For fuel storage tanks full of sludge and fuels that are dark and subpar, fuel polishing is often recommended. There are probably a thousand entities across the country that market the ability to do “fuel polishing”. A thousand companies breed intense competition, sometimes leading to cutting corners and stretching the truth when it comes to what fuel polishing will and will not do.
In order to answer the original question, you have to know what it is. Fuel polishing is mechanical fuel processing in order to clean out sludge, water, asphaltenes, and other non-fuel solids, to clean up the fuel and make or keep it usable. At its core, it's fuel filtration to remove contaminants.
To that end, fuel polishing is not rocket science, nor should it be difficult to understand. The fuel is filtered and processed to try and return it as far back to its original pristine condition as the situation allows.
There’s certainly value in doing that. But what effect does fuel polishing have on the most common problem areas that stored fuels have? If we understand the kind of filtration process many polishers use, we can understand what they are doing. A good fuel polisher will typically employ a three-stage filtration system. The fuel is drawn out of the tank, cycled through multiple kinds of filters, and returned to the tank.
Engine manufacturers and industry stakeholders are getting more strict with fuel cleanliness. For a long time, this meant having in-spec test results for tests like Water and Particulate. Now we're seeing ISO ratings being referenced more often. The ISO rating is an index expression for the number of particles in fuel that are of different sizes. Specifically, the ISO 4406 rating system specifies the number of particles equal to or larger than a given size in microns.
You see the ISO rating expressed in terms of three numbers.
For example, an ISO rating of 18/16/13 means there are a certain number of particles 4 microns or larger, a smaller count of particles 6 microns or larger, and even fewer particles 14 microns or larger in each milliliter of fluid.
Each number in the ISO cleanliness code reflects the concentration of particles that are equal to or larger than the specified size threshold (e.g., 4 µm, 6 µm, or 14 µm). The ISO code uses a logarithmic scale to represent particle counts:
An ISO code of "1" indicates that there are 0 to 1 particle(s) per milliliter for that size range.
As you go up each ISO number, the particle count range roughly doubles. This means that an ISO code of "2" would correspond to 1 to 2 particles per milliliter, and an ISO code of "3" would correspond to 2 to 4 particles per milliliter, and so on.
However, the doubling doesn’t correspond directly to "2 to the power of n." Instead, each ISO number corresponds to a predefined range of particle counts, with each step up the scale representing approximately twice as many particles as the previous step:
As of 2024, many engine manufacturers and fuel injector makers have increasingly stringent recommendations for ISO cleanliness codes for diesel fuel. These recommendations are primarily driven by the need to protect modern, high-pressure common rail (HPCR) fuel injection systems, which are highly sensitive to particulate contamination. These systems operate at extremely high pressures and have very fine tolerances, making them vulnerable to damage from even small particles.
Below are the most recent typical ISO Cleanliness Recommendations for various types of injector systems:
1. High-Pressure Common Rail (HPCR) Systems:
3. General Diesel Engines:
There are also ISo recommendations for injector OEMs:
They make these recommendations because they know that adhering to these ISO cleanliness recommendations can prevent premature wear, reduce the risk of injector failures, and minimize engine downtime. Some manufacturers even tie their warranty coverage to the use of fuel that meets specific ISO cleanliness standards, meaning that failing to maintain these standards could void the warranty.
Since the recommendations for ISO ratings are getting stricter, fuel polishing to remove particulates to do this is becoming more important.
Good fuel polishing actually treats storage tank solid contiminants in two different ways. For the layer of particulate settled on the bottom of the tank, they'll try to suck as much of that out as possible. For the particulate in the fuel (including the particulate that gets kicked up in the process), it gets removed by going through two stages of that three-stage filtration.
The first is a "gross particulate" filter designed to get the large particulates like dirt and rust. These are usually bag filters with larger micron ratings. The second is a fine particulate filter that gets particulate out down to single digit micron sizes.
One caveat to this is that fuel polishing by itself will not do anything to keep sludge and particulate from coming back. For that, they need to incorporate some kind of quality chemical stabilizer, since sludge and asphaltene formation is a chemical reaction issue.
Water is deadly to stored fuel, both by itself and through the fact it leads to bigger microbe issues. Quality fuel polishing should incorporate water separation filters that remove water from the fuel. The fuel polishing technician will also manually suction out as much of the tank’s water bottoms as possible. So it’s reasonable to say that quality fuel polishing does remove water.
One thing fuel polishing does *not* work to do is get rid of microbial infestations in fuel tanks. Fuel polishing can break up and remove biomass sludge formations which can be associated with microbe activity. And don’t get us wrong, it’s important to be able to do that. But it’s dangerous to confuse the ability to do that with the idea of actually getting rid of the microbes in the tank. With fuel polishing alone, the microbes are still in the tank. And they will come back unless you do some other things.
Indeed, answering this question could be a kind of litmus test for you to know if a fuel polisher is going to solve your problem. If they insist you don't need to use a biocide to get rid of a microbe problem, steer clear of them.
The only way for a fuel polisher (or you) to completely solve a microbe problem in a fuel tank is to incorporate biocide treatment in their process. Biocides are the only things that will kill fuel microbes and keep them from coming back.
But for a biocide to be effective, it has to be applied early enough in the process and given enough time to work. Many fuel polishers prefer to go to the job, filter, and fuel, and get out of there. And that’s a problem because you can’t rush that kind of thing if you want to do it right.
For the main things it claims to do, yes it does. But some fuel polishers are better than others. A fuel polisher that combines quality polishing to clean the fuel with the right chemical treatments to keep the fuel problems from coming back – they’re going to be your best bet.