Economics
What tank bottoms are actually worth
Sludge is priced as a disposal problem. Most of it is recoverable hydrocarbon, and the difference between those two framings is the whole business case.
Ask a terminal operator what is at the bottom of a crude tank and the answer is usually a volume and a disposal cost. Ask what is in it, and the answer changes: paraffin, asphaltenes, water, solids — and a large fraction of recoverable oil.
That distinction matters because it changes who pays. When tank bottoms are treated as waste, the operator pays to remove them and pays again in lost product. When they are treated as stock, the same material is separated, the oil goes back into inventory, and only the residue is disposed of.
The reason the first framing persists is that conventional cleaning cannot do much else. Manual entry and vacuum removal take the material out as a mixture. Once it leaves site as waste, the hydrocarbon in it is gone.
In-situ biological treatment inverts the sequence. The sludge is broken down inside the tank, the phases separate, and the oil is drawn off before anything is classified as waste. What remains for disposal is a fraction of the original volume.
None of this is free, and the recovery depends on the product, the temperature and the depth of the sludge. But the question a tank owner should be asking is not what cleaning costs per tonne. It is how much of what is down there comes back.