Process Sequencing in Livestock Wastewater Treatment: Integrating Activated Sludge and Electrochemical Oxidation

Combining biological treatment with electrochemical oxidation (EO) may provide an effective strategy for high-strength livestock wastewater by coupling biodegradable organic compounds removal with oxidation of more persistent contaminants. This study compared activated-sludge biological treatment (B) and boron-doped diamond EO in both sequences (EO + B and B + EO) using livestock wastewater with a chemical oxygen demand (COD) of approximately 16 g O2 L−1. B treatment was evaluated at different hydraulic retention times and EO at predefined COD removal targets. EO + B achieved 87–94% COD removal with lower specific energy consumption, whereas B + EO consistently achieved approximately 99% COD removal and greater removal of dissolved organic carbon, phosphorus, several metals, and solids. Lowering the organic load before EO did not reduce the stage-specific energy demand of EO polishing. Process order also altered nitrogen speciation: EO + B resulted in lower final ammonium and nitrate concentrations but nitrite accumulation, whereas B + EO promoted nitrate formation. Salmonella spp. and Escherichia coli were not detected after either sequence. B + EO provided deeper physicochemical polishing but promoted substantial chlorate and perchlorate formation, whereas EO + B was less energy demanding and avoided final oxychlorine accumulation. These distinct water-quality profiles support fit-for-purpose selection of treatment sequences according to the intended reuse application.