Comparison of acoustic and hydrodynamic cavitation based hybrid AOPs for COD reduction of commercial effluent from CETP

The present work investigates the treatment of commercial effluent obtained from Common Effluent Treatment Plants (CETP) using acoustic cavitation (AC) and hydrodynamic cavitation (HC) based hybrid AOPs. Comparison of different hybrid AOPs viz. H₂O₂, Fe²⁺/H₂O₂, Fe²⁺/H₂O₂/Air, Fe²⁺/H₂O₂/S₂O₈^2- and Fe²⁺/H₂O₂/S₂O₈^2-/Air in combination with both AC and HC has been performed in terms of extent of chemical oxygen demand (COD) reduction and kinetic rate constants. The best results of COD reduction as 95.2% and 97.28% were obtained for AC/Fe²⁺/H₂O₂/Air and HC/Fe²⁺/H₂O₂/Air systems respectively at Fe²⁺/H₂O₂ ratio of 0.1 and pH of 2 within 60 min of treatment under conditions of ultrasonic power dissipation as 150 W, inlet pressure for HC as 4 bar (as applicable depending on process) and temperature of 30 ± 2 °C. Slightly lower efficacy was established for the combination approach involving AC or HC coupled with Fe²⁺-activated S₂O₈^2- and H₂O₂ yielding COD reduction of 82.9% and 86.93% for the AC/Fe²⁺/H₂O₂/S₂O₈^2-/Air and HC/Fe²⁺/H₂O₂/S₂O₈^2-/Air systems respectively at Fe²⁺/H₂O₂/S₂O₈^2- ratio of 1:40:17.5. Cost estimation on the basis of cavitational yield performed on the AC and HC based treatment systems revealed economical nature of HC based treatment. Kinetic studies were also performed by fitting the experimental data with pseudo first order kinetic model (PFOKM), generalized kinetic model (GKM) and Behnajady-Modirshahla-Ghanbery kinetic model (BMGKM). It was demonstrated that GKM provided best fitting for all the experiments whereas BMGKM was most suitable for Fenton based reactions. It was clearly established that complex CETP effluent can be effectively treated using the combined approaches based on HC with potential for larger scale operation.