A Pilot-Scale Study on Iron-Driven Autotrophic Denitrification Enhanced by CF-Fe-CS Cathode for Low C/N Wastewater Treatment
Municipal wastewater treatment plant (WWTP) effluent in China typically exhibits a low carbon-to-nitrogen (C/N) ratio, necessitating substantial external carbon addition for conventional heterotrophic denitrification, which incurs high costs and secondary pollution risks. Nitrate-dependent ferrous oxidation (NDFO) offers a promising alternative, yet suffers from unsustainable iron sources and surface passivation. This study constructed a pilot-scale electrochemical-biological coupled system (CF-Fe-CS/NAFO) with an effective volume of 200 L and a treatment capacity of 100 L/d, incorporating a composite cathode (5% apparent filling rate of carbon felt-iron-chitosan, CF-Fe-CS). A constant potential of -1.0 V (vs. Ag/AgCl) was applied to the cathode to achieve in-situ electrochemical reduction of Fe(III). During stable operation from day 16 to 60 with a hydraulic retention time (HRT) of 48 h and synthetic influent containing 15 mg/L NO3-N, effluent NO3-N remained below 6.5 mg/L, achieving total nitrogen (TN) removal of 50-60%, whereas the control reactor (no applied potential) exhibited effluent NO3-N above 12 mg/L and TN removal below 20%. From day 61 to 74, treating real secondary sedimentation tank effluent (influent NO3-N: 15.29 mg/L), the system reduced effluent NO3-N to 6.06 mg/L, maintaining TN removal above 50%. From day 74 to 98, as HRT was sequentially reduced from 48 h to 24, 12, 6, and 3 h, effluent NO3-N increased to 9.5, 11.9, 13.6, 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 system achieves long-term, stable, advanced nitrogen removal from WWTP secondary effluent without external organic carbon.