MICROBIAL CONTAMINATION and ASSOCIATED CORROSION in FUELS, DURING STORAGE, DISTRIBUTION and USE

Edward C. Hill1,a and Graham C. Hill1,b

1 ECHA Microbiology Ltd, Unit M210 Cardiff Bay Business Centre, Titan Road, Cardiff CF24 5EJ, UK

athill@echamicrobiology.co.uk, bghill@echamicrobiology.co.uk Keywords: Microbially Induced Corrosion, MIC, Fuel Microbiology, Biocides, On-site tests, Sulphate Reducing Bacteria, Microbes. Abstract. Microbial contamination and growth in distillate fuels has been described for seventy years. The consequences have ranged from fouling of filters and injectors, to engine malfunction and damage, fuel gauge malfunctions and aggravated corrosion of engines, fuel tanks, equipment and facilities. The types of microbes present vary with the differences in fuel composition and differences in storage and use conditions. Anti-microbial strategies have traditionally included prevention by ‘good housekeeping’ and ‘fire-brigade’ applications of biocides when there are operational problems. Since 2002, first the aviation industry and later fuel suppliers and some militaries, have used simple on-site microbiological tests to monitor fuel and fuel systems and use the results to take remedial actions before operational problems occur. This paper will review our latest knowledge of microbially influenced corrosion and of the new anti-microbial strategies which are being successfully implemented to prevent it. Introduction The recognition of microbial growth in fuels, and associated fouling, spoilage and corrosion, goes back about seventy years. For much of this time, when incidents were reported, ‘fire fighting, measures were taken, but there was limited fundamental research. As the chemistry of fuels and their additives changed, and storage and distribution practices changed, microbiological problems appeared and disappeared. There has been considerable scope for re-inventing the wheel. Some historical (and personal) highlights are given below: 1939 Gas producing bacteria cause explosion in kerosene tank 1941 Bacteria in storage tanks degrade aviation gasoline and kerosene 1950’s Sulphate Reducing Bacteria (SRB) in aviation gasoline tanks cause fuel pump failures due to sulphide corrosion in Hastings, Valetta and Canberra aircraft. Bacteria cause fuel filter plugging in Boeing B 47 and KC 97 aircraft (USAF). US B52 crashes. 1960’s Numerous reports of fungal spoilage and corrosion in aircraft kerosene tanks (Fury, Boeing and Lockheed aircraft) - predominantly Cladosporium resinae but many other fungi, and bacteria, also significant. Biobor JF introduced as a dedicated anti-microbial fuel treatment biocide. 1970’s Widespread microbial spoilage and corrosion problems reported in fuel for ships, road vehicles, agricultural vehicles, trains, power generators, heating systems etc. 1980’s More of the same widespread problems, but aviation fuel problems become less common as awareness and housekeeping standards improve. 1990’s onwards. Professional microbiologists leave petroleum industry leading to less in-house scrutiny of procedures and practices, and less research. Suspect stocks of long term stored fuel reach market, including garage fore-courts. More and more problems with marine and automotive diesel, F76 and aviation kerosene, from a wide spectrum of microorganisms, including yeasts. Some problems with heavy fuels. More difficult to trace sources of contamination due to sharing of facilities, pipelines, tankers, barges etc. In many ways, this latter period has been a low point in our control of microbiological problems in distillate fuels, and this has been exasperated by new microbiological problems with unleaded

gasoline and biofuels. Despite warnings that microbes do not like lead, and would be happier digesting unleaded gasoline than leaded gasoline, when prolific microbial growth began to appear in UL gasoline, the industry was largely taken by surprise. Likewise, professional advice that using natural components which nature had put together in biofuels, would be an invitation to microbes to take them apart again, went unheeded; biodiesel is particularly susceptible to microbial contamination. There has fortunately been one incredibly high point in this period, which was achieved when serious operational problems of fouling malfunction and corrosion in aircraft and marine gas turbines became a major issue. Airlines, experiencing differing and often conflicting advice from OEM’s and biocide suppliers, turned to the International Air Transport Association for technical help. I.A.T.A. put together a Working Group of fuel system engineers from OEM’s and airlines, fuel quality staff from fuel suppliers, and industrial microbiologists, and in 2002 they issued a landmark publication ‘Guidance Material on Microbiological Contamination in Aircraft Fuel Tanks’ [1]. The strategies prescribed by I.A.T.A. were made possible by the commercial development in 1998 of the first on-site quantitative test for microbes in fuel [2], which allowed on-site microbiological monitoring to take place instead of the previous erratic and time consuming process of sending samples to a competent laboratory, if one could be found. The I.A.T.A. ‘Guidance Material’ described microbiological problems, identified high risk operations and areas, gave advice on taking samples and described on-site tests for fuel drawn from aircraft tank drains. The test results were coupled to limit values which defined negligible, moderate and heavy contamination, and prescribed remedial measures. In brief, a moderate level of contamination called for addition of an approved biocide to one fuel uplift, and heavy contamination required a tank clean prior to the biocide treatment. Validation of the success of anti-microbial procedures was by a re-test. This strategy has been largely successful and has lead to similar concepts being developed by fuel suppliers and some military services, with of course variations to allow for different fuels, fuel systems and sampling points, and for additional test methods and different limit values and remedial procedures. These issues will be described in more detail later. In 2005 FSAW issued a warning notice to airlines about microbial problems, after a large number of power loss incidents including aborted take-offs [3]. This paper aims to highlight and update our current knowledge of microbial growth, spoilage and fouling, which occurs in distillate fuels, and will refer in particular to how this is related to microbially influenced corrosion. It will also emphasise the success of new anti-microbial strategies based on the results of on-site testing and suggest how these strategies might be extended. Growth of Microbes in Fuels. Four broad groups of factors can be recognized as important in determining the nature, rate and extent of microbial growth in wet fuels, namely the nature and capabilities of the microbial population, the physical parameters of their environment, the nutrients which they can access (largely the fuel and its additives) and the presence and availability of water. Types of Microorganisms. Bacteria, yeasts and moulds can all be found growing in wet fuels. It is not surprising that different authors describe consortia of different microbes, as every niche where growth occurs will exhibit different physical characteristics. Even fuels of the same specification will contain different types and ratios of hydrocarbon components, and possibly contain additives with differing chemistries. These niches are dynamic in these respects and we should expect that the microbial consortia will also change. Microbes themselves will also change their environment by removing some fuel/additive molecules, producing acids and other products, removing oxygen, and solubilising carbon compounds. Only relatively few microbial species are capable of degrading hydrocarbons, different species degrading different chain lengths and molecular configurations - but many other species grow with them, feeding on degradation products. Only a small proportion have been characterised and

identified and the expense and time to do this is rarely justified. The exception is that when trying to trace sources of contamination, detailed characterisation of the dominant microbes in a problem situation is a useful tool, as the same identifying ‘fingerprint’ can be sought in possible sources of contamination. It can be said that there are broad differences between some microbial consortia, and these will be dictated by the chemical and physical characteristics of the niche where they occur (see later). For example, recent work in USA has revealed that the microbes associated with Avtur – FSII are markedly different than those associated with Avtur without FSII. In our experience, yeasts now occur much more often than they did a few years ago, and the incidence of Cladosporium resinae (now Hormoconis resinae) is much reduced. We find the same fuel used in helicopters and fixed wing aircraft will sustain different microbes, as the temperature cycles and condensation rates during flight are markedly different. One or many types of microbe may be involved in a fouling/corrosion incident, simultaneously or in succession, but we know very little about the precise role played by each species. We do know that fouling is a matter of the amount of slimy polymer which microbes produce as well as their numbers, but so far efforts to identify the conditions which stimulate slime production and to manipulate them to reduce fouling, have not been very successful. The Physical Environment. In general warmth stimulates growth and many microbes prefer a near neutral pH. However, some microbes are adapted to extremes of both temperature and pH. The oxidation/reduction (redox) potential is also important; anaerobic (oxygen hating) microbes require a negative redox potential. Some microbes need light and this prevents them from growing in fuel. Nutrient Availability. We are what we eat – a truism for humans and also for microbes. A simple chemical dry weight analysis of microbial biomass tells us what they have ‘eaten’ but not which format their diet has been available in. Typical Dry Weight Analysis of Microbial Biomass

Carbon 50% Oxygen 20% Nitrogen 14% Hydrogen 8% Phosphorus 3% Sulphur 1% Potassium 1% Sodium 1% Calcium 0.5% Magnesium 0.5% Chlorine 0.5% Iron 0.2% Trace elements 0.3%

These elements must have been available in the microbial environment and could have been present in the fuel, fuel additives, contaminating water, ‘dirt’, dead microorganisms, sub-lethal traces of biocides etc. Nitrogen and particularly Phosphorus are often in short supply in the fuel/water environment and are termed growth limiting - absence will restrict microbial growth. Inadvertently adding a growth limiting nutrient to a fuel, for example as an additive, could result in a burst of microbial growth.

Oxygen can also be considered as a nutrient. Aerobic microbes need oxygen, anaerobic microbes hate it and facultative microbes tolerate oxygen rich and oxygen depleted conditions. Water Requirements for Growth. Free water is essential for active microbial growth. In the absence of free water microbes may survive but be inactive, possibly in the form of spores. Even free water must be ‘available’ water which microbes can adsorb. If it contains a high concentration of solutes it will have a high osmotic pressure and this will prevent or impede water adsorption, depending on the type of organism. In general yeasts and moulds can cope with high osmotic pressure better than bacteria – which is why moulds can grow on the surface of sweet marmalade. FSII anti-icing additive in Avtur – FSII has a particular role in influencing water availability, as it migrates into free water creating a high osmotic pressure and this is slowly anti-microbial; continuous use at high concentrations is necessary for a significant affect. Water in fuel may come from sea water carry over from cargoes, rain water, ground water or from condensation. This water will usually start as free bottom water or as condensate on surfaces but it may later become a water-in-fuel dispersion. Microbes make more water as a by-product of hydrocarbon degradation. Growth will initially be at the fuel/water interface (where nutrients from the fuel are plentiful), in the free water bottom and on wet surfaces (biofilm) – but growth then becomes dispersed in the fuel phase, often in water droplets, particularly when the fuel is agitated. Time Scales. It should not be assumed that growth in fuel/water is as rapid as microbial growth in our food or during disease, even in the most ideal circumstances. A time scale of several weeks is normal from the initial contamination of the fuel to the onset of an operational\ problem. Of course there have been instances when fuel bunkered or uplifted is so heavily contaminated that operational problems occur immediately. Operational Problems Due to Microbial Growth These have been referred to in a host of publications [4, 5, 6, and 7] and will only be summarised here. It will be expected that different types of microbe may contribute to different fouling and spoilage problems, but this is rarely considered when investigating incidents, except corrosion incidents. Microbes can cause:

• Rapid filter clogging; slime on pipes, tanks etc. [3]. • Fouling of Fuel Quantity Indicator probes by hydrated microbial slime with serious over-

estimates of fuel volume in storage and use. • Fuel injector malfunction. • Fuel flow variations, which increase engine wear and may heat distort gas turbine casings

resulting in contact with turbine blades. • Interference with fuel/water separation due to the production of microbial surfactants. • Severe corrosion under microbial slime or from corrosive fuel.

One or more of these symptoms should prompt a microbiological investigation. The corrosion problems referred to are amplified in the following section. Microbially Influenced Corrosion (MIC) Metals affected by MIC include iron and steel, stainless steel, AISI 300 series materials containing 8-35% nickel (Type 304 are more susceptible than Type 316), aluminium and its alloys and copper and its alloys. Some microbes also attack stone and concrete.

There are many mechanisms by which microbial growth and microbial activity can accelerate normal electro-chemical corrosion mechanisms and other mechanisms. These are summarised below:

1. Localised in situ production of corrosive products particularly strong acids (e.g. sulphuric acid, nitrous and nitric acids), organic acids, sulphide, etc. Sulphide, produced from Sulphate Reducing Bacteria (SRB) in tank bottoms, is directly corrosive and also dissolves in fuel and causes fuel to be corrosive – copper/silver coupon tests fail, fuel pumps fail.

2. Microbes in biofilm use up oxygen locally, which produces steep oxygen gradients. This causes electron flow and anodic pitting.

3. Microbes degrade protective coatings such as paints, chrome primers, some plastics, rubber, oxide films etc.

4. Microbes degrade and inactivate organic and inorganic corrosion inhibitors. 5. Microbes and microbial slimes are polyanionic and are believed to adsorb metal ions, which

migrate from the metal surface leaving a pit in the surface. 6. Some microbes can depolarise metal surfaces. 7. Some microbes can accelerate stress corrosion. 8. Some microbes can cause hydrogen embrittlement of some alloy steels and possibly other

metals. It is not intended to describe all of these various mechanisms in this paper, but the generation of the corrosive product sulphide (mechanism 1) and the degradation of protective coatings (mechanism 2) are of particular importance in MIC in fuel tanks and will be addressed in detail. Mechanism 1, Sulphide corrosion. Sulphide corrosion is due to Sulphate Reducing Bacteria, SRB. The principle mechanism is:

• SRB growing in biofilm on a steel surface generate sulphide (hydrogen sulphide and sulphide ions)

• Sulphide reacts with ferrous irons to form iron sulphides • Iron sulphide film (Mackinawite) develops on the steel surface and this is initially

protective. This Mackinawite transforms e.g. into Smythite and/or Pyrrhotite. The iron sulphide film fractures and the sulphide is now cathodic to the steel exposed by the fracture. Metal exposed to air is at +200mv, but to ferrous sulphide is -400mv

• Active corrosion cells are established around fractures and pits develop. There are other contributing SRB corrosion mechanisms e.g depolarisation of steel by dehydrogenase enzymes, possibly stress corrosion and possibly hydrogen permeation and cracking.. It has already been pointed out that sulphide dissolved in fuel, produced by a local infection of SRB, can cause corrosion wherever that fuel is used. SRB are anaerobic (oxygen hating) and corrosion is therefore prevalent where there is an oxygen deficiency, such as in the bottom of slow turnover fuel tanks, and also in bilges and ballast water compartments on ships. Most SRB cannot attack hydrocarbons themselves, or they do so only slowly, and they flourish best in biofilm consortia of other microbes, which includes aerobic hydrocarbon oxidisers. These associated microbes not only produce organic acids which are essential nutrients for SRB, but they also remove oxygen, thus creating an oxygen gradient across the biofilm, the lower part of which is anaerobic and this stimulates SRB. Obvious signs of SRB growth and activity are:

• Smell of bad eggs • Blackening of water and sludge • Hydrogen sulphide detected in the fuel

• Visible pits (often under a brown crust) in steel and steel alloys; these are conical pits with concentric terraced edges. Pits are graphite grey in colour when first exposed.

• Copper and alloys, silver and aluminium show blackening. As well as the corrosion hazard there is a health hazard from SRB activity which has sometimes had fatal consequences. Hydrogen sulphide produced by SRB is more toxic than hydrogen cyanide and can and does kill in enclosed spaces. Mechanism 3, Degradation of protective coatings. Degradation of protective coatings can be by degradation of protective, naturally occurring, metal oxide films, e.g. yeasts can remove oxide film from stainless steel in breweries and the steel then corrodes. Some microbes degrade applied paints, rubber, some plastics etc. Attack has been reported on bitumen, polyethylene, polyvinyl chloride, polyurethane, fibre reinforced polymers, glass fibre, phenolics, epoxies, polyesters, polyethers. In brief:

• Complex polymers are difficult to attack. • Simpler polymers (e.g. some polyurethanes are attacked at “loose ends” of molecules. • Some fillers are easily attacked - plastic may become porous. • Some plasticisers are attacked – plastic may become brittle

The degraded coatings may exhibit blistering, delamination, leaching, increased porosity, strength loss and/or loss of adhesion, which enables microbes to access the metal surface. To minimize the microbial degradation of applied coatings, attempts have been made to impregnate coatings with biocide, but conventional biocides leach from the coating and there has usually been only a short term protective effect. Other strategies should be considered, such as making the coating so ‘slippery’ that biofilm cannot adhere or applying a surface substantive, non-soluble biocide. Several MIC mechanisms can occur at the same time or in succession. Some of these mechanisms have only been demonstrated in the laboratory. A time scale of several months is normal from the initial contamination of the fuel/water to the onset of a corrosion problem. Corrosion of Aircraft Fuel Tanks. The mechanisms in the above list which cause corrosion of aluminium alloys in aircraft fuel tanks are 3, 1 and 2. Mechanism 3 is necessary before the microbes can corrode the metal. Mechanism 1 attack in aircraft is usually due to organic acids which solubilise aluminium. Mechanism 2 occurs because uplifted fuel is highly oxygenated and so is any underlying water, but a steep oxygen gradient is established between the fuel/water and the airframe metal under an oxygen consuming biofilm, and anodic pitting occurs. Corrosion in Steel Tanks. The corrosion mechanisms here, in fuel storage tanks and in bilge and ballast tanks, are likely to be 3 and 1. Mechanism 1 is due to the activity of Sulphate Reducing Bacteria. This can be a vicious corrosion mechanism in tanks, as it is relatively fast and often dramatic. We have records of an 11 mm steel tank bottom being perforated by SRB pitting attack, and, after steel replacement, being perforated again just over a year later. Although sulphate from sea water is normally reduced by SRB, sulphur and many other sulphur compounds can also be reduced. Corrosion in Water Tanks and Cooling Water Systems. Although not a fuel related issue, for added information microbial corrosion here is by mechanisms 1, 2 and 3 and additionally, in cooling water systems, by mechanism 4. Both nitrite based and organic corrosion inhibitors are degraded by some microbes, often in a matter of days, and the systems are then subject to normal corrosion processes.

Anti-microbial Strategies Industry Guidance. The industry guidance for aircraft operators from I.A.T.A. referred to earlier has been reissued (2nd Edition 01 February 2004) [1] and is supplemented by I.A.T.A. sponsored training sessions worldwide. The I.A.T.A. Technical Fuels Group meets regularly to review microbiological issues. Aircraft Maintenance Manuals now include I.A.T.A. Guidance, adapted for the particular needs of the O.E.M. Other organisations have also issued guidance, particularly:

• Energy Institute / Institute of Petroleum has issued ‘Guidelines for the Investigation of the Microbial Content of Petroleum Fuels and for the Implementation of Avoidance and Remedial Strategies’ (2006) [8], and this is accompanied by a training CD “Microbes in Fuels”.

• ASTM USA. Fuel and Fuel System Microbiology: fundamentals, diagnosis and contamination, ASTM Manual Series; MNL 47, (2003) [9] .

These documents cover all fuels and the contents will not be addressed in detail in this paper.

There are various in-house initiatives, civilian and military, which are based on regular microbiological monitoring on-site and early intervention. A range of test kits are available which can detect microbial contamination in fuel, in water, specifically detect SRB, specifically detect microbes which reduce nitrite corrosion inhibitor, and one test kit which assays biocide concentrations. One package of tests which is commercially available is the ECHA/EXXONMOBIL MICROBE–LAB for use at fuel terminals and it is distributed by Warner Lewis. It comprises a range of test kits plus pH test strips, sample bottles, sterilising wipes, pipettes, Manuals and a Training CD ROM. The concept which is promulgated in all of these Guides and initiatives is that ‘good housekeeping’ will reduce the risk of microbial fouling and corrosion, and that if microbial contamination does occur, and is detected and treated at an early stage of development, there will be:

• No operational problems • Anti-microbial chemicals (biocides) if added to fuel will be more effective (no need to

penetrate thick biofilm). Note that adding biocide to heavily contaminated fuel leaves sticky dead microbial residues which can cause severe fouling; advice is always to clean heavily contaminated tanks before using biocide.

• Less downtime (no tank cleaning) • Less disruption to operations • More cost effective

On-site test kits. Monitoring with these has become an integral part of anti-microbial strategies and has been mentioned several times in this paper. They have been described in some detail in recent reviews [5, 10] and as an example; one is illustrated in Figure 1 below.

Test kits should be able to produce results which can indicate that:

• There is an existing level of microbial contamination which will cause operational problems. • There is a low level of contamination which could grow to cause a future operating problem. • Imported fuel is contaminated and is a potential hazard to my equipment/facility. • If I am a fuel supplier, I am exporting potentially hazardous fuel. • The anti-microbial procedures which I am using are successful.

The microbes which cause fouling and corrosion in fuels can be quantitatively detected with:

• The MicrobMonitor2 test ( NATO SN Code 6640-99-834-3573) for fuel and water. • The Sig Sulphide test (NATO SN Code 99-666-5919), used if SRB activity is suspected. • The Sig Nitrite test, which detects the microorganisms responsible for the degradation of

nitrite corrosion inhibitors.. Other test kits are commercially available. There is no perfect test and all have advantages and disadvantages. When test results exceed limit values, anti-microbial procedures are usually instituted, and these will normally include the application of a biocide. Sampling. The exact location of the sampling point and the time of sampling will influence the contamination detected and hence the test result. These factors have been adequately dealt with elsewhere [5, 6, 8, 9, and 10]. Biocides. Biocides are anti-microbial chemicals - they can also injure or kill animals and plants. Some general properties are:

• At high concentrations they kill; at low concentrations they slow down or prevent growth • Microbes suspended in water/fuel are easier to kill than microbes on surfaces • Adequate concentration and application time are critical – if there are survivors they may

become biocide resistant • Biocides must be regulatory compliant, safe in use and when discharged as waste.

For fuel use:

• Biocides must be combustible, and compatible with metals, seals, additives etc., and must be approved by OEM’s

• Biocides are normally added to fuel but are sometimes used in tank bottom water if the fuel itself cannot be treated.

• Aviation kerosene is normally ONLY biocide treated on the aircraft, to prevent the possibility of ‘double dosing’.

Biocides can be used in fuel to:

Figure 1.

MicrobMonitor2 Test for microorganisms in fuel or associated water

A nutritive gel in a flat glass bottle is inoculated with 0.5 ml of fuel and incubated. ‘Colonies’ of microorganisms appear in the gel (red spots) and are counted.

• Decontaminate a heavily contaminated tank or system; manual cleaning is normally also necessary

• Kill low level growth to stop it developing further. • Prevent growth, using a low continuous concentration, of any microbes which access the

facility. Preventive use in fuel is difficult to implement successfully and is not endorsed by I.A.T.A. Two biocides are listed by I.A.T.A. as suitable for use in aviation kerosene, namely Biobor JF and Kathon FP1.5. There are current regulatory and other problems with their worldwide use in aviation fuel and in some other fuel applications..

• Biobor JF. Since 01 September 2006, the Biocidal Products Directive prevents its use in Europe. It is generally not suitable for automotive and marine fuels

• Kathon FP 1.5. Contains chlorine. The 19TH BI-M SCH V Regulations (GERMANY) 1992 – prevented use of any automotive fuel additives containing chlorine and these regulations are now extended to aviation fuel.

The important chemical characteristics of these two biocides are listed below. Chemical Composition of Biobor JF: Substituted Dioxaboranes 95% Petroleum Naphtha 5% Dose Concentration in aircraft fuel 270 ppm w/w; Soak time 36 -72 hours Chemical Composition of Kathon FP 1.5: 5-Chloro-2-methyl-4-isothiazolin-3-one (CMIT) 1.15% 2-Methyl-4-isothiazolin-3-one (MIT) 0.35% Dipropylene Glycol 90.00% Water 5.85% Magnesium Chloride 0.95% Magnesium Nitrate 1.70% Dose Concentration in aircraft fuel 100 ppm w/w; Soak time 12 -24 hours There is an urgent need for another approved aviation fuel biocide There are other biocides available for other fuels and there have been two recent publications which compare biocide effectiveness in marine diesel [11, 12]. One biocide favourably evaluated in these publications was GrotaMar71 (NATO Stock No 6840-12-370-0293). It is used for treating marine/automotive fuels; it is fast acting, completely combustible, and freely soluble in both fuel and water. There are many engine builder approvals but it does not yet have aircraft OEM’s approvals.

As mentioned earlier, anti-icing additives are fuel biostats rather than biocides; that is they will slow or prevent growth but may not eradicate an existing problem. Thus they need to be used continuously to be effective. Low concentrations can stimulate microbial growth. If disinfecting the tank water bottom rather than the fuel is the favoured option, the biocide selected must have water solubility but no fuel solubility and must be amenable to detoxification by dilution or chemical neutralisation before the water phase is discharged to waste. Controlling Microbial Growth During Fuel Storage, Distribution and Supply. This is done to protect the functionality and integrity of the facility as well as to ensure supply of on spec. ‘clean’ fuel.

Most fuel suppliers employ Good Housekeeping procedures which: • Minimise significant microbial contamination of fuel and equipment, e.g. by preventing

cross contamination • Minimise ingress and accumulation of water • Optimise cleansing by settling

Some microbes which grow in storage/distribution and cause problems may not normally grow in user tanks e.g. Sulphate Reducing Bacteria (SRB) may grow well in semi-stagnant storage tanks but do not grow well in high turnover (oxygenated) user tanks. Nevertheless, it is prudent to avoid passing any microbial contamination in fuel downstream. Although there are no recognised microbiological specifications or limit values for fuel supplied, it must be ‘fit for purpose’, which could be interpreted as being fit for use at the time and also not being the source of contamination which could develop later into an operational problem. The opinion of the Energy Institute (formerly Institute of Petroleum) microbiology committee is that rigid global microbiological specifications are not appropriate but there is Industry Guidance on Norms and Limits. Some fuel cargoes are traded with a microbiological limit value – usually applied to a composite fuel sample, but this parameter is more often measured in the discharge port tanks and not on the ship. Many fuel suppliers develop “In house” Guideline Limit Values based on laboratory or on-site tests. Pre-defined limits may indicate system status and/or fuel quality: Normal, Warning, and Action Limits can be:

• Specific to individual operations or sample types • Specific to the sampling point • Based on operational experience • Updated with experience • For specific microbes e.g. SRB

There is no industry consensus for limits or actions - YET Actions taken to ‘clean’ contaminated fuel may be physical actions. As these avoid the use of toxic chemicals, they are user friendly and have little environmental impact. They have the disadvantage that they do not decontaminate the facility or tank in which infected fuel is stored or used and there is no ongoing downstream affect. The simplest physical method is gravitational settlement; the principles of this are governed by Stoke's Law. This determines the "Terminal Velocity" (VS) of a falling particle, i.e. the maximum vertical velocity which a particle attains before drag restricts further acceleration, and this is dependant on the viscosity of the fluid and the particle’s density and diameter. The density of microbes and microbial debris varies from 0.9 - 1.3 gm/cm3; most wet microbial particles approximate to 1.05gm/cm3, considerably greater than the density of fuel. Settling rates will thus typically be 0.18cm h-1 for an individual bacterium and up to 460 cm h-1 for a microbial aggregate 100µm diameter (just visible). Because of this density difference between microbes and fuel, centrifugal devices will also remove microorganisms. Filtration is an acceptable fuel clean up procedure and has been implemented on a very large scale. However a final filter porosity of 1µm may be needed for satisfactory results on aviation fuel and this will be costly in time and filter changes. All normal filters will allow a small proportion of the incident particles to pass, so the greater the fuel contamination the more microbes will remain in the filtered fuel. Other physical methods such as heat, UV, Gamma and Micro-wave irradiation, have been proposed and evaluated but are not normally practical or completely effective.

Alternative or supplementary to physical treatment methods are biocidal treatments of the fuel or the tank water bottoms, subject to the restrictions described earlier. If biocide treated fuel passes downstream, the recipient will probably ‘benefit’ from residual anti-microbial activity but if he has a heavily contaminated facility the unexpected dead microbial slimes which develop may be unwelcome. Additionally, there may be environmental issues when tank bottom water containing biocide is disposed to waste. Nevertheless, there are many occasions when automotive and marine fuel in intermediate and final storage tanks is successfully biocide treated. Other anti-microbial strategies have been developed for disinfecting filter-water separators and for disinfecting empty tanks and pipelines. Controlling Microbial Growth in End-user Facilities and Equipment. On-board or on-site microbiological monitoring can be used to set limit values and guide anti-microbial strategies, most of which have been described in the previous section. In general the use of biocides is simpler as their application and disposal is all in the hands of the end-user. However, biocide use in F76 water displaced tanks will inevitably raise environmental impact issues. In many marine cases, the end-user will have more than one fuel tank available, and the ability to test, select and use the cleanest, whilst settling, filtering, centrifuging or biocide treating any suspect tanks. Special attention should be given to day tanks/settling tanks; a test on a drain sample will give an early indication that the fuel on board is contaminated. Bunkered fuel can also be tested although in practice fuel bunkered from different compartments/tanks will vary in its level of contamination. Even the same compartment will show different levels of contamination at different times during the transfer. The significance of the results will have to be interpreted after the bunkering has been completed. Conclusions During the last decade, the availability of safe and simple on site microbiological tests has enabled the civil aviation industry to monitor their aircraft fuel tanks for microbiological contamination and to take remedial measures before operational problems. Pressure has been increasing on fuel suppliers to deliver clean fuel to end-users, as end-users cannot operate limit values for microbial contamination in their equipment or facility, unless the fuel supplied to them is consistently ‘clean’. Many fuel suppliers are now monitoring their facilities, following in-house protocols, as there is no industry consensus on how this should be done. Some militaries are also carrying out microbiological monitoring of their ship and aircraft fuel and applying arbitrary limit values. However, it is currently difficult to ensure that clean fuel is supplied for military aircraft, ships and vehicles when they are using other nationality terminals and refuellers. There is good reason for cooperating militaries to work towards common microbiological standards and common anti-microbial strategies, to achieve the success in controlling fouling and corrosion which is now progressively apparent in the civil aviation industry.. References [1] Anon: Guidance Material on Microbiological Contamination in Aircraft Fuel Tanks, Int.

Air Trans. Ass., Montreal & Geneva, 2nd Edn. (2004). [2] E.C. Hill: Use of Thixotropic Biopolymers as an Alternative to Agar for the Cultivation of

Microorganisms on Solid Media, in: Polymer Degradation and Stability 59, Elsevier. (1998), 121 – 128.

[3] FSAW Bulletin 05-08A, (14 June 2005) (FAA Tracking No. 04.215). [4] C.C. Gaylarde, F.M. Bento and J. Kelley: Microbial Contamination of Stored Hydrocarbon

Fuels and its Control; Revista de Microbiologia, Brazil (1999), 1 – 10. [5] E.C. Hill and G.C. Hill: Detection and Remediation of Microbial Spoilage and Corrosion in

Aviation Kerosene – from Refinery to Wing, Proc. 7th Int. Conf. on Stability and Handling of Liquid Fuels, Graz, (24-29 September 2000).

[6] E.C. Hill and G.C. Hill: Investigations of Microbially Influenced Corrosion in Ships and Aircraft. Proc. UK Corrosion, Cardiff, (2002), 1 – 10.

[7] Microbes in the Marine Industry: edited by E.C. Hill. Inst. Mar Eng. & Scientists, London,

(2006). [8] Guidelines for the Investigation of the Microbial Content of Petroleum Fuels and for the

Implementation of Avoidance and Remedial Strategies’ edited by E.C. Hill, Energy Institute, London, (July 2006).

[9] Fuel and Fuel System Microbiology: edited by F.J. Passman, ASTM Manual 47 (2003). [10] E.C. Hill: Detection and Control of Microbiological Growth in Aviation Fuel During

Distribution and in Aircraft – Will Today’s Solutions Solve Tomorrow’s Problems? Proc. 6th Int. Coll. ‘Fuels’, Esslingen, (January 2007), 163 – 168.

[11] M. Turley: Beating the Bug: Motor Boat Monthly, (November 2002), 60 – 63. [12] A. Wood: Diesel Bug – the Ultimate Test: Practical Boat Owner 484 (April 2007), 58 – 62. Authors: Edward C. Hill and Graham C. Hill, ECHA Microbiology Ltd., Cardiff UK Tel +44 (0)2920496321; Fax +44 (0)2920493671 email; thill@echamicrobiology.co.uk; ghill@echamicrobiology.co.uk Microsoft Office Word Version 2003 © ECHA Microbiology Ltd. 2007