freezing temperatures, as well as minimising condensation build-up inside the tank itself.

Strong winds and plastic tanks

Wind can accelerate heat loss in a fuel tank and lead to rapid cooling of the stored fuel. What’s more, as fuel levels drop and the tank weighs less, strong winds could move it out of place which can damage the tank itself and lead to an oil spill.

Top tip: We recommend checking that your tank is positioned securely on its base ahead of the winter – this includes making sure it’s central and, on a level, even base. Keeping your fuel topped up ahead of the winter will also reduce the risk of your tank position being compromised.

Heavy rainfall

Old tanks and poor maintenance can cause rainwater to get inside your fuel tank if seals are corroded or faulty, openings are left open, vents are damaged, or the tank integrity has been compromised by perforations, cracks or holes in the tank.

These wet conditions create the perfect conditions for diesel bug to thrive and quickly propagate, eating your fuel’s hydrocarbons and leaving a waxy sludge or biomass behind.

Top tip: During your routine tank inspections, it’s important to look for signs of damage to avoid water getting in and ensure hatches and caps are tightly closed.

Fuel contamination issues in spring

Spring is the perfect time to ensure your fuel tank is in good condition. After a winter full of heating buildings and fuelling equipment, a build-up of sludge and residue is highly likely – which if left untreated, can cause severe damage to your fuel system and equipment.

Bacteria, viruses, fungi and yeasts can multiply at a rapid rate in the right conditions; these are generally having due to having water (from condensation) in the fuel (hydrocarbons) and generally our UK milder temperatures.

Fluctuating temperatures

It’s not just during the cold months when your fuel tank is at an increased risk from diesel bug: you should also take extra care during the spring. The cool nights paired with warm days can cause a layer of frost to appear on the outside of your fuel tank. Like with our road vehicles, this has usually disappeared by late morning; however, this fluctuation in temperature means the same conditions have most likely occurred inside your tank too.

As your tank heats up during the day and cools down at night, this can cause condensation to form and ultimately lead to a build-up of water inside. These droplets appear on the inner walls and because water is heavier than most fuels, they will eventually settle on the bottom of the tank.

As previously discussed, water creates the ideal conditions for bacteria and other harmful contaminants to develop and in time, degrade the quality of your fuel. Diesel bug is one of the most common microorganisms that can grow in these conditions which deteriorate your fuel and lead to sludge build-up which can damage your engine.

Top tip: As water usually appears on the bottom of your tank, it can be difficult to see during your routine checks, so it’s important to invest in regular professional fuel tests. Engineers have the skills and equipment to look for condensation and other issues such as diesel bug and sludge build-up.

Steel tank corrosion

Steel tanks are a great choice to store bulk fuel as they’re sturdy and can withstand impacts that would shatter a plastic tank. They’re also non-porous and boast a much longer lifespan than plastic tanks. However, their properties also increase the risk of corrosion from the weather, particularly heat and UV light.

Over time, corrosion reduces a tank’s integrity and eventually penetrates the tank’s structure, instigating tank bottom perforation. This can cause cracks and holes in the surface which can lead to oil spills and leaks into the environment.

Corrosion tends to take shape from the inside of your tank, which means the magnitude of the problem isn’t usually known until problems with the periphery of your tank transpire, which by this point, they are often too complex to fix.

As well as the increased risk of oil spills, tank corrosion also leads to an increase in oxidation of the hydrocarbon ingredients which can cause sludge, diesel bug and fungi to build-up.

Top tip: Regular fuel tests and tank cleans by a professional company are key in securing the future of your fuel supply throughout the year.

How to tell if your fuel is contaminated

Clean, high-quality fuel is key in ensuring the smooth running and long lifespan of your fuel system and its associated equipment or machinery. If you spot any of the below physical symptoms, you must get it checked right away to avoid expensive replacements:

  • Engine/machinery start and stop issues
  • Tank corrosion
  • Blocked or slimy filters that need regularly changing
  • Rotten egg-like smell
  • Increased fuel consumption
  • Vehicle parts wearing out prematurely
  • Fuel discolouration
  • Frozen or rusting pipework

How to prevent diesel fuel contamination

Good maintenance and regular professional checks are key to reducing the risk of contamination of your stored fuel. A proactive approach as part of a planned preventative maintenance programme will help you avoid the high costs of reactive, emergency scenarios.

At Beesley Fuels, we not only carry out fuel testing, but we specialise in industrial tank cleaning too, allowing us to provide an all-in-one solution to protect your business fuel needs.

Simply call us today on 0330 123 1144 to speak to our team of fuel experts regarding our wide range of services to manage diesel bug and other contaminants.

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