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Important reasons not to ignore biomass in fuel storage tanks

Posted by: Erik Bjornstad

Biomass is a general term used to describe the biological by-products that microbes like bacteria, fungus and molds produce during their lifecycles. In fuel storage settings, it’s accurate to say that anytime you find fuel microbes, you’re also going to find the presence of biomass. And when you find it, it’s important to do something about it because biomass plays a larger role in fuel and tank problems than you might give it credit for.

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Home Sweet Home For Microbes

The conventional logic from fuel professionals for years has been that since microbes need free water to grow, you’ll find the most microbial activity at the interface between the fuel layer on top and the water layer on the bottom. But that’s not, strictly speaking, true.

Frame the question a different way. If they had their choice, where would microbes most like to live in fuel storage tanks? Is it at the fuel-water interface? No, actually, microbes most prefer to live on solid surfaces behind a layer of biomass.

Go into a fuel storage tank and look more closely at the biomass or biofilm deposits that you’ll inevitable find in certain places. In fact, shrink yourself to a microscopic size and go deeper, into the biomass. You’ll find that it’s a really complex structure that houses thousands of different kinds of microbes. And they’re all co-existing symbiotically with each other, helping each other grow and thrive. Almost like a complex colony. It’s no wonder microbes prefer to live in and behind biomass.  They can thrive a whole lot more than jsut growing and multiplying on their own, out in the open.

This leads to a couple of key reasons which it’s important to have tank biofilms cleaned and removed when they’re found.

The Protection They Need

The most pressing reason why biofilms and biomass need to be removed from fuel storage tanks is they offer protection to microbes and keep biocides from working properly.  Bacteria and other microbes like to live in and behind biofilm formations because they offer a protective shield from the action of biocides.

The best practices for treating microbial contamination are to add the recommended dosage of biocide to the fuel.  Biocide will kill microbes when it comes into contact with them.  However, biocides can have problems penetrating biomass formations. If the biocide can’t come into contact with the microbes, it can’t kill them.

If you’ve ever treated a fuel system with biocide and thought you had eliminated microbial contamination, only to have the problem come back in a couple weeks, this is likely what happened. The biocide killed all of the microbes it could get to, but the microbes in and behind the biofilms in the system were protected and simply reinoculated the system at a later time (like when a future fuel drop dislodged some of the biofilm).

So the best practice now is to have the biofilm removed (or use a biofilm dispersant that helps break up the formation) before adding biocide.

Biomass Linked To Tank Corrosion (Gradients and Osmosis, Oh My!)

There’s another reason why getting rid of tank biomass can be critical to the tank’s long term health. Not only do they hide and house damaging microbes, biofilm formations can be a key link to tank corrosion.

Biofilm formations on tank surfaces can do it in a couple of ways.  First, they help facilitate the formation of electro-potential gradients. Electrons will move across the tank surface, and when there’s biofilm on top of the metal surface, these gradients (differences in electrical potential that cause electrons to move from one place to the other) form a lot more easily. Anytime you have this kind of electron movement, corrosion reactions happen more easily and happen in an accelerated manner.

The other way is they trap oxygen. Picture a tank surface with a chunk of biomass attached to it. Now picture that it’s located right by an interface of fuel and water in the tank. Lots of tanks have this exact kind of scenario.  Believe it or not, fuel and water dissolve different amounts of oxygen in them – fuel can dissolve 5-10x more water than water can.  When you have fuel and water layers next to each other, some of the oxygen in the fuel is going to want to move into the water – the principle of osmosis (things like to move from areas of high concentration to areas of low concentration).

So what happens when this oxygen moves next to some biomass? Many times, the biomass will trap pockets of this oxygen, creating a small area of high oxygen concentration – a perfect place for corrosion to happen through the formation of acids like carbonic acid.

The takeaway here is that it’s important to have fuel storage tanks cleaned on at least an annual basis. Given the key role that biomass plays in microbial fuel contamination and tank corrosion, it’s bad practice not to.

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This post was published on March 8, 2019 and was updated on March 8, 2019.

Topics: Fuel Storage, Fuel and Tank Services