Autotrophic denitrification using Fe(II) as an electron donor: A novel prospective denitrification process.

As a newly identified nitrogen loss pathway, the nitrate-dependent ferrous oxidation (NDFO) process is emerging as a research hotspot in the field of low carbon to nitrogen ratio (C/N) wastewater treatment. This review article provides an overview of the NDFO process and summarizes the functional microorganisms associated with NDFO from different perspectives. The potential mechanisms by which external factors such as influent pH, influent Fe(II)/N (mol), organic carbon, and chelating agents affect NDFO performance are also thoroughly discussed. As the electron-transfer mechanism of the NDFO process is still largely unknown, the extensive chemical Fe(II)-oxidizing nitrite-reducing pathway (NDFOchem) of the NDFO process is described here, and the potential enzymatic electron transfer mechanisms involved are summarized. On this basis, a three-stage electron transfer pathway applicable to low C/N wastewater is proposed. Furthermore, the impact of Fe(III) mineral products on the NDFO process is revisited, and existing crusting prevention strategies are summarized. Finally, future challenges facing the NDFO process and new research directions are discussed, with the aim of further promoting the development and application of the NDFO process in the field of nitrogen removal.

Effects of sponge iron dosage on nitrogen removal performance and microbial community structure in sequencing batch reactors.

The application of sponge iron (SI) carriers can improve the biochemical treatment performance of sequencing batch reactors (SBR) during wastewater treatment. This study used SBR reactors to explore the effects of SI dosage on the nitrogen removal performance and reactor stability and microbial community structure under low temperature and ultra-low load. In contrast to conventional SBR, the average removal rate of total nitrogen (TN) in the biological sponge iron system (BSIS) was increased by 5.38 % for 45 g/L, 18.93 % for 90 g/L, and 13.52 % for 135 g/L, respectively. The nitrogen removal performance and reactor stability showed the best performance under the SI dosage of 90 g/L. The addition of SI formed the anaerobic-anoxic-aerobic microenvironments, which facilitate the propagation of denitrifying bacteria (Saccharimonadales, Hydrogenophaga) and iron bacteria (Rhodoferax and Acinetobacter) in the BSIS. This study provides a new insight on the application of SI in the wastewater treatment.

Use of sponge iron as an indirect electron donor to provide ferrous iron for nitrate-dependent ferrous oxidation processes: Denitrification performance and mechanism.

Sponge iron (SI) can serve as an indirect electron donor to provide Fe(II) for the nitrate-dependent ferrous oxidation (NDFO) process, producing OH- and magnetite. The SI-NDFO system mainly uses Fe(OH)2 as an electron donor, achieving a TN reduction rate of 0.42 mg-TN/(gVSS·h) for a period of at least 90 days. The enrichment of iron-oxidizing bacteria and the competition of iron-carbon micro-electrolysis for reaction sites on the surface of SI are the main reasons for the improvement of total nitrogen removal efficiency (TNRE). With an influent NO3--N concentration of 50 mg/L and a SI concentration of 50 g/L (at pH 5.0 and 30 °C), the TNRE reached a maximum level of 38.28%. In addition, reducing the pH environment was found to improve the denitrification efficiency of the SI-NDFO system, although denitrification stability was also reduced as a result. Overall, the SI-mediated NDFO process is a promising technique.