Are the buckets made from metal? In that case you can burn out the remaining oil. But that’s something you should only do if you know what you‘re doing obviously. German garbage disposal sites have special oil containers where you can get rid of larger amounts. Is there something similar where you are?
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Plastic buckets.
Been trying to find some service that collects used oil but no success yet, i live in Mexico so its very hard to find info about this especially on my region (very rural).
This may not be useful to you, but in my part of the U$ the autopart stores collect old oil.
You may be able to get in touch with a nearby auto repair center and see how they dispose of it. Also your government's environmental departments or ministries may have some resources on their website.