Growth of micro-organisms in petroleum products has been recorded since 1895, causing fouling, malfunction and corrosion in storage tanks, equipment, pipelines, filters and engines.

The microbes feed on the fuel which enables them to multiply and form colonies. Microbial contamination is most commonly seen as sludge that forms in the bottom of storage tanks and accumulates on filters. This sludge affects engine operations in several ways:

The life cycles of bacteria of a variety of types, both aerobic and anaerobic, produce corrosive acids and sticky sugars that further impinge on engine and system functions:

Petroleum diesels typically hold 60ppm of suspended water. At this level, the water is almost insignificant. Provided the fuel is regularly used and replenished the aforementioned microbial contamination problems do not occur. Bio-diesels typically hold up to 25 times more suspended water than petroleum diesel.
The worldwide introduction of bio-diesel in recent years, in pursuit of conserving fossil fuel, carbon neutral targets and energy security, has brought a mariner's fuel problem on land: bio-diesel is a perfect habitat for vigorous microbial growth. The Lloyds Register published a technical paper (Paper No.4, Session 1994-95) highlighting critical levels of bacteria in fuel. Less than 500cfu (colony-forming units) per litre is acceptable. At the 500-1000cfu level, Lloyds warns "microbial proliferation occurring" and at over 1000cfu "microbial proliferation; operational problems; investigate thoroughly; use anti-microbial treatment". In petroleum diesel the critical levels beyond 500cfu would rarely be seen outside of marine or long term storage situations. Bio-diesel is a different matter, the higher water content provides the microbes with a very attractive environment.
Consequently the amount of exposure to condensation, leaks or time in undisturbed storage before proliferation begins, is dramatically reduced. Today's biofuels deteriorate much faster than their petroleum forebears. Colony Forming Units of bacteria grow on the water-fuel interface becoming heavier until they drop out of the fuel to the bottom of the tank where bio-films and sludge quickly form. The bacteria feeds on the fuel but the microbes do not eat 'all' of the fuel, they break down the carbon chains which reduces the combustible properties. This leads to:


Aviation jet fuels are subject to the same problems which are the reason for the aviation industry's rigorous fuel management regimes. Gasoline - petrol - has, until the introduction of bio-content, been the only fuel unaffected by microbial contamination but petrol containing bio-ethanol suffers too. Universal use of biofuels is at least as profound a change to fuels as the switch to unleaded petrol in the 1990's. Fuel quality management is no longer something that rests only with refineries, ship owners and airlines.

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