Are There "bugs" In My Fuel?
Microorganism contamination of fuel oil (commonly referred to as diesel "bugs") is an old problem faced by the operator of all diesel fueled engines. This includes planing speed sportfishing and cruising yachts that rapidly consume large quantities of fuel, to trawlers, long range cruisers and sailboats that are more efficient and keep fuel for longer periods with less turnover. All diesel fuel systems have the potential for contamination, and unprotected systems will continue to provide life support to the bugs once they are introduced into the system.
Microorganisms need water and food to survive, and they live at the interface between the water and fuel in a tank, using diesel fuel as their food.

 Fuel For Food?
Diesel fuel is an organic compound made up of hydrogen and carbon, and is actually a good food source for these microscopic organisms. Within a fuel tank, there is water for germination, carbon for food, dissolved oxygen and sulfur for respiration, and trace elements for growth. May not sound good to you, but to the diesel bugs, it's filet mignon!

  There's no water in my tank!
A well managed fuel system has components to remove and control possible water contamination, because within the marine environment it's a tough job. There are many ways for water to find its way into your fuel tank. For example, the fuel you just took onboard could be contaminated with water. Also, airborne water vapor can condense in partially empty fuel tanks. Other sources of water entry into the fuel system include leaks through faulty fuel tank fittings, or spray through fuel tank vents during rough sea conditions. Even the best designed and maintained systems can still have enough water to support a healthy bacteria colony.

  Evidence of the "bugs"
Virtually all fuel has some bacteria life however it may not be that apparent to the naked eye. These microbes start off as small microscopic single, separate living cells. At this point, they can pass though any size filter medium, and injector nozzles. However, their growth can be very rapid. Given the right conditions, a single cell weighing one millionth of a gram can grow into a bio-mass of slimy algae mats weighing 10 kilograms (22 pounds) within twenty four hours. By the time you can visually see evidence of growth, the infestation problem is quite severe. Not all growth happens so suddenly. In some situations, growth occurs slowly, creating problems during development.
There are many different types of organisms that make up what we refer to as diesel "bugs". Generally, there are aerobic types; requiring oxygen for metabolism, and anaerobic types that don't. The aerobic types are responsible for the algae and slimy mats, while the anaerobic bacteria, known as SRB's (sulfur reducing bacteria) produce hydrogen sulfide as a by-product of their metabolism. Combined with moisture, strong sulfurous acids are formed, which are corrosive to metals. Evidence of this type of bacteria is usually seen a black grit resembling coffee grinds. The following are some telltale symptoms of microbial contamination:

Slimy material - in filters, tanks, and other fuel system components
Black grit - in filters, and water separator bowls
Corrosion - to fuel system components due to acids produced
 

The diesel fuel itself, since it is degraded, also shows symptoms:
Color - fuel is darker, and can be more opaque
Odor - a rotten egg smell due to hydrogen sulfides

Solutions International, Inc.