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Microbial Bacteria in Diesel Fuel

The Problem 

Microbial colonies can proliferate in any fuel tank. Microbes are present everywhere, but without food (fuel) and water they cannot multiply. When there is free water in the tank, the microbes have everything they need to grow, fouling fuel and damaging tanks in the process. Warm weather and the presence of biofuels will accelerate this process. Once these microbes have gained a foothold in your fuel tank, they can be hard to eliminate. By some estimates, a microbial colony can consume up to 1% of your fuel investment, while destroying the rest. The first step to eliminating this problem, of course, is to diagnose the problem and its severity.

Microbes Need Food & Water

Microbial colonies proliferate at the interface between fuel and free water that has settled to the bottom of the tank. This creates a “rag layer” which gives them everything they need to thrive. Warm temperatures will accelerate the growth of microbial colonies. Microbial growth can occur in any diesel fuel. Biodiesel, being made from plant and animal fats, makes especially good food for these bugs and contributes to the increased incidence of biological growth problems seen in recent years. Bugs can grow in petro diesel as well. Stagnant fuel is especially at risk.

Degraded Fuel Becomes Unusable

With time the microbial colony proliferates beyond control. This leads to acid formation, rust, corrosion and filter plugging. Fuel degrades to the point that it can form a slimy sludge that is unusable as fuel.

This process can occur in a bulk storage tank or in a piece of equipment that is left idle for a long period of time. To the right is a classic example of what is called filter “leopard spotting”, which requires the filter to be exposed to both microbes and water. The black spots are microbial colonies. Live or dead, microbes will clog filters and damage fuel systems. It is important to eliminate bugs completely and permanently. 

Microbial Growth vs. Algae

Microbial colonies, sometimes incorrectly referred to as algae, are actually bacteria or fungus. Algae needs light to live and grow, there is no sunlight in a closed fuel tank so algae cannot survive. ​

The Solution

Microbial contamination varies in appearance, but looks very different from typical dirt. Most people don't realize they have a problem with their system until they experience filter plugging. By this time the situation is far more severe than just a few microbes. 

Diagnosing Microbial Growth

One easy indicator may be found in your used fuel filters. Open up a clogged filter, if it is coated with stinky, slimy, black filth; then you probably have a microbe problem. ​One ASTM test method for diagnosing microorganisms in fuel is described in ASTM D7463-08. This test detects the presence of Adenosine Triphosphate (ATP) in fuel and fuel/water mixtures. ATP is an energy-bearing molecule found in all living cells. If ATP is present, it means that microbes are living in your diesel tank. The test helps estimate the concentration of microorganisms, which is useful in diagnosing a microbial problem, validating the effectiveness of the treatment, and for ongoing monitoring of your fuel supply. The advantages of this test method are that it is fast and easy. The user can do it and see the results in about 10 minutes. 

Other tests that can be done by users may be more complex and require several days before producing results. It should be noted that regardless of the test method you choose, one sample may not provide the complete picture. The best way to get a good idea of the severity of any biological problem is to take samples from more than one location and at a variety of different depths. This is especially true when tank design makes it difficult to examine areas where there is water. Depending on the size of your fuel investment, you may wish to call in an expert or purchase a commercially available kit to do the testing yourself. A quick search for "microbial growth test kit" will yield a number of options.

Eliminating Microbial Colonies

Microbes are everywhere. But when they have acclimated to your fuel system, half-measures will probably not eliminate the problem. If you don't remove the system's free water and substantially reduce the active colony, they will simply grow back. You never truly sterilize a fuel system, but you can reduce and control biological activity to a level where it is not a problem. Numerous good biocides are available for proactive use. Do not use what many call "maintenance dosing". These are usually sub-lethal and actually make things worse. A better proactive strategy is to periodically treat the system with a kill dose. The time frame for this can be determined by a systematic testing program to determine how frequently your system gets re-infested. Also, it is very important to institute a vigorous water removal program. This alone will significantly reduce future contamination issues. 

For bad infestations, we recommend a multi-step process: 

  1. Drain the tank of all free water.
  2. Use a high dose of a reputable biocide to "shock" the contaminated tank
  3. Follow this with a thorough "man down" tank cleaning, scouring the sides
    and bottom to remove all remnants.
  4. Prevent reoccurrence by using a periodic kill dose of biocide on a regular
    basis to protect a clean system.
  5. Install a highly efficient filter system at the outlet of your tank and/or at your dispenser to ensure that microbes (alive or dead) are not passed along into your equipment where they can quickly clog your onboard filters and form engine deposits. 

Be sure to follow dosing instructions carefully and retest your tank periodically, to be sure that your anti-microbial regimen is working. Some equipment operators choose to skip the tank cleaning step due to downtime, cost considerations or the inability to physically enter the tank. In this case, be aware that you may load up a large quantity of filters with dead microbes before the tank flushes itself out. Microbes cannot reproduce without water. If your tank is properly maintained and contains no free water, then microbes will not grow. 

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