Processes used in the treatment of a petroleum refinery wastewater included initial API oil separator to be followed by dissolved air flotation and an extended aeration system. The API oil separator removed most of the settleable solids and oil. The residual oil varied from around 680 mg/l to 104 mg/l and was further treated with chemical coagulation, flocculation and dissolved air flotation which remove more than 80 % of the oil and grease. Mean COD after dissolved air flotation was around 970 mg/l and it was lowered to around 378 mg/l using the extended aeration biological system at 15-day HRT. The final effluent COD/BOD ratio was high indicating the presence of a high percentage of refractory organic compounds.

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