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Excessive sludge production is cost prohibitive and a major concern in biological treatment of petroleum refinery wastewater. Thus, it is essential to identify operational conditions for treatment systems that result in low sludge production of the system, while maintaining its high removal performance. This study assesses the feasibility of using contact stabilization process for secondary treatment of refinery wastewater through a step by step analysis. The mixed liquor dissolved oxygen (DO) and the rate of activated return sludge (RS) were selected as operational parameters governing the optimum performance of the system. A total number of 32 individual experiments were conducted on a pilot plant under four different aeration phases (DO) and eight RS percentages. The analyses investigated the biokinetic coefficients, observed removal efficiencies, and the amount of produced sludge to identify suitable operational …