**5. Biological contamination and biofilm formation**

Microbial contamination of fuels is a major problem that affects all types of fuels, however because of the demanding specifications and use requirements for jet fuel is a much more important type of contamination. Jet fuel is a very harsh environment; however, many different types of microorganisms have found ways to metabolized different components of the fuel. The paraffinic and some other hydrocarbons are a food source for bacteria but some aromatic compounds and most sulfur containing compounds are toxic to microorganisms [22]. Microorganisms are generally found at the water-fuel interface, indicating that water removal is always a part of microorganism control. The effects of microorganisms growing in fuel are quite diverse, from the filter clogs in the fueling system, to problems in flight, to biocorrosion problems in the tanks and associated fueling system components and hydrogen sulfide corrosion [23].

Microorganisms that can metabolize a wide range of aromatic and aliphatic hydrocarbons have been identified and studied. The metabolism of aromatic compounds involves initial attack on side chains to oxidize the compound at the benzylic position and in subsequent steps open the aromatic ring [24]. The metabolism of paraffinic hydrocarbon is known to occur under both aerobic and anaerobic conditions and there have been several mechanisms proposed [25]. All of the mechanisms appear to convert the hydrocarbons initially to fatty acids either through an oxidative mechanism or through a mechanism that begins with the addition of fumarate, followed by a series of steps to give carbon dioxide and fatty acids [26]. The various anaerobic bacteria depend upon either sulfate or nitrate as an oxidant in their metabolism.

In order to survive, microorganisms have mechanisms that protect them from the toxic compounds such as formation of biofilms, secretion of surfactants and regulation of efflux pumps [27]. Biological contamination, especially where colonies or films can clog filters, reduce fuel flow among other effects. Biological contamination also changes the composition of the fuel, as the microorganisms metabolize their preferred hydrocarbons, typically paraffinic compounds, leaving the more difficult to metabolize. Some of the effects of microorganisms on fuel and fuel storage systems are shown in **Table 4**.

A number of different types of microorganisms have been isolated from air force fuel tanks, including *Micrococcus*, *Bacillus*, *Staphylococcus*, *Sphingomonas* and *Discosphaerina fagi* [28]. Microorganisms are typically present in fuels, but require the presence of water in order to grow. Regular removal of water and the use of biocides can minimize their growth [29]. Numerous types of microorganisms have been isolated from fuel systems, including moulds, bacteria, yeasts and fungi. The microorganisms require water for growth and they feed on nutrients in the fuel. Many microorganisms can metabolize the hydrocarbons, while others feed on partially degraded fuel, the trace heteroatom containing contaminants in the fuel and the additives in the fuel [30].

The types of microorganisms present in the fuel frequently depend on storage conditions and length of storage. Normally, aerobic microorganisms are dominant; since there is a constant supply of oxygen saturated fuel. During long-term fuel storage however, the oxygen is quickly used and anaerobic microorganisms flourish. In many cases, bacteria are able to use sulfate which results in the formation of hydrogen sulfide. This toxic and foul smelling gas attacks the steel of the bottom plate. Hydrogen sulfide also dissolves in the fuel and the fuel can become aggressive toward steel, silver and copper alloys [31]. Anaerobic bacteria are a particular problem in marine environments where salt water provides an abundant supply of sulfate that the anaerobic bacteria convert to hydrogen sulfide which leads to foul smelling and toxic fuel and significant biocorrosion of steel [32].


**Table 4.** The effects of microorganisms on fuel system components.

**5. Biological contamination and biofilm formation**

**Table 3.** Comparison of the classes of polar compounds found in various jet fuels.

**aminonaphthalene**

3656 Jet A X X X X X 3658 Jet A X X X X X

4336 JP-8 X X X X X

5098 JP-8 X X X X X

2747 Jet A X X X X

3773 JP-8 X X X X

4177 JP-8 X X X X

**Fuel Type Phenols Quinoline/**

202 Flight Physics - Models, Techniques and Technologies

2959 Jet A X

sulfide corrosion [23].

metabolism.

Microbial contamination of fuels is a major problem that affects all types of fuels, however because of the demanding specifications and use requirements for jet fuel is a much more important type of contamination. Jet fuel is a very harsh environment; however, many different types of microorganisms have found ways to metabolized different components of the fuel. The paraffinic and some other hydrocarbons are a food source for bacteria but some aromatic compounds and most sulfur containing compounds are toxic to microorganisms [22]. Microorganisms are generally found at the water-fuel interface, indicating that water removal is always a part of microorganism control. The effects of microorganisms growing in fuel are quite diverse, from the filter clogs in the fueling system, to problems in flight, to biocorrosion problems in the tanks and associated fueling system components and hydrogen

**Indoline/**

**tetrahydroquinoline**

**Aniline/ pyridine** **Pyrroles/satd. indoles**

Microorganisms that can metabolize a wide range of aromatic and aliphatic hydrocarbons have been identified and studied. The metabolism of aromatic compounds involves initial attack on side chains to oxidize the compound at the benzylic position and in subsequent steps open the aromatic ring [24]. The metabolism of paraffinic hydrocarbon is known to occur under both aerobic and anaerobic conditions and there have been several mechanisms proposed [25]. All of the mechanisms appear to convert the hydrocarbons initially to fatty acids either through an oxidative mechanism or through a mechanism that begins with the addition of fumarate, followed by a series of steps to give carbon dioxide and fatty acids [26]. The various anaerobic bacteria depend upon either sulfate or nitrate as an oxidant in their

In order to survive, microorganisms have mechanisms that protect them from the toxic compounds such as formation of biofilms, secretion of surfactants and regulation of efflux pumps [27]. Biological contamination, especially where colonies or films can clog filters, reduce fuel flow among other effects. Biological contamination also changes the composition of the fuel, Microorganisms have been shown to be highly adaptable in their ability to use different food sources. It has been shown that several adaptations are needed for Pseudomonas aeruginosa to metabolize jet fuel [33]. When jet fuel is the available carbon source, the bacteria alter their metabolism through transcriptional regulation to favor the use of paraffinic hydrocarbons of the C11–C13 length as a food source [34]. These same transcriptional changes require biofilm formation for bacterial growth.

The introduction of ultralow sulfur and low sulfur fuels introduces new complexities in biofilm formation. It is known that sulfur compounds normally found in petroleum based fuels are natural lubricity improvers, antioxidants and antimicrobial agents [35]. Studies have shown that the removal of the sulfur compounds does not appear to alter the bio-corrosion properties of the fuel under anaerobic conditions [36], however microorganisms have also been shown to rapidly deplete the corrosion inhibitor/lubricity improver (CILI) additives from the fuel [22].

On approach to reduce microorganism growth in fuels is to incorporate biocides in the fuel formulation. There are very few biocides that have been approved for use in Jet A, but are not allowed in the military jet fuels JP-4, JP-5 and JP-8. In Jet A, the allowed biocides are Biobor™ and Kathon™. Kathon has a sulfur heterocyclic compound as the active ingredient [37] and Biobor has a boron containing compound [38].
