The Triton

Engine Room

Even well maintained fuel tanks have microbial growth


Microbial contamination of diesel fuel is a fact of life. It is not a matter of “if”; it is just a matter of “when”.

As a general rule, whenever diesel and water come into contact in a fuel tank microbes will start to proliferate. Whether it is bacteria, mold or other fungi, unless this growth is held in check the result will be clogged filters, tank sludge and injector failure.

Water is heavier than diesel fuel and therefore accumulates at the bottom of a tank. No matter how well maintained the fuel tanks are aboard a yacht, there is usually some amount of free water sitting on the tank bottom. This water can come from a variety of sources:

  •  Freshly refined fuel contains some water. As the fuel cools down after cracking, this water separates out.
  •  During transport from the refinery via tankers and barges, ballast water can get mixed with the fuel.
  •  Humidity in the air condenses out on the walls of a fuel tank and drips down to the tank bottom. If there is dew on the grass or a car windshield in the morning, then there is dew in the fuel tank. That’s why the axiom “always keep your fuel tanks topped up” is so important to follow. When a tank is full, there is little room for humid air to condense onto the walls of the tank.
  •  Rain, sea spray and water from washdowns can enter deck fuel fillers and tank vents.

And of course there is always a possibility of a catastrophic event, that is, the new deckhand accidentally putting the water hose in the diesel tank port.

The water at the bottom of the tank serves as a perfect medium in which microorganisms can live. And the hydrocarbons in the fuel are a tasty food source for many species of microbes. The result is a proliferation of bacteria and fungi feeding at the fuel/water interface and large colonies floating in the free water below that interface. In addition, microbes can adhere to the tanks walls and can grow fast enough to quickly coat those walls with a slime of organisms.

In the initial stages of microbial contamination, aerobic bacteria use the oxygen dissolved in the water for respiration. Once this supply of oxygen is depleted, anaerobic bacteria take over. These sulfate-reducing bacteria do not need oxygen to survive; instead, they convert the hydrogen and the sulfates in diesel fuel to supply the energy needed for their life cycle. The waste product of this bacterial conversion is hydrogen sulfide, the stuff that smells like rotten eggs. But worse than the smell is the corrosive effect that this compound has on metal fuel tanks and engine parts.

When fuel tanks are severely contaminated with water, bacteria and mold, a complete tank cleaning is needed. This is a more extensive process than fuel polishing. First, all the sludge and water sitting at the bottom of the tank must be sucked out, stored and disposed of in an environmentally friendly manner as these wastes are biohazards to the marine environment.

The next step is to top off the tank with clean fuel, which assures that all the tank walls and baffles are covered with fuel. This fuel serves as the medium to disperse a fuel additive. The fuel additive to be used at this point should have biocides to kill the microorganisms and solvents to loosen the microbial colonies from the walls, baffles and tank bottom. This step is critical, for if any live fungi and bacteria remain after the tank cleaning process, then any new fuel will become inoculated and subsequently contaminated by these surviving microbes.

The final step, which I call “dialysis for your diesel”, is a multi-pass filtration process that agitates the fuel and removes organic and inorganic debris floating in the fuel, as well as any emulsified water. This extensive filtration process, also called fuel polishing, removes particles down to a diameter of three microns or less. To put this in perspective, a human hair has a diameter of 80 microns and particles between five and 10 microns in diameter can cause problems in fuel systems for main engines and generators.

Once the tank is cleaned and the fuel is polished, an additional dose of fuel additive is mixed into the tank to help guard against future microbial contamination. Then each time the fuel is topped off, a new dose of additive is mixed with the fuel to control any contaminants in the fresh fuel.

At a minimum, fuel tanks should be cleaned on an annual basis. Ideally, they should be cleaned every six months. Vigilance is the watchword to combat fuel contamination, and trouble-free diesel engine operation is the reward that you reap.

Capt. Jeff Werner has been in yachting for more than 20 years on private and charter yachts, both sail and power. He is an instructor for RYA, MCA, USCG and US Sailing courses and owns Diesel Doctor ( Comments on this column are welcome at


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