Airborne bacteria and fungi can readily enter fuel tanks through air vents, and multiply very fast in this bottom water phase. These micro-organisms do in fact need the presence of water in order to multiply, but given a very small initial quantity, they can actually produce their own supply of water by feeding off the fuel. This is particularly likely to happen in diesel and gas oil tanks where it is impossible to exclude water altogether. Water enters these tanks by various methods, such as through condensation, rain water or ground water leakage or even with the fuel delivery.

Microbiological growth will often produce black sludge but the worst type of micro-organisms are the sulphate reducing bacteria (SRBs). These produce acidic by-products which can corrode fuel tanks and systems, with the potential to cause severe damage.

There is also some evidence that the incidence of contamination is increasing. Though the reasons for this are not clear, the availability of lower grade fuel in certain locations is being put forward. Local weather conditions are often overlooked as the cause of this contamination producing condensation in cold periods followed by rapid growth in following warm periods. Analysis of Contamination >