## 1 Introduction  
The surge in demand for food, water, and energy, driven by the expansion of the global economy and a growing population [1], has led to an increase in nitrogen pollution in water bodies [2, 3]. Ammonium nitrogen (NH4+-N) [4] and nitrate nitrogen (NO3-N) pollutants contribute to eutrophication and pose significant risks to human health [5]. While both chemical and biological methods are used for wastewater denitrification, biological methods are generally favored due to their efficiency and cost-effectiveness [6]. Traditional biological denitrification technologies, including the anaerobic-anoxic-oxic (A2/O) process [7], sequencing batch reactor (SBR) systems [8], and the oxidation ditch [9], are challenged by issues such as inconsistent performance and high energy consumption [10]. Iron is an essential trace element for microbial growth, playing a crucial role in promoting microbial proliferation [11]. It is also a component of key enzymes [12], further stimulating microbial activity during enzyme synthesis [13]. Additionally, iron is a vital part of electron transfer cytochromes [14], which are indispensable carriers for intracellular oxidative electron transfer in microbial cells [15]. Iron is increasingly used in wastewater treatment, with zero-valent iron being the most prominent form. The combination of zero-valent iron and microorganisms not only meets the energy needs of the microorganisms but also enhances their environmental adaptability and treatment efficiency [16]. [PERSON] [17] found in their study on the characteristics and mechanism of the influence of organic carbon on Fe(0)-AD that the addition of zero-valent iron promoted the enrichment of iron-oxidizing bacteria (such as _Sphaerotilus_ and _Ferriplasetus_) and autotrophic denitriying bacteria (such as _Hydrogenophaga_ and _Hyphomicrobium_), thereby significantly enhancing the denitrification effect. [PERSON] [18] found in their study on the influence of zero-valent iron on microbial nitrate removal that under the coexistence of zero-valent iron and microorganisms, nitrate could be almost completely removed, and the addition of zero-valent iron promoted the enrichment of denitriying functional bacteria such as _Methylotenera_ spp. and _Pseudomonas_ spp. [PERSON] [19] used iron-polyurethane foam composite carriers as fillers to investigate the microbial community structure in a sequencing batch activated sludge reactor and found that the functional microorganisms had a relatively high abundance of aerobic heterotrophic denitriying bacteria, _Rhodobacter_, and anoxic heterotrophic denitriying bacteria, _Haliangium_. [PERSON] [20] added iron chips to a sequencing batch activated sludge reactor on simultaneous nitrification and denitrification (SND), and results showed that iron chips significantly improved nitrogen removal and increased the relative abundance of aerobic denitriying bacteria, such as _Thauera_, _Theromonas_, _Rhodobacter_, and _Hyphomicrobium_, within the SND system. Sponge iron (SFe) is a special type of zero-valent iron (ZVI) with a loose and porous structure, mainly composed of ZVI (accounting for more than 90%) and containing a small amount of carbon and other impurities [21]. Due to its large specific surface area and high chemical activity, sponge iron is widely used in the field of water treatment [22]. The biological sponge iron system (BSIS) is a composite system formed by introducing SFe into activated sludge in a specific manner [23], which organically combines iron-carbon microelectrolysis (IC-ME) and the activated sludge process [24]. During the corrosion process of sponge iron, Fe\({}^{2+}\) is released.