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Bacteria in diesel is a well known problem to anyone who works with diesel engines, because it is the Number 1 cause of engine breakdowns.
Diesel is an organic fuel.
It provides an ideal environment for microscopic fungi, yeast and bacteria to feed and grow. There is:
- dissolved water for germination
- carbon for food
- oxygen and sulfur for respiration
- trace elements for growth and propagation.
As many as 27 varieties of bacteria are responsible for the majority of problems with diesel engines and their performance.These many differing types of bacteria infect systems and form bio-films on steel surfaces. Accelerated corrosion occurs wherever the bio-film settles, usually in the form of pits or crevices. Unlike general corrosion, it is an attack on a very specific area.
No one knows when they receive contaminated diesel, but once contaminated diesel enters the fuel system, it is very difficult to eradicate.
Acquired from the air and any moisture, during tank filling and/or expansion and contraction of storage tanks, the bacteria cover themselves in a protective film (slime) to protect against biocides and lie dormant in the minute crevices of the metal, rubber and polyurethane coatings of the fuel tanks and fuel systems.
Then, when water is present [a droplet is a lake to a microbe] and the environment hits the right temperature range, they begin reproduction in the fuel water interface.
Microscopic in size, they can develop into a mat easily visible to the naked eye very quickly. A single cell, weighing only one millionth of a gram can grow to a biomass of 10 kilograms in just 12 hours, resulting in a biomass several centimetres thick across the fuel/water interface.
Each specie has its own characteristics.
BACTERIA: utilise hydrocarbons and reproduce asexually by binary fission; swelling in size as they feed, they then separate into two cells. In this way, microbes double their numbers every 20 minutes, one spore converting to 262,144 in 6 hours.
SULPHATE REDUCING BACTERIA (SRB): are a specific group of bacteria utilising simple carbon, not hydrocarbons, and require the activity of other microbes in a consortium. Aerobic (in the presence of oxygen) or anaerobic (without oxygen) bacteria have a combined effect. The aerobic bacteria (sulphate oxidising) create a film to consume the oxygen first. This allows the anaerobic (sulphate reducing) bacteria to thrive.
SRB's reduce sulphates to produce hydrogen sulphide [a lethal gas]. They are directly involved with many microbial corrosion reactions and can cause sulphide souring of stored distillate products. Their action changes the Ph creating an acidic environment, conducive to accelerated corrosion. They attach themselves to the steel as a film and go to work. They derive their nutrition from the surrounding environment and multiply. They are particularly difficult to deal with and produce a sludgy by-product with a strong sulfur odour similar to rotten eggs [hydrogen sulphide].
IRON REDUCING BACTERIA: also contribute to corrosion, eating steel and reducing ferrite to an oxide through a chemical reaction .
YEASTS: prefer acidic environments, such as produced by SRB's. They bud on the parent cell, eventually separating. Reproduction takes several hours.
FUNGUS: grow in the form of branched hyphae, a few microns in diameter, forming thick, tough, intertwined myciel mats at fuel/water interfaces.
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