Simultaneous nitrification and denitrification (SND) were coupled with a denitrifying phosphorus removal (DPR) to achieve simultaneous nutrient and carbon removal. With influent chemical oxygen demand (COD), ammonia-N (NH₄⁺-N), and total phosphorus (TP) concentrations of 250, 50, and 8 mg/L, the SND-DPR coupled system achieved stable nutrient removal efficiency of COD, NH₄⁺-N, TN and TP were 91.8 ± 1.7%, 88.4 ± 1.8%, 64 ± 3.3% and 99.2 ± 0.6%, respectively. Enhancing the C/N ratio strengthened the storage of intracellular polymers and provided sufficient intracellular carbon sources for phosphorus uptake. The nutrient removal efficiency reached the highest level at a C/N ratio of 5, and no advantage was observed after increasing the C/N ratio to 7. Nutrients were mainly removed during the aerobic stage at a low DO concentration as well during the anoxic stage, which helped achieve concurrent nitrification and denitrification by ordinary heterotrophic organisms (OHOs), promote denitrifying and aerobic phosphorus removal, and conserve organic carbon demand and energy consumption for aeration. The system was limited for DO in the aerobic stage at a low DO concentration, resulting in a deficiency in electron acceptors (O₂ and NO₃^–N) and limiting the subsequent promotion of phosphorus uptake and TN removal. The limited DO content in the low DO stage was the key factor involved in enhancing the nutrient removal efficiency along with the increasing influent C/N ratio. GRAPHICAL ABSTRACT

A multi-anode mi60crobial fuel cell (MA-MFC) was developed to investigate simultaneous nitrification and denitrification (SND) in the bio-cathode. As the chemical oxygen demand to nitrogen (COD/N) ratio of the cathode was increased from 0 to 4.5, the electricity-producing quantity ranged between 498 C and 543 C and the attained total nitrogen (TN) removal rate reached 12.07 g TN·m⁻³·d⁻¹, resulting in a TN removal efficiency of 78.8% under the target COD/N ratio of 3.5. The removal of pollutants in series and parallel, open-circuit and closed-circuit were compared, respectively. The removal rates of TN, NH₄⁺-N, and cathode and anode COD were all higher in the parallel connection configuration than in the series configuration. In parallel connection, the TN removal rate reached 14.4 g TN·m⁻³·d⁻¹, which was 1.9 times that in series connection. Compared with the open-circuit system, the removal rate of TN in the closed-circuit system was improved by 17.8%, which could be ascribed to electrochemical denitrification. The results of high-throughput sequencing confirmed and clarified the presence of autotrophic denitrification and heterotrophic denitrification, including aerobic denitrification, when the MA-MFC had been operated for 18 months. Highlights