Like any other organic fluid, fuels and oils are susceptible to natural deterioration. Outbreaks of microbial contamination of fuels develop periodically and can often be traced to poor quality supplies coming on to the open market. Modern refining techniques, storage and transportation systems all contribute to contamination of the fuel.

Oxidation, chemical incompatability, water and microbes all contribute to the process of polymerization and stratification; this results in poor fuel quality, tank sludge, bio film and acids. Diesel, Aviation Fuel and Bio Fuels are hygroscopic, meaning that it absorbs water from the air. The existence of this water provides a breeding ground for microbial growth. Left untreated, these micro organisms will multiply, contaminating fuel lines and clogging filters and injectors, causing poor performance and high emissions. Over time, these micro organisms will also break down the fuel making it harder to burn.

The bacteria both aerobic and anaerobic (microorganisms), produce highly corrosive chemicals like organic acids, including sulphurous (H3PO3) and sulphuric acid (H3PO4). These chemicals will cause severe degradation of storage tank coatings, corrosion of metal tank surfaces and injection equipment. The effect of the microbiological contaminants on fuel injection equipment is to quickly impair performance and dramatically increase wear rates. This brings about poor combustion conditions, loss of engine power and reduced service life of components.

For microbial population and their associated byproducts, fuel quality rapidly deteriorates and problems like haziness, failure to meet specifications, corrosion, filter plugging and additive degratation etc. can be occured.

Fuel Haziness
Microbial surfactants can stimulate the suspension of water in fuel causing it to become hazy and causing failure of fuel water separators.

The primery cause of haziness is an increase in the water content of the fuel resulting from the production of biosufactants. These are byproducts of microbial growth and alter the surface tension at the fuelor water interface. As a consequence the solubility of water in the fuel is increased.

Although the microbes need very little amount of water to grow, in practice there is often sufficient water in the bottom of large storage tanks and even in the fuel tanks of road vehicles such as trucks and buses for some microbial growth to occur. The microbes tend to be most active at the interface between the water and fuel and also on internal tank and system surfaces. When the tank contents are disturbed, for example when the tank is refilled, the microbes become suspended in the bulk fuel where they cause fouling and may be passed on down the distribution chain to contaminate facilities downstream. Costs for decontaminating facilities are high. Prevention of problems by good housekeeping and regular monitoring is most definitely more cost effective than the "fire brigade treatment" of problems and the consequential losses due to system and equipment failures.

Degradation of Additives
Certain additives, which rich in nitrogen(N2) and phosphorous(P), encourage microbial growth. In the process the additives are degraded and consequently their effect is lost.

Microbially Induced Corrosion (MIC)
Growth of the sulphate reducing bacteria(SRB) in fuel tanks can cause sulphide spoilage of fuels. The fuel becomes corrosive and will fail sulphide limit specifications.

Hydrogen sulphide(H2S) is produced by SRB. This enters solution and is highly corrosive, causing severe pitting of the fuel tanks and pipework.

Sludge Formation
Microbial debris is deposited on the tank bottom where it forms a layer of sludge. This sludge creates an environment which favours MIC. It may also become contaminated with viable microorganisms and unless removed will act as a reservoir of infection every time the tank is used.

Filter Plugging
Biopolimers are formed during microbial growth, which are gummy products. Along with microbial and other debris, they are deposited on filters and pipes leading to reduce flow rates and blockages. At end user level this can have serious consequences causing engine damage and in extreme cases complete failure.

Fouling, and Other factors influencing microbial contamination
Fouling by slimes produced by bacteria, yeasts or moulds can cause severe filter plugging, blocking of fuel lines and injectors and consequently then cause excessive wear and failure of engines and system components. Fuel fouled by microbial slimes can fail particulate specifications.

If there is a low turnover of the stored fuel, such as in a strategic reserve, contamination is much more likely to develop. Poorly maintained or outdated storage facilities also present greater opportunities for contamination.

Although microbial spoilage and contamination occurs in a wide range of fuel types, some have been found to be more susceptible than others. For example, straight chain paraffins tend to be more readily degraded than aromatics and olefins.

In addition to the microbial contamination at the fuel water interface there is a sessible population attached to the tank walls. This is frequently overlooked. Unless treated it will act as an innoculum reservoir for future contamination.

Until now the most widespread method of dealing with microbiological contamination has been by the addition of a suitable biocide to the contaminated system. This method however brings with it its own raft of problems.