Synergistic energy recovery and removal of organics and ammonium from moderate-strength wastewater through an MEC–flue gas–anammox system

The treatment of wastewater characterized by high levels of NH₄⁺-N and a moderate organic strength with a low ratio of chemical oxygen demand (COD) to total nitrogen, poses significant challenges to conventional biological processes. In this study, an energy-harvesting microbial electrolysis cell (MEC)–flue gas–anaerobic ammonium oxidation (anammox) system was proposed to treat manure-free piggery wastewater (707 ± 8 mg/L COD and 321 ± 3 mg/L NH₄⁺-N). The MEC first effectively removed wastewater organics and recovered chemical energy in the form of H₂ (with trace CH₄ production). Flue gas-derived nitrogen oxides were subsequently absorbed by MEC effluent, providing a sustainable nitrite source for the anammox process. The residual NO₃⁻-N generated from anammox metabolism and flue gas absorption could be further removed through hydrogenotrophic denitrification using in situ MEC-derived H₂. The integrated system eliminates the need for energyintensive aeration (required in aerobic processes) while also addressing the inefficiency of anaerobic treatment in handling moderate-strength wastewater. The system demonstrated an outstanding performance, with effluent concentrations of COD < 40 mg/L, NH₄⁺-N < 0.8 mg/L, NO₂⁻-N < 0.5 mg/L, and NO₃⁻-N < 0.8 mg/L. Moreover, the MEC enabled the continuous production of H₂ (0.50–0.91 m³/(m³·d)) and CH₄ (0.01–0.14 m³/(m³·d)) with an energy recovery efficiency of ∼249% (the ratio of the energy content of the produced H₂ and CH₄ to the input electrical energy). This study provides an energy-effective strategy for the highly efficient treatment of moderate-strength wastewater. (Figure presented.)