Key Takeaways & Executive Findings
- •• • Tidal flow operation significantly enhanced NH4+-N removal (P<0.001) by achieving reoxygenation capacity up to 450 g·(m2·d)−1, far exceeding conventional subsurface flow (≤100 g·(m2·d)−1), enabling robust nitrification and creating dynamic aerobic/anoxic microzones for simultaneous nitrification-denitrification. • • Iron-loaded biochar (BC-TF) achieved an average TN removal of 86.03%, significantly higher than other groups (P<0.001), attributed to its high specific surface area (341.293 m2·g−1) and porosity (75%) that concentrate nutrients, slow-release carbon supply, and electron shuttle capability enhancing denitrification. • • Iron-loaded biochar mediated Feammox pathway (anaerobic ammonium oxidation coupled to iron reduction), directly converting NH4+-N to N2 or NO2−, and the tidal flow's idle period regenerated Fe(II) to Fe(III), ensuring long-term iron cycling and sustained performance. • • Pilot-scale validation showed tidal flow increased effluent DO to 5.50 mg·L−1 (VFW) and 5.12 mg·L−1 (HSFW), improving TN removal by >30% (P<0.001) compared to continuous flow, with effluent meeting Class V of GB 3838—2002, confirming practical feasibility.
Abstract
To address low nitrogen removal efficiency in wastewater treatment plant (WWTP) effluent due to insufficient carbon sources and weak reoxygenation in conventional constructed wetlands (CWs), a tidal flow-subsurface flow integrated CW using iron-loaded biochar (BC-TF) as substrate was developed, with zeolite-based CW as control. Simulated wastewater experiments, water quality monitoring, nitrification/denitrification intensity assays, and high-throughput sequencing were employed. Results showed that tidal flow operation significantly enhanced removal of total nitrogen (TN) and ammonia nitrogen (NH4+-N), and increased nitrification intensity. Addition of iron-loaded biochar significantly improved TN and nitrate nitrogen (NO3−-N) removal, with BC-TF achieving an average TN removal of 86.03%, significantly higher than other groups (P<0.001). Microbial analysis revealed Proteobacteria, Actinobacteria, and Bacteroidetes as key phyla; iron-loaded biochar increased microbial abundance and diversity in tidal flow wetlands, while tidal flow alone reduced bacterial diversity. Pilot-scale experiments confirmed that tidal flow increased dissolved oxygen and nitrogen removal. This study is the first to combine iron-loaded biochar with tidal flow-subsurface flow CWs, systematically revealing the synergistic nitrogen removal mechanism of 'iron-loaded biochar-tidal flow-microorganisms', clarifying the role of iron-nitrogen coupling, and validating engineering applicability via pilot tests. The combination enhances reoxygenation, supplements carbon sources, and optimizes microbial community structure, effectively improving deep nitrogen purification of WWTP effluent, providing technical reference for tailwater treatment.
1. Introduction
Municipal wastewater treatment plant (WWTP) effluent, characterized by low carbon-to-nitrogen (C/N) ratios and residual nitrogen, poses a significant threat to receiving water bodies. Constructed wetlands (CWs) offer a green and low-cost solution, yet their efficacy is hampered by two critical bottlenecks: insufficient carbon sources for denitrification and weak oxygen transfer limiting nitrification. Conventional subsurface flow CWs typically achieve reoxygenation capacities below 100 g·(m2·d)−1, insufficient for complete ammonia oxidation. Tidal flow operation, by cyclic flooding and draining, entrains air into the substrate, achieving reoxygenation up to 450 g·(m2·d)−1, thereby overcoming oxygen limitation. However, carbon deficiency remains unresolved.
Iron-loaded biochar presents a multifunctional substrate that addresses both bottlenecks. Its high specific surface area (341.293 m2·g−1) and porosity (75%) adsorb and concentrate nutrients, while the biochar matrix serves as a slow-release carbon source. Iron oxides facilitate electron transfer and mediate the Feammox pathway, enabling anaerobic ammonium oxidation. This study integrates iron-loaded biochar with tidal flow-subsurface flow CWs, systematically evaluating nitrogen removal performance, microbial community dynamics, and pilot-scale feasibility. The combination of enhanced reoxygenation, supplemental carbon, and optimized microbial ecology offers a promising solution for deep nitrogen removal from WWTP effluent, addressing the critical need for sustainable wastewater polishing.
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HU Anqi, MA Jingsheng, JIANG Yongcan, LIU Guanglong (2026). Mechanisms and Pilot-Scale Validation of Iron-Loaded Biochar-Based Tidal Flow Constructed Wetlands for Enhanced Deep Nitrogen Removal from Wastewater Treatment Plant Effluent. Chinese Journal of Environmental Engineering. https://doi.org/10.12030/j.cjee.202509001
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Frequently Asked Questions
What is the long-term stability of iron-loaded biochar under repeated tidal cycles, and does iron leaching pose a risk to effluent quality?
The study indicates that tidal flow's idle period regenerates Fe(II) to Fe(III), maintaining iron availability. However, specific iron leaching rates were not reported. Long-term stability would require monitoring over extended operation, but the design aims to minimize leaching by retaining iron in the biochar matrix. Effluent iron concentrations should be monitored to ensure compliance with discharge standards.
How does the BC-TF system perform under variable influent C/N ratios typical of real WWTP effluent, and what is the optimal operational strategy?
The study used simulated wastewater with controlled conditions. Real effluent C/N ratios may fluctuate, potentially affecting denitrification. The biochar's slow-release carbon and adsorption capacity provide buffering, but adaptive management (e.g., adjusting tidal frequency or adding external carbon) may be needed. Pilot tests under real conditions are recommended to assess robustness.
What is the cost comparison between iron-loaded biochar and conventional zeolite substrates, including production and replacement costs?
Iron-loaded biochar production involves pyrolysis of biomass and iron impregnation, which may be costlier than zeolite. However, its higher nitrogen removal efficiency (86.03% vs. control) could reduce land footprint and operational costs. A life-cycle cost analysis is necessary to quantify economic benefits, but the study suggests the technology is economically viable for deep nitrogen removal.
How does the microbial community structure in BC-TF differ from that in zeolite-based tidal flow wetlands, and what are the key functional microorganisms?
BC-TF showed higher microbial abundance and diversity. Key phyla included Proteobacteria, Actinobacteria, and Bacteroidetes. At genus level, Bradyrhizobium was enriched, possessing ammonia oxidation, denitrification, and iron reduction capabilities, crucial for iron-nitrogen coupling. Co-occurrence network analysis revealed 62.59% positive correlations, indicating functional synergy.
What is the scalability potential of this system for full-scale WWTP effluent treatment, and are there any hydraulic or clogging issues?
Pilot-scale tests confirmed feasibility, with TN removal improved by >30% compared to continuous flow. However, long-term operation may face clogging due to biofilm growth and particulate accumulation. Tidal flow's draining phase helps mitigate clogging by promoting substrate aeration. Further research on media replacement and maintenance protocols is needed for full-scale application.
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