Technical notes of interest to Marine Engineers

Microbial contamination of fuels

Compiled by: Petro Canada Technical Services, November 1986

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In the last few years, the number of reported fuel problems attributed to microbial contamination has increased. This increase has occurred as more companies stored larger quantities of fuel for longer periods of time. Microbial growth in fuel storage systems can lead to a variety of problems, including filter plugging and corrosion.

The key to minimizing the occurrence and effects of microbial contamination is by prevention through good fuel maintenance and housekeeping practices.

Microbial contamination of fuel, though not a new problem, is more common in metal working industries, which use water-soluble oils as cutting fluids. The refining process essentially sterilizes all hydrocarbon fluids. The fuels can become contaminated soon after leaving the refinery. Microbes can infest a fuel tank along with a new fuel delivery, with ground water run-off, or even with airborne particles. These microorganisms exist rather harmlessly in moisture-free fuel, passing through fuel systems without any operational problems. However, in the presence of water, the microorganisms begin to metabolize (grow and reproduce). The rate of metabolism depends on how well the environment suits the particular microorganism's needs. Some bacteria are able to double their population every 20 minutes, and this speed illustrates how rapidly fuel-fouling problems may arise.

In field systems, microbial fouling of fuel is difficult to predict. Each fuel tank presents a unique environment for microbial growth. The growth of a large colony of microorganisms in a fuel system can cause several problems. The first, and usually most obvious, is fuel filter plugging with a greenish-black or brown slime, frequently accompanied by a foul odour. The slimy, string-like colony can also plug sharp bends in fuel lines, fuel meters and other restrictions. These microorganisms can also cause corrosion due to the acid by-products some of them produce. It is also possible, if the microorganisms pass through the fuel filter, that they will form deposits and cause damage in the fuel pumps and injectors. Microbial contamination can also cause swelling or blistering of any rubber surface (washers, hoses, connectors) that comes in contact with the fuel.

When it has been established that there is a problem with microbial fuel Contamination and action must be taken, there are several approaches. The best solution to a problem of this nature is by prevention. Microbial organisms can enter the fuel through so many different routes that keeping them out is not a practical solution. However, keeping fuel systems clean and dry can prevent growth of these microorganisms. Preventive procedures include frequent "sticking" of tanks with water detecting paste, pumping out water bottoms where necessary, preventing rain and surface water entering tanks, and draining fuel/water separators regularly. Sediment from dirt and rust should also be minimized.

When microbial contamination is a recurring problem, a biocide can be used to chemically sterilize the fuel system. There are two general classes of biocides: water-soluble and fuel soluble, and each offers advantages depending on the system to be treated. Biocides are generally only mildly toxic but should be handled carefully. In cases of ingestion or physical contact, follow the manufacturer's recommendations and seek medical aid.

Although most of the microbes that will live in fuel tanks are common organisms to which humans are constantly exposed, contact should be avoided. When a fuel tank is highly contaminated and physical cleaning is necessary, workers should use breathing protection, and the microorganisms should be kept wet to minimize airborne contact. Disposal of the sludge and water removed from the fuel tanks should be done according to local health regulations. Never place these materials in sanitary sewage systems, since they can kill bacteria used in sewage treatment. Never place them in storm sewers or surface water streams since they can kill fish and other aquatic organisms.

The most common problem associated with exposure to these microbes is dermatitis, which for some people can be quite serious. Any exposed skin should be thoroughly washed with warm, soapy water. Any ingestion of the microbes or exposure to broken skin should be considered serious. It is recommended that if this happens, the worker should be taken to a doctor, along with a sample of the microbes.