Microbes take advantage of just about every food source on the planet, including fuel. They only need water and food to proliferate. Microbes include bacteria, yeasts and moulds.

They tend to prefer to live at the fuel/water interface, living in the water and feeding on the fuel. They also have a tendency to ‘hide’ in the lower flow areas of a fuel system where the lack of turbulence allows them to thrive in relative safety.

Microbes in small quantities do little harm to the fuel system. It is when they become established that problems like blocked filters start to show, affecting engine efficiency and ultimately posing operating risks to crew and other waterway users. When really established, over a period of time, these microbes begin to corrode the fuel system. They can wear injectors and even produce holes in stainless steel tanks. At this advanced stage, fuel system maintenance is likely to require specialist intervention.


Water is likely to be present in all fuel systems (storage and operational). It may exist as free water, visible as a separate layer below the fuel or as a hazy layer between the water and fuel layers. The hazy layer consists of microscopic droplets of water suspended in fuel. This is known as absorbed water. The condensation process liberating the water is precipitated by the temperature changes involved in normal operations, including the effect of prevailing weather conditions.

Characteristics of sulphur & bio-diesel

The storage and management of fuel of the new specification, with lower sulphur content and a percentage of bio-diesel, requires additional precautions at all points of the supply chain as well as in operational use. The role played by these 2 components in fuel is described briefly below:


Sulphur has 2 key characteristics affecting storage and operational use. Firstly, it enhances fuel lubricity, so without it engine components tend to wear faster. Fuel consumption may also be higher due to increased friction. It also appears to have a bio-static effect, so may suppress the growth of the diesel bug. The science behind this observation is under investigation but what is clear is that the combination of lower sulphur and increased levels of FAME has enhanced the potential for bug problems.

Bio-diesel (FAME)

Bio-diesel may come from a variety of natural sources ranging from chip shop waste and abattoir residue to specifically grown and managed crops like jatropha and palm oil. Whatever its source, it is more hygroscopic than traditional fuels. In other words it attracts and holds water, so it is more prone to produce condensation issues in fuel systems. Extra water in the fuel system may have adverse effects on seals, flexible pipes and gaskets. It can affect the performance of the rubber and synthetic seal compounds used in pump seals and flexible fuel pipes. It also improves conditions for the growth of the diesel bug.

‘Cleansing’ effect

The inclusion of bio-diesel in the fuel provides a ‘cleansing’ effect in mature fuel systems. The effect results in deposits inside tanks, pipework and pumps becoming detached. This detached debris will mainly be trapped in fuel filters, but micron sized particles can pass through the pump and reach the injectors. The injectors can then become corroded as a result. The DfT recommends that farmers replace equipment filters after the first 2 or 3 tank refills in case any deposits of contaminants have been dislodged. A blocked filter is considerably less costly than a fuel pump or injectors. Were either of these to fail there may be warranty issues.

The preventative maintenance approach – good housekeeping

Good housekeeping and fuel system management are the antidotes to the diesel bug. For marine operators this includes:

  • Filling tanks, prior to vessel lay-up over winter, to reduce the chance of condensation
  • Draining water regularly from the bottom of the tank, including the hazy layer (diesel has a lighter Specific Gravity than water and therefore floats above it)
  • Testing regularly for diesel bug (at least annually)
  • Treating as appropriate to test findings – including Biocide, Fuel Polishing, Fuel System re-design


The chosen test will provide a reading to indicate the level, and in some cases type, of any contamination present in the fuel or storage system. The test result will guide the selection of the remediation strategy.
Low Contamination Levels may be dealt with by appropriate application of a biocide followed by retesting after a short period of time, say 7 days to ensure effectiveness of application. (Has it worked?).

Higher Levels of Contamination may require the emptying and mechanical cleaning of the tank. Then the whole system including tank lining must be treated with biocides. In some cases tank re-lining or replacement may be required. In many cases the removed fuel may be recovered by professional filtering and polishing followed by an appropriate biocide treatment. The cost effectiveness of this treatment will depend on the volumes of fuel involved. In many applications, boats particularly, this may prove to be a difficult and expensive exercise highlighting the need for an effective tank maintenance regime alongside the regular engine servicing schedule.


Conidia Bioscience would like to acknowledge the contributions from all the October 2010 Marine Fuel Forum Presenters to the content of this booklet plus the UK Major Ports Group and the following contributors:
Chris Parkin Cleaner Fuels & Vehicles Department, UK Department for Transport
Hugh Tucker United Kingdom Petroleum Industry Association (UKPIA)