Filtration / Polishing is used for emergency generator fuel storage to maintain fuel quality during the long storage life of the fuel. Most fuel system used for transportation, turn over their fuel on a regular basis, weekly or monthly. Generator fuel systems can have fuel storage that is present for many years. The large storage quantities needed for the design generator run times dwarfs the amount of fuel used in testing, maintenance, and minor power interruptions.
As diesel fuel ages it degrades in 2 ways: particulate formation, and biological growth.
Particulates form as fuel ages when certain components of the fuel start to come out of solutions and form solids. The particulates will increase the wear on engine components, and so filtering is needed. Engines typically have inlet filter units, however it is recommended that the gross particulate formation be removed at the tank, to extend the life of the engine filters, and to prevent the potential for a filter blockage.
Water enters a fuel tank from either (a) water from off-site fuel storage systems and delivery trucks, that are inadvertently introduced into the tank, (b) water from condensation of air drawn into the tank as the fuel level is lowered.
Any water introduced into the tank will settle to the bottom of the tank. The fuel-water interface at this point is a condition that promotes biological growth. This growth can be problematic especially because it can accelerate. Organic material from the fuel tank will very quickly plug the inlet filters at the engine.
Filtration / Polishing systems include water coalescers to remove water from that tank, and in this way minimize the potential for organic growth. Bio-cide additives to the fuel tank are still recommended.