Filtering out the bad stuff in stored fuel

The most reliable way to restore problem fuel is to address both sides of the instability process: the chemistry happening inside the fuel and the physical contamination that builds up as a result. Chemical treatment targets the causes of degradation—using proven biocides, stabilizers, and dispersants to halt microbial growth, slow oxidation, and break apart the precursors of sludge.

But chemistry alone can’t remove what has already formed. That’s where mechanical processing comes in. Effective fuel restoration always involves some level of filtration and circulation to physically extract the insoluble material—sludge, asphaltenes, microbial biomass, corrosion particulates—that accumulate when fuel becomes unstable. Combining these approaches creates a hybrid solution that not only fixes the current problem but also resets the fuel so it can remain reliable in storage.

When you bring in a partner to help resolve stored fuel issues, the quality of their mechanical filtration process becomes just as important as the chemical treatments they apply. Filtration isn’t a simple box-checking step—it determines how much harmful material is physically removed before that fuel ever reaches an engine or generator. High-quality mechanical processing should reliably capture the full spectrum of insoluble contaminants: degraded asphaltenes, sludge particulates, microbial biomass, corrosion residue, and other solids that accumulate in aging fuel. The more thoroughly these contaminants are removed, the lower the risk of injector fouling, filter plugging, and system failure down the line.

So evaluate your partner’s filtration capabilities with the same seriousness you apply to their chemistry. Ask specifically about their micron ratings—how fine their filters can actually capture material—and their beta ratings, which indicate the filter’s true efficiency at catching particles of a given size. Both numbers matter, and reputable vendors should be able to explain how their filtration train, filter sizes, and flow rates match the conditions in your tank and the cleanliness your equipment requires. If a potential partner can clearly articulate how their process achieves the level of particle control needed for reliable fuel, you’re likely dealing with someone who will deliver consistent, measurable results.

Absolute filter rating and beta rating

One way you can tell a fuel-polishing professional knows their craft is that they can clearly explain the difference between a filter’s micron rating and its beta rating.

The micron rating—often called the absolute rating—describes the size of the largest opening in the filter media. In theory, it represents the largest particle that could pass through the filter. But this number alone tells you almost nothing about how effectively the filter captures particles at that size. Two filters with the same micron rating can still perform very differently because micron rating measures pore size, not filtration efficiency.

That’s where the beta rating becomes essential. Based on the ISO 16889 test standard, the beta rating indicates how efficiently a filter removes particles of a specific size by comparing how many particles of that size enter the filter versus how many exit. It directly expresses the filter’s capture efficiency for that particle size.

For example, if you start with 1,000,000 five-micron particles per milliliter of fuel before filtration and end up with 1,000 particles of that size afterward, the filter would have a beta rating of 1,000 at 5 microns. That corresponds to 99.9% efficiency, because only 0.1% of particles at that size passed through. Higher beta ratings indicate better filtration performance, and filters naturally achieve higher beta ratings with larger particles because they are easier to capture.

Knowing the beta rating allows you to determine the filter’s true efficiency, which is why it is every bit as important as knowing the filter’s absolute (micron) rating. A knowledgeable partner should be able to explain both and show how their filtration approach meets the cleanliness requirements of your fuel system.

ISO Ratings do matter

It might be useful, going into a potential partnership, to know your fuel’s current ISO cleanliness rating and the rating it needs to reach. Most modern engines now specify a target ISO rating for the fuel they require. A rating might look like 18/16/13, where each number corresponds to the maximum allowable concentration of particles at progressively smaller size ranges. In this example, the counts apply to particles ≥4 microns, ≥6 microns, and ≥14 microns.

A knowledgeable partner should be able to tell you what ISO rating your engine is calling for, where your stored fuel stands today, and what level of filtration and circulation is required to bring the fuel down to that specification.

You want to work with a partner whose mechanical filtration removes small particles at high efficiency, especially as today’s common-rail diesel engines have extremely tight tolerances. Fuel filters used to be rated between 10 and 30 microns; now it’s common to see 3–5 micron secondary filters because manufacturers must protect components with tolerances as tight as one or two microns. That means particulate control is no longer optional—it is integral to engine health.

And this is why ISO ratings matter more now than they ever have. More and more engine manufacturers are beginning to specify minimum ISO cleanliness levels directly in their fuel requirements, because the ISO code is a direct measure of particulate contamination—the very contaminant most likely to damage high-precision fuel systems. If you’re bringing in someone to safeguard your fuel, make sure their processes meet the standards necessary to safeguard your modern, expensive engines.

You may be interested in these related posts:

• Choosing a partner who uses high-quality chemicals for your stored fuel
• Fuel Storage Issues Explained: Why Stored Fuel is Easily Contaminated
• Stored fuel more than 6 months old? Here's what you need to do
• Diesel Filtration: New Recommendations