Ammonium (NH₄⁺) oxidation is crucial for nitrogen (N) removal, contributing to regional and global N cycles, but is regarded as limited to a few biological pathways. A novel pathway for NH₄⁺ oxidation and the N cycle is provided in the microbial anaerobic NH₄⁺ oxidation coupled with Fe(III) (ferric iron) reduction, called ferric ammonium oxidation (Feammox). Over the past few years, Feammox, which results in significant loss of N in natural environments, has been detected widely in both terrestrial and aquatic ecosystems. Researchers have revealed various Feammox pathways, end products of nitrate (NO₃^-), nitrite (NO₂^-), and gaseous nitrogen (N₂), and the interactions within the nitrogen and iron-cycle-related microbial communities, which might offer some novel alternative processes for wastewater treatment. However, there are substantial variations among different studies in terms of the key functional microorganisms. The underlying mechanisms of Feammox, as well as the effect of environmental factors, remain poorly understood. In this review of the emerging process, we detail the end-products and microbes involved in the Feammox process and discuss possible mechanisms and the main influential factors. In particular, we assess the potential applications in wastewater treatment based on previous experimental studies and highlight knowledge gaps and future research opportunities.

Integrating denitrifying anaerobic methane oxidation (DAMO) with Anammox provides alternative solutions to simultaneously remove nitrogen and mitigate methane emission from wastewater treatment. However, the practical application of DAMO has been greatly limited by slow-growing DAMO microorganisms living on low-solubility gaseous methane. In this work, DAMO and Anammox co-cultures were fast enriched using high concentration of mixed sludges from various environments, and achieved nitrogen removal rate of 76.7 mg NH4+-N L-1 d-1 and 87.9 mg NO3--N L-1 d-1 on Day 178. Subsequently, nitrogen removal rate significantly decreased but recovered quickly through increasing methane flushing frequency, indicating methane availability could be the limiting factor of DAMO activity. Thus, this work developed a novel Membrane Aerated Membrane Bioreactor (MAMBR), which equipped with gas permeable membrane for efficient methane delivery and ultrafiltration membrane for complete biomass retention. After inoculated with enriched sludge, nitrogen removal rates of MAMBR were significantly enhanced to 126.9 mg NH4+-N L-1 d-1 and 158.8 mg NO3--N L-1 d-1 by membrane aeration in batch test. Finally, the MAMBR was continuously fed with synthetic wastewater containing ammonium and nitrite to mimic the effluent from partial nitritation. When steady state with nitrogen loading rate of 2500 mg N L-1 d-1 was reached, the MAMBR achieved total nitrogen removal of 2496.7 mg N L-1 d-1, with negligible nitrate in effluent (~6.5 mg NO3--N L-1). 16S rRNA amplicon sequencing and fluorescence in situ hybridization revealed the microbial community dynamics during enrichment and application. The high performance of nitrogen removal (2.5 kg N m-3 d-1) within 200 days operation and excellent biomass retention capacity (8.67 kg VSS m-3) makes the MAMBR promising for practical application of DAMO and Anammox in wastewater treatment.

Nitrate/nitrite-dependent anaerobic methane oxidation (n-DAMO) coupling to Anaerobic ammonium oxidation (Anammox) provides an opportunity for simultaneous nitrogen removal and methane emissions mitigation from wastewater. However, to achieve high nitrogen removal rate in such a process remains a critical challenge in practical application. This work investigated the interactions between n-DAMO and Anammox in membrane biofilm reactor (MBfR) and then developed operational strategies of MBfR for high rate nitrogen removal from landfill leachate. Initially, influent containing nitrate and ammonium facilitated the development of n-DAMO and Anammox microorganisms in MBfR, but nitrogen removal performance is hard to be further improved even deteriorated. Detailed investigations of interactions among n-DAMO and Anammox microorganisms confirmed that extra addition of nitrite into MBfR fed with nitrate and ammonium not only stimulated the activities of Anammox bacteria, but also enhanced the activities of n-DAMO archaea from 172.3 to 356.9 mg NO3--N L-1 d-1. Functional gene analysis also indicated that mcrA and hzsA genes increased after nitrite addition. Based on this finding, influent containing NO3-, NO2- and NH4+ enabled nitrogen removal rates of MBfR increase from 224.9 to 888.2 mg N L-1 d-1. Finally, nitrate in the influent was gradually replaced with nitrite to mimic the effluent from partial nitriation of landfill leachate, but maintain the nitrate availability for n-DAMO archaea through increasing nitrate production from Anammox. These operation strategies enabled MBfR achieve the steady state with a nitrogen removal rate of 6.1 kg N m-3 d-1. Microbial community analysis revealed n-DAMO archaea, n-DAMO bacteria and Anammox bacteria jointly dominated the biofilm, and their relative abundance dynamically shifted with feeding regime. This work provides promising operational strategies for high rate of nitrogen removal from landfill leachate through integrating n-DAMO and Anammox process.