I am in the same pickle...
I have POL (i.e. Petroleum, Oils and Lubricants) algal contamination. 100% sure... I did a microscopic examination.... you can see the cells!
The following will let you know exactly how big of a pickle I am in and my proposed plan of attack.
I fill my tractor fuel tank after each use to prevent water condensation. I still got algae.
The critters REQUIRE water to thrive and are anaerobic as they grow in the water layer UNDER the oil. I am pretty sure it is in my bulk fuel tank (275 gal Danby home fuel oil upright steel tank). It is located in my shop and never exposed to the weather. About once a year I fill it to a max of 200 gallon per fill. Actually... I top it off with about 170 gallons to the 200 gallon level.That gives at least 75 gallons worth of atmospheric free board above the fuel. I have a sealed fuel delivery system. I pump oil from the bulk tank with a dedicated hard piped diesel pump and regular gasoline fill nozzle... like you see at a gas station. I have a Fram can filter between the bulk tank and the pump. Given what I know now about the critters cell size,,,, the filter is useless as tits on a boar hog as it ONLY captures clumps.
Now this is where I think I screwed up.
The bulk tank is vented directly to the atmosphere through a regular 2" screened vent cap 24/7, like a regular fuel tank 24/7.
Me and the bulk tank reside in Maryland where the humidity can range from the low 30%'s to like now 94%. The corresponding temperatures can range from like 46* F overnight (like a couple days ago) to 92*F right now.
This is how the water is getting in the bulk tank. Continuous water vapor condensation is believe this is the root of my algae problem.
The solution, first super clean the tank with some sort of oscillation spray nozzle by drawing the fuel level to around 20 gallons, enough where I can still draw with the pump without cavitation. Dose the tank with a good diesel biocide, tie wrap a sock filter something like a pillow case or actual sports sock to the delivery nozzle stick it in the tank (2" threaded opening)and turn on the pump ... let her circulate. Gotta get as much of the clumps out as you can.
Once I get everything in the system as clean as I will discard the 20 gallons of circulation fuel and then refill with a fresh 200 gallons of fuel.
To alleviate the condensation in the bulk tank I will fabricate a desiccator on the tank vent line. The desiccator anhydrous granules are whiter when you first install it. As they absorb water vapor from air entering the tank... it will change color like to a pink. That's when you know to replace it with fresh granules. The granules can be reused... you can spread them out on a cookie sheet, pop them in your wife's oven at about 425*-450* F till they turn white again... then you are off to the races!
There is one more thing I plan to do.... replace the kubota fuel with a Racor fuel/water and a ball valve in the fuel supply line from the tank.
I still have no idea what Biocide to use to clean and dose my bulk tank with.
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What do you all think... any other ideas... suggestions