Dissimilar iron reduction coupled with anaerobic oxidation (Feammox) is a common pathway to oxidize ammonia nitrogen in natural environment, which however has been rarely reported in sewage treatment. In this study, the sludge digester liquor was used as the NH₄⁺-N source to study the NH₄⁺-N removal during Feammox driven by adding ferrihydrite under anaerobic conditions. The results showed that after 48 days of anaerobic incubation, the NH₄⁺-N removal efficiency and total nitrogen removal efficiency were 72.4% and 57.5%, respectively, while the NH₄⁺-N removal efficiency and total nitrogen removal efficiency in the reactor without adding ferrihydrite were 7.6% and 2.5%, respectively. The continuous production of Fe(Ⅱ) was detected in the ferrihydrite-added reactor. After feeding with isotope-labeled ¹⁵NH₄⁺-N as the NH₄⁺-N source, ³⁰N₂ was detected in the headspace of the ferrihydrite-added reactor, which proved the occurrence of Feammox. Real-time quantitative PCR(qPCR) results showed that Acidimicrobiaceae A6 expressed actively. High-throughput 16S rDNA showed that a variety of iron-reducing bacteria were significantly enriched. After simultaneously adding ferrihydrite and humus into the reactor, the removal efficiency of NH₄⁺-N further increased to 88.35%, and the removal efficiency of total nitrogen reached 73.75%. The analysis of the composition and morphology of iron oxides added in the reactors showed that humus weakened the passivation rate of iron oxides, which was beneficial for ferrihydrite to maintain activity and continue to participate in Feammox.