Biodiesel, hogwash. No, really, hogwash.

In China, hogwash cooking oil is an underused feedstock for biodiesel.  According to the article, some 60 million tons of hogwash is produced annually in China, but instead of being recycled for biodiesel, it is reprocessed illegally and sold back to restaurants.

Doctors in China are concerned that this practice may be a vector of a number of diseases and is a possible carcinogen.  Hogwash oil is produced from rotten pork and peroxided oil, which is then used repeatedly for frying.