What's the correct diesel biocide dose for a bulk storage tank?

The dispenser filter on your 10,000-gallon bulk tank plugged twice this month, the bottom sample came up with a dark layer under the fuel, and the drum of biocide on the shelf has a label that reads like a tax form. Now someone has to decide how much goes in.

Get the dose wrong in either direction and you pay for it. Too little leaves survivors in the water layer and the problem comes back in a few weeks. Too much wastes product, can push ultra-low sulfur diesel toward its sulfur limit, and puts you outside the label. This post walks through how to size the dose for the fuel you have, how to tell a shock dose from a maintenance dose, and what has to happen after the biocide goes in.

How do you calculate the biocide dose for the fuel in the tank?

Measure the fuel in the tank, convert the label rate to that volume, and dose that number. A 10,000-gallon tank holding 6,500 gallons gets a 6,500-gallon dose. Treating to capacity on a half-full tank puts the fuel at roughly twice the label concentration, which is both wasteful and off-label.

Biocide labels state the rate in parts per million by volume, and the arithmetic is simple once you anchor it to 100 gallons. Bellicide's shock rate of 200 ppm works out to 2.5 fl oz per 100 gallons of fuel. On the 6,500 gallons above, that is 65 times 2.5, or 162.5 fl oz, which is about 1.3 gallons of product. The maintenance rate of 100 ppm is 1.25 fl oz per 100 gallons, so the same tank takes about 81 fl oz. A 16 oz bottle covers 640 gallons at the shock rate; a 1-gallon jug covers 5,120 gallons, which is why bulk tank operators buy in gallons and drums.

The number that trips people up is the stick reading. Pull it the same day you dose, after any delivery has settled, and keep the figure in the tank log next to the dose. When the ATP result comes back in a month, you want to know what concentration the fuel was at, not what you meant it to be.

Shock dose or maintenance dose: which one does your tank need?

A shock dose is for a tank with a confirmed active microbial population; a maintenance dose is for keeping a clean tank clean. The test that tells you which one you have is ATP-by-filtration, which returns a quantified microbial count in hours and costs roughly $50 to $100 per sample against $500 and up for a lab culture (Bell FTS service data).

Dosing without that result is guessing, and both guesses are expensive. A maintenance dose on a tank that needed a shock leaves the colony alive and your filter spend climbing. A shock dose on a clean tank spends double the product for nothing. The symptoms that point toward a shock are familiar to anyone who runs a bulk tank: a run of plugged dispenser or engine filters, a slimy or dark layer in the bottom sample, a bottom-water reading on the paste stick, or fuel that has gone hazy. Any one of those plus a positive ATP result means shock.

Bell FTS built its program around this sequence, assess then treat, because the test result sizes the dose and the dose sets the budget. The sampling and ATP cadence that keeps a bulk tank on maintenance dosing is laid out in the guide to long-term diesel fuel storage.

Why does the EPA label set both the ceiling and the floor?

A fuel biocide is a registered pesticide under FIFRA, and federal law makes it a violation to use one in a manner inconsistent with its labeling (U.S. EPA, Pesticide Labels). The label rate is the legal range, and it is also the tested range: the manufacturer proved kill at that concentration, and nothing is proven above or below it.

Underdosing is the common failure. Survivors at the fuel-water interface repopulate the tank, and the operator concludes the biocide "didn't work" when the biocide never reached label concentration. Overdosing has its own cost beyond the invoice. Bellicide's thiocyanate chemistry contributes about 4.8 ppm of sulfur at the 200 ppm shock dose, which is tolerable against the 15 ppm cap for ULSD under ASTM D975, but a doubled dose on slow-turn fuel starts eating into that margin. For a tank where sulfur contribution matters, ClearKill contributes none; its label range runs from 50 to 200 ppm for prevention up to 500 to 1,000 ppm for a shock, so read the ClearKill label rather than carrying the Bellicide numbers over.

One more reason to stay on the label: when a customer or an auditor asks what went into the tank, "the label rate on the dated log entry" is an answer. "A bit extra to be safe" is not.

When and how should the biocide go into a bulk tank?

Add the biocide to the tank just before a delivery, or circulate the tank after dosing, so the product reaches the water phase and the tank bottom where the microbes live. A dose poured into the fill port of a still tank sits in the top layer of fuel and does little for the colony 10 feet below it.

Microbes in a diesel tank do not float around in the fuel. They live at the interface between the fuel and the bottom water, and in the biofilm on the tank floor and walls. Bellicide and ClearKill are dual-phase, formulated to partition into both the fuel and the water, but partitioning requires contact. The turbulence of a 7,000-gallon drop is the best mixing a bulk tank ever gets, which is why the standard practice is to dose ahead of the truck. On a tank that is not expecting fuel, a polishing rig or the tank's own circulation loop does the same job.

Biofilm is the other reason a correct dose sometimes falls short. A mature biofilm shields the organisms underneath it, and the biocide kills the surface and leaves the core. Bell FTS pairs a shock dose with Tank Treatment SDF in that situation, a sludge and biofilm dispersant that breaks the film so the biocide can get behind it. Where the tank bottom carries years of sludge, chemistry alone will not clear it and a tank cleaning is the honest recommendation.

What happens after a shock dose?

A shock dose kills the colony and leaves everything it was made of in the tank. Dead biomass plugs filters exactly the way live biomass does, so plan on fuel polishing after a shock treatment and a follow-up ATP test about 4 weeks later to confirm the kill held.

Operators who skip the polishing step tend to call a few weeks later convinced the biocide failed, when what they are seeing is the aftermath of a successful kill moving through the filters. Polishing circulates the fuel through water separation and 5 to 10 micron final filtration at 5 to 50 GPM, pulling the dead material and the free water out of the tank. With the water gone, the tank loses the habitat that started the problem, and the maintenance dose has a fair chance of holding.

From there the tank goes onto a maintenance program: dose each delivery at 100 ppm if the fuel sits longer than 30 days, stabilize slow-turn inventory with Dee-Zol Life at 1:2,000, check the bottom for water monthly, and run ATP quarterly so you have a trend line instead of a surprise. The Bell FTS Fuel Secure subscription packages that testing cadence per tank.

Bellicide or ClearKill for a bulk ULSD tank?

Both are EPA-registered, broad-spectrum, dual-phase biocides formulated to reach complete kill within about 2 hours and hold it for 4 weeks or more. The choice comes down to sulfur, corrosion, and which product is registered in your state.

Bellicide is the general-purpose answer: thiocyanate chemistry, a 200 ppm shock and 100 ppm maintenance rate, and registration in 42 states. ClearKill is the ULSD specialist: MBT chemistry with zero sulfur contribution, a filming amine corrosion inhibitor that protects against microbially influenced corrosion, and a pour point of -39°C for cold sites. It is registered in 13 states. For a slow-turn bulk tank of ULSD, especially one that has shown corrosion on the bottom, ClearKill is the stronger fit where it is available; for everything else Bellicide does the job.

Treat rates are Bell Performance label specifications. Always confirm against the label on the container you are using.