Diesel fuel does not stay in the condition it was delivered in. Left in a tank long enough – particularly a backup generator tank that sees infrequent use – it picks up water, grows microbial contamination, and develops particulate sludge that standard filtration cannot handle. Fuel polishing is the process that corrects this. It is not routine maintenance for every tank, but for the tanks where it is needed, skipping it creates real equipment risk.
What fuel polishing actually does
Fuel polishing is an on-site filtration process that circulates stored diesel through a series of filters and separators to remove water (both free water and emulsified water suspended in the fuel), particulates (rust, tank scale, and solid contaminants), microbial growth (bacteria and fungi that grow at the fuel-water interface and produce sludge and acids), and oxidation byproducts (dark, gummy compounds that form as diesel ages).
The process runs on-site without draining the tank. Equipment connects to the tank’s supply and return ports, circulates the fuel through filtration stages, and returns clean fuel to the tank. Depending on tank size and contamination level, a polishing job can run from a few hours to a full day.
When polishing is actually needed
- Low turnover rate: Tanks that hold fuel for 12 months or more without significant drawdown. Backup generator tanks are the primary example.
- Visible contamination: Dark fuel, visible sediment, or a strong sour odor indicate degraded fuel.
- Water intrusion history: Tanks with known condensation issues or breached vents accumulate water at the bottom, which promotes microbial growth.
- Filter plugging: Generators that clog fuel filters more frequently than expected are often drawing from a tank with elevated particulate levels.
- Post-incident recovery: Following a fuel quality failure or a generator that stalled during an actual outage, polishing is typically required before returning to service.
- Pre-inspection or compliance requirements: Some facilities polish on a scheduled basis as part of their NFPA 110 maintenance program.
What polishing does not fix
Fuel polishing removes contamination but does not restore chemically degraded fuel to its original specification. Severely degraded diesel may need to be removed and replaced rather than polished. A polishing provider can test the fuel before the job and advise whether polishing will be sufficient or whether disposal and fresh fuel is the correct answer.
Polishing also does not repair tank corrosion, seal breached vents, or address water intrusion sources. If the tank has structural or sealing issues, those need to be corrected before or alongside polishing, or the contamination will return.
How often to polish
There is no universal interval. The factors that drive cadence are tank age and condition, fuel turnover rate, climate and condensation exposure, prior contamination history, and regulatory or compliance requirements at the facility. Annual polishing is a reasonable baseline for backup generator tanks at critical facilities.
Fuel testing before you decide
A fuel quality test provides objective data before committing to polishing. Draw a sample from the bottom of the tank, where contamination concentrates, and send it to a laboratory for analysis against ASTM D975 standards. Results typically come back within a few days and show water content, particulate levels, microbial presence, and oxidation indicators. This is the correct first step for any tank where you suspect contamination but have not confirmed it.
Finding a polishing provider
Fox Fuel does not provide fuel polishing services. If your tank requires polishing, hire a qualified tank service provider – typically a mobile fueling company or a diesel mechanic with polishing equipment. Once polishing is complete, Fox Fuel can resume fuel delivery and monitoring.
Questions about fuel delivery for your facility?
Call (215) 659-1616 or get a quote online. Fox Fuel serves commercial accounts across Pennsylvania and New Jersey from our Willow Grove location – family-owned since 1981.