Oils that have become unsuitable for their original purpose due to the presence of impurities or loss of original properties after use in a wide range of industrial or commercial activities, such as engineering, power generation and vehicle maintenance. Waste oils should be properly disposed of, or treated in order to be reused.

Solvents & waste oils are one of the five waste streams considered in this assessment next to wastewater sludge, filter cake from wastewater treatment, paint and ink residues, and fluff.