Diesel tank cleaning and fuel polishing: when and how

By Austin, fuel-system technician8 years in the tradeUpdated October 31, 2026.

Fuel polishing pumps the fuel through filters and water separators and back into the tank, so you keep the fuel. Tank cleaning removes the sludge, water, and growth from the tank itself. Polish when the fuel is worth saving and the contamination is light. Clean when there's heavy sludge, the water keeps coming back, or filters keep plugging after treatment. Either way, nobody should climb into the tank.

On this page

Signs a tank needs attention
Polishing or cleaning?
How fuel polishing works
How tank cleaning works
Never climb into a fuel tank
After the job
FAQ
Sources

Signs a tank needs attention

What you see	What it points to
Filters that clog often, or a dispenser that slows after a delivery	Contamination stirred up from the bottom (CRC Report 672)
Rust or slime on a used filter	Corrosion or microbial growth (CRC Report 672)
Water that keeps coming back after you drain it	A leak, a bad cap or vent, or a water layer you can't reach
Slime, mats, or heavy sediment in a bottom sample	Growth or sludge on the tank bottom (see how to test diesel fuel)
Filters still plugging weeks after a biocide treatment	Dead growth still coming off the walls; a badly infected tank usually needs the bottom material removed mechanically (Practical Sailor)

Polishing or cleaning?

	Fuel polishing	Tank cleaning
What it does	Filters and dewaters the fuel while it stays in the tank	Removes the fuel, then the water, sludge, and growth from the tank
Good for	Light contamination, water and particles in the fuel, standby fuel you want to keep usable	Heavy sludge, recurring water, a microbial problem that treatment didn't fix
Limits	Doesn't remove sludge stuck to the walls and bottom	More downtime and cost; the fuel has to go somewhere while it's done

For a small farm tank with bad fuel, it's often simpler to have the fuel and bottoms pumped out and start over. Your fuel supplier can often pump the tank out (Cenex).

Field note: When I find a bad tank, the usual call is to have it pumped and cleaned. When one gets opened up, I see all of it: sludge, rust scale, and microbial mats.

How fuel polishing works

A polishing unit pulls fuel from the tank, runs it through a filter and water separator, and returns it to the tank, over and over. Fuel polishing is used to clean and filter fuel in tanks that are contaminated or at risk of it; a typical unit has a primary filter and water separator, a pump, and a secondary filter and water separator (U.S. Patent 7,883,627).

Draw from the bottom. That's where water and sediment are. The goal is to move fuel across the bottom, not just through the middle of the tank.
Drain the separator. The water it catches has to be emptied and disposed of properly.
Test before and after so you know it worked (see how to test diesel fuel).

How tank cleaning works

Tank cleaning is a contractor job with a set sequence. OSHA's guidance on tank cleaning covers a pre-cleaning site review, degassing and vapor control, removing the product, disposal, and a safety check before the tank goes back into service (OSHA).

Remove the usable fuel, usually by pumping it through filters into temporary storage. Removing product calls for controlling ignition sources and bonding and grounding the equipment (OSHA).
Remove the water and sludge from the bottom.
Clean the interior, from outside the tank where possible, or by trained entrants under a confined space permit.
Check the tank before it goes back in service. OSHA calls for a recommissioning safety check using a written checklist signed by qualified people (OSHA).
Return or replace the fuel, and change the filters.

Ask where the waste goes. Tank-bottom water is petroleum-contaminated and can count as hazardous waste. Don't let it go on the ground or down a drain (Missouri DNR). A contractor should tell you how sludge and water will be handled.

Field note: Owners draining tank bottoms onto the ground is something I still run into. Don't do it. Catch it in a closed, labeled container and get it disposed of properly.

Never climb into a fuel tank

"Because they may contain hazards when taken out of service, most storage tanks will be considered Permit-Required Confined Spaces" (OSHA). The hazards include vapors, hazardous gases, and an atmosphere with too little or too much oxygen, which can cause serious injury or death (OSHA).

What counts as a confined space: a space big enough to enter and work in, with limited ways in and out, and not meant for continuous occupancy. Tanks are the first example the rule gives (29 CFR 1910.146).
A hazardous atmosphere includes flammable vapor over 10% of its lower flammable limit, or oxygen below 19.5% or above 23.5% (29 CFR 1910.146).
On farms: OSHA's confined space standard doesn't apply to agricultural operations, but OSHA points to it as the guide for preventing deaths, and the General Duty Clause still requires a workplace free from recognized hazards (OSHA Fatal Facts No. 16). OSHA's farm example is a worker who died from a hazardous atmosphere inside a storage tank that wasn't marked as a confined space (OSHA).

If a tank needs interior work, hire a contractor with a confined space program and rescue plan.

After the job

Let it settle. Mixing or circulating a tank kicks sludge up off the bottom; one fuel treatment maker says to let fuel settle at least 8 hours before use (Bell Performance).
Expect to change filters for a while, especially after a biocide treatment (CRC Report 672).
Keep water out so you don't have to do this again soon: tight caps and vents, and water checks before and after deliveries (CRC Report 672). See water in your diesel tank.
Treat for microbes if they were the problem. See diesel bugs and biocides.

FAQ

Can I clean my own fuel tank?

You can drain water off the bottom and change filters yourself. Don't go inside the tank. Most storage tanks are permit-required confined spaces, even when they're out of service (OSHA). Interior cleaning is a job for a contractor with a confined space program.

Will an additive clean my tank?

Some multifunctional additives contain detergents that lift sludge and corrosion deposits (Gilbert). That loosened material ends up in your filters, so expect to change them. An additive won't remove a heavy sludge layer or the water under it.

How often should a tank be cleaned?

There's no set schedule for a farm or fleet tank. Clean it when the signs below show up, and keep water out so it doesn't need cleaning often.

Sources

This guide is general information. Tank entry and waste disposal are regulated; follow OSHA, your state, and your contractor's procedures.