Performance and Resilience of Oxidation Ditch and Step-Feeding Anoxic–Oxic Processes during Extreme Rainfall Events: Evaluation and Optimization Strategies for Wastewater Treatment Plants

Extreme rainfall poses significant challenges to the operational stability of wastewater treatment plants (WWTPs). This study used full-scale plant data to evaluate the robustness and nutrient removal performance of oxidation ditch (OD) and step-feeding anoxic–oxic (AO) processes during an extreme rainfall event, with a sludge correction model applied to account for mixed-liquid suspended solids (MLSS) variations. The results showed that influent characteristics fluctuated markedly during the extreme rainfall event, with influent chemical oxygen demand (COD) decreasing to below 20 mg / L and influent total nitrogen (TN) dropping to less than 3.5 mg / L at peak rainfall, whereas influent total phosphorus (TP) exhibited a short-term peak during the initial rainfall period. Under these highly diluted and low C / N conditions, the OD showed impaired nitrogen removal, with effluent TN increasing to 18.54 mg / L after the rainfall, whereas the step-feeding AO system maintained stable effluent TN and TP below 6.03 and 0.20 mg / L , respectively, which was attributed to preanoxic buffering and controlled returned activated sludge (RAS) concentration ( 18,740 mg / L ). A dynamic MLSS correction model was established to separate the effects of recycle flow and RAS on MLSS. These results highlight the superior resilience of step-feeding systems under extreme flow and pollutant shocks, offering practical insights to enhance WWTP robustness under extreme rainfall events.