Snake oil and Fuel additives

Battle of Diesel Bug - Choose Your Weapon!

Enzymes v Biocides

With so many diesel-bug additives available, which one is the best? In this blog we share some facts on the two types of treatment available; enzymes and biocides. 

Why Do We Need Them Anyway?

Good quality modern diesel is dosed with additives at the refinery.  In an ideal world they last a maximum of 6 months.  Marine is far from ideal, by the time you fill your tank the diesel could have been in the supply chain for over 2 months, so already degrading.  Plus, during its journey from
refinery to tank there are many opportunities for water to mix with the fuel, where there is water there is diesel bug.


Enzymes are mixed with an alcohol usually glycol.  A Google search quickly reveals the action of enzymes is not simple, in a nutshell they act as a catalyst reacting with the microbes, bacteria, yeast & moulds, removing & altering some of the proteins making them unstable.

Enzymes are very fragile & easily destroyed through heat and pressure as they are drawn through the engine and returned to the tank. This is especially true in marine engines that can return up to two thirds of the fuel back to the tank. The role of the alcohol is to absorb the water & therefore the habitat the diesel bugs rely on.

Generally the treatment works, the enzymes inhibit diesel bug - but don’t kill it.  The water absorbed into the fuel from the alcohol is passed through the engine. This can be a major issue; Older engines could easily handle the water but it is critical that today’s modern common-rail engines use only very clean & very dry fuel, any water in suspension or otherwise can have a
serious  effect on the fuel system potentially leading to expensive breakdowns.  As the Enzymes do not kill diesel bugs, tanks should be constantly dosed or the bugs will proliferate once the water returns, as it always will


Biocides kill Diesel Bug. There are two types, one is used neat and the other is supplied with a glycol carrier - see above caution on water absorption.  The neat biocide will not remove water; this must be removed with a water separator.  As biocides kill the bugs, when the water returns the bugs cannot breed because they are dead.

In summary enzymes inhibit bugs, biocides kill them.  The enzymes and one of the biocides contain alcohol which absorbs the water into the fuel and can cause serious damage to a modern engine.  If there is more water in the bottom of the tank than the alcohol can absorb then it will remain there. The best and safest method to remove water is through a water separator.

MarShip UK produce a Biocide, the neat version.   We could produce an enzyme product but do not as we believe they are neither as effective nor permanent as biocides.  We always recommend a water separator is fitted.

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Peter Weide Director MarShip UK

Peter Weide is MD of MarShip UK, a UK based company specializing in optimising the efficiency of marine diesel engines. Advising on maintaining the cleanliness of Air, Fuel and Lubricating oil, we regularly recommend solutions to operators and appear regularly in industry press, with our full range of diesel additives DieselAid we can offer solutions for most operating conditions and wont advise them if you don’t need them.

Prolonging Efficiency in Your Diesel Engine
Enzyme diesel fuel Treatment
Enzymes are Biological Caterlysts
Biocides are a chemical substance or microorganism intended to destroy an organism.