Enrichment of Feammox microflora utilizing in-situ iron in excess activated sludge: substrate metabolism, gene variation, and microbial communities.

In anaerobic ammonium oxidation coupled with ferric iron reduction (Feammox) culture systems, the Feammox process coexists with nitrate-dependent ferrous iron oxidation (NDFO). To enrich Feammox microflora, Fe-rich excess activated sludge (EAS) was used as the inoculum and iron source, and NDFO was used to convert Fe in EAS to Fe, providing an iron source for Feammox microorganisms. Feammox microflora were obtained after 74-d cultivation, when the NH-N removal capacity was 2.87 ± 0.75 mg N/g dry sludge·d. During the enrichment process, the absolute and relative abundances of Feammox-related genes acm significantly increased 19.3- and 21.9-times higher than those in raw sludge, respectively. Moreover, the absolute abundance of acm was significantly positively correlated with those of Amx and nosZ. The relative abundance of acm was significantly positively correlated with those of Amx, Geo, and nosZ. Acidobacteriota and Nitrospirota abundances increased significantly. Functional profiling of the microbial communities showed that the carbon and iron metabolism were remarkably promoted. Genes involved in carbon fixation, ferric-associated reactions, and iron uptake and transport were clustered. This study provides a new approach for Feammox microorganism enrichment, avoids the problem of sludge mineralization caused by continuous addition of an iron source, and provides a new approach for recycling EAS.