Bacteria can be planktonic, i.e. free-floating, or sessile, i.e. attached to surfaces as a biofilm. Tests of water samples taken to evaluate bacterial concentrations only detect planktonic cells.
A biofilm is an aggregate of microorganisms that adhere to each other and/or to a surface. A biofilm is often referred to as slime; however, not everything described as slime is a biofilm. The microbes produce a matrix of extracellular polymeric substances (EPS) generally comprising extracellular DNA, proteins and polysaccharides.
Once established, biofilms are very difficult to remove entirely, and any residual cells can quickly regenerate the colony.
Water forms condensation in tanks, and this creates droplets on the walls and in pipelines, and accumulates at the tank bottom. These droplets contain microbes, which can metabolise nutrients and generate a slick substance visible as bio sludge in the tank.
In severe cases, a slimy biofilm can build on the tank walls. Within this gel-like mass, the microbes are shielded, and can proliferate unchecked. If part of the biofilm breaks off and falls into the diesel, the fuel will be inevitably contaminated. Even a small amount of biofilm contains countless microbes that can derive nutrients for growth from diesel. It is therefore crucial to regularly inspect tanks, and evaluate the need for cleaning.
The only way to remove a biofilm is mechanically, i.e. by means of a thorough tank cleaning. Biocides of all types are only effective against bacteria and moulds on the surface of biofilms – they are unable to penetrate further. As a result, microbes within the biofilm are protected, and can continue to grow even after biocide treatment.
A reactive approach to biofilms is insufficient – and even dangerous. Microbes can cause severe corrosion in a very short time, attacking both steel and aluminium. Pitting corrosion is a widespread problem found, for example, on ships – and can result in fuel tank leaks.