Gutter Oil

In China, the cooking oil is the essential element for every meal. Everyday Chinese consume many oil. Therefore, some of the bad guy try to save the cost to product the cheaper oil. Gutter oil is a cheap way to produce cooking oil, which is wasted or used oil came from the gutter. Those "oil" will mix will another chemical to make the taste and smell become better. After all the proceed,the oil will be repackaged and sold to small restaurants at below market prices. These gutter oil is full of chemical elements  and may cause careers or other side-effect to the health.

Actually this kinds of oil is also common in western country, but used in industrial , not cooking.In 2014, Boeing company try to use the gutter oil to be the bio-fuel for the aircraft. It can be save the cost and make the environment become better.

Reference:
https://www.youtube.com/watch?v=vp92BlLLVEI