The Dreaded Diesel Bug: What It Is and How to Prevent It

Diesel bug isn’t a creepy crawly that lives inside fuel tanks, but a microbial sludge that grows between the fuel and the layer of water at the bottom of the tank when this water is in the fuel for too long. The thick sludge can then clog machinery and filters, causing costly engine repairs and downtime for machinery.

So, what causes diesel bug and how does water get into fuel in the first place?

Diesel is hygroscopic, which means the fuel absorbs and holds onto water at a molecular level. This can lead to free, bound, and emulsified water in the fuel. You can read more about these 3 types of water contamination in diesel fuel in our article here.

The amount of water that is absorbed by the fuel varies with temperature, air humidity and the biodiesel content. Condensation can happen inside the tank when the temperature changes outside, and the water droplets then fall into the fuel, mixing with its molecules or settling at the bottom of the tank.

Rainwater can find its way into the tank and fuel too if there are loose cap fittings or cracks in the seals or tank walls, so parts need to be checked regularly.

Diesel bug has the appearance of sludge or slime, which is black or brown in colour. What it looks like could be likened to the mould you can get in your shower plug. It gives off a bad odour similar to rotten eggs due to the microbial content.

Your fuel may have diesel bug if you notice a brown sludge residue on your equipment and if filters start blocking quicker than usual due to the sludge. Without a filter the sludge would get into the engine of your machinery and cause considerable damage, so replacing blocked filters as soon as possible is essential to protect your equipment.

If you have noticed diesel bug symptoms and have found sludge forming in your tank, what is the best way to remove it?

Once diesel bug is inside your tank, the use of fuel tank filters alone is not going to solve your issue as you need to work on killing the bug and cleaning the sludge from your tank.

Biocides are a type of diesel fuel additive that can be used to treat diesel bug. It is added to the fuel tank directly and controls the bacteria and sludge formation, as well as dispersing existent diesel bug sludge.

If you have a large problem, your tank may need to be professionally cleaned and your remaining fuel be polished to remove the diesel bug. Fuel polishing removes water, sludge and microbial contamination through filtration and circulation in a cleaning process, that needs to be done by a professional. This is recommended as part of an annual fuel maintenance programme to ensure fuel is kept at ISO standards, of a EN590 quality standard.

Prevention is the best cure when it comes to diesel bug, so removing any water in the fuel will stop the sludge growing in the first place. The water can be removed from the fuel, using fuel tank filters, the use of a fuel recirculation unit or fuel tank water absorbers.

A fuel tank filter removes water from the fuel entering your vehicle/machinery but does not remove its presence from the fuel tank. To remove it from the tank you can use a recirculation unit which takes the water out of the fuel and puts it back into the tank.

If you can see a layer of free water at the bottom of your fuel tank, you may have a sludge valve at the bottom of the tank which you can use to drain away the free water. If your tank has no valve, you may need to have the water removed with a pump. This will not work if your fuel has bound or emulsified water, however. Let’s look at the best way to prevent this type of water contamination from happening.

Tank Dryers remove contaminated water in fuel with a highly absorbent silica gel. They are placed into a tank and absorb a small amount of water, which means it may not be suitable for large fuel tanks or containers.

A fuel tank water absorber usually only removes free water from the bottom of a fuel tank, but Aquafighter is the only fuel tank water absorber on the market that removes suspended and emulsified water as well. This stops the diesel bug being able to form which would cause engine and equipment damage. The Aquafighter product range reduces the water content levels down to less than 75ppm, meeting and exceeding the EN590 diesel fuel standards of 200ppm. It is placed inside the fuel tank through the tank opening and simply pulled out and disposed of once full.