Key Takeaways & Executive Findings
- •• • The CF-Fe-CS/NAFO system achieved effluent NO3-N below 6.5 mg/L and TN removal of 50-60% at HRT 48 h, versus control effluent >12 mg/L and <20% removal, proving electrochemical Fe(III) reduction sustains NAFO without external carbon. • • Treating real secondary effluent (influent NO3-N 15.29 mg/L), the system lowered effluent NO3-N to 6.06 mg/L with TN removal >50%, and nitrite/ammonium accumulation <0.6 mg/L, demonstrating robustness to complex matrices. • • Reducing HRT from 48 h to 24, 12, 6, and 3 h increased effluent NO3-N to 9.5, 11.9, 13.6, and 14.6 mg/L, with TN removal dropping to 32.6%, 19.9%, 10.9%, and 5.8%, respectively, quantifying kinetic limits for design. • • SEM confirmed chitosan gel forms a uniform coating on carbon felt-iron, preventing iron oxide agglomeration and enhancing microbial attachment; electrochemical analysis showed charge transfer resistance of 3.284 Ω·cm², enabling rapid Fe(III)/Fe(II) cycling; microbial community analysis revealed enrichment of Gallionellaceae, Thiobacillus, Sediminibacterium, and Thauera, forming a synergistic denitrifying consortium.
Abstract
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.
1. Introduction
Municipal wastewater treatment plants (WWTPs) in China face stringent discharge limits, necessitating advanced nitrogen removal from secondary effluent, which is characterized by a low carbon-to-nitrogen (C/N) ratio. Conventional heterotrophic denitrification requires external organic carbon addition, escalating operational costs and risking secondary pollution. Iron-driven autotrophic denitrification (NAFO) offers a sustainable alternative, but its practical application is hindered by unsustainable iron sources and surface passivation, which limit long-term efficacy.
This study addresses these bottlenecks by integrating a carbon felt-iron-chitosan (CF-Fe-CS) composite cathode into an electrochemical-biological system. The applied potential (-1.0 V vs. Ag/AgCl) enables in-situ electrochemical reduction of Fe(III) to Fe(II), providing a continuous electron donor for NAFO. The pilot-scale system (200 L) demonstrates stable nitrogen removal from real secondary effluent without external carbon, offering a viable solution for low C/N wastewater treatment.
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LÜ Tonghui, AN Facai, SUN Dezhi (2026). A Pilot-Scale Study on Iron-Driven Autotrophic Denitrification Enhanced by CF-Fe-CS Cathode for Low C/N Wastewater Treatment. Journal of Environmental Engineering Technology. https://doi.org/10.13205/j.hjgc.202604008
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Frequently Asked Questions
What is the long-term stability of the CF-Fe-CS cathode under continuous operation, and what mechanisms prevent iron passivation?
The system operated stably for 98 days. SEM analysis showed that the chitosan gel forms a uniform coating on the carbon felt-iron surface, inhibiting iron oxide crystallization and agglomeration. Electrochemical analysis revealed a charge transfer resistance of 3.284 Ω·cm², indicating rapid and reversible Fe(III)/Fe(II) cycling, which sustains Fe(II) availability and prevents passivation.
How does the system perform under varying hydraulic retention times (HRT), and what is the optimal HRT for practical application?
At HRT 48 h, effluent NO3-N was below 6.5 mg/L with TN removal of 50-60%. Reducing HRT to 24, 12, 6, and 3 h increased effluent NO3-N to 9.5, 11.9, 13.6, and 14.6 mg/L, with TN removal dropping to 32.6%, 19.9%, 10.9%, and 5.8%, respectively. For deep nitrogen removal, HRT ≥48 h is recommended; shorter HRTs may be acceptable if effluent standards allow higher NO3-N.
What is the energy consumption and cost comparison against conventional heterotrophic denitrification?
The study does not provide direct energy or cost data. However, eliminating external carbon source addition reduces chemical costs and secondary pollution. The applied potential of -1.0 V (vs. Ag/AgCl) likely incurs moderate electricity costs, but the system's long-term stability and high removal efficiency may offset operational expenses. A detailed techno-economic analysis is recommended.
How does the system handle real wastewater matrices with potential inhibitors or competing electron acceptors?
During treatment of real secondary effluent (influent NO3-N 15.29 mg/L), the system achieved effluent NO3-N of 6.06 mg/L with TN removal >50%, and nitrite/ammonium accumulation <0.6 mg/L. This indicates robustness to complex organic matter and potential inhibitors. The microbial community, enriched with Gallionellaceae, Thiobacillus, Sediminibacterium, and Thauera, likely contributes to resilience.
What are the scale-up bottlenecks from pilot (200 L) to full-scale (e.g., 10,000 m3/d)?
Key bottlenecks include electrode fabrication at scale, uniform potential distribution across large cathodes, and maintaining low charge transfer resistance. The pilot's 5% filling rate and 200 L volume provide a basis for design, but full-scale systems may require modular configurations and optimized electrode spacing. Further research on long-term fouling and electrode replacement is needed.
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