Diesel bugs and biocides: how to spot microbial growth and treat it
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"Diesel bugs" are bacteria and fungi that live in the water at the bottom of a fuel tank and feed on the fuel. Get the water out first, then use a biocide to kill what's left, then expect to change filters as the dead growth comes loose. Microbes concentrate where fuel meets water (Bell Performance), so without water they can't take hold.
It's more common than most owners think. In one 2018 analysis of 3,855 tank-bottom samples, about 20% tested positive for microbes (Gilbert, National Tanks Conference).
Signs you have a microbe problem
The first sign is usually the filter, not the tank. Slimy or mucous-like buildup on fuel filters and needing to change them more often are the earliest warnings (Power Service).
- Filters plugging early, with dark slime or sludge on the old filter (CRC Report 672)
- Water on the tank bottom, confirmed with water-finding paste (see water in your diesel tank)
- Dark, hazy fuel or visible stringy material in a clear-jar sample (Bell Performance procedures)
- Rust and corrosion in the tank and fittings; microbes produce acids that eat metal (Gilbert)
How to test for it
Sample from the bottom of the tank, not the top. A sample from the top can look clean while the water layer is full of growth. Pull from about 3 to 6 inches off the bottom, plus any water layer (Bell Performance).
| Test | Speed | Good for | Limits |
|---|---|---|---|
| Culture test (dip slide or petri dish) | 48 to 72 hours or more | Cheap; shows whether growth is present | Only detects organisms that grow on that medium |
| Quick yes/no test (immunoassay) | Minutes | Fast check in the field | Tells you if a threshold is passed, not how bad it is |
| ATP test | About 5 minutes | Gives a number you can track over time | Misses dormant spores |
Source: Bell Performance, citing ASTM D6469. Test kits: fuel microbe test kits on Amazon.
How often? Quarterly testing is a reasonable baseline for stored fuel, and more often after you've had a problem (Bell Performance). For most farm tanks, a test whenever you find water or slime is the practical minimum.
How biocides work
A biocide kills the microbes. It doesn't remove the water they live in, the sludge they've made, or the bodies they leave behind.
- They're pesticides. Any product sold as a biocide has to be registered with EPA, and using it differently from the label is a violation of federal law (Practical Sailor; Biobor JF label). Look for the EPA registration number on the bottle. A "fuel treatment" with no registration number isn't a biocide.
- Water-soluble vs. fuel-soluble. Water-soluble products reach the water layer where microbes live and give a faster kill. Fuel-soluble products blend more easily through the fuel and are often better for maintenance dosing (Practical Sailor). Labels differ on where to add them: Biobor JF says to blend it into the fuel only, not the water bottom (Biobor JF label).
- Two doses. Most labels give a higher "shock" dose for a contaminated tank and a lower maintenance dose for a clean one (Power Service Bio Kleen; Biobor JF label).
- Dead growth still causes trouble. Killing the microbes releases slime and dead cells from the tank walls, and that surge can plug filters (Practical Sailor). A badly fouled tank usually needs the bottom cleaned out mechanically, not just treated.
Treating a contaminated tank, step by step
Drain, dose, let it settle, drain again, then watch your filters and retest. This follows the sequence industry presenters recommend for a positive test (Gilbert).
- Remove the water bottom. Pump or drain from the low end until water-finding paste stays brown. Removing excess water is standard practice before treating (Biobor JF label). Keep the water in a closed, labeled container; don't dump it (Missouri DNR).
- Add the shock dose for the whole tank. Follow the label for a contaminated tank. Some makers say to figure the dose on a full tank even if it's partly filled (Bell Performance procedures). Biobor's label says to start when the tank is at least half full, never empty (Biobor JF label).
- Mix it in. Add it just before a delivery, or circulate the tank, so it reaches all the fuel (Bell Performance procedures).
- Let it settle. Bell's procedure calls for at least 8 hours so dead microbes fall to the bottom (Bell Performance procedures). Follow your label's wait time.
- Drain the bottom again. Pull off the dead material and any water that settled out (Gilbert).
- Change filters, and keep spares. Expect more frequent filter changes for a while after treatment (CRC Report 672). Fill-Rite filters on Amazon.
- Retest. Test again over the following weeks to confirm it worked (Gilbert). If growth comes back fast or sludge is heavy, have the tank cleaned.
What the labels say
Dose from the label on your bottle; these are what two common biocides publish.
| Product | Shock dose (contaminated tank) | Maintenance dose (clean tank) | Notes |
|---|---|---|---|
| Power Service Bio Kleen Check price on Amazon | 1 oz per 20 gallons; 1 gallon per 2,500 gallons | 1 oz per 55 gallons; 1 gallon per 7,000 gallons | Sold as a quick-kill biocide (Power Service) |
| Biobor JF Check price on Amazon | 270 ppm (2.6 oz per 100 gallons) | 135 ppm (1.3 oz per 100 gallons) | EPA Reg. No. 65217-1; blend into the fuel, not the water bottom; never exceed 1,000 ppm (Biobor JF label) |
Keeping microbes from coming back
Keep water out, and they can't grow. Everything in water in your diesel tank applies:
- Check for and drain water monthly and after deliveries.
- Keep caps, gaskets, and vents sealed, and don't let the tank sit low for long.
- Use a water-blocking dispenser filter and look at old filters for slime.
- Consider a maintenance dose of biocide if you've had a problem before. Bell's procedure for ultra-low sulfur diesel tanks calls for a maintenance dose every 90 to 120 days (Bell Performance procedures).
FAQ
Is diesel algae really algae?
No. It's bacteria and fungi living in the water layer and feeding on the fuel (Cenex; Gilbert).
Can I just add biocide and skip draining the water?
The biocide may kill what's there, but the water stays, and growth comes back once the biocide is used up. Removing water is standard practice (Biobor JF label; Veeder-Root).
Why are my filters plugging worse after I treated?
That's dead growth coming loose from the tank walls. Change filters as needed, drain the bottom after the product settles, and it should clear up (Practical Sailor; CRC Report 672).
Is a fuel additive the same as a biocide?
Not unless it has an EPA registration number. Only registered products can claim to kill microbes (Practical Sailor).
Sources
- Biobor JF, EPA-accepted label (EPA Reg. No. 65217-1)
- Power Service: Bio Kleen diesel fuel biocide
- Bell Performance: Fuel tests that predict failure (ASTM D6469 and microbial testing), 2026
- Bell Performance: Fuel storage tank treatment procedures
- Riley Gilbert, Underground Storage Tank Corrosion: Problems and Solutions, National Tanks Conference (2018)
- Coordinating Research Council: CRC Report 672 (2016)
- Practical Sailor: Diesel biocides take on contaminated fuel
- Veeder-Root: Why water needs to be removed continuously
- Cenex: Warning signs your diesel is water-contaminated
- Missouri DNR: Management of petroleum storage tank wastes
This guide is general information. Biocides are pesticides: read and follow the label, and wear the protection it calls for.