Peter Weide, Director of Marine 16, considers diesel bug fuel treatments – enzymes or biocide. Which one and how do they work?

Gone are the days of filling up your fuel tank and forgetting about it.

Good quality modern diesel is dosed with additives at the refinery, and in an ideal world they last a maximum of six months. Marine is far from ideal as by the time you fill your tank the diesel could have been in the supply chain for more than two months, already degrading with many opportunities for contamination with water during its journey from refinery to tank, and where there is water, there is diesel bug.

With so many additives available, we take a closer look at the two types of diesel bug treatments available: enzymes and biocides.


Enzymes are mixed with an alcohol, usually glycol. In a nutshell enzymes act as a catalyst reacting with the microbes, bacteria, yeast and moulds, removing and altering some of the proteins making them unstable. Enzymes are very fragile and 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.

Generally the treatment works as the alcohol enables the fuel to absorb the water thereby removing the habitat diesel bug needs to thrive whilst enzymes inhibit growth, although do not kill it.

There is a major issue, however, in that the absorbed water into the fuel then passes through to the engine.  Although older engines could generally handle the water, notwithstanding the fuel systems can still rust. It is essential today’s modern common-rail engines use only very clean and very dry fuel; any water in suspension or otherwise can have a serious effect on the fuel system potentially leading to expensive breakdowns. Additionally, as the enzymes do not kill diesel bug, 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 while others are supplied with a glycol carrier (see previous caution on water absorption). The neat biocide will not remove water, which must be removed with a water separator. However, as biocides kill the bugs, when the water returns the bugs cannot breed because they are dead.

In summary enzymes inhibit bugs, while biocides kill them. The enzymes and some biocides contain alcohol which absorbs the water into the fuel and can cause serious damage to a modern engine. Marine 16 produce a biocide, the neat version. They do not produce an enzyme product as they believe they are neither as effective nor permanent as biocides.

It is important to know that if there is more water in the bottom of the tank than the alcohol can absorb, then it will remain there. Water is the single biggest contaminant in diesel, and the best and safest method to remove water is via the drain plug. Marine 16 recommend fuel tanks are drained from the drain plug regularly. If a drain plug is not fitted a Diesel Dipper® can reach the bottom of the tank through a Dip Tube and will run independent of the engine ensuring all water is removed from the tank bottom.

Peter Weide, Director of Marine 16, formally a chief engineer, has 20 years’ marine experience with Mobil Oil marine fuels and lubricants (ExxonMobil), ship repair yard director and head of sales with marine engine manufacturer Wartsila.

01666 817 577

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