Response Surface Methodology-Assisted Optimization of Operating Parameters in a Laboratory-Scale Contact Stabilization Process Treating Synthetic Food-Processing Wastewater

The contact stabilization (CS) process is a promising technology for wastewater treatment, yet its performance under elevated organic loading remains poorly understood. This study investigated the effects of cycle time, sludge retention time (SRT), and influent chemical oxygen demand (COD) on pollutant removal and sludge characteristics in a laboratory-scale CS reactor treating synthetic food-processing wastewater. Single-factor experiments showed that cycle time and SRT were dominant. Reducing cycle time from 12 h to 6 h maintained COD removal while improving NH4+-N (ammonia nitrogen) and TP removal, but 2 h led to substantial deterioration. Robust performance was maintained at SRT 4 d, whereas severe biomass washout occurred at 1 d. Influent COD displayed an optimal range near 500 mg·L−1. A Box–Behnken design was employed for response surface modeling. Multi-objective optimization established a robust operating window of cycle time 7–10 h, SRT 4–5.5 d, and influent COD 300–500 mg·L−1, achieving >65% COD removal, >55% TN removal, and stable settleability (SVI <100 mL·g−1). These findings elucidate CS process behavior under elevated organic loading and provide practical engineering guidance.