Wastewater treatment is considered an important supporting process in petroleum refining. The four types of refinery wastewater include cooling water, process water and steam, storm water, and sanitary sewage water. The refinery stages that produce the most wastewater are desalting, distillation, thermal and catalytic cracking, and coking. The most polluted wastewater is the process water that comes into direct contact with petroleum fractions; thus it cannot be simply treated in municipal wastewater treatment plants. Storm water is considered contaminated as a result of incidental exposure to pollutant sources and accidental spills during refinery reactions. The refinery’s cooling water and sanitary sewage water will probably not require much treatment before sending it to public water facilities where municipal wastewater is treated. Different wastewater streams are usually not mixed even if it reduces the load on treatment units. This is because different wastewater streams have different components and toxicities. The wastewater’s contamination level depends on its usage in the petroleum refinery. Generally, the pollutants in the streams include hydrocarbons, particularly toxic aromatic compounds such as benzene. Also, the wastewater streams include other heteroatom compounds mercaptans, amines, phenols, and cyanides, dissolved gases (H₂S and NH₃), and acids (H₂SO₄ and HF). Such components require much more treatment than municipal wastewater. In addition, environmental policies such as the Clean Water Act and Safe Drinking Water Act require refineries to effectively treat their wastewater.

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