at one time i had an unlimited supply of poly 30gal barrels which held polyphosphate an additive put into drinking water to seal old pipes to prevent iron from leaching into the water.
i had thought about using one as a fermenter but couldn't bring myself to brew 20 or more gallons of the same beer.
it would've worked out great. i had installed a hose bib on one to drain just above where i imagined the yeast cake would be.
now they are doing time as rain barrels.
sadly my supply dried up too.
i would think the barrels would be difficult to clean.