Abstract: This study systematically diagnoses and analyzes the causes of unstable nitrogen removal efficiency in the wastewater treatment system of a refinery during actual operation. The wastewater treatment system of this enterprise features a complex process flow, comprising multiple sets of treatment units for oily, saline and chemical wastewater. The research identifies the AO (anoxic-oxic) process as the weak link in the system’s nitrogen removal. This is attributed to inhibitory impacts on nitrifying bacteria and insufficient denitrification resulting from fluctuations in influent COD load, alkalinity deficiency and imbalance in dissolved oxygen distribution. From multiple dimensions including water quality characteristics, process parameters and microbial ecology, Analyzes the bottleneck of nitrogen removal. Furthermore, optimization strategies centered on enhancing biological nitrogen removal in the front-end process are proposed. These include optimizing operational parameters, precise dosing of carbon sources and alkalinity, aiming to improve the overall nitrogen removal efficiency and operational economic efficiency of the system.

Key words: refinery wastewater, biological nitrogen removal, nitrification, denitrification, AO process