Enhanced disinfection and pathogen removal mechanisms in bio-electrocoagulation (BEC) systems: A mechanistic and performance analysis

The safe reuse of treated wastewater for agriculture is critically impeded by the physical encapsulation of viable pathogens within sludge flocs, a phenomenon scientifically characterized as the "Trojan Horse" effect. This mechanism shields microorganisms within the coagulant matrix, allowing for their concealed transport and potential downstream reactivation rather than true elimination. While Bio-Electrocoagulation (BEC) systems offer integrated treatment, the mechanistic distinction between true pathogen inactivation (via electrochemical/chemical oxidation) and mere physical sequestration (floc entrapment) remains poorly quantified. This study investigates these disinfection pathways within a continuous-flow hybrid BEC system to validate its reliability for restricted irrigation. A modified BEC reactor, integrating aluminum-based electrocoagulation (EC), a fixed-film biofilter, and a terminal sand filter, was operated using raw municipal wastewater characterized by high chloride content (~309 mg/L). Performance was evaluated under varying applied voltages (22-30 V), flow rates (1-5 L h⁻¹), and pH conditions (3-9) to decouple the contributions of sweep flocculation, electro-chlorination, and electroporation. The system achieved consistent total coliform removal of >99.9% (>3-log reduction). Mechanistic analysis suggests that disinfection is consistent with predominant action by electrochemical oxidation rather than physical enmeshment. This is supported by the system's robust performance across the pH 3-9 spectrum; despite the inhibition of aluminum hydroxide floc formation at extreme pH levels, pathogen inactivation remained stable, a finding consistent with the dominant role of in-situ generated active chlorine and ROS. Furthermore, a "disinfection plateau" observed at charge loadings >1600 C L⁻¹ indicated a shift from charge-controlled to mass-transfer-limited kinetics. The terminal "bio-sand" barrier proved essential for process synergy, polishing turbidity to 99.4% and ensuring the physical exclusion and containment of floc-entrapped biomass. Operational protocols including regular backwashing of the sand filter and controlled sludge withdrawal from the EC chamber were implemented to manage accumulated solids, thereby mitigating the risk of pathogen reactivation and ensuring the risk was reduced, not merely relocated. A risk-bounding analysis based on residual chlorine, contact time, and organic precursor estimates suggests low DBP formation potential under the optimized operational envelope (∼1600 C L⁻¹), and a mitigation strategy incorporating charge loading control and optional dechlorination is proposed. Although the specific energy consumption (9.9 kWh m⁻³; EE/O = 3.19 kWh m⁻³ log⁻¹) exceeds conventional disinfection methods, the BEC system operates as a "universal treatment" unit, simultaneously removing 81% of COD, 98% of TSS, and effectively disinfecting high-strength influent. Normalized energy analysis and sensitivity assessment indicate that with scale-up optimization and renewable power integration, operational costs could decrease to $0.35–0.50 m⁻³, making it competitive for decentralized reuse. The study confirms that BEC provides a resilient, multi-barrier solution for decentralized agricultural water reuse, shifting the paradigm from simple separation to effective inactivation and removal.