<P> Because grease skimming involves higher viscosity hydrocarbons, skimmers must be equipped with heaters powerful enough to keep grease fluid for discharge . If floating grease forms into solid clumps or mats, a spray bar, aerator or mechanical apparatus can be used to facilitate removal . </P> <P> However, hydraulic oils and the majority of oils that have degraded to any extent will also have a soluble or emulsified component that will require further treatment to eliminate . Dissolving or emulsifying oil using surfactants or solvents usually exacerbates the problem rather than solving it, producing wastewater that is more difficult to treat . </P> <P> The wastewaters from large - scale industries such as oil refineries, petrochemical plants, chemical plants, and natural gas processing plants commonly contain gross amounts of oil and suspended solids . Those industries use a device known as an API oil - water separator which is designed to separate the oil and suspended solids from their wastewater effluents . The name is derived from the fact that such separators are designed according to standards published by the American Petroleum Institute (API). </P> <P> The API separator is a gravity separation device designed by using Stokes Law to define the rise velocity of oil droplets based on their density and size . The design is based on the specific gravity difference between the oil and the wastewater because that difference is much smaller than the specific gravity difference between the suspended solids and water . The suspended solids settles to the bottom of the separator as a sediment layer, the oil rises to top of the separator and the cleansed wastewater is the middle layer between the oil layer and the solids . </P>

Joint treatment of raw industrial wastewater and domestic sewage