Microbiology technical bulletin sheet 1:

Microbial growth in diesels and other fuels containing fatty acid methyl esters (FAME)

www.energyinst.orgEI Microbiology Committee - May 2011Copyright © 2011 by the Energy Institute a professional membership body incorporated by Royal Charter 2003 Registered charity number 1097899.

IntroductionIndustry experience has highlighted that diesel fuels containing fatty acid methyl esters (FAME) can have an increased susceptibility to microbial growth. Microbial growth by bacteria and fungi in diesel storage tanks, distribution facilities and end-user tanks can lead to contamination of the diesel with microbial particulates (biomass) which can cause severe filter blocking problems and blocking of fuel lines. Significant operational issues have already been experienced by some fuel retailers, where rapid onset of pump filter blocking has been encountered. Fuel filter blocking and fuel starvation problems have also been experienced by some truck and bus fleets, although, to date, operational problems attributed to microbial contamination of diesel in private cars have been extremely rare.

The Energy Institute (EI) has undertaken research to identify the factors that allow microbiological contamination and growth and hence to help guide effective strategies to mitigate that growth. The first part of the research was the EI Literature review Implications of biofuels on microbial spoilage and corrosion within the fuel distribution chain and end use (May 2008). The second part, laboratory research on which some of these recommendations were based, will be reported in due course. This technical bulletin provides an update to best practice recommended in the 2nd edition (2008) of the EI Guidelines for the investigation of the microbial content of petroleum fuels and for the implementation of avoidance and remedial strategies. These EI documents are available from www.energypublishing.org.

Which fuels are affected?Automotive diesels

Automotive diesel specification EN590 currently allows up to 7 % FAME. EI research shows that automotive biodiesels containing above 2 % FAME are significantly more susceptible to microbial growth, notably by fungi. The rate at which growth occurs and the overall extent of growth increases with increasing FAME concentration.

Field evidence suggests that the most extensive problems associated with microbial growth occur in diesel where FAME is blended at less than 20 % but greater than 2 %. There is some evidence that when FAME blend concentration exceeds approx. 20 % in diesel, providing excessive amounts of visible water are not present, then microbial growth may actually be reduced or inhibited. This is because FAME has a water scavenging effect. Microbes can only grow when free water is present and when high concentrations of FAME are present, water is dissolved in the fuel and is no longer available for microbial growth. This is the subject of on-going EI research. Tests of field samples often show storage tanks containing B50 or B100 are free of significant microbial contamination. However, if excessive amounts of water are present, then very heavy levels of contamination may be found even in B50 or B100.

www.energyinst.orgEI Microbiology Committee - May 2011Copyright © 2011 by the Energy Institute a professional membership body incorporated by Royal Charter 2003 Registered charity number 1097899.

Marine diesels and boiler and agricultural fuels

New legislation in the EU (EU Directive 2009/30/EC, effective 1 January 2011) has mandated the use of effectively zero sulphur diesels (i.e. <10 ppm) for a number of applications, including vessels operating on inland waterways and non-road mobile machinery. Consequently, fuel sold for these applications can be dual graded with automotive diesel and could contain FAME.

Significant concentrations of FAME may be present in diesel supplied for inland or leisure marine applications through deliberate blending, or through inadvertent cross contamination with FAME or automotive diesel in distribution and will be prone to similar issues of increased susceptibility to microbial growth as discussed above. In leisure marine applications the problem can be exacerbated by difficulties in keeping fuel tanks and systems free of water. Fuel for these applications may also be kept for significantly longer periods by end-users, providing increased opportunity for microbial growth. Some marine fuel specifications (e.g. ISO 8217 DMA) allow only de-minimis concentrations of FAME (<0,1 %) and some suppliers are also able to offer guarantees that diesel will be supplied free of FAME. Where an operator perceives a significant risk of microbial growth or contamination with water or where it is known fuel will be stored for in excess of three months, it is recommended purchase of FAME-free fuel is considered.

Aviation fuels

FAME is not currently permitted as a deliberate addition to aviation turbine fuels but on occasion FAME may be present in trace amounts due to cross contamination with FAME-containing products during distribution. Currently aviation turbine fuel specifications restrict FAME content to a maximum of 5 ppm although there is a current proposal to allow up to 100 ppm of FAME in aviation fuel. Emergency procedures exist to enable limited continued flying operations if FAME is detected up to 30 ppm but this is strictly on a case by case basis. EI research has shown that at these low concentrations there is no significant increased susceptibility to microbial growth.

What characteristics of FAME make it susceptible to microbial growth?Nutrient

FAME is a nutrient which is readily utilised by microbes for growth. Some studies in scientific literature show that most FAMEs are about twice as rapidly biodegraded as mineral diesel.

Capacity to hold water

Microbes must have free available water to grow. The majority of microbiological growth problems occur when water is present either as a discrete layer at the base of the tank or as a visible haze in the fuel.

However FAME has properties which enable it to absorb, disperse and dissolve water. As noted above, when high concentrations of FAME are present in diesel (e.g. >20 %) microbial growth may be impeded because the fuel scavenges water from tanks and systems making it no longer available for microbial growth. However, this only holds true when the amount of water present does not exceed the capacity of the FAME to dissolve water. Although FAME can dissolve more water than mineral diesel, it is quite possible that in some systems, non-visible free water will be present as well as visible free water.

Studies show that in biodiesels some free water may be held in suspension as micro-droplets in the diesel. This dispersed undissolved water is available to microbial growth. Consequently, whereas previously in mineral diesels microbial growth was restricted to pockets of accumulated water on the tank floor, or films of condensate water on tank and pipeline surfaces, microbial growth may occur in biodiesels throughout the fuel phase. Additionally, because water is held in suspension in the fuel it is not as easy to remove it from tanks by routine draining from sumps. This water may not be visible as the droplets are so small; to all intents and purposes it will look clear and bright.

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EI recommendations for facilities handling fuels containing FAME

Storage tanks at terminals and depots

• Wherever possible, allow tank contents to settle for a minimum of 24 hours after product receipts (or three hours per metre of product height) before putting tanks back in service. After settling, drain off from tank bottoms as described below. This will not only help minimise the extent of microbial growth in a tank but also reduce the chances of delivering contaminated fuel from the tank.

• Drain water from tanks routinely (e.g. weekly but dependent on turnover) regardless of water dip readings. Do not wait until a certain level of water (e.g. 10 mm depth is commonly used) is present in tank bottoms before you instigate tank draining. Traditional water measuring techniques (tank dipping) may not provide as reliable an indication of the extent of free water present in biodiesels as they do for mineral diesels. Any water in the bottom of the tank indicates the possibility of significant amounts of water suspended in product. Even if water is not detected in the tank bottoms by dipping, sufficient water may be present for microbial growth. Tank bottoms may need to be drained more rigorously, and it may be necessary to drain off both water and some product. Suspended water in the fuel may give it a hazy appearance. It may be appropriate to install or designate small recovery tanks for collection of drained product and water; these can then be settled, water drained off and then product recovered back to storage (this should be subject to an acceptable microbiological test rating).

• Routinely test bottom samples or drain samples from storage tanks for microbial contamination (on-site tests or lab analysis). A monthly test would be a typical monitoring frequency. We advise categorising three levels of contamination (light, moderate or heavy). Light requires no action other than continued monitoring. Moderate requires investigation and more diligent attention to draining or extra product settling and heavy contamination (ideally subject to a retest to confirm the result) would instigate remedial action (e.g. tank cleaning and/or biocide treatment, depending on circumstances and company policy). Further guidance on testing and defining light, moderate and heavy contamination is available in the EI guidelines referenced above and also from some of the manufacturers of test kits described in Annex A of that document.

EI Microbiology Committee - May 2011Copyright © 2011 by the Energy Institute a professional membership body incorporated by Royal Charter 2003 Registered charity number 1097899.

What additional measures can be taken to prevent occurrence of microbial growth?It can be exceedingly costly and disruptive to operations to decontaminate systems which have become heavily colonised by microbial growth. Treatment with fuel biocides is possible but biocides can be ineffective at penetrating thick microbial slimes (bio-films) on tank and pipeline surfaces. Often effective treatment can only be achieved if the system or tank is cleaned prior to treatment or with repeated biocide application. Some locations within tanks and fuel lines can be difficult to clean effectively. Any use of fuel biocides should consider the compatibility of the biocide with the fuel, other fuel additives and fuel system components. Some biocides are not approved in fuel used for certain applications (e.g. gas turbine power generators). Due regard should be given to the acceptability of the biocide to fuel users or distributors and suppliers downstream of the point of application. Biocides may initially cause an increase in particulate levels in fuels as they kill and disperse microbes attached to tank and system surfaces.

Rather than treat a contamination when significant operational or fuel quality problems occur, it is preferable to prevent microbial growth occurring in the first place. The EI recommends a number of additional measures in the routine maintenance and operation of facilities handling fuels containing FAME, and specifically where there is a known or perceived risk of microbial growth.

www.energyinst.orgEI Microbiology Committee - May 2011Copyright © 2011 by the Energy Institute a professional membership body incorporated by Royal Charter 2003 Registered charity number 1097899.

For further information on the EI’s Microbiology Committee and technical work in this area please contact Kerry Sinclair (technical@energyinst.org).

Details of EI microbiology publications are available at www.energypublishing.org.

The information contained in this microbiology technical bulletin is provided as guidance only and while every reasonable care has been taken to ensure the accuracy of its contents, the Energy Institute cannot accept any responsibility for any action taken, or not taken, on the basis of this information. The EI shall not be liable to any person for any loss or damage which may arise from the use of any of its publications.

Retail sites

• Have tanks checked regularly for presence of water and, if appropriate, have this removed. Specialist contractors can assist in removing microbial contamination, cleaning fuel tanks and fuel lines and treating with biocides. Contractors can be used on a routine preventative basis where there is a history of operational problems such as plugging of pump filters and reduced flow at pumps. Experience suggests problems are encountered more commonly in spring and summer and contractors might be utilised accordingly.

Vehicles and small end user tanks

• In the majority of cases, microbial contamination problems encountered in vehicles are associated with a specific problem with a diesel supply tank (e.g. at a fleet depot). Hot fuel return on modern common rail direct injection vehicles will in most cases kill any growth that has reached the fuel tank although vehicles fitted with dual tanks have experienced in-vehicle growth. Attention should be paid to the maintenance and water removal from diesel supply tanks at depots. If problems persist, treatment can be considered. Expert advice should be taken for suitability, dose rates and frequency of use. Ideally biocide will be added to the supply tank but direct addition to vehicle or end-user tanks is also possible, bearing in mind that if tanks are contaminated, biocide treatment can initially cause an increase in fuel particulates and additional filter changes may be required.

• Through simple precautions and good housekeeping, microbial problems can usually be reduced or prevented.