Microbiological Infestation of Diesel Fuel, or ‘Diesel Bug’ occurs when bacteria, which occur naturally in water present in the fuel are allowed to grow. In order for the bacteria to grow, they must have favorable conditions – stagnant fuel, a warm environment, water to live in and food (which in this case is the diesel fuel itself).

The most common cause of infestation is water which has settled in the bottom of a fuel tank, whether this is at the supplier or on-board the vessel. The water may come from condensation accumulating in a slack tank, due to temperature changes i.e. hot days and cool nights or sea water contamination in the fuel supply chain.

Normally, the first sign of microbiological infestation of the diesel on-board is filter plugging, which can then lead to fuel starvation and engine failure. The filter and its housing may be found covered with a sludge or slime, which is produced by the bacteria. Other symptoms may include corrosion of the fuel system, damage to fuel pumps and injectors due to a reduction in the natural lubricating effect of the diesel on running gear and increased fuel consumption.

Prevention is better than cure, therefore care should be taken when loading and storing diesel fuel. Fuel should only be bunkered from approved suppliers, who ensure their fuel conforms to ISO 8217 standards, have a good turn over of fuel and undertake regular maintenance of their fuel storage and supply systems to ensure they remain in good order. Regular draining of fuel tanks on-board to remove settled water, and a tank cleaning routine to ensure tanks remain clean and free of sludge are good engineering practice, and will reduce the chances of an infestation. Treatment of bunkered diesel fuel with a diesel biocide may also be undertaken – adding of a maintenance dose in accordance with the manufacturers recommendations each time that fuel is bunkered will help control bacteria in fuel.

Should microbiological contamination be suspected, a sample of fuel should be taken from the tank, sent for analysis and expert assistance sought. In order to restore the fuel oil system, all the contaminated fuel should be drained and discarded. Hot washing of fuel tanks with a disinfectant followed by thorough drying should be undertaken. Filter housings and other accessible fittings should be removed, and deposited sludge or slime manually cleaned. Clean diesel fuel may then be loaded, and a high initial dose of biocide treatment added, before thoroughly flushing the system with the treated fuel to ensure all contaminated diesel is removed. New filter elements may then be fitted, and the system returned to service.

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