Different Types of Fuel Contamination

Unfortunately, fuel contamination is a common problem when fuels such as diesel are left in fuel storage tanks. Therefore, being aware of the different types of contamination will help you to take steps to prevent it in the first place. From regular testing to professional tank checks, this guide has everything you need to know.

What is fuel contamination?

Essentially, fuel contamination is caused by bacteria, rust, algae, sediment or even water that has mixed with your fuel. It’s difficult to spot and if left untreated, even the smallest amounts can cause long term chaos for your business, which is why regular management is key to mitigating the risk of expensive problems within your organisation.

What causes fuel contamination?

Contamination problems are increasing due to the mandated increase in FAME (Fatty Acid Methyl Ester) content in both red and white diesel in order to reduce CO2 emissions. FAME is the generic chemical term for a form of biodiesel which is predominantly produced from renewable energy sources and used cooking oils. FAME attracts moisture which can lead to contamination issues and much more serious complications such as clogged filters if ignored. Although FAME is the most common, there are other forms of contamination.

How to know if your fuel is contaminated

If you’re storing fuels and oils within your own fuel storage tanks on-site. You should be aware of the signs that your fuel may be contaminated, in order to ensure you keep them at their peak condition. Below, we’ve listed three of the signs to look out for.

Appearance Changes

When we test your fuel, our engineers will take samples from the top, middle and bottom of the tank. Generally, fuel should be clear and bright. If it’s been affected by contamination, it will be cloudy or dark or even have visible debris suspended in the sample.

If the fuel is cloudy, this indicates that water is present in your tank. Water is the most damaging contaminant found in fuel and needs to be rectified as soon as possible.

If the fuel is dark in colour, then asphaltenes are probably beginning to form within your tank which can thicken your fuel and eventually cause blockages in engine filters.

Vehicle or equipment performance is affected

Another key sign of fuel contamination is if your vehicle or fuel system is beginning to perform poorly. If you’re used to driving a certain vehicle, you may notice that it’s driving a little differently, for example you may be having issues with involuntary changes in speed or accelerating.

You should also be aware of the different types of smoke that your vehicle may emit.

Tank condition changes

You should also regularly check your fuel tank as it allows you to catch any fuel contaminants before they begin to spread and cause issues for your business. Your first point of call should be to see if any filtering systems are clogged as a result of sludge buildup. Another good indicator is if there’s a layer of sludge at the bottom of your fuel tank. However, this can be difficult to spot without a professional company doing a full tank inspection. You can however use the sludge and water outlet to drain your tank if you have one fitted.

Which material is best for your storage tank, metal or plastic?

Metal fuel storage tanks

Steel storage tanks bring high levels of security to help prevent syphoning of your fuel. They are also highly durable and long-lasting; however, the biggest issue with metal storage tanks is that they can start to show signs of corrosion and are more vulnerable to corroding than plastic tanks. Fortunately, metal storage tanks can often be repaired and are often available in larger sizes than plastic tanks.

Plastic fuel storage tanks

Plastic fuel storage is a cheaper option than metal tanks, but they can bulge or get holes in and are less secure than steel fuel storage tanks. The good news for those of you with a plastic tank is that signs of damage or corrosion happens firstly on the outside of the tank so you can act before it damages your fuel. Plastic tanks have a better resistance to heat too but they will weaken over time if left in direct sunlight.

Not only is it important to check the inside of your tank but you also need to look for signs of damage on the outside, such as corrosion of steel tanks or bulging and holes in plastic tanks. These issues need to be treated immediately because if ignored, fuel can eventually perforate the tank walls and leak into the surrounding environment.

The different types of fuel contamination in a plastic tanks?

There are many different types of contamination that your fuel can suffer from, but some of the more common ones can include:


These can cause the most issues with your fuel and come from dust or dirt which can corrode your fuel systems. This type of damage will require a professional to treat it.


These are formed when the diesel is heated under high pressure fuel injection systems. These systems operate at around six times the pressure of conventional fuel systems. In turn, asphaltenes are created which are small pieces of tar that are around 2 microns in size.

Microbes (Diesel Bug)

Diesel fuel can contain up to 20% biodiesel; the UK mandates 7% which is really good for the environment but isn’t so good for companies that store fuel for extended periods of time. Biodiesel holds and attracts water more than other fuels which can provide the perfect growth conditions for diesel bugs and other bacteria. Plus, the build up of water within the fuel can cause resins to stick to the side of your fuel tank.

How can you identify fuel contamination?


Particulates can get into your fuel in a variety of ways, including dirt or sand which can enter into fuel tanks in open cracks, causing damage to your engine.

Particulates will be visible as small fragments of debris that are suspended in clear fuel, especially if your fuel has been agitated.


Mould is recognisable as fungal or mossy growth, most often found on the inner walls of your tank or even suspended in your fuel. It can be damaging to fuel pumps and filters, and can even lead to blocked pipework.

If you notice a build up of mould within your fuel system, your fuel has probably become contaminated by microbial growth and will need to be treated by a fuel professional.


Sludge is a viscous, dark and often smelly gel-like substance that accumulates on the base of your fuel tank beneath your fuel. It is the by product of diesel bug growth as it consumes FAME and other compounds present in your fuels, and poses a risk not only to your pumps and engines but can also accelerate tank erosion.

Sludge can accumulate at the bottom of your fuel tank from a combination of particulates and mould. This can damage both your fuel and your system.

Concerned about fuel contamination?

We offer a free site survey as well fuel sampling to all businesses*. Using our free survey will allow you to understand the maintenance services you require and help you to prevent any potential issues.

If you’re wanting to learn more about your fuel then our fuel testing and sampling FAQs will be able to give you answers to questions we’re regularly asked.

If you have any questions or would like to have your fuel tested by us you can call our fuel contamination experts today on 0330 123 1144 to learn more about how we can help you.

*Limited to one survey per business.

Free Online Quote

Free Online Quote

The benefits of using Beesley Fuels

24 – 48 hour Standard Delivery

We deliver on our promise, with 92% of our orders successfully delivered within 24 hours.

24/7 Emergency Delivery

We can come to your rescue with our emergency deliveries, helping you avoid unplanned downtime.

Sustainable Fuelling Solutions

We don't just supply fuel, we have the tools and expertise to help you fuel your business responsibly.

Personal Account Managers

With us, you’ll have one point of contact throughout, who will know your fuel needs inside out.