Enhancing anammox resistance to salinity stress through waste iron scraps addition: Performance and microbial community dynamics.

This study investigated the potential of waste iron scraps (WIS) to improve salinity resistance in anaerobic ammonium oxidation (anammox) systems. Two reactors (R1 was used as a control and R2 was modified with WIS) were operated under increasing salt stress (0-2.75 %). At 2 % salinity, R2 attained a nitrogen removal rate (NRR) of 4.4 ± 0.2 kg/(m3·d), 93 % greater than R1 (2.3 ± 0.3 kg/(m3·d)). When salinity escalated to 2.75 %, R1 collapsed completely, while R2 maintained stable performance with NRR of 1.8 ± 0.2 kg/(m3·d). Microbial network analysis revealed WIS significantly strengthened interspecies cooperation, fostering synergistic interactions among functional communities. High salinity (2-2.75 %) markedly reduced Candidatus_Kuenenia abundance in both reactors. However, potentially novel anammox bacteria, Brocadiaceae_unclassified, remained dominant (52.1 %) in R2, suggesting its critical role in sustaining nitrogen removal under hypersaline conditions. Mechanistically, WIS likely enhanced microbial salt tolerance through continuous iron release, which upregulated osmoregulation pathways, iron transport systems, and energy metabolism. These findings demonstrated WIS as a cost-effective strategy for enhancing anammox processes in saline wastewater treatment.