Fuel polishing and tank cleaning

[Van]

I think it referes to heavier fuels diesel etc..

Biological slime is caused by the degradation of fuels through the action of bacteria such as Pseudomonas aeruginosa, or fungi such as Cladosporium resinae and Candida tropicalis. Water in the fuel supplies nutrients and plays an important role in the metabolism of the micro-organisms, making it easier for growth to accelerate in the hydrocarbon phase. Middle distillates originating from naphthenic crude oils are particularly sensitive to this phenomenon because of their tendency to solubilize relatively large quantities of water during the refining process. At temperatures above 20°C these type distillates can hold up to 300 ppm or more of soluble water. However, once the fuel temperature decreases (usually long after leaving the refinery) the water becomes less soluble, and comes out of solution as a haze or opacity. The formation of free water droplets thus initiates conditions for the onset of bacterial activity.