Water is commonly found in diesel fuel due to condensation, handling and environmental conditions.
Diesel fuel tanks are always subject to water condensation because diesel fuel, unlike gasoline, has no vapor pressure to displace air. When a fuel tank is warm, the air expands and is forced out. As the tank cools at night, humid air is sucked back into the tank and water condenses out on the cooler tank walls. (One reason to keep diesel fuel tanks topped off if possible.)
Additional water can enter fuel systems through an open fill port, a defective tank, or an open drum during rain. Or it can simply be transferred from another water contaminated tank.
The presence of water in diesel fuel systems causes a variety of problems. Water rusts steel and iron components, forming loose particles of iron oxide. These rust particles can quickly clog fuel filters. Micron sized and smaller rust particles may pass though fuel filters to reach injectors, scoring surfaces and ruining fuel injection spray patterns.
Standing water at the bottom of a fuel tank provides an excellent environment for a wide variety of soil bacteria, entering through tank openings and during dispensing. Fuel and water form an interface that provides a comfortable home where bacteria can feed on diesel fuel in a moist environment. These bacteria form a slime layer that often breaks free to rapidly clog fuel filters and disperse bacteria throughout the fuel system. Living bacteria pump out acids as a waste product, further corroding and damaging fuel system components.
It must be noted that water in a fuel tank that gets sloshed around or simply pumped off the bottom, leaves the tank as visible floating droplets. Floating water droplets are fairly easy to remove from fuel using a quality fuel/water separator. Once some water and fuel pass through a pump, they form a stable emulsion, which is often very difficult to remove. Water dispersed this way may just pass through the fuel system to be burned. But more likely some will settle in, and corrode sensitive components.
Good to know: Using biodiesel at 5% makes removing water more difficult. At 20%+ biodiesel concentration, water removal can be much more difficult to near impossible.
Finally, any water that makes its way through to the fuel injectors reduces the lubrication properties of the diesel fuel along the way. This leads to galling, premature wear, damaged injection tips, and further corrosion of sensitive fuel system mechanical components.
Many primary filtration devices do not have the capability to remove water efficiently, leaving the engine prey to pump and injector damage and reduced efficiency. It is therefore essential to effectively separate water from the fuel prior to the final stages of solid particulate filtration.