Gutter oil is actually a general concept, which is a general term for all kinds of inferior oil in people's lives. Waste oil can be divided into two categories:

One is gutter oil in a narrow sense, that is, the oily floating objects in sewers or the leftovers and leftovers (commonly known as swill) in hotels and restaurants are simply processed and refined;

The second is the oil produced by processing and refining inferior pork, pig offal and pig skin.