How can you defend your diesel supply against microbial contamination? If you’re a user or supplier of diesel fuels, microbial contamination can pose a serious threat to your business. Following ASTM best industry practices is ideal to maintaining a quality fuel maintenance program:

  • ASTM D6469 – Standard Guide for Microbial Contamination in Fuels and Fuel Systems,
  • ASTM D7464 – Standard Practice for Manual Sampling of Liquid Fuels for Microbiological Testing,
  • ASTM D8070 – Screening of Fuels for Microbial Contamination by Lateral Flow Immunoassay.

Performing ASTM recommended fuel sampling and testing methods for onsite assessment of microbial contamination, leads to better decision making when determining whether to treat your tank with biocide and/or other cleaning methods. Implementing preventative sampling and testing procedures is imperative in maintaining the reliability of mission-critical power operations.

Microbiological Contamination in Fuels – Tank Environment Issues

Middle distillate fuels such as diesel, biodiesel, and heating products can be affected by contaminants like water, particulates (e.g., rust and dust), other fuel types such as gasoline, and microbial contamination. For microbial contamination to develop in fuel storage tanks, there has to be water. Water accumulation in tanks can be generated by the condensation of ambient air moisture. Accumulation of free water in the bottom of the tank will provide the needed oxygen for the microorganisms to grow at the fuel-water interface. Diesel fuel is particularly at risk of microbial contamination due to its properties. In the last 10+ years Fatty Acid Methyl Ester (FAME), also called biofuel, have been added to diesel. Biodiesel blends are more susceptible to microbial contamination because it attracts and holds a greater amount of water compared to conventional diesel fuel.

As stated, all microbes (bugs) require the presence of water to grow and proliferate in fuel systems. Consequently, microbes such as aerobic bacteria generally concentrate in the bottom water (“water phase”) within a fuel tank and in the fuel/water interface, feeding on the fuel as nutrients and using water as a source of oxygen to proliferate. For the most accurate determination of microbial contamination within a fuel system, it is recommended to test at least some of the water phase from a fuel tank, when present. If you are conducting regular water draining it is strongly advised to test the water sample drained from the tank before disposal.

Within a fuel phase, there may be a limited amount of suspended water available to sustain the growth of microbial communities, especially so for aviation fuels. Fuel-only samples (where no free water phase is collected from the tank) are heterogeneous by nature regarding the suspension and distribution of microorganisms, meaning there are various levels of microbial presence throughout the sample. This can lead to variable results where multiple fuel-only samples are taken from the same fuel tank. As a result, a fuel-only sample is less likely to be representative of the actual tank conditions, as in most of these cases there will be either a free water phase, pockets of water, or condensation present somewhere within the tank.

Fuel Sampling

A microbiological test is only as good as the sample which has been taken. It is recommended that industry standards and guidance materials such as ASTM D7464, ASTM D6469 are followed.

To collect a sample that accurately represents the current condition of your fuel, it is important to sample directly from the storage tank. Be sure to pull samples from problematic areas that are often hard-to-reach and/or are toward the bottom of the tank. It is in these areas fuel will often degrade due to stagnation and contaminants will collect after falling out of suspension. If your fuel system has multiple sources, be sure to test all locations to ensure the fuel quality is consistent and contamination has not spread beyond the original point of testing. Practically for most standard tanks, a sample should be taken from the lowest point to obtain a bottom free water sample. This is generally most likely to give the most representative result of microbiological contamination.

It is important that a clean sampling container (HDPE Sampling Containers are industry standard) is used to minimize the risk of cross-contamination. If using the same sampling equipment for multiple samples each item used should be cleaned prior to re-use with no less than 70% alcohol wipes (or other sterilization methods) and left to dry before reuse.

It is often recommended that a minimum sample of 1 Litre (roughly ¼ gallon) should be drawn initially for both visual and microbiological testing. In an ideal world taking samples from different parts of the tank would be widely practiced obtaining the most representative result given organisms are not evenly spread.

Once the sample is taken, the container should be sealed and properly labeled with at least the date, time, asset, and location details to reduce potential confusion if taking multiple samples and/ or transporting the samples to off-site facilities.


Regular fuel testing is critical in being able to manage an energy source. If unnoticed, contamination in a fuel tank can compromise an entire fuel system. This can leave room for vulnerabilities and unnecessary lapses in system integrity. Fuel can fall out of the desired specification characteristics in a matter of months or even weeks, requiring frequent testing to ensure your fuel remains optimal for use.

If conducting different microbial tests for comparative purposes, it is vital that the same sample is used, at the same time, irrespective of how long each test takes to provide a result. Delaying testing times between different tests will impact the results each test will provide and your comparison will be invalid.

A testing regime developed over time to optimize testing frequency will reduce the risk of major impact from microbiological contamination. Click here to learn more about the FUELSTAT on-site test kit.