ABSTRACT
The recovery of partial nitritation and anammox (PN/A) process after starvation remains challenging. Substrate limitation disrupts microbial activity and delays the restoration of nitrogen removal performance. Therefore, PN/A nitrogen removal performance was evaluated in a 20‐L laboratory‐scale continuous stirred‐tank reactor (CSTR) containing sponge carriers across four operational phases. The four phases included 3‐month starvation, gradual substrate reintroduction under suspended‐growth conditions, transitional stabilization with sponge media, and attached‐growth recovery under higher nitrogen loading without external acetate supplementation. The laboratory reactor was operated at a nitrogen loading rate of 0.4–0.7 kg N/(m
3
·d), dissolved oxygen below 0.1 mg/L, and without organic carbon addition. The nitrogen removal efficiency, microbial community composition, and microbial markers of the laboratory‐scale reactor were compared with those of two well‐operated full‐scale systems in Korea and the United States. Total nitrogen removal efficiency exceeded 90% across all three systems when the ammonia‐to‐nitrite ratio (NH₄
+
:NO
2
−
) was maintained between 1:1 and 1:1.5 at low BOD/TN ratio of < 0.05. Next‐generation sequencing (Illumina MiSeq, 16S rRNA amplicon profiling at a sequencing depth of 41,120–57,097 reads/sample) was employed to determine the relative abundance of nitrogen‐transforming microbial communities, specifically the anammox group, across the three systems. The results showed that the anammox bacteria in all three systems belonged to
Candidatus
Brocadia. Following gene‐specific PCR amplification of the anammox community, denaturing gradient gel electrophoresis (DGGE) identified
Candidatus
Brocadia fulgida. This species exhibits high environmental adaptability and is commonly detected in full‐scale systems operated for > 5 years. Molecular analysis indicated that
Candidatus
Brocadia fulgida persisted in the system throughout the 3‐month starvation period, facilitating the re‐establishment of the attached‐growth PN/A process. The successful recovery of the laboratory‐scale reactor yielded nitrogen removal performance comparable to that of the full‐scale PN/A systems, demonstrating that the Brocadia group serves as a keystone taxon during process recovery.