Revealing the intrinsic mechanisms for accelerating nitrogen removal efficiency in the Anammox reactor by adding Fe(II) at low temperature

Fe(II) is an essential trace element for anaerobic ammonium oxidation bacteria (AAOB) metabolism, and can improve the nitrogen removal efficiency of anaerobic ammonia oxidation (Anammox). Here we operated two identical expanded granular sludge bed (EGSB) reactors at low temperature (15 ± 3 °C) for 154 days. Reactor 1 (R1) received additional Fe(II) (0.12 mmol/L) during the late startup phase, while reactor 0 (R0) served as the control and did not receive extra Fe(II). Nitrogen removal in R1 became stable at 55 d of operation, ten days earlier than R0. The nitrogen removal rate (NRR) of R1 was 1.64 kg N m−3 d−1 and its TN removal rate was as high as 89%, while R0 only reached 75%. The addition of Fe(II) was further beneficial to aggregation and stability of the granular sludge, and the used sludge of both reactors showed enrichment for AAOB populations compared to the inoculum, for instance, increased abundance of Candidatus-Kuenenia and in particular of Candidatus-Brocadia (from 0.17% to 10.10% in R0 and 7.79% in R1). Diverse microbial species and complex microbial network structure in R1 compared to R0 promoted the coupled denitrogenation by Anammox, dissimilatory nitrate reduction to ammonium (DNRA), nitrate-dependent Fe oxidation (NDFO), and ferric ammonium oxidation (Feammox). In addition, the microbial community in R1 was more resistant to short-term low temperature (2–7 °C) starvation, illustrating a further positive effect of adding Fe(II) during the startup phase of an Anammox reactor.