Water and oil sometimes combine (emulsify) during industrial processes. The resulting emulsions are either oil-in-water (o/w) or water-in-oil (w/o), depending on which material is dispersed in the other. Either type of emulsion may also contain other contaminating materials (solids, dirt, metal particles, emulsifiers, cleaners, soaps, solvents, and so on). Each producing industry has a typical emulsion that requires treatment. Oil separation and removal can be divided into two processes: gravity separation of non-emulsified oil and chemical treatment and separation of emulsified oil.