c - a dangerous good which is classified as harmful for the health and for the environment. But why are there living organisms found in the tank? Actually, there are well adapted microorganisms which can live and grow in such a harmful environment.
By microorganism bacteria, yeasts and moulds are meant. These microorganisms or in colloquial language microbes, are present everywhere. They are in the air, in the soil, in the water and also on our skin. They are not always a problem und some of them are even very useful, e.g. in the food industry. Only the total amount of the microbes might make it a problem in the fuel tank.
Under optimal conditions the microbes double within 20 minutes. After additional 20 minutes there are 4 then 8 -16 32 - 64 ... After 10 hours there are 1 073 741 824 microorganisms.
The rapid proliferation explains the enormous build-up of so-called bio sludge in contaminated fuel tanks. This bio sludge blocks the fuel filter more and more and prevents a good flow. The flow of the fuel might stop completely and no fuel can reach the engine anymore.
Microbes need water. Water is the source of life! They also need food and warmth to feel comfortable. As we also know now microbes can adapt very good to new living conditions. They have learnt how to extract the nutrition they need out of the diesel fuel. With the addition o f bio diesel in the fuel it is even easier for microbes to exploit the fuel as food since bio fuel is very good biodegradable. The temperature in the tank is given and often cannot be influenced, i.e. when a storage tank is outside.
But where is the water coming from? Diesel fuel contains up to 7 Vol% bio-diesel according to the latest DIN EN 590 norm, According to the specification up to 200 mg/kg water is allowed. 200mg/kg is not a lot, but considering the size of a bacteria or yeast of only some µm, the little amount of water in the tank has the size of a huge lake. The lake already provides very good living conditions. Diesel fuel itself can bind up to 60 mg/kg water which means it is not available for microbes. But the remaining 140 mg/kg water is available for microbes. Taking condensation water into account, the lake becomes slowly an ocean. In the past when mineral oil diesel did not contain bio component, the water sank to the bottom, because water is heavier than diesel fuel. So the water could be drained from the lowest point of tank.
Keeping the tank dry is a good measurement to remove the source for microbes. But nowadays bio diesel has to be mixed into the fuel to fulfil regulations. Bio diesel can have up to 500 mg/kg water, but it can bind up to 5000 mg/kg water. This is good because large amounts of water are bonded and then not available for microbes. No microbial contamination cases have been reported in 100 % bio diesel. The problem starts when only a certain percentage of bio diesel is mixed into the mineral oil fuel because the small bio diesel amount cannot dissolve all the excess water. In addition the bio diesel acts like an emulsifier and distributes fine water drops in the fuel. Draining the clear water from the tank bottom has become very difficult.