A pilot-scale study on iron-driven autotrophic denitrification enhanced by CF-Fe-CS cathode for low C/N wastewater treatment

The effluent from municipal wastewater treatment plants （WWTPs） is characterized by a low carbon-to-nitrogen （C/N） ratio in China. Achieving deep nitrogen removal using traditional heterotrophic denitrification processes requires substantial external carbon source addition， leading to high operational costs and potential secondary pollution. Nitrate-dependent ferrous oxidation （NDFO） presents a promising alternative denitrification technology. However， it faces bottlenecks such as unsustainable iron sources and surface passivation. In this study， a pilot-scale electrochemical-biological coupled system （CF-Fe-CS/NAFO） was constructed， featuring a built-in composite cathode （5% apparent filling rate of carbon felt-iron-chitosan， CF-Fe-CS）， an effective volume of 200 L， and a treatment capacity of 100 L/d. A constant potential of -1.0 V （vs. Ag/AgCl） was applied to the cathode to achieve the in-situ electrochemical reduction of Fe（Ⅲ）. In the test reactor， during stable operation from day 16 to 60 with a hydraulic retention time （HRT） of 48 h and synthetic wastewater containing 15 mg/L NO3--N， the effluent NO3--N concentration remained consistently below 6.5 mg/L， with a total nitrogen （TN） removal efficiency of 50% to 60%. In contrast， the control reactor （without an applied potential） showed an effluent NO3--N concentration above 12 mg/L and a TN removal efficiency below 20%. During continuous operation from day 61 to 74 using real secondary sedimentation tank effluent from a WWTP （influent NO3--N： 15.29 mg/L）， the system reduced the effluent NO3--N to 6.06 mg/L， maintaining a TN removal efficiency above 50%. From day 74 to 98， as the HRT was sequentially reduced from 48 h to 24 h， 12 h， 6 h， and 3 h， the corresponding effluent NO3--N concentrations increased to 9.5 mg/L， 11.9 mg/L， 13.6 mg/L， and 14.6 mg/L， with TN removal efficiencies of 32.6%， 19.9%， 10.9%， and 5.8%， respectively. These results demonstrate that the CF-Fe-CS/NAFO electrochemical system can achieve long-term， stable， and advanced nitrogen removal from WWTP secondary effluent without the need for an external organic carbon source.