Electrocoagulation and Hydrodynamic Cavitation, a Hybrid Process for Efficient COD Removal From Rice Mill Effluent

Rice mill effluent (RME) typically exhibits high chemical oxygen demand (COD), total organic carbon (TOC), total suspended solids (TSS), and turbidity, which adversely affect the efficiency of cavitation-based advanced oxidation processes (AOPs). In this study, electrocoagulation (EC) was employed as a preliminary treatment to improve effluent characteristics prior to hydrodynamic cavitation (HC) treatment. The EC operating parameters were maintained constant throughout all experiments. In the secondary treatment stage, multiple HC strategies were systematically evaluated, including standalone HC, HC with air injection, HC with air and hydrogen peroxide (H₂O₂), and HC with air and sodium persulfate (Na₂S₂O₈). EC significantly improved RME characteristics by reducing TSS, turbidity, COD, and TOC, thereby enhancing effluent clarity. The improved effluent quality facilitated enhanced mass transfer, stable bubble formation and collapse, and improved cavitation efficiency during the subsequent HC process. Oxidant-assisted HC significantly enhanced COD removal efficiency. Optimum dosages of H₂O₂ (10 g/L) and Na₂S₂O₈ (3.0 g/L) achieved COD removal efficiencies of 88.8% and 90.7%, respectively. Apparent pseudo-first-order kinetic analysis confirmed enhanced degradation kinetics under optimum oxidant-assisted conditions. The optimum H₂O₂-assisted HC process achieved a kinetic rate constant of 6.99 × 10^-3 min^-1, whereas Na₂S₂O₈-assisted HC exhibited comparatively higher degradation kinetics with a rate constant of 9.28 × 10^-3 min^-1. CFD analysis further confirmed the formation of intense low-pressure cavitation zones downstream of the venturi, supporting the experimentally observed enhancement in oxidation performance. Additionally, the synergistic interaction between EC and HC processes was confirmed through a synergy coefficient greater than unity. The laboratory-scale integrated EC-HC process demonstrated a promising approach for RME treatment. However, further validation through continuous-flow operation, comprehensive water quality assessment (including TOC reduction and toxicity evaluation), energy optimization, and detailed techno-economic analysis is required to establish its practical applicability.