Online Quote Call Now

Fuel Contamination: Distillate Fuel Storage, Maintenance and Recommendations

If your business relies on diesel fuel, fuel contamination caused by water, dust and dirt and microbes (diesel bug) can cause major problems.

Ensuring that your tank is properly and regularly maintained is vital. The condition of your tank can have a massive effect on fuel quality and poor maintenance can encourage contaminants and bugs to grow inside the system.

You have a legal duty to ensure your system is regularly maintained and as low-risk as possible to help prevent health and safety hazards and oil spills.

Unsure what to look for? Download our Tank maintenance checklist

Fuel contamination

The quality of stored fuel is proportional to the condition of the storage vessel and the level of housekeeping undertaken. Typically, under ideal cool storage conditions where precautions are taken to eliminate entrainment of dust, dirt and water contaminants from the tank, diesel fuel can be stored for anywhere between 6 months and 1 year without significant degradation.

What fuels can become contaminated?

Middle distillate fuels such as refined diesel, jet fuel and marine gas oil are at great risk from contamination due to the presence of FAME.

FAME is readily biodegradable, and as such, is a good nutrient source for microbes. Couple this with FAME’s ability to absorb atmospheric moisture, you have then created an ideal home in which bugs can live and multiply.

FAME-free fuel, such as Hydrotreated Vegetable Oil (HVO fuel) is only slowly and partially biodegradable and is not hygroscopic, so it’s therefore much more resistant to microbial attack.

Potential sources of fuel contaminationThe problemThe consequence
Dust and Dirt

Particulate matter can enter fuel tanks at the point of delivery due to unclean fittings used for discharge.

Air bourne dirt and dust can enter through breather pipes and debris can also accumulate due to internal corrosion of the tank itself.

The Worldwide Fuel Charter specifies fuel with an ISO Cleanliness Level of 18/16/13 at the point of delivery. In practice, each time fuel is moved it will pick up further particulate contamination necessitating downline filtration.

Excessive levels of particulates need to be removed by filtration to prevent abrasive wear to fuel pumps and injectors.


Low levels of water can exist either dispersed within the fuel, or as free water which will collect at the bottom of the storage vessel. The obvious source of ingress is from rainwater leaking into the tank from corroded or loose fittings and lids.

Atmospheric moisture can also enter storage tanks through breather pipes. As fuel is drawn out, moist air enters the tank which then condenses and accumulates as free water. In addition, both regular diesel and gas oil now contain up to 7% FAME (fatty acid methyl ester) which is hygroscopic and attracts moisture into the fuel.

The Worldwide Fuel Charter specifies a maximum dissolved water content of 200 ppm. In practice, fuel stored under good conditions is usually well below this value and typically below 100 ppm.

Free water in storage tanks, however, provides a home for microbes, which, given a source of sustenance in the form of fuel, can multiply exponentially causing degradation of both the fuel and the storage tank itself.


In addition to dust and dirt, air bourne contaminants include spores of fungi, yeasts and bacteria.

There are many different species of microorganisms that can live in fuel, such as in the water phase at the fuel-water interface feeding on the fuel. The main culprit is Hormoconis Resinae (H Res) which is commonly referred to as ‘Diesel Bug’.

This microbe is a filamentous fungus which requires only low water levels to survive and grow into long multi-cellular filaments capable of blocking filters.

If unchecked, these organisms can form thick, slimy biofilms which adhere to tank surfaces. During new fuel deliveries, which can be broken up and dispersed, infecting other areas of the tank. Biofilms can also excrete acids causing corrosion to metallic tanks.

Fuel contamination storage method recommendations

  • Storage vessels should be kept in good condition, leak-free and well maintained
  • Tank drains or sumps should be periodically drained to remove any accumulation of water and sediments
  • Best practice is to keep fuel tanks as full as possible to minimise the headspace available for moisture-laden air
  • Avoid accidental contamination when refilling storage tanks
  • Filter the fuel each time it’s moved
  • Tanks should be cleaned and inspected every five years, irrespective of fuel condition

How do I detect the different fuel contaminants?

Health checks

Prevention is always better than a cure. Therefore, identifying potential issues at an early stage through sample analysis can prevent downtime and failure.

Establishing a fuel monitoring program where samples of fuel are taken at regular intervals will determine the condition of the fuel. Any issues will manifest themselves at the dead bottom of the storage vessel.

Tank bottom samples can be obtained by the use of a sample ‘thief’ and compared visually with a sample from the upper layer of fuel for obvious signs of contamination.

Laboratory analysis will confirm water content, ISO cleanliness and microbial activity and should be carried out at least once a year.


If contamination is identified through sample analysis, then fuel polishing and tank cleaning may be necessary. In some cases, this may involve circulating the tank contents through the polishing rig. Up to 6 passes may be needed by this method or alternatively, the tank contents can be polished onto vehicles.

The debris, water and sludge can then be removed from the base of the tank and the tank subsequently cleaned prior to refilling.

If microbial contamination is present, this can be treated with proprietary biocide. The method of treatment will depend upon the degree of infestation.

Fuel which is to be stored for extended periods can oxidise and degrade, forming peroxides, organic acids and sticky gums. The by-products of oxidation can cause acid erosion of metallic tanks. Biodiesel blends are less oxidation stable than petroleum diesel and it would be wise to consider the addition of proprietary additives to improve the storage life of such fuels.

If you suspect contamination, we have a number of different environmental services available to try and rectify your fuel. Our environmental division offers free invaluable technical support through our two onsite leading chemists. Call 0330 123 1444 for more information and to speak to one of the team.

tonnes of net CO2 reduced across our transport fleet from using HVO

Read more

Our Journey to Net Zero Carbon


7 Reasons to Choose Crown Oil


Unrivalled Service from Start to End


Online Quote