Application of multi-electron donor autotrophic denitrification (MEDAD) substrate in constructed wetlands for treating low carbon-to-nitrogen ratio wastewater: A feasible framework

Constructed wetlands (CWs) offer a sustainable solution for treating low carbon-to-nitrogen (C/N) ratio wastewater, yet their nitrogen removal efficiency is often constrained by insufficient organic electron donors. This review systematically evaluates the mechanisms, efficacy, and challenges of autotrophic denitrification in CWs using inorganic electron donors, including sulfur-, iron-, hydrogen-, and manganese-based systems. Sulfur-based systems (e.g., S-0, pyrite) achieve nitrate removal rates exceeding 90 % but risk sulfate accumulation and acidification, while iron-based substrates (e.g., siderite, steel slag) enhance simultaneous nitrogen (71 % TN removal) and phosphorus removal (93 % TP removal) via Fe2+/Fe3+ redox reactions. Hydrogenotrophic denitrification exhibits high efficiency (>97 % NO3--N removal) but faces practical hurdles in H-2 utilization. Crucially, multi-electron donor autotrophic denitrification (MEDAD) systems, integrating substrates like pyrite-steel slag composites, demonstrate synergistic benefits: TN and TP removal efficiencies improve by 40-54 % and 19 %, respectively, while stabilizing pH and mitigating phytotoxicity. The interplay between wetland vegetation and MEDAD substrates is highlighted, with root exudates (e.g., organic acids, sugars) potentially regulating microbial denitrification pathways. However, challenges persist, including byproduct management (e.g., sulfates, Fe (OH)(3) passivation), substrate longevity, and scalability. This study proposes a feasibility framework for MEDAD-CW integration, emphasizing substrate optimization, microbial community control, and system engineering. By addressing these factors, MEDAD-CWs can achieve efficient, stable treatment of low C/N ratio wastewater, advancing their application in eutrophication mitigation and water quality restoration.