Water can dissolve in fuel, especially under warm and humid conditions. During cool-down this water can separate from the fuel and together with condensation from the air the water appears in the tank as condensation water. This condensation water is collected at the tank bottom and builds up water drops at the tank wall and in the pipelines. Microbes are in this water drops. They are building proteins and make the separation of water and fuel difficult because an emulsion has been formed. Diesel fuel is usually bright yellow and clear but after a while it becomes brownish and turbid. As a result of the metabolism microbes also form a slimy substance which becomes visible as bio sludge in the tank.
In very heavy cases build up, slimy bio films at the tank wall can be found. In this gel like bio films the microbes can proliferate without any trouble. If parts of the bio film are falling off the wall into fresh diesel, the new diesel will inevitably be contaminated. Loads of microbes in tiny parts of bio film will find new sources for growth in the fresh diesel. So it is crucial to do a regular tank inspection and tank cleaning.
Bio films can only be removed by mechanically treatment during a systematic tank cleaning. A biocide, no matter which one, would only treat the surface of the bio film against bacteria and moulds. Microbes hiding under the jelly bio film are very well protected against the biocide and therefore can survive the treatment. They can continue to grow in the bio film.
But this is not enough! Microbes are able to cause severe corrosion in a very short time. Both steel and aluminium can be attacked by microbes. Pitting corrosion is a wide-spread problem and often found on ships and vessels. The result can be leakage in fuel tanks.
Especially tanks with low turnover are at risk for microbial contamination with bio film formation and corrosion. To prevent this it is highly recommended to monitor the hygiene of the tank regularly to detect microbial contamination in time. Do not wait until bio sludge is visible with the naked eye.