Application of ultrasonication and hybrid bioreactor for treatment of synthetic textile wastewater

This study explores the influence of ultrasonic pretreatment on the performance of a hybrid bioreactor treating synthetic textile wastewater. The study primarily focuses on assessing the decolorization and chemical oxygen demand (COD) reduction efficiency of a hybrid bioreactor. The hybrid bioreactor provided anaerobic-aerobic sequential treatment to the ultrasonically pretreated textile wastewater containing two new generation diazo dyes namely, reactive black 5 (RB5) and reactive red (RR120), along with other chemical constituents. Ultrasonic pretreatment produced approximately 52% color and 48% COD reduction efficiency at an optimized power of 200 W and 180 min sonication time. For the hybrid reactor treating ultrasonically pretreated textile wastewater, a maximum of 98% COD and 92% color removal efficiencies were obtained at an HRT of 32 h [organic loading rate (OLR)=0.74 kg COD m^-3d^-1] and 24 h (OLR=1.0 kg COD m^-3 d^-1), respectively. Significant levels of aromatic amines formation were observed in the anaerobic phase. However, those were effectively eliminated in the subsequent aerobic phase inside the hybrid bioreactor. Volatile fatty acids to bicarbonate alkalinity (VFA/B-alk.) ratios, one of the health indicators of the bioreactor operation, were found in the range of 0.23-0.32 at different hydraulic retention times (HRTs), thus indicating the adequate buffering capacity for stability of the bioreactor. The study demonstrates the importance of ultrasonic pretreatment for mineralizing organic and inorganic impurities present in textile wastewater, which significantly improves the performance of subsequent hybrid bioreactor.