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Petroleum refining is an essential step in converting crude oil into useful products such as fuels and petrochemicals. The refining process usually results in generating huge amounts of wastewater that must go through a combination of treatment steps to reduce the concentrations of the different contaminants to acceptable discharge levels. Wastewaters from petroleum refining come from different sources in the refinery, including cooling water, process water and equipment rinse drains. Although petroleum refining utilises large amounts of water for cooling, it does not come into direct contact with process oil streams and therefore contains fewer contaminants than process wastewater. The latter is usually derived from desalting crude oil, steam stripping operations, pump gland cooling, reflux drum drains and boiler blow-down. It is usually highly contaminated as it often comes into direct contact with oil during the refining process.

Petroleum refineries typically utilise primary and secondary wastewater treatment for the separation of oil, water and solids, using an American Petroleum Institute (API) separator or a coalescing plate interceptor (CPI). The API separator provides simple gravity separation on a continuous flow basis. As the wastewater slowly flows through the separation chamber, the oil rises to the surface of the water while the solids settle to the bottom. The CPI separators improve on conventional gravity separation by having a much smaller distance for the droplets to rise, which is accomplished by having coalescing plates in the separation chamber. The oil droplets rise to the next highest plate instead of the surface of the water. After contacting the plate surface, the small droplets coalesce into larger droplets. As the buoyant forces on these droplets become greater than the attraction to the plate’s surface, the droplets break free and rise quickly to the surface. After the CPI, the separated wastewater goes through physical separation methods such as the use of a series of settling tanks or the use of dissolved-air flotation (DAF) to remove emulsified oils. DAF forms fine bubbles by dissolving air into the wastewater at about 3 bar then releasing the pressure. Chemical coagulants such as alum (Al₂(SO₄)₃.18H₂O) are usually added to the wastewater to combine small dispersed particles into larger agglomerates that can be removed by sedimentation, air flotation or filtration. Coagulation can also be accomplished by in situ generation of coagulants through electrocoagulation, which involves the electrolytic oxidation of an appropriate anode material.¹

Characteristics of Petroleum Refinery Wastewater
Wastewater generated by chemical and petrochemical plants, including petroleum refineries, is often characterised by high concentrations of aliphatic and aromatic petroleum hydrocarbons that could have detrimental and harmful effects on plant and aquatic life if discharged without treatment. Petroleum refineries are usually classified as either hydroskimming, which include distillation, reforming and desulphurisation units, or complex, which includes a catalytic cracking unit in addition to the basic units. The main contaminants of refinery wastewater include phenols, polycyclic aromatic hydrocarbons (PAHs), ammonia and heavy metals. Phenols are considered to be among the most hazardous, and there are certainly the most difficult to remove. The United Arab Emirates (UAE) has one of the most stringent environmental regulations, especially those in terms of discharge levels. A list of concentrations of major contaminants in a typical petroleum refinery wastewater after preliminary treatment and the discharge limits for selected countries is given in Table 1.

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