At the end of Casey Linker’s shift as a cook at a pub in Columbus, Ohio, she did something she suspected she wasn’t supposed to do.

The kitchen’s refrigerator was packed. To create room, she pulled out a large plastic tub of bacon fat, in a container she couldn’t throw away. “I let it thaw out and I put it down the drain,” said Ms. Linker, 22 years old.

The...