Microbes in fuel are a fact of life for people who store fuel for any amount of time. And with the onset of ultra-low sulfur diesel (where the bacteria-killing sulfur is no longer present), they're an even bigger problem than before.
The only thing that will kill microbes in diesel fuel and keep them from spreading is a diesel fuel biocide (like Bellicide). But you can't just dump a biocide into a storage tank and then "set it and forget it".
They're not complicated but there are some small important steps you should always remember to take in order to make sure that biocide does everything you expect it to.
Fill the diesel tank after you've added the biocide
This is somewhat optional, depending on if you've got fuel delivery schedules to contend with. But the ideal is to add biocide to the remaining fuel in the tank, then fill the tank up to capacity with more fuel. You will have to add enough biocide to get the recommended treat rate for all the fuel you're going to finish with. For example, Bellicide needs 1 ounce per 40 gallons in order to shock and kill an existing fuel infection. If you have 400 gallons of infected fuel in a tank that normally holds 4,000 gallons, you want to add enough Bellicide to shock treat all 4,000 gallons (which is 100 ounces, or about 3 quarts). Then, when you fill the tank up to 4,000 gallons, it helps to mix the biocide in. Also important, it ensures that biocide makes contact with all the tank surfaces up to the top. Bacteria and microbes can live on these surfaces very easily.
Recirculate the fuel 30-60 minutes Or more
How long for depends on the volume of fuel treated. Larger volumes should circulate longer. Circulation of the fuel is essential, no, critical, to the biocide working properly because it's the best way to make sure it's completely blended and completely reaching all the areas where bacteria are living. A diesel biocide won't work if it can't contact the bacteria. Just adding biocide on top of fuel without circulating is going to prove much less effective.
Let the diesel fuel settle before using
This lets the treated diesel settle in the tank, and allows any already-dead microbe bodies to settle at the bottom. You may even want to drain the sump at this point to clear any of them out.
Replace filters for a few days after fuel treatment
This has nothing to do with biocide effectiveness but does matter in that the biocide is going to kill all the microbes in the fuel, and their dead bodies are going to plug the fuel filters. There's nothing you can do about it, and it's nobody's filter. It's just the way it is. Change the filters a little more often and you'll be good to go.
And don't forget.... if you're storing diesel fuel for a longer period of time, a fuel stabilizer is a good idea. You want to look for a fuel antioxidant that will interrupt the chemical reactions in the fuel that result from exposure to water and air. Not only that, but remember that any microbes you killed will have been secreting acidic byproducts into the fuel that will accelerate its breakdown over time. Adding a fuel stabilizer like Dee-Zol Life along with the biocide will provide your best protection for the stored diesel fuel over time.
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This post was published on June 6, 2013 and was updated on November 17, 2017.