Performance Of Anammox Process Research Articles

Inhibition, Recovery and Stabilization of MBR-Anammox Process: Enhancing Nitrogen Removal by Biochar-supported Nano Zero-valent Iron Adoption

Anaerobic ammonium oxidation (Anammox) is recognized as a promising option to energy-neutral wastewater treatment, yet difficulties encountered in practice due to its poor stability. This work investigated the effects of biochar-supported nano zero-valent iron (nZVI-BC) on nitrogen removal performance of anammox process. Three membrane bioreactors (MBRs) suitable for slowly growing anammox bacteria was operated for 100 days, undergoing startup, dissolved oxygen (DO) inhibition, performance recovery and stabilization periods. The results showed that nZVI-BC adoption could improve the nitrogen removal efficiency (NRE) by 5.5–27.3 % and 3.1–20.7 % and nitrogen removal rate (NRR) by 5.4–28.6 % and 3.2–22.9 %, compared with control group and biochar adoption, respectively. Throughout the experimental periods, nZVI-BC was conducive to enrich anammox bacteria and increased the functional gene copies associated with anammox metabolism. Meanwhile, co-occurrence network analysis promulgated that nZVI-BC boosted the robustness and interaction of microbial community. Furthermore, the electrochemical analysis of the extracellular polymeric substance (EPS) revealed that nZVI-BC adoption augmented the extracellular electron transport (EET) process of anammox biomass. Metagenomic sequencing revealed that nZVI-BC reinforced the complete dissimilatory nitrate reduction to ammonium (DNRA) pathway in Candidatus Brocadia, the dominant anammox bacteria, potentially reducing nitrate production and enhancing nitrogen removal. The study manifested the possibility of nZVI-BC to improve the anammox robustness and nitrogen removal performance, and provided operational guidance for its engineering application.

Metabolomics and molecular simulation reveal the responding mechanism of anammox consortia to perfluorooctanoic acid by regulating metabolic network

Perfluorooctanoic acid (PFOA) is ubiquitous in aquatic environment, posing serious threats to human health, while the potential of anaerobic ammonium oxidation (anammox) process in treating PFOA-containing wastewater and the response mechanism of anammox consortia to PFOA remain unknown. In this study, the comprehensive effects of PFOA on anammox granules and their interaction mechanism were investigated based on molecular simulation and metabolomic analysis. The environmental concentrations (≤2.0 mg/L) of PFOA had no impact on the anammox process performance. However, microbial consortia structure and functional gene abundances changed under the exposure of PFOA. By contrast, Prokaryotes were more sensitive to PFOA than eukaryotes. In addition, PFOA regulated microbial metabolic pathways, including amino acid metabolism and nucleotide metabolism, by changing the cell energy allocation strategy. Molecular docking simulation suggested that the effects of PFOA on key proteins in anammox bacteria were significantly different. These findings provide insights into the interaction between anammox consortia and PFOA, and further promote the implementation of anammox process to treat fluorine-containing wastewater.

Biochar-mediated Anammox granular sludge: Formation/characteristics of pitaya type granular sludge and enhanced performance of Anammox process

Traditional Anammox granular sludge (AnGS) has disadvantages such as long formation time and low structural strength. Biochars have the potential for particle formation and structural stability of AnGS. Herein, different biochars (peach, coconut, jujube, and litchi biochars) were added to four expanded granular sludge bed (EGSB) reactors to investigate changes in sludge characteristics during Anammox start-up and stable Anammox granulation. Results showed that reactors with peach and coconut biochars achieved rapid start-up of Anammox, which were 15 days faster than that of the control reactor. Scanning electron microscopy (SEM) results showed the formation of the pitaya type AnGS with biochar as the crystal nucleus. Compared with the control, pitaya type AnGS particularly enriched the large-sized particle (1.0–3.0 mm), and had a higher specific Anammox activity (SAA) at the same particle size, along with an increase in mechanical strength (by ∼13%) and the relative abundance of functional bacteria (by 28–109%).

Holdup of sub-granules towards enhanced production of anammox sludge in a dual circulation sludge bed reactor

The performance of anammox process largely depends on the anammox sludge as the functional core. Enhancing the production and retention of anammox sludge in the reaction system was a hot while pressing issue to be addressed for the application of the anammox process. In this study, a novel reactor named Ducan (Dual Circulation Anammox) was developed to achieve promoted production of anammox sludge, and the mechanism was revealed meanwhile. It was found that the Ducan reactor could improve the sludge production rate by 96.2 % compared with the control group. The enhanced formation of new-born anammox granular sludge (AnGS) by holding up the wash-out anammox sludge was believed to be the main factor contributing to this result. Furthermore, the typical structure of the new-born AnGS was characterized and the sub-granule (SG) on a 100 μm scale was revealed as its direct structural unit. Vulnerable disassembly of aged AnGS was the main source of SG due to the weak binding force of AnGS caused by the formation of large cavities inside. By setting a unique structure of dual-systolic tubes in the Ducan reactor, the SG was endowed with reduced surface roughness and increased EPS protein with hydrophobic amino acids and a looser structure. These properties made it easier for SG to contact each other and assembled into new-born AnGS with big size, which could then settle back to the reactor and result in enhanced production of anammox sludge. The results in this study were expected to offer a viable reactor towards enhanced production of AnGS, and shed light on addressing the issue of sludge shortage during the extensive application of anammox technology.

Insights into regulating influent nitrogen load to restore autotrophic nitrogen removal performance of a two-stage reactor

To better understand performance recovery and its mechanism, after the deterioration of an anaerobic ammonia oxidation (anammox) process, experiments were conducted in a UASB reactor. A two-stage anammox reaction was conducted in the reactor, and long-term continuous operation was conducted for 202 d. The mechanism of performance recovery was studied by adjusting the total nitrogen (TN) concentration and hydraulic retention time (HRT) of the influent. The results showed that after the long-term continuous operation, the TN removal efficiency decreased from 88.22 % before deterioration to 11.68 %, and then recovered to 80.63 %. The nitrogen loading rate decreased from 2.25 kg-N/m3/d before deterioration to 0.26 kg-N/m3/d and then recovered to 2.79 kg-N/m3/d. Nitrogen removal load (NRR) decreased from 1.98 kg-N/m3/d to 0.26 kg-N/m3/d before deterioration and recovered to 2.25 kg-N/m3/d. Microbial diversity was analyzed by high-throughput sequencing. During the operation, the dominant anammox strains changed from Candidatus Kuenenia (24.19 %) before deterioration to Candidatus Brocadia (11.90 %) after recovery. The influent TN concentration and HRT were the key factors that affected performance recovery. Reducing the influent TN concentration and shortening the HRT can effectively avoid the inhibition of anammox by the substrate and enable the rapid recovery of nitrogen removal performance. As the microorganisms in the reactor recovered their activity through enrichment, the influent TN gradually increased, and produced NRR higher than the pre-deterioration level, so the reactor performance was fully restored. This study provides a scientific support for recovering the two-stage anammox process performance after deterioration.

Recovery of anammox process performance after substrate inhibition: Reactor performance, sludge morphology, and microbial community

Most of the current studies have focused on the inhibition of anaerobic ammonium oxidation (anammox) by substrates, however, little attention has been paid to the recovery process of the reactor after inhibition. Therefore, we investigated the changes in reactor performance, granular sludge structure, and microbial community during the recovery phase after being inhibited by a high nitrogen load for 15 d. The nitrogen removal rate of the reactorwasrestored to pre-inhibition levels after 75 d of recovery, and the stoichiometric ratio converged to the theoretical value. The surface of the granular sludge developed into a broccoli-like structure, and the Ca and P contents of the granules increased from 6.88% and 4.39% to 24.42% and 13.88%, respectively. The abundance of the anammox bacterium Candidatus brocadia increased from 5.86% to 12.10%, and network analysis indicated that SMA102 and SBR1031 were positively correlated with the occurrence of Candidatus brocadia.

Unravelling the importance of Ca2+ and Mg2+ as essential in anammox culture medium

The mechanism and nitrogen removal performance of anammox process under different concentrations of Ca2+ and Mg2+ were explored from the perspective of molecular biology analysis based on the metabolic changes of the second messenger cyclic diguanylate (c-di-GMP). After 100-day operation, reactor with 98 mg/L Ca2+ and 30 mg/L Mg2+ achieved a higher anammox performance with an average total nitrogen (TN) removal efficiency of 85.8%. Under the Mg2+concentration of 30 mg/L, a higher Ca2+ could accelerate anammox process by promoting the amplification of Candidatus Brocadia (0.62%) and production of Diguanylate cyclase (DGC-s: 6.54 × 108 copies/μL DNA) which function was to synthesize c-di-GMP. While under the Ca2+concentration of 49 mg/L, Mg2+ concentration at appropriate rang could promote the degradation process of c-di-GMP. Since Ca2+ had positive linear relationship with TN removal (R2 = 0.96), a higher Ca2+ concentration is recommended in the culture medium. This study provided a potential method for optimization of anammox process.

Unraveling nitrogen removal and microbial response of marine anammox bacteria-dominated consortia to Mo(VI) addition in nitrogen-laden saline wastewater treatment

Enhanced performance of anammox process has attracted public attention in treating saline wastewater with marine anammox bacteria. The addition of some key metal elements would enhance metabolic activity in wastewater treatment. Herein, responses of marine anammox bacteria (MAB) to Mo(VI) addition were explored in nitrogen-laden saline wastewater treatment. The experiment was conducted at a low temperature (15 ± 1 °C). With the optimal Mo(VI) concentration of 2.1 mg/(g VSS), the substrate removal rate reached the maximum of 1.41 kg N/(m3·d). Specific anammox activity and substrate conversion rate increased by 16.0% and 36.9%, respectively. The lower ΔNO2--N/ΔNH4+-N indicated the presence of the potential electron acceptor. Moreover, the relative abundance of Cadidatus Scalindua increased by 4.18% during this experiment. Dynamic analysis showed that the remodified Logistic model was proper to analyze nitrogen removal with Mo(VI) addition. An appropriate Mo(VI) dosage (0.5–4 mg/L) would shorten the lag time of anammox process greatly.

Study on Start-up Performance of Anammox Process

Anammox, as one of the most revolutionary new denitrification technologies so far, has broad application prospects. In order to explore the start-up process of the anaerobic ammonia oxidation process, the experiment was conducted to domesticate anaerobic sludge in an up-flow anaerobic sludge bed reactor. The results showed that the reactor reached a stable operation stage from the 95th day to the 112th day. The removal rate of NH4 +-N and the removal rate of NO2 −-N were stabilized at 94.12% and 97.70%, respectively. According to the removal of NH4 +-N and NO2 −-N in the reactor, the start-up process can be divided into 4 stages, and there is denitrification and synergistic anaerobic ammonia oxidation in the reactor.

Start-Up of Anammox SBR from Non-Specific Inoculum and Process Acceleration Methods by Hydrazine

Biological nutrient removal from wastewater to reach acceptable levels is needed to protect water resources and avoid eutrophication. The start-up of an anaerobic ammonium oxidation (anammox) process from scratch was investigated in a 20 L sequence batch reactor (SBR) inoculated with a mixture of aerobic and anaerobic sludge at 30 ± 0.5 °C with a hydraulic retention time (HRT) of 2–3 days. The use of NH4Cl, NaNO2, and reject water as nitrogen sources created different salinity periods, in which the anammox process performance was assessed: low (<0.2 g of Cl−/L), high (18.2 g of Cl−/L), or optimum salinity (0.5–2 g of Cl−/L). Reject water feeding gave the optimum salinity, with an average nitrogen removal efficiency of 80%, and a TNRR of 0.08 kg N/m3/d being achieved after 193 days. The main aim was to show the effect of a hydrazine addition on the specific anammox activity (SAA) and denitrification activity in the start-up process to boost the autotrophic nitrogen removal from scratch. The effect of the anammox intermediate hydrazine addition was tested to assess its concentration effect (range of 2–12.5 mg of N2H4/L) on diminishing denitrifier activity and accelerating anammox activity at the same time. Heterotrophic denitrifiers’ activity was diminished by all hydrazine additions compared to the control; 5 mg of N2H4/L added enhanced SAA compared to the control, achieving an SAA of 0.72 (±0.01) mg N/g MLSS/h, while the test with 7.5 mg of N2H4/L reached the highest overall SAA of 0.98 (±0.09) mg N g/MLSS/h. The addition of trace amounts of hydrazine for 6 h was also able to enhance SAA after inhibition by organic carbon source sodium acetate addition at a high C/N ratio of 10/1. The start-up of anammox bacteria from the aerobic–anaerobic suspended biomass was successful, with hydrazine significantly accelerating anammox activity and decreasing denitrifier activity, making the method applicable for side-stream as well as mainstream treatment.

Enhancing nitrogen removal performance of anammox process after short-term pH and temperature shocks by coupling with iron-carbon micro-electrolysis

Abstract Nitrogen contamination is regarded as a major threat to terrestrial net productivity and environmental sustainability. Anaerobic ammonium oxidation (anammox) is a cost-effective and promising technology in wastewater treatment. In this study, the effects of pH/temperature and their short-term shocks on a hybrid system of anammox coupled with iron-carbon micro-electrolysis (IC-ME) were investigated. Results indicated that the hybrid system significantly enhanced the total nitrogen (TN) removal and expanded the stable pH operating range (TN removal efficiencies over 50%) from pH 6.5–8 to pH 5.5–8.5. Free nitrous acid (FNA) in acid environment, when its concentration was higher than 396.36 ± 5.51 μg/L, was thought to be one critical degradation inhibitor. After suffering a short-term (one day) pH shock, the hybrid system exhibited the improved performance of both stability and nitrogen removal, mainly because Fe corrosion maintained the pH value by IC-ME. More diverse nitrogen removal processes promoted by IC-ME enhanced the TN removal of hybrid after high temperature shocks. However, the susceptibility and inferior adaptability of anammox bacteria were considered to inhibit the nitrogen removal performance of the hybrid and individual anammox system. Microbial analysis identified Candidatus Kuenenia as the major anammox genus, accounting for 3.61% of the microbial community biomass, and the genus of SM1A02 might be one novel potential strain that help the anammox process.

Particle size-dependent behavior of redox-active biochar to promote anaerobic ammonium oxidation (anammox)

Nitrate (NO3−) accumulation in effluent decreases the biological nitrogen performance of anammox process for wastewater treatment. Redox-active biochar has been demonstrated as an effective additive to improve microbial NO3− reduction via redox cycling of the surface functional groups. However, their redox reactivity toward anammox consortia remains unknown. In this study, we investigated the effects of corn stover-derived biochars with different particle sizes (10–30 μm (SP), 200–500 μm (MP) and 2–5 mm (LP)) on nitrogen removal performance of anammox reactors under continuous operation with varying nitrogen loading rates over the long term. The average nitrogen removal efficiencies of anammox reactors amended with SP-, MP- and LP-biochar were 97.9%, 97.5% and 87.9%, respectively, which were significantly higher than that of the control (80.5%). Effluent NO3− concentration was reduced by 40.7–45.2% with presence of biochar as compared to the control, which may be attributed to microbial interaction with Chloroflexi-affiliated heterotrophic denitrifying bacteria. The electrochemical analysis of the sludge extracellular polymeric substances (EPS) based on cyclic voltammetry and electrochemical impedance spectrum revealed that the extent of biochar to increase the electron-transport ability of EPS decreased with the biochar particle size (SP > MP > LP). Furthermore, SP-biochar outperformed LP-biochar for promoting the proliferation of anammox bacteria Candidatus Brocadia and increasing the abundance of anammox-associated functional genes (hzsA, hdh, nirS and napA) in the sludge. Together, these findings showed the stimulatory effect of redox-active biochar for anammox consortia and the particle size-dependence, which may potentially assist in operating anammox processes with high nitrogen removal performance.

Calcium nitrate as a bio-stimulant for anaerobic ammonium oxidation process

This study explored the role of calcium nitrate as a bio-stimulant for anaerobic ammonium oxidation (anammox) process. The anaerobic sequencing batch reactor was firstly inoculated with malodorous river sediment and only fed with calcium nitrate until no marked endogenous release of ammonium in effluent (Phase 1). Subsequently, nitrite and ammonium were supplied to test the performance of anammox process (Phase 2). During the operation of Phase 1, the effluent ammonium increased firstly and then decreased. Additionally, continuous nitrite (about 1.54 mgN/L) was observed in the effluent. The microbial analysis showed the simultaneous increase of the relative abundance of heterotrophic denitrifier Denitratisoma and sulfur autotrophic denitrifier Thiobacillus from 0.15% to 5.37% and 0.21% to 4.19%, respectively. Besides, 15N isotopes trace and qPCR results showed that the contribution of anammox to total nitrogen (TN) removal increased from 3.07% to 27.6%, and that the anammox functional gene hzsB increased from 1.37 × 105 to 2.90 × 106 copies/g. These results indicated that calcium nitrate may induce partial mixotrophic denitrification (heterotrophic and sulfur autotrophic denitrification) to provide nitrite as electron acceptor for anammox, thus promoting the occurrence of anammox. In Phase 2, rapid ammonium and TN removal were accomplished in the initial operation with the reduction efficiency of 80.1% and 90.0%, respectively. The relative abundance of anammox bacteria Candidatus_Brocadia significantly increased from 0.01% to 7.15% during the operation of Phase 2. These findings further confirmed the above deduction. Taken together, calcium nitrate can be a promising bio-stimulant for anammox process by promoting the coupling of mixotrophic denitrification with anammox.

Stable nitrogen removal by anammox process after rapid temperature drops: Insights from metagenomics and metaproteomics

This study investigated the impacts of rapid temperature drops on anammox process performance and the metabolism of its core microbial populations through proteomic analysis. Over a 50-day period, the temperature of an up-flow granular bed anammox reactor was stepwise decreased from 35 °C to 15 °C and resulted in repeated transient increases in effluent nitrite concentrations. At 15 °C, a nitrogen removal rate of 2.71 ± 0.23 gN/(L·d) was maintained over 100 days operation. Total AnAOB population abundance (20.9%±4.9%) and AnAOB protein abundances (75.7% ± 3.3%) remained stable with decreased temperature. Key proteins of Ca. Brocadia for nitrogen metabolism, as well as for carbohydrate metabolism and primary metabolite biosynthesis were less expressed at 15 °C than 35 °C, while several proteins of heterotrophic Chloroflexi spp. involved in carbohydrate and metabolites metabolisms were expressed to a higher degree at 15 °C. Overall, metabolism of AnAOB responded at a higher degree to low temperatures than that of heterotrophs.

Role and application of quorum sensing in anaerobic ammonium oxidation (anammox) process: A review

Anaerobic ammonium oxidation (anammox) is a promising biological nitrogen removal process, due to its advantages of high efficiency and low cost. However, problems remain in the application, such as long startup period and susceptibility to environmental variations. Quorum sensing (QS), as a means of bacterial communication, attracts more attentions in regulating aggregation behavior and microbial density. Many physiological characteristics of anammox bacteria have been confirmed to be associated with QS, including specific anammox activity, growth rate and the production of extracellular polymeric substances (EPS), which directly affect the performance of anammox process. Therefore, a comprehensive understanding of the QS in anammox process is prerequisite. This work systematically reviewed the role and application of QS in the anammox process with the focus on mechanism. Additionally, current challenges and research needs were proposed.

Inhibition factors and kinetic model for anaerobic ammonia oxidation in a granular sludge bioreactor with Candidatus Brocadia

Varying pH and high levels of nitrite have major inhibitory effects on anaerobic ammonium oxidizing bacteria (AnAOB) and are known to interfere with the anaerobic ammonia oxidation (Anammox) process. In this study, we evaluated the effect of pH and nitrite on Anammox process performance through a predictive kinetic model. A series of batch experiments were performed in a stably operating Anammox sequencing batch reactor system with “Candidatus Brocadia” as the dominant AnAOB taxon (>95%), by varying the nitrite concentration from 40 to 378 mg/L and the pH from 6.7 and 8.5, while maintaining the NH4+-N concentration at 54.6 ± 5.6 mg/L. Dual substrate kinetics including a pH inhibition function was able to predict Anammox activity at higher NO2−-N concentrations, with a nitrite inhibition constant of 210 mg/L. A synergistic negative effect on Anammox activity was observed when combining pH and nitrite stressors. Anammox activity was completely lost at low (6.7) and high (8.5) pH conditions, but resumed after adjusting to an optimal pH (7.5 ± 0.2). We found that AnAOB bacteria are susceptible to both high nitrite concentrations and fluctuating pH levels. Based on experimental evidence and modeling, pH mediates nitrite inhibition in the Anammox process due to the accumulation of free nitrous acid. Our model can predict the outcome of different operating scenarios and help select optimal operational strategies for Anammox implementation.

Medium shift influence on nitrogen removal bacteria: Ecophysiology and anammox process performance

In this study, we focused on the proportion of particular bacterial groups and changes in microbial community structure in relation to the anammox process parameters and the feeding medium strategy in the Sequencing Batch Reactor (SBR). In order to present an insight into the microbial dynamics while feeding medium shift from synthetic wastewater to landfill leachate, fluorescent in situ hybridization (FISH), Real Time PCR, PCR – DGGE (Polymerase Chain Reaction–Denaturing Gradient Gel Electrophoresis) and Reverse Transcription PCR-DGGE analysis were used. Feeding medium change has the strongest impact on relative abundance of denitrifiers and representatives of Planctomycetes. The relative abundancy of specific genes for all investigated nitrogen removal bacterial groups dropped after landfill leachate implementation. However, anammox consortium were able to adapt to the new reactor operating conditions and time for adaptation was estimated at the level of 90 days.

The performance and microbial community in response to MnO2 nanoparticles in anammox granular sludge

The response of anaerobic ammonium oxidation (anammox) process and granular sludge in the upflow anaerobic sludge blanket reactor was analyzed under long-term exposure to MnO2 nanoparticles (NPs) (1–200 mg L−1). The addition of 200 mg L−1 MnO2 NPs had a significantly positive effect on nitrogen removal and this system exhibited excellent performance, with a total nitrogen removal efficiency of 93.1%. Moreover, the specific anammox activity enhanced with increasing concentrations of MnO2 NPs up to the maximum value of 657.3 ± 9.3 mg TN g−1 VSS d−1 under MnO2 NPs concentration of 200 mg L−1. This value was approximately 1.6-fold higher than that of the reactor in the absence of MnO2 NPs. The extracellular polymeric substances and settling velocity were both increased with MnO2 NPs addition. Meanwhile, the high-throughput sequencing results revealed that MnO2 NPs increased the relative abundance of dominant bacteria (Candidatus Kuenenia) from 17.3% at the absence of MnO2 NPs to 23.9% at 200 mg L−1 MnO2 NPs, which resulted in a higher efficiency of biological nitrogen removal on the anammox system. These results indicated that MnO2 NPs enhanced nitrogen removal performance of anammox process.

Start-up and operational performance of Anammox process in an anaerobic baffled biofilm reactor (ABBR) at a moderate temperature

A lab-scale anaerobic baffled biofilm reactor (ABBR) was used as a novel reactor to start up Anammox process at a moderate temperature around 20 °C and an innovative filling module was adopted as support material. Quick start-up of Anammox process from the aerobic activated sludge was achieved after 47 days operation. The max nitrogen loading rate and nitrogen removing rate attained 1.00 kg N m−3 d−1 and 0.90 kg N m−3 d−1 after 161 days operation. Scanning electron microscope photographs showed that the structure as well as the states of the micro-aggregates (micro-aggregates sticking on a non-woven fiber, entangling non-woven fibers and enwrapped by non-woven fibers) enhanced biomass retention for Anammox bacteria. Microbial community analysis showed that Anammox bacteria were effectively enriched with Candidatus Brocadia, Candidatus Jettenia and Candidatus Kuenenia being the main Anammox species in the mature biofilms. This contributed to the excellent Anammox operation performance at the moderate temperature.

Coupling of partial nitritation and anammox in two- and one-stage systems: Process operation, N2O emission and microbial community

Advances in research and technological development in the field of wastewater treatment encourage the implementation of engineered autotrophic nitrogen removal (ANR) systems based on the coupling of partial nitritation (PN) and anaerobic ammonium oxidation (anammox). Such processes can be conducted in two independent dedicated reactors (i.e., two-stage system) or, alternatively, in the same reactor under limited aeration (i.e., one-stage system). In this investigation, both configurations were successfully tested using the sequencing batch reactor (SBR) technology. Processed wastewater was supernatant from a sewage sludge anaerobic digester containing about 1 g NH4+-N/L and 0.3 g PO4-P/L. Pre-conditioning of the supernatant through dilution and magnesium phosphates (e.g., struvite) precipitation favored the anammox process performance under both configurations. The N loading rate (NLR) applied in the PN reactor was ≤1.3 g N/(L·d) with nitrite production efficiencies of about 48%, whereas the N removal rate (NRR) in the anammox reactor was 0.43–0.56 g N/(L·d). On the other hand, in the one-stage reactor, the NRR was approximately 0.27 g N/(L·d). Estimated emissions of nitrous oxide (N2O) in such bioreactors ranged from 0.4 to 4.6% of the N loaded. Comparatively, similar NRRs were achieved for both reactor configurations but the one-stage system used a smaller reaction volume, ran at a lower NLR, and emitted N2O at a lower rate than the two-stage system. The microbial community in both systems was dominated by aerobic ammonium-oxidizing bacteria of the genus Nitrosomonas and the anammox species Ca. Brocadia sinica.