Water Environment Research

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.

AbstractXylitol was first applied to enhance nitrogen removal from saline wastewater through “Candidatus Brocadia sinica”‐dominated anammox process under low temperature. The reactor was maintained at 15°C, and the salinity of wastewater was 35 g/L. Ammonium removal rate (ARR) and nitrite removal rate (NRR) were stable at around 0.27 kg/(m3 d) without xylitol addition. As an osmotic pressure regulator and cryoprotective agent, optimal ARR and NRR were 0.51 kg/(m3 d) and 0.63 kg/(m3 d) at 0.3 mM xylitol. At the addition of 1 mM high‐dosage xylitol, there existed dissimilatory reduction in nitrate to ammonium nitrogen and heterotrophic denitrification in the reactor. Remodified logistic model was suitable to simulate removal process with xylitol addition. As a result, xylitol dose should be controlled within 0.3 mM, which greatly promoted the nitrogen removal from saline wastewater under low temperature.Practitioner points Xylitol could be used as osmotic pressure regulator and cryoprotective agent to enhance nitrogen removal. The optimal dose was achieved at 0.3 mM xylitol for “Candidatus Brocadia sinica” in low‐temperature saline wastewater. High‐dosage xylitol could interfere with nitrogen removal efficiency due to the presence of DNAR and HB. Remodified logistic model was suitable for the analysis and prediction of nitrogen removal process with xylitol addition.

A comparison of the quantification of a specific microbial group in activated sludge by fluorescent in‐situ hybridization, coupled with either direct microscopic counting or flow cytometry, was performed using an enhanced‐biological‐phosphorus‐removal, sequencing‐batch reactor. The population dynamics of Candidatus Accumulibacter phosphatis (Cand. A. phosphatis) was evaluated during two separate runs of the reactor. With the operational conditions used, Cand. A. phosphatis was enriched until a failure in the pH controller eliminated its ecological advantage. As a result, the comparison of quantification techniques included Cand. A. phosphatis concentrations as low as 11% and as high as 96% of the total cells in the samples. The analysis demonstrated that, regardless of the particular limitations of each technique, both provided similar results when the activated‐sludge flocs were easily dispersed. However, when the activated‐sludge samples contained flocs that were difficult to disperse, flow cytometry failed to provide quantitative results.