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How to: Remove Water from Diesel

Keep wondering how water gets in your fuel tank? Water comes from the supplier, insecure tank fittings and the atmosphere in the form of condensation. Let's look at these three ingress points.

The Supplier

Fuel changes hands, and tanks, as much as 7 times before you receive it. After refining it is stored, transported, and transferred each time the fuel is traded. Be that in tanks, on ships, barges, or road tankers, to finally being received into your tank. At each point the fuel picks up contaminates in the form of debris and the biggest contaminant, water. The debris can easily be filtered out, but the water is more difficult to remove so can often end up in your tank.

Insecure Tanks Fittings

Although self-explanatory we hear a lot of stories of water in tanks that after investigation has come from loose filler caps, perished or missing gaskets, broken tank breather pipes, sounding pipes etc.


As the tank heats during the day and cools at night the fuel expands and contracts expelling air and drawing it back into the tank, this moisture laden air readily condenses on the tank wall and falls to the bottom as free water. It is insidious, a very small amount every single day, worse on some days better on others but non the less it very slowly builds up on the bottom of the tank. this is made worse If the fuel contains Biodiesel (FAME). Biodiesel is 30 times more hygroscopic than standard diesel meaning it readily absorbs water from the atmosphere. depending on which country you are in, condensation is by far the biggest source of water.

What is so harmful about the water?

Water in the bottom of a diesel tank is by far the single greatest contaminant in a fuel tank and will quickly lead to many problems:

  • It forms a habitat for Diesel Bug.

  • It reduces the lubricity in the fuel.

  • It accelerates the degradation of the fuel.

  • It produces a mild acid layer at the interface

  • It helps the agglomeration of asphaltenes.

  • it increases risk of resins, super hard very small particles that can pass through filters wrecking fuel pumps.

  • It rusts the tanks and fuel system and in extreme cases, when absorbed in the fuel, can turn to super-heated steam and blow the top of the fuel injectors.

  • Diesel Bug in the water can turn to anaerobic bacteria that can eat through 1/2-inch steel plate.The resins look like sand, can be almost as hard and can pass through filters wrecking fuel pumps.

Diesel Bug

The most common form of sludge in the tank will be from Diesel Bug, the generic name given to moulds, yeast and bacteria. Removing the water is the first step to ensuring this microbial infection is minimized. Be careful of using diesel bug treatments that come with an alcohol carrier. The alcohol absorbs the water into the fuel so that it can pass through the engine. That was fine a few decades ago but can cause serious damage in modern high speed common rail engines, the more common treatments are Enzymes, there is more information here on the difference between Enzymes Vs Biocides. The only real way to remove the bug is to kill it with a biocide as in DieselAid® B and remove the water with a Diesel Dipper®.

......So what can be done?

.................Remove the water !!

Use the drain plug on the tank at least once a week. Or let the Diesel Dipper do it for you, whilst you're relaxing at sea and the contents of the bottom of the tank get picked up and deposited in the Dipper.

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