Enhancing the nitrogen removal efficiency of a new autotrophic biological nitrogen-removal process based on the iron cycle: Feasibility, progress, and existing problems

Partial nitrification (PN)-Anammox is considered to be the optimal nitrogen-removal process for municipal sewage following organic carbon and phosphorus recovery. However, under the mainstream sewage conditions of low temperature and low-ammonium concentration, the growth of nitrite-oxidizing bacteria (NOB) in the PN process cannot be suppressed, leading to the production of a large amount of NO3−-N. The biochemical processes of oxidizing NH4+-N with Fe3+ (anaerobic ammonium oxidation coupled with ferric iron reduction, Feammox) and reducing NO3−-N with Fe2+ (nitrate-dependent anaerobic ferrous-oxidation, NDAFO) in an anaerobic environment are well known and have been found to coexist with Anammox. The continuous addition of Fe in the individual NDAFO or Feammox process leads to the problem of sludge mineralization, which limits the in-depth study of its conversion process and the use of these processes in industrial applications. If Feammox and NDAFO are coupled, the cyclic utilization of Fe2+/Fe3+ can achieve continuous operation of Feammox and NDAFO, and NH4+-N and NO3−-N can be converted simultaneously, which avoids sludge mineralization and removes NO3− generated during the PN process. When the above processes are coupled with Anammox, the conversion of NH4+-N to N2 can be enhanced. For this reason, this paper reviews and summarizes relevant research on the progress of a single NDAFO or Feammox process, and discusses the feasibility and existing problems of their coupling to realize the autotrophic removal of nitrogen. It is expected that through this relevant summary, new ideas will be provided for the clean and energy-saving removal of nitrogen from wastewater.