Abstract

Nitrogen is an essential element for life but its excessive release into the environment in the form of reactive nitrogen causes severe damage, including acidification and eutrophication. One of the main sources of nitrogen pollution is the use of fertilizers in agricultural soils. Feammox is a recently described pathway that couples ammonium (NH4+) oxidation with iron (Fe) reduction. In this study, the enrichment and bioaugmentation of anaerobic sludge under conditions that promote Feammox activity were investigated. The first enrichment stage (E1) achieved 28% of ammonium removal after 28 days of incubation, with a production of 30 mg/L of Fe2+. E1 was then used as inoculum for two enrichments at 35 °C with different carbon sources: sodium acetate (E2) and sodium bicarbonate (E3). Neither E2 nor E3 showed significant NH4+ removal, but E2 was highly effective in iron reduction, reaching Fe2+ concentrations of 110 mg/L. Additionally, an increase in nitrate (NO??) concentration was observed, which may indicate the occurrence of this pathway in the Feammox process. The Monod kinetic model, analyzed using AQUASIM software, showed a good fit to the experimental data for NH4+, NO3?, and Fe2+. Sequencing analysis revealed the presence of phyla associated with Feammox activity. Although there was only a slight difference in NH4+ removal between the bioaugmented and non-augmented control sludge, the bioaugmented sludge was statistically superior in nitrate production and iron reduction. This study provides valuable insights into the enrichment and bioaugmentation of the Feammox process potential large-scale wastewater treatment applications. © The Author(s), under exclusive licence to Springer Nature Switzerland AG 2025.