The Diesel Bug

The Effects of Microbial Contamination

The physical effects of microbiological contamination (Diesel bug) are the formation of sludge’s, bio-films (slimes) and surface or interfacial scums.

Virtually all fuel has some bacterial life however it may not be that apparent to the naked eye. These microbes start off as small microscopic single, separate living cells. At this point, they can pass though any medium size filter or injector nozzles and be burnt up within the engine causing no damage. These bacteria live on the fuel / water interface, using the water to breath & breed. Given the right conditions a single cell weighing one millionth of a gram can grow into a bio-mass weighing 10 kilograms within twenty four hours. By the time you can visually see evidence of growth, the infestation problem is quite severe. Not all growth happens so suddenly. In some situations, growth occurs slowly, creating problems during development.

Pictures of live microbiological contamination (the diesel bug) in a filter removed from a Diesel pumping unit and picture of the inside of the Diesel pumping unit.

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There are many different types of organisms that make up what we refer to as diesel "bugs". Generally, there are aerobic types; requiring oxygen for metabolism, and anaerobic types that don't. The aerobic types are responsible for the algae and slimy mats, while the anaerobic bacteria, known as SRB's (sulphur reducing bacteria) produce hydrogen sulphide as a by-product of their metabolism. Combined with moisture, strong sulphurous acids are formed, which are corrosive to metals. Evidence of this type of bacteria is usually seen a black grit resembling coffee grinds. The following are some telltale symptoms of microbial contamination:

Slimy material - in filters, tanks, and other fuel system components

Black grit - in filters, and water separator bowls

diesel bug

Corrosion - to fuel system components due to acids produced

The diesel fuel itself, since it is degraded, also shows symptoms:

Color - fuel is darker, and can be more opaque

Odor - a rotten egg smell due to hydrogen sulfides


Fuel for Food?

Diesel fuel is an organic compound made up of hydrogen and carbon, and is actually a good food source for these microscopic organisms. Within a fuel tank, there is water for germination, carbon for food, dissolved oxygen and sulphur for respiration, and trace elements for growth. May not sound good to you, but to the diesel bugs, its fish and chips! Especially with the introduction of Biodiesel which is a blend of old cooking oil with Diesel.


Microbial Contamination

Consequences of using contaminated fuel.

1. Encouraging growth of further contamination

2. Reduced filter life (fuel filter clogging and blockage)

3. Coalescer malfunctions

4. Engine wear due to variations in fuel flow

5. Corrosion of the fuel system

6. Corrosion of engine fuel injectors

7. Damage to in-line instruments

8. Corrosion of the fuel/storage tanks and lines

9. High fuel consumption

10. Blockage of pipes & valves

11. Increased water content (damage to components from water)

12. Breakdown of diesel (poor lubricity)

13. Health problems (Endotoxin producing bacteria)

14. Total engine failure (starvation of fuel)

Ultimately, performance suffers and fuel consumption and maintenance costs increase, but perhaps the most critical concern is the real potential for blockages in the fuel system which cause engine failure while underway - with potentially devastating consequences

The most common response from a diesel engine owner or a fleet manager is "I do not have a Diesel bug problem.'


What clogs fuel filters?
What causes Fuel pumps to prematurely malfunction?
What is the residue left on injectors?

A problem to some has to be major repairs or a total engine starvation leading to a shutdown. Given time this may well happen. But by then it is far too late! Preventative maintenance is the key.

Others in the know are aware that once diesel has left the refinery it becomes contaminated by water, condensation, the fuel tanks themselves, or even the air. Almost all Diesel has some microbial contamination; by addressing the ongoing issues pre-emptively, with regular fuel sampling, maintenance & inspection of storage tanks and fuel dispensers, the fitting of a duo fuel conditioner in the suction line of the fuel pump, the fitting of water/particle filters to the outlet of the fuel dispenser a major problem can be eradicated.