Biofilm-driven multi-stage anaerobic-aerobic process for high-strength petrochemical wastewater treatment

High-strength petrochemical wastewater typically shows poor biodegradability, making stable compliance difficult with biological treatment alone. In this study, an integrated train combining coagulation-Fenton oxidation pretreatment with a biofilm-driven multi-stage anaerobic-aerobic process was developed. The Fenton pretreatment was optimized by response surface methodology, and the downstream system comprised an anaerobic biofilter, multi-stage biological contact oxidation, followed by hydrolysis-acidification/contact oxidation and clarification. Results indicated that the Initial pH was the most influential factor for Fenton performance. Under optimized conditions (pH 2.20, H₂O₂ dosage 4.5 mL/L, H₂O₂/Fe²⁺ molar ratio 20), pretreatment achieved 51.9% COD removal. At steady operation (Day 28), overall COD decreased from 3740 mg/L to 239.2 mg/L (93.6% cumulative removal). Anaerobic biofilter provided stable COD removal of 20.2-23.5% with an optimal temperature window of 25-35°C, while the multi-stage biological contact oxidation maintained 64.1-80.0% COD removal and was buffered under higher loading by extending reaction time/increasing hydraulic retention time. Biofilm stability was supported by MLSS of 4,151 mg/L and SVI of 75.9 mL/g in the multi-stage reactor (Day 30). Overall, coagulation-Fenton served as an influent-shaping module, complementing the anaerobic-aerobic biofilm process to achieve robust removal of high-strength refractory organics.