Combined Ozonation and Aerobic Biodegradation: An Effective Dual-Stage Approach for Treating Reactive Azo Dye Wastewater

This study investigates dual-stage treatment of reactive azo dye wastewater using ozonation as pre-treatment followed by aerobic biodegradation. Two commercially important dyes were used: CI Reactive Black 5 (RB5, di-azo) and CI Reactive Violet 5 (RV5, mono-azo), both at 2000 mg/L in a semi-batch reactor. Parameters monitored throughout treatment included pH, absorbance at λmax, conductivity, COD, TOC, and BOD₅. Ozonation achieved near-complete colour removal within 60–80 minutes and transformed both dyes from essentially non-biodegradable (BOD₅/COD ≈ 0) to readily biodegradable (BOD₅/COD ≈ 0.47–0.49). Subsequent aerobic treatment achieved over 90% reduction in both COD and TOC. RV5 (mono-azo) degraded consistently faster than RB5 (di-azo) across all metrics. The combined approach is economically and ecologically advantageous: ozonation breaks recalcitrant molecules into biodegradable fragments, while aerobic treatment completes organic removal at lower cost than extended ozonation alone.