What's the best way of disposing rendered fat.

[sir pball]

12. Speaking as a hobbyist soapmaker

Trash it, if you aren't going to cook with it.

Home rendered fat is only usable for soap after multiple washes with boiling water, until it's snow-white and odorless. The tiniest impurity will make your soap smell funny at best, and go rancid in mere weeks at worst. I've tried it a few times, pork shoulder does yield a ton of fat; it's not worth the effort for the return.

Plus in order to safely make soap, you need to know what kind of fat it is, and if it's a mixture of e.g. chicken and pork, you need to know the exact ratios. Unreacted lye is a catastrophically bad thing to have in soap.