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Addressing "Algae" in Diesel Fuel

Posted April 30, 2019 by Caleb Courville

There is nothing worse than opening your tank and seeing diesel fuel contaminated with “algae“.

If this fuel were ever to be used, it could wreak havoc on your fuel system.

You may be asking yourself how do I get rid of this “algae” and prevent it for good?

Luckily, we have the answers to those very questions.

In this article, we’ll explain what the “algae” really is, where it comes from, common methods to combat it, and best practices to prevent it for good.

Part 1:

Understanding the Problem

A Common Misnomer

In order to address “algae” in diesel fuel we must first understand the problem in its entirety.

It was a common misconception that the dark sludge growing in your tank is “algae”.

As a result, many still refer to it as such.

What you are actually dealing with is microbial growth.

How do we know this?

For starters, your fuel tank is far too dark.

Algae are plant organisms and they cannot survive without sufficient sunlight.

On the other hand, there are plenty of microbes (bacteria and fungi) that reside in your diesel fuel.

Graphic depicting different cartoon microbes

When water present in the diesel separates into a distinct layer below the fuel (phase separation), you may notice a dark layer begin to form.

A term often coined to describe this phenomenon is the “diesel bug”.

Graphic depicting the development of fuel contaminates in diesel fuel overtime

The interface between the diesel fuel and the water creates the perfect breeding ground for various bacteria and fungi to thrive.

The microbes live and proliferate in the water while consuming the hydrocarbons in the diesel fuel.

In time, the accumulation of microbes will form a visible biomass (rag layer) between the water and diesel fuel.

By-products and dead cells from the growing microbial communities also fall towards the bottom of tank to create a viscous sludge.

The Effects of Microbial Growth

The sludge by-product of microbial growth, when churned up, can clog any engine filter with ease.

A clogged engine filter, especially one clogged at a time of importance, can cause serious problems.

For example, data centers relying on diesel generators for backup power may experience unexpected downtime due to clogged filters.

This can result in costs of hundreds to tens of thousands of dollars a minute for maintenance.

On a smaller scale, clogged filters on boats can often leave owners stranded.

“The diesel bug may start on a microscopic level, but it is clear it can lead to macroscopic consequences.”

Part 2:

Common Solutions

Common Methods to Prevent “Algae” in Diesel Fuel

There are a lot of opinions on how to best prevent “algae”/microbial growth in fuel.

Some will push for the use of biocides which use hazardous chemicals to kill most of the microbes directly.

Others prefer fuel additives that pull water back up into the fuel to help prevent conditions (phase separation) best suited for microbial growth.

At AXI International, we understand that neither of these solutions are perfect for every situation, but recognize they embody the two methodologies for controlling microbial growth in diesel fuel.

You can either kill them directly or prevent the very conditions they thrive in.


Although initially effective, biocides are not the end all solution to microbial growth and sludge formation.

Placing health, safety, and environmental concerns aside, frequent use of biocides can create resistant microbes that no longer die upon application.

This is due to the fact that it is nearly impossible to sterilize diesel.

As a result, the surviving microbes, through means of natural selection, will exhibit increasingly resistant traits that will eventually render biocides ineffective.

Graphic depicting a biocides eventual ineffectiveness on microbes in diesel fuel

Part 3:

Best Practices

Removing Microbial Sludge

If microbial growth has progressed to a point that it is noticeable, no treatment mentioned will effectively remove the sludge.

In order to restore the fuel, you must employ a mobile fuel polishing service or system.

Mobile fuel polishing systems work by circulating the fuel out of the tank for filtration.

Graphic depicting periodic fuel polishing with a mobile fuel polishing system

These systems are designed to effectively remove both small and larger contaminates like sludge and can prevent microbial growth through the removal of water.

Beyond restoring the fuel, it is not recommended to use mobile fuel polishing as a long term preventative measure to “algae“/microbial growth when compared to other options.

Preventing Microbial Growth

In some cases, fuel additives that pull water up into the fuel by the means of an emulsifier can be an appropriate response to preventing microbial growth.

The additives address the problem at its source.

Without water, microbes cannot proliferate in the fuel.

By pulling the water into the fuel, it will eventually vaporize in the engine and exit out the exhaust.

In cases involving a Tier-4 engine, water emulsifiers will only create more problems.

Tier 4 engines feed fuel into the combustion chamber at extremely high pressures with very little tolerance for fuel contaminants.

Internals of an injector tips nozzle

Emulsified water, being much larger than the 2-4 micron injector openings, can cause abrasive wear and eventual failure of the injector tips.

Due to this reality, Tier 4 engines require a more technical solution.

Tier-4 Preventative Solutions

For Tier-4 engines, fuel maintenance systems are the best approach to prevent “algae”/microbial growth in the diesel fuel.

Fuel maintenance systems are permanent installations that work on a programmed schedule to regularly pull fuel from the tank to filter out contaminants.

Graphic depicting the installation of an enclosed and compact fuel maintenance system for automated fuel filtration

Unlike fuel polishing systems, fuel maintenance systems are better at maintaining the fuel as opposed to restoring or remediating it from a highly contaminated state.

By the same effect of fuel polishing systems, fuel maintenance systems prevent “algae“/microbial growth through the removal of water.

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