Startup and oxygen concentration effects in a continuous granular mixed flow autotrophic nitrogen removal reactor

A slow-release oxygen composite based on sulfur/CaO2 for sustained in-situ ammonia degradation form farmland drainage

The in-situ nitrification process continuously requires a stable supply of oxygen. However, the application of conventional oxygen-releasing materials is limited by its high alkalinity and rapid oxygen release rates. In this study, a novel sulfur-based slow-release oxygen material (SOSM) was designed to address these challenges. SOSM releases oxygen through the decomposition of CaO2 and maintains pH balance with sulfur (S0). An 88-day continuous flow experiment for microbial degradation of ammonia nitrogen (NH4+-N) was conducted with SOSM as a carrier. The results showed that the dissolved oxygen (DO) remained above 8 mg/L during 15 d, with oxygen being released following Fickian diffusion. S0 is oxidized by sulfur bacteria, forming a CaSO4 precipitate within the material, while hydrogen ions (H+) are generated to counteract the alkalinity caused by CaO2. The continuous flow experiments indicated that nitrification with SOSM occurred in three distinct phases with the following NH4+-N removal efficiency: the domestication phase (1-21 d, 86.2%), the stabilization phase (22-76 d, 93.4%), and the deterioration phase (77-88 d, 60.5%). The enrichment of Proteobacteria and Actinobacterota promoted NH4+-N removal when oxygen was abundant, while the enrichment of Acidobacteriota facilitated active sulfur cycling. The system was dominated by nitrification and supplemented by the complete autotrophic nitrifying anaerobic ammonia oxidation (CANON) process. The SOSM developed in this study can effectively address the critical issues of in-situ agricultural drainage remediation and expand the application scope of nitrification technology. It offers a novel approach to biological nitrogen removal treatment technology, especially nitrification technology to remove NH4+.

Upflow blanket filter anammox (UBFA) system treating low-nitrogen wastewater: high-efficient nitrogen removal, granules formation, N2O emission, and microbial succession

This research provides an important approach for low-nitrogen wastewater treatment through anaerobic ammonium oxidation (Anammox), and Anammox granule sludge (AnGS) in the Upflow. Blanket Filter Anammox (UBFA) system through shortening the hydraulic retention time was successfully cultivated. The percentage of medium granules (1.0-2.0 mm) with the highest Anammox activity increased from 0 to 28.5%, and the proportion of flocs (0-200 μm) reduced from 84.5% to 17.6%. Through the multidimensional analysis of AnGS, the relationship between AnGS and EPS secretion, low SVI, high PN/PS, multiple filamentous bacteria, and AnAOB were explored. Microelectrode tracing tests demonstrated that the main anammox reaction active layer was 0-1500 μm, and the highest activity was observed at 200-400 μm, whereas denitrification activity and N2O production were mainly distributed in the granules deep layer of 1500-2500 μm. The research showed that Candidatus Brocadia and Candidatus Kuenenia were the predominant anammox species in the UBFA system, while the abundance of AnAOB was higher in medium granules.

Metatranscriptomics provides an in-depth perspective on the resistance and detoxification of anammox bacteria to dissolved oxygen in a pilot CANON process

In the completely autotrophic nitrogen removal over nitrite (CANON) process, the conflicting oxygen requirements of anammox and ammonium-oxidizing bacteria often lead to retardation in anammox activity. However, our study achieved stable nitrogen removal with a maximum capacity of 1096 g-N/m3/d in a 20 m3 CANON reactor under long-term intensive aeration. The anammox bacteria unusually distributed in the outer layer of the biofilm and demonstrated remarkable oxygen tolerance. Their activity only declined by 18.5 % under 2.0 mg/L of dissolved oxygen. When anammox bacteria encountered oxygen exposure, they adopted some strategies. Metatranscriptomics revealed that Candidatus Kuenenia, the dominant anammox species in our system, downregulated its gene expressions involved in carbon metabolism and oxidative phosphorylation. This may reduce electron leakage that combines with O2, thereby minimizing the generation of reactive oxygen species (ROS). By contrast, the secretion of extracellular proteins and conversion of O2·- were upregulated to eliminate ROS promptly. This behavior endowed Ca. Kuenenia with a unique oxygen detoxification pathway: O2·- were initially converted to H2O2 by superoxide dismutase SOD2 and superoxide reductase dfx (major role), followed by reduction to H2O via non-heme chloroperoxidase cpo (a newly recognized mechanism in the oxygen detoxification of anammox) and catalase katE. These results expanded the current knowledge of anammox alleviating oxidative stress.

Shift in microorganism and functional gene abundance during completely autotrophic nitrogen removal over nitrite (CANON) process

Nitrification-denitrification process has failed to meet wastewater treatment standards. The completely autotrophic nitrite removal (CANON) process has a huge advantage in the field of low carbon/nitrogen wastewater nitrogen removal. However, slow start-up and system instability limit its applications. In this study, the time of the start-up CANON process was reduced by using bio-rope as loading materials. The establishing of graded dissolved oxygen improved the stability of the CANON process and enhanced the stratification effect between functional microorganisms. Microbial community structure and the abundance of nitrogen removal functional genes are also analyzed. The results showed that the CANON process was initiated within 75 days in the complete absence of anaerobic ammonium oxidizing bacteria (AnAOB) inoculation. The ammonium and nitrogen removal efficiencies of CANON process reached to 94.45% and 80.76% respectively. The results also showed that the relative abundance of nitrogen removal bacterial in the biofilm gradually increases with the dissolved oxygen content in the solution decreases. In contrast, the relative abundance of ammonia oxidizing bacteria was positively correlated with the dissolved oxygen content in the solution. The relative abundance of g__Candidatus_Brocadia in biofilm was 15.56%, and while g__Nitrosomonas was just 0.6613%. Metagenomic analysis showed that g__Candidatus_Brocadia also contributes 66.37% to the partial-nitrification functional gene Hao (K10535). This study presented a new idea for the cooperation between partial-nitrification and anammox, which improved the nitrogen removal system stability.

Microbial community evolution and individual-based model validation of biofilms in single-stage partial nitrification/anammox system

In this study, matrix degradation, microbial community development, and distribution using an individual-based model during biofilm formation on carriers at varying depths within a single-stage partial nitrification/anammox system were simulated. The findings from the application of individual-based model fitting, fluorescence in situ hybridization, and high-throughput sequencing reveal the presence of aerobic bacteria, specifically ammonia-oxidizing bacteria, as discrete particles within the outer layer of the carrier. Facultative anaerobic bacteria exemplified by anaerobic ammonia-oxidizing bacteria, are observed as aggregates within the middle layer. Conversely, anaerobic bacteria, represented by denitrifiers, are enveloped by extracellular polymeric substances within the inner layer. The present study extends the application of individual-based model to the formation of polyurethane-supported biofilms and presents valuable avenues for the design and advancement of pragmatic engineering carriers.

Insights into the rapid start-up of an inoculated municipal sludge system: A high-height-to-diameter-ratio airlift inner-circulation partition bioreactor based on CFD analysis

The utilization of municipal sludge as a seed sludge for initiating the autotrophic nitrogen removal (ANR) process presents a challenge due to the negligible abundance of anaerobic ammonia-oxidizing bacteria (AnAOB). Here, a computational fluid dynamics model was used to simulate sludge volume fraction and sludge particle velocity. A high-height-to-diameter-ratio airlift inner-circulation partition bioreactor (HHAIPBR) was operated for 175 d to enrich AnAOB from municipal sludge, and the performance of the ANR process was investigated. The start-up period of HHAIPBR inoculated with municipal sludge required approximately 69 d. A high nitrogen removal performance, with a mean total nitrogen removal efficiency of 82.1%, was obtained for 1 month. The simulation results validated the presence of sludge circulation and revealed the distribution characteristics of dissolved oxygen inside the reactor, further supporting the promotion of sludge granulation via the high height-to-diameter ratio. Nitrosomonas (3.31%) of Proteobacteria and Candidatus Brocadia (6.56%) of Planctomycetota were dominant in the HHAIPBR. This study presents a viable approach for the industrial cultivation of anammox sludge and the rapid start-up of the partial nitritation-anammox system.

Simultaneous partial nitrification, Anammox and nitrate-dependent Fe(II) oxidation (NDFO) for total nitrogen removal under limited dissolved oxygen and completely autotrophic conditions

Sustainable nitrogen removal from wastewater at reduced energy and/or chemical consumptions is challenging. This paper investigated, for the first time, the feasibility of coupled partial nitrification, Anammox and nitrate-dependent Fe(II) oxidation (NDFO) for sustainable autotrophic nitrogen removal. With NH₄⁺-N as the only nitrogen-containing compound in the influent, near-complete nitrogen removal (a total of 97.5 % with a maximal total nitrogen removal rate of 6.64 ± 2.68 mgN/L/d) was achieved in a sequencing batch reactor for a 203-d operation without organic carbon source addition and forced aeration. Anammox (predominated by Candidatus Brocadia) and NDFO bacteria (such as Denitratisoma) were successfully enriched, with total relative abundances up to 11.54 % and 10.19 %, respectively. Dissolved oxygen (DO) concentration was a key factor affecting the coupling of multi (ammonia oxidization, Anammox, NDFO, iron-reduction, etc.) bacterial communities, resulting in different total nitrogen removal efficiencies and rates. In batch tests, the optimal DO concentration was 0.50-0.68 mg/L with a maximal total nitrogen removal efficiency of 98.7 %. Fe(II) in the sludge not only competed with nitrite oxidizing bacteria for DO to prevent complete nitrification, but promoted the transcription of NarG and NirK genes (10.5 and 3.5 times higher than the group without Fe(II) addition) as indicated by the reverse transcription quantitative polymerase chain reaction (RT-qPCR), resulting in increased NDFO rate (by 2.7 times) and promoted NO₂^--N generated from NO₃^--N, which back fed the Anammox process, achieving near-complete nitrogen removal. The reduction of Fe(III) by iron-reducing bacteria (IRB) and hydrolytic and fermentative anaerobes enabled a sustainable Fe(II)/Fe(III) recycling, avoiding the need in continuous Fe(II) or Fe (III) dosage. The coupled system is expected to benefit the development of novel autotrophic nitrogen removal processes with neglectable energy and material consumptions for the treatment of wastewater with low organic carbon and NH₄⁺-N contents in underdeveloped regions, such as decentralized rural wastewaters.

Challenges of aerobic granular sludge utilization: Fast start-up strategies and cationic pollutant removal

Aerobic granular sludge (AGS) is a self-aggregated microorganism consortium with pollutant removal properties. The aim of this work is to study and review the application of aerobic granules for water treatment with special focus on new applications and methodologies. Carbon-nitrogen containing pollutants are the classic targets of AGS technology. Carbon and nitrogen removal of AGS are classified as a biodegradation process. More recently, the AGS granules have been studied as sorbent materials for wastewater treatment. In particular, the sorption of cationic pollutants has been studied through biosorption and bioaccumulation mechanisms without distinguishing when one or the other process is involved. AGS conformation made them suitable for complex wastewater treatment. Indeed, several studies have demonstrated the removal of polyvalent cationic pollutants even with higher capacity than conventional sorbent materials. However, this was achieved almost exclusively for synthetic substrates, with single cation evaluation and using in some cases only qualitative measures. For successful industrial AGS application in complex substrates, it is necessary to evaluate and demonstrate the technology in real industrial conditions and reduce the currently long start-up times which limits its utility. Two new strategies have been proposed: autoinducer molecules and the production of artificial granular from common active sludge with commercial alginate. Finally, the increase of research on AGS cations assimilation properties will allow a new point of view, where granules will be materials for the recovery of valuable metals from industrial wastewater streams.

Achieving mainstream anammox in biological aerated filter by regulating bacteria community structure

Although mainstream partial nitrification-anammox (PN-A) is a highly efficient and sustainable wastewater treatment process, it is difficult to achieve and stabilize due to the competition among functional bacteria. In this study, achieving one-stage mainstream anammox via regulating bacteria community structure was studied in a lab-scale biological aerated filter (BAF). The results showed that high free ammonia with 89.57 mg/L, nitrite nitrogen (NO₂^--N) competition between anammox bacteria (AnAOB) and nitrite oxidizing bacteria (NOB), and backwash regulated the bacteria community structure. After backwash, Candidatus Kuenenia became the dominant bacteria and the relative abundance increased to 5.56 %. In BAF, one-stage mainstream anammox with total nitrogen (TN) being lower than 15 mg/L in the effluent was achieved using lag-time of bacteria activity recovery caused by alternating operation of high and low ammonia nitrogen (NH₄⁺-N), which have great potential applied in municipal wastewater treatment plants (MWWTPs).

A quantified nitrogen metabolic network by reaction kinetics and mathematical model in a single-stage microaerobic system treating low COD/TN wastewater

A single-stage intermittent aeration microaerobic reactor (IAMR) has been developed for the cost-effective nitrogen removal from piggery wastewater with a low ratio of chemical oxygen demand (COD) to total nitrogen (TN). In this study, a quantified nitrogen metabolic network was constructed based on the metagenomics, reaction kinetics and mathematical model to provide a revealing insight into the nitrogen removal mechanism in the IAMR. Metagenomics revealed that a complex nitrogen metabolic network, including aerobic ammonia and nitrite oxidation, anammox, denitrification via nitrate and nitrite, and nitrate respiration, existed in the IAMR. A novel method for solving kinetic parameters with high stability was developed based on a genetic algorithm. Use this method to calculate the kinetics of various reactions involved in nitrogen metabolism. Kinetics revealed that simultaneous partial nitritation-anammox (PN/A) and partial denitrification-anammox (PDN/A) were the dominant approaches to nitrogen removal in the IAMR. Finally, a kinetics-based model was proposed for quantitatively describing the nitrogen metabolic network under the limitation of COD. 58% ∼ 67% of nitrogen was removed via the anammox-based processes (PN/A and PDN/A), but only 7% ∼ 12% and 1% ∼ 2% of nitrogen were removed via heterotrophic denitrification of nitrite and nitrate, respectively. The half-inhibition constant of dissolved oxygen (DO) on anammox was simulated as 0.37 ∼ 0.60 mg L^-1, filling the gap in quantifying DO inhibition on anammox. High-frequency intermittent aeration was identified as the crucial measure to suppress nitrite-oxidizing bacteria, although it has a high affinity for DO and NO₂^--N. In continuous aeration mode, the simulated NO₃^--N in the IAMR would rise by 39.6%. The research provides a novel insight into the nitrogen removal mechanism in single-stage microaerobic systems and provides a reliable approach to practicing PN/A and PDN/A for cost-effective nitrogen removal.

Dual inner circulation and multi-partition driving single-stage autotrophic nitrogen removal in a bioreactor

The single-stage autotrophic nitrogen removal (ANR) process is impeded by a long start-up cycle and unstable operation performance. In this study, an airlift inner-circulation partition bioreactor (AIPBR) was operated continuously for 215 days to explore methods of strengthening the performance and stable operation of the single-stage ANR system. AIPBR start-up period took around 38 days, the total nitrogen removal efficiency was > 85% on day 35. With the decrease of hydraulic retention time and the increase of aeration rate, the nitrogen removal rate increased to 0.85 ± 0.02 kg-N/m³/day. The sludge morphology gradually changed into dark-red floc-coupled granular sludge. Nitrosomonas (9.95%) and Candidatus Brocadia (6.41%) were dominant in the sludge. During long-term operation, AIPBR achieved the dual inner circulation of sewage and sludge and then formed effective dissolved oxygen and sludge partitions to provide a suitable growth environment for various functional bacteria, promote synergy between them, and strengthen the ANR performance.

Evaluation of the joint effects of Cu2+, Zn2+ and Mn2+ on completely autotrophic nitrogen-removal over nitrite (CANON) process

The completely autotrophic nitrogen-removal over nitrite (CANON) process has merits in energy saving and consumption reducing, thus being considered as an attractive alternative over the common denitrification technology. In this study, the effects of three common heavy metals (Cu²⁺, Zn²⁺ and Mn²⁺) in wastewater to the CANON process were evaluated comprehensively. A central composite design with response surface methodology was utilized to investigate the joint effect of these three metal ions on the nitrogen removal performance of CANON process. In accordance with the determined optimal dosage in batch tests, four bioreactors were established with different amounts of heavy metal dosage in long-term operation, which determined the optimal concentrations for Cu²⁺, Zn²⁺ and Mn²⁺ to be 0.25, 0.81 and 1.00 mg/L, respectively. However, the optimal dosing level determined in batch tests showed no promotion during long-term experiment. This indicated that the actual concentration of heavy metals in bioreactors during long-term operation could be higher than expectation, leading to the difference between short-term tests and long-term experiment. The distribution of metal ions revealed that Mn²⁺ was mainly absorbed in anammox bacteria cells while Cu²⁺ and Zn²⁺ were mostly identified inside AOB cells. Moreover, the addition of heavy metals consistently showed positive effects for the relative abundance of AOB, while only a low level of dosage could promote the abundance of anammox bacteria. Furthermore, a mathematical model was established to simulate the CANON system considering the impacts of heavy metals, which was calibrated and validated using independent dataset in this study.

High-rate nitrogen removal in a continuous biofilter anammox reactor for treating low-concentration nitrogen wastewater at moderate temperature

The high-rate nitrogen removal in a continuous biofilter anammox reactor (CBAR) was investigated to treat low-concentration nitrogen wastewater. Shortening hydraulic retention time (HRT) gradually could restart CBAR and accumulate anammox bacteria effectively in the reactor, where the carmine anammox granular sludge and biofilm were coexisted well. It spent 21 days to restart CBAR completely after it had been idle for 116 days. Meanwhile, the total nitrogen removal rate remained stable at 86.42% accompanied with a total biomass concentration of 26.02 g-SS/L in 0 ~ 20 cm zone under nitrogen loading rate of 4.25 ± 0.10 kg-N/(m³·day), HRT of 20 min and 25 ℃. In addition, the specific anammox activity of biomass exceeded 0.28 g-N/(g-VSS·day) in 0 ~ 20 cm zone, which was related with a high relative abundance of Candidatus Brocadia (>30%) in the same zone. Thus, it is a feasible approach to adopt CBAR to treat low-concentration nitrogen wastewater efficiently.

One-stage partial nitrification and anammox process in a sequencing batch biofilm reactor: Start-up, nitrogen removal performance and bacterial community dynamics in response to temperature

A one-stage partial nitrification and anammox (PN/A) process was started up and operated under varying temperatures in a lab-scale sequencing batch biofilm reactor. The start‑up phase took 110 days with an intermittent aeration strategy, and the removal efficiencies of ammonia‑nitrogen and total nitrogen were found to be 92.22% and 76.07%, respectively. The total nitrogen removal efficiency (NRE) increased by 9.49% when temperature decreased from 30 °C to 25 °C, but declined by 83.84% from 25 °C to 20 °C. The PN process was inhibited and subsequently limited the nitrogen removal performance at 20 °C. When temperature returned to 28 °C, the NRE recovered to 67.27%, but it was still lower than the value before the decrease in temperature (79.40%). Microbial community analysis showed that the predominant ammonia oxidation bacteria and anammox bacteria were Nitrosomonas and Candidatus Kuenenia, respectively. Nitrosomonas grew, while the relative abundance of Candidatus Kuenenia increased as temperature decreased and vice versa.

Unveiling performance stability and its recovery mechanisms of one-stage partial nitritation-anammox process with airlift enhanced micro-granules

The performance stability and its recovery mechanisms of a partial nitritation-anammox process were investigated. A one-stage airlift enhanced micro-granules (AEM) system was operated for 650 days continuously to treat 50 mg-NH₄/L wastewater. During the stable stage, a high nitrogen removal efficiency of 72.7 ± 8.4% lasting for 230 days was successfully achieved under 0.28 L/min aeration rate and 0.10-0.20 mg/L dissolved oxygen (DO) concentration. A microbial consortium with good granularity appeared in red. The specific activity of anammox and ammonia oxidation increased to 1.02 and 0.93 g-N/g-VSS/d, respectively. Meanwhile, the microbial analysis showed the AEM system shifted the dominant microflora from Proteobacteria to Planctomycetes in which Candidatus Brocadia abundance reached a high of 35.0%. The results reveal that the long-term airlift-aeration promoted granulation and further enhanced activities, the abundances of anammox bacteria, and suppressed nitrite-oxidizing bacteria. Optimizing the DO control is also critical for stability increment and process recovery.

The prediction of partial-nitrification-anammox performance in real industrial wastewater based on granular size

To date, the partial nitrification-Anammox (PN-A) granular sludge size has been exclusively analyzed in synthetic substrates. In this work, different ranges of granular size of PN-A sludge were studied at low oxygen concentration using real industrial wastewater as, well as a synthetic substrate. The granular sludge was characterized by the specific nitrification activity (SNA), specific anammox activity (SAA), and granule sedimentation rate. The relative abundance of the bacterial consortium was assessed for each range of diameters through the fluorescence in situ hybridization (FISH) technique. SNA exhibits a direct association with the specific surface of granules, which proves the importance of the outer layer in the nitrification process. Even more critical, the flocculent sludge allowed the stability of the nitrifying activity. The SAA showed different performances faced the real industrial and synthetic substrates. With the synthetic substrate, the SAA decreased at higher diameter ranges, whereas with the industrial substrate, the SAA increased at higher diameter ranges. This situation is explained by the oxygen protection in the sludge maintained with industrial wastewater. The relative abundance of heterotrophic bacteria increased from 9.6 to 22%, due to the presence of organic matter in the industrial substrate. The granular sedimentation rate increased with the diameter of the granules with a linear correlation (R² > 0.98). Thus, granular sizes can be selected through sedimentation rate control. A linear correlation between SAA and granular sludge diameter ranges was observed. With this correlation, an error of less than 11% in the prediction of SAA was achieved. The use of diameter measurement and granular sedimentation rate as routine techniques could contribute to the control and start-up of PN-A reactors. In the same sense, organic matter present in defined concentrations, can be beneficial for the granular sludge stability, and thus, for nitrogen removal.

Key factors governing the performance and microbial community of one-stage partial nitritation and anammox system with bio-carriers and airlift circulation

A one-stage airlift internal circulation biofilm reactor was continuously operated for 668 days to treat 50 mg/L of ammonia wastewater to pursue the long-term stability of partial nitritation and anammox (PNA) process. The operational performance and microbial community structure of the biofilm and the flocs were investigated. A nitrogen removal efficiency (NRE) of 70% was obtained successfully at a dissolved oxygen (DO) of 0.05-0.15 mg/L by regulating aeration rate. The microbial analysis indicated Candidatus Brocadia (29.5%) and Nitrosomonas (6.8%) were dominant in both biofilms and flocs. It was found that DO control and aeration rate were the key factors in performance stability, and a stable performance could be recovered and maintained under oxygen-limiting conditions. Further, the achievement of activated ammonia oxidation bacteria (AOB), dominated anammox bacteria (AMX), suppressed NOB, and controlled heterotrophic bacteria (HB) in the biofilms played a major role in the long-term stable operation.

Sludge ratio affects the start-up performance and functional bacteria distribution of a hybrid CANON system

To investigate the effect of sludge ratio on the hybrid CANON system, autotrophic nitrogen removal sludge was inoculated with different granule/floc ratios to initiate the CANON system, and maintained the sludge ratio during the operation process. The start-up performances were compared, and the distribution characteristics of functional bacteria were investigated. The results show that the Equivalent system (granules:flocs = 1:1-1:1.5) successfully started-up on day 19, and the nitrogen removal rate (NRR) reached 0.299 kgN m^-3·d^-1 on day 63. At the same time, it was less affected by the load shock than High-granules and High-flocs systems. Therefore, the Equivalent system had the strongest start-up performance. The activities of the functional bacteria conformed to spatial heterogeneity, unlike the abundance. With the increased floc proportion, the difference in the activity and abundance of anaerobic ammonium-oxidizing bacteria (AAOB) between the granules and flocs increased, while there was a decrease in the difference in aerobic ammonium-oxidizing bacteria (AOB). However, the abundance of Nitrosomonas in the granules was higher than in the flocs when the proportion of flocs was higher than 50%. This study provides new ideas and insights for the fast start-up of the CANON system and can conform to the varying needs of engineering applications.

Conversion of full nitritation to partial nitritation/anammox in a continuous granular reactor for low-strength ammonium wastewater treatment at 20 °C

The feasibility of converting full nitritation to partial nitritation/anammox (PN/A) at ambient temperature (20 °C) was investigated in a continuous granular reactor. The process was conducted without anammox bacteria inoculation for the treatment of 70 mg L^-1 of low-strength ammonium nitrogen wastewater. Following the stepwise increase of the nitrogen loading rate from 0.84 to 1.30 kg N m^-3 d^-1 in 320 days of operation, the removal efficiency of total inorganic nitrogen (TIN) exceeded 80% under oxygen-limiting conditions. The mature PN/A granules, which had a compact structure and abundant biomass, exhibited a specific TIN removal rate of 0.11 g N g^-1 VSS d^-1 and a settling velocity of 70.2 m h^-1. This was comparable with that obtained at above 30 °C in previous reports. High-throughput pyrosequencing results revealed that the co-enrichment of aerobic and anaerobic ammonium-oxidizing bacteria identified as genera Nitrosomonas and Candidatus Kuenenia, which prompted a hybrid competition for oxygen and nitrite with nitrite-oxidizing bacteria (NOB). However, the overgrowth of novel NOB Candidatus Nitrotoga adapted to low temperatures and low nitrite concentration could potentially deteriorate the one-stage PN/A process by exhausting residual bulk ammonium under long-term excessive aeration.

Exploring the optimized strategy in the nitritation-anammox biofilm process for treating low ammonium wastewater

The main challenge for achieving the simultaneous nitritation, anammox and denitrification (SNAD) process is to optimize the concentrations of nitrite and dissolved oxygen (DO). This study explored the performance of SNAD biofilm reactor under three operational strategies. At Stage 1, 2 and 3, the average concentrations of DO were 0.7, 2.7 and 5.2 mg/L, respectively. The peak concentrations of NO₂^--N in the sequencing batch reactor (SBR) cycle were 5.3, 6.0 and 2.7 mg/L, respectively. The average removal rates of total inorganic nitrogen (TIN) were 0.30, 0.42 and 0.22 kg N/m³/d, respectively. Protein (PN) was the dominant extracellular polymeric substance (EPS) content on the SNAD biofilm. The PN concentration remained stable while the polysaccharide (PS) concentration changed rapidly under different operational strategies. High-throughput sequencing analysis indicated that high DO and long aeration period condition could lead to a slight decrease in the abundances of denitrifying bacteria and anammox bacteria.

Optimization of the intermittent aeration to improve the stability and flexibility of a mainstream hybrid partial nitrification-anammox system

Intermittent aeration is favorable for maintaining a long-term sewage partial nitrification-anammox (PN/A) process but the underlying mechanism is not yet fully understood. In this study, mainstream PN/A was established in an integrated fixed film activated sludge (IFAS) PN/A reactor and nitrite oxidization bacteria (NOB) activity was continuously suppressed. The suppression of NOB was significantly affected by the dissolved oxygen (DO) concentration during the aeration period as well as the duration of anoxic period. NOB was more suppressed in the hybrid system under a low DO level (0.5 mg/L) than under a high DO level (1.5-1.8 mg/L). Meanwhile, shortening the anoxic time from 40 to 20 min and keeping low DO during the intermittent aeration cycle could still suppress NOB activity, increasing the nitrogen removal rate by 40%. Biomass segregation was also enhanced by low DO, which favors the NOB inhibition in IFAS PN/A system. Overall, under an optimized intermittent aeration, a stable and high nitrogen removal efficiency (80-89%) with a nitrogen removal rate of 0.101 kg-N/(m³·d). This study is useful to supports the application of PN/A in sewage treatment.

Achieving simultaneous nitrogen and antibiotic removal in one-stage partial nitritation-Anammox (PN/A) process

Partial nitritation-Anammox (PN/A) process has been recognized as a sustainable process for biological nitrogen removal. Although various antibiotics have been ubiquitously detected in influent of wastewater treatment plants, little is known whether functional microorganisms in the PN/A process are capable of biodegrading antibiotics. This study aimed to investigate simultaneous nitrogen and antibiotic removal in a lab-scale one-stage PN/A system treating synthetic wastewater containing a widely-used antibiotic, sulfadiazine (SDZ). Results showed that maximum total nitrogen (TN) removal efficiency of 86.1% and SDZ removal efficiency of 95.1% could be achieved when treating 5 mg/L SDZ under DO conditions of 0.5-0.6 mg/L. Compared to anammox bacteria, ammonia-oxidizing bacteria (AOB) made a major contribution to SDZ degradation through their cometabolic pathway. A strong correlation between amoA gene and SDZ removal efficiency was found (p < 0.01). In addition, the degradation products of SDZ did not exhibit any inhibitory effects on Escherichia coli. The findings suggest that it is promising to apply the PN/A process to simultaneously remove antibiotics and nitrogen from contaminated wastewater.

Start-up of single-stage partial nitritation-anammox micro-granules system: Performance and microbial community dynamics

By manipulating influent nitrogen load and DO concentration in bulk liquid, the start-up and performance of a new micro-granule based partial nitritation-anammox process was investigated in a continuous stirred tank reactor (CSTR). Under the condition of nitrogen loadings from 0.3 to 1.4 kgN /m³/d and DO <0.21mg/L, the single-stage partial nitritation-anammox (SPNA) system was successfully started, with a nitrogen removal of 76.2%. Meanwhile, the oxygen utilization efficiency by ammonium oxidizing bacteria (AOB) increased in the system with the increase of influent ammonia loading rate. Micro-granules with an average diameter of 0.25 mm were formed. Sludge granulation was promoted by increasing influent nitrogen load, and there was a positive correlation between nitrogen load, extracellular polymeric substances (EPS) content and sludge particle size. Ca. Kuenenia became the dominant anaerobic ammonium oxidizing bacteria (AnAOB) in the SPNA system. As the dominant AOB genera, Nitrosomonas coexist with Ca. Kuenenia in the micro-granules.

The occurrence and role of Nitrospira in nitrogen removal systems

Application of the modern microbial techniques changed the paradigm about the microorganisms performing nitrification. Numerous investigations recognized representatives of the genus Nitrospira as a key and predominant nitrite-oxidizing bacteria in biological nutrient removal systems, especially under low dissolved oxygen and substrate conditions. The recent discovery of Nitrospira capable of performing complete ammonia oxidation (comammox) raised a fundamental question about the actual role of Nitrospira in both nitrification steps. This review summarizes the current knowledge about morphological, physiological and genetic characteristics of the canonical and comammox Nitrospira. Potential implications of comammox for the functional aspects of nitrogen removal have been highlighted. The complex meta-analysis of literature data was applied to identify specific individual variables and their combined interactions on the Nitrospira abundance. In addition to dissolved oxygen and influent nitrogen concentrations, temperature and pH may play an important role in enhancing or suppressing the Nitrospira activity.

Characterization of the start-up of single and two-stage Anammox processes with real low-strength wastewater treatment

In order to promote the application of anaerobic ammonium oxidation (Anammox) for municipal wastewater treatment, single and two-stage Anammox processes were started up for real low-strength wastewater treatment under similar conditions for the comparison. Results showed that the anaerobic baffled reactor (ABR)-Nitritation-Anammox and the ABR-Completely Autotrophic Nitrogen removal Over Nitrite (CANON) process took 75 days and 101 days to start up with a total nitrogen removal rate of 86-92% and 81-87% under steady state, respectively. The 16 S rRNA sequencing analysis revealed that the phylum of Proteobacteria dominated in CANON system and Anammox system with the relative abundance of 35.39% and 15.27%, respectively. Phylogenetic analysis showed that Anammox species, related to Ca. Brocadia Sinica JPN1 and Ca. Kuenenia stuttgartiensis, dominated in these two systems, respectively. The nitrogen removal performance of two-stage process was 5% higher than that of single stage process, while the start-up period and dominated Anammox species were different.

Quick start-up and stable operation of a one-stage deammonification reactor with a low quantity of AOB and ANAMMOX biomass

In this study, a quick start-up of one-stage deammonification in an immobilized aerobic ammonium oxidizing bacteria (AOB) and anoxic ammonium oxidizing (ANAMMOX) bacteria up-flow reactor (IAAR) was successfully achieved. With the aid of gel layers, AOB and ANAMMOX bacteria had excellent spatial distribution, theoretically meeting dissolved oxygen requirements for the simultaneous processes of aerobic and anaerobic ammonium oxidizing. The results indicated that an IAAR containing 0.4 g-VSS L^-1 immobilized biomass achieved a nitrogen removal rate (NRR) of 0.53 kg-N m^-3 d^-1 after only 10 days of operation and subsequently reached a maximum nitrogen removal rate (NRR_max) of 3.73 kg-N m^-3 d^-1. The micro-profiles of DO and pH were measured using microelectrodes to help understand the stratification of the microbial processes inside the gel layers. The distribution of AOB and ANAMMOX bacteria within the gel layers was verified using fluorescence in situ hybridization (FISH) analysis. The community distribution in the FISH three-dimensional images closely corresponded to the micro-profiles of DO concentration and pH, enabling rapid adaptation and stable operation of the reactor seeded with a quite low quantity of biomass.

Performance and microbial community of the completely autotrophic nitrogen removal over nitrite process with a submerged aerated biological filter

Stable performance is a technical problem in the completely autotrophic nitrogen removal over nitrite (CANON) process with one single stage, which needs to be addressed. In the current work, a laboratory-scale submerged aerated biological filter (SABF) with a 3-L working volume was introduced into the CANON process to enhance its stable performance for 290 days under the following conditions: temperature of 30 ± 1 °C and dissolved oxygen (DO) level of 0.2-0.8 mg·L^-1. The results showed that the average ammonium nitrogen removal efficiencies (ANRE) and total nitrogen removal efficiencies (TNRE) were 97.4% and 75.7%, respectively. A 16S rRNA gene high-throughput sequencing technology confirmed the phyla Proteobacteria and Planctomycetes as the ammonium oxidizing bacteria (AOB) and anaerobic ammonia-oxidizing bacteria (AnAOB) of this CANON process with SABF, respectively. The major contributor to nitrogen removal was the genus Candidatus Brocadia, in Brocadiae. The aim is to present an effective strategy as a reference for the design of full-scale plant for the CANON process.

The role of COD/N ratio on the start-up performance and microbial mechanism of an upflow microaerobic reactor treating piggery wastewater

This study investigated the role of COD/N ratio on the start-up and performance of an upflow microaerobic sludge reactor (UMSR) treating piggery wastewater at 0.5 mgO₂/L. At high COD/N ratio (6.24 and 4.52), results showed that the competition for oxygen between ammonia-oxidizing bacteria, nitrite-oxidizing bacteria and heterotrophic bacteria limited the removal of nitrogen. Nitrogen removal efficiency was below 40% in both scenarios. Decreasing the influent COD/N ratio to 0.88 allowed achieving high removal efficiencies for COD (∼75%) and nitrogen (∼85%) due to the lower oxygen consumption for COD mineralization. Molecular biology techniques showed that nitrogen conversion at a COD/N ratio 0.88 was dominated by the anammox pathway and that Candidatus Brocadia sp. was the most important anammox bacteria in the reactor with a relative abundance of 58.5% among the anammox bacteria. Molecular techniques also showed that Nitrosomonas spp. was the major ammonia-oxidiser bacteria (relative abundance of 86.3%) and that denitrification via NO₃^- and NO₂^- also contributed to remove nitrogen from the system.

Total inorganic nitrogen removal during the partial/complete nitrification for treating domestic wastewater: Removal pathways and main influencing factors

Achieving nitrite accumulation was considered as the prerequisite of ANAMMOX, which hindered the application of ANAMMOX. In this study, total inorganic nitrogen (TIN) removal during the partial/complete nitrification was studied in a lab-scale sequencing batch reactor (SBR) for treating domestic wastewater. The results showed TIN was removed by denitrification, ANAMMOX and N₂O emission during the partial/complete nitrification. AOB, AOA, Nitrobacter (NB), Nitrospira (NS), AnAOB and DNB were coexisted in the partial/complete nitrification. The microbial competition among these functional communities determined the type of nitrification, TIN removal and pathways. Since low DO concentrations benefits Nitrospira growth, the partial nitrification was damaged. After long-term operation, AOB gradually accommodated the low DO concentration. When V_max,AOB (the maximum specific reaction rate of AOB) higher than V_max,NOB (the maximum specific reaction rate of NOB), a part of nitrite was reduced by DNB and AnAOB. Therefore, TIN was removed during the complete nitrification.

Fast start-up of the CANON process with a SABF and the effects of pH and temperature on nitrogen removal and microbial activity

The long start-up time of the completely autotrophic nitrogen removal over nitrite (CANON) process is one of the main disadvantages of this system. In this paper, the CANON process with a submerged aerated biological filter (SABF) was rapidly started up within 26 days. It gave an average ammonium nitrogen removal rate (ANR) and a total nitrogen removal rate (TNR) of 94.2% and 81.3%, respectively. The phyla Proteobacteria and Planctomycetes were confirmed as the ammonia oxidizing bacteria (AOB) and anaerobic ammonium oxidation bacteria (AnAOB). The genus Candidatus Brocadia was the major contributor of nitrogen removal. pH and temperature affect the performance of the CANON process. This experimental results showed that the optimum pH and temperature were 8.0 and 30 °C, respectively, which gave the highest average ANR and TNR values of 94.6% and 85.1%, respectively. This research could promote the nitrogen removal ability of CANON process in the future.

Effect of dissolved oxygen on nitrogen removal and the microbial community of the completely autotrophic nitrogen removal over nitrite process in a submerged aerated biological filter

Dissolved oxygen (DO) is a crucial parameter of the completely autotrophic nitrogen removal over nitrite (CANON) process. This study determined the nitrogen removal performance and microbial community of the CANON process in a laboratory-scale submerged aerated biological filter (SABF) over a DO concentration range from 0 to 1.2 mg·L^-1. The results showed that the optimum DO (0.2-0.3 mg·L^-1) corresponded to an average ammonium nitrogen removal efficiency of 93.4% and a total nitrogen removal efficiency of 81.0%. A 16S rRNA gene high-throughput sequencing technology confirmed that the phyla Proteobacteria and Nitrospirae enriched, whereas the phylum Planctomycetes was inhibited with increasing DO concentration. At the genus level, the increase of DO concentration resulted in the enrichment of genera Dok59 and Nitrospira, but restrained the genus Candidatus Brocadia. This research can be used to improve the nitrogen removal ability of the CANON process in an SABF in the future.

Start-up of the completely autotrophic nitrogen removal over nitrite process with a submerged aerated biological filter and the effect of inorganic carbon on nitrogen removal and microbial activity

Good start-up and performance are essential for the completely autotrophic nitrogen removal over nitrite (CANON) process, and inorganic carbon (IC) is also important for this process. In this study, a lab-scale submerged aerated biological filter (SABF) was adopted for the CANON process. A 16S rRNA gene high-throughput sequencing analysis showed that the phyla Proteobacteria and Planctomycetes were the dominant microorganisms and that the genus Candidatus Brocadia functioned as the nitrogen remover. The effect of IC on the nitrogen removal was analyzed. The results showed that the optimum concentration ratio of IC to nitrogen (IC/N) was 1.2, which produced the highest average ammonium nitrogen removal rate (ANR) and total nitrogen removal rate (TNR) values of 95.5% and 80.3%, respectively. The average AOB and AnAOB activities were 2.45 mg·L^-1·h^-1 and 3.57 mg·L^-1·h^-1, respectively. This research could promote the nitrogen removal ability of the CANON process with a SABF in the future.

State of the art of aerobic granulation in continuous flow bioreactors

In the wake of the success of aerobic granulation in sequential batch reactors (SBRs) for treating wastewater, attention is beginning to turn to continuous flow applications. This is a necessary step given the advantages of continuous flow treatment processes and the fact that the majority of full-scale wastewater treatment plants across the world are operated with aeration tanks and clarifiers in a continuous flow mode. As in SBRs, applying a selection pressure, based on differences in either settling velocity or the size of the biomass, is essential for successful granulation in continuous flow reactors (CFRs). CFRs employed for aerobic granulation come in multiple configurations, each with their own means of achieving such a selection pressure. Other factors, such as bioaugmentation and hydraulic shear force, also contribute to aerobic granulation to some extent. Besides the formation of aerobic granules, long-term stability of aerobic granules is also a critical issue to be addressed. Inorganic precipitation, special inocula, and various operational optimization strategies have been used to improve granule long-term structural integrity. Accumulated studies reviewed in this work demonstrate that aerobic granulation in CFRs is capable of removing a wide spectrum of contaminants and achieving properties generally comparable to those in SBRs. Despite the notable research progress made toward successful aerobic granulation in lab-scale CFRs, to the best of our knowledge, there are only three full-scale tests of the technique, two being seeded with anammox-supported aerobic granules and the other with conventional aerobic granules; two other process alternatives are currently in development. Application of settling- or size-based selection pressures and feast/famine conditions are especially difficult to implement to these and similar mainstream systems. Future research efforts needs to be focused on the optimization of the granule-to-floc ratio, enhancement of granule activity, improvement of long-term granule stability, and a better understanding of aerobic granulation mechanisms in CFRs, especially in full-scale applications.

The transformation from anammox granules to deammonification granules in micro-aerobic system by facilitating indigenous ammonia oxidizing bacteria

Granular deammonification process is a good way to retain aerobic and anaerobic ammonia oxidizing bacteria (AOB and anammox bacteria) and exhaust flocculent nitrite oxidizing bacteria (NOB). In this study, to facilitate indigenous AOB growth on anammox granules, by stepwise reducing influent nitrite, anammox granules were effectively transformed into deammonification granules in a micro-aerobic EGSB in 100 days. Total nitrogen removal efficiency of 90% and nitrogen removal rate of 2.3 g N/L/d were reached at stable deammonification stage. High influent FA and limited oxygen supply contributed suppression for Nitrospira-like NOB. In transition stages, Proteobacteria and Chloroflexi were always dominated. Anammox abundance decreased, while AOB abundance grew fast. Anammox bacteria and AOB were dominated by Brocadia fulgida and Nitrosomonas europaea, respectively. Denitrification activity and bacteria existed although without influent organic. The final AOB abundance was about 4.55-13.8 times more than anammox bacteria abundance, with almost equal potential activities.

Single-stage autotrophic nitrogen removal process at high loading rate: granular reactor performance, kinetics, and microbial characterization

For the possible highest performance of single-stage combined partial nitritation/anammox (PNA) process, a continuous complete-mix granular reactor was operated at progressively higher nitrogen loading rate. The variations in bacterial community structure of granules were also characterized using high-throughput pyrosequencing, to give a detail insight to the relationship between reactor performance and functional organism abundance within completely autotrophic nitrogen removal system. In 172 days of operation, a superior total nitrogen (TN) removal rate over 3.9 kg N/(m³/day) was stable implemented at a fixed dissolved oxygen concentration of 1.9 mg/L, corresponding to the maximum specific substrate utilization rate of 0.36/day for TN based on the related kinetics modeling. Pyrosequencing results revealed that the genus Nitrosomonas responsible for aerobic ammonium oxidation was dominated on the granule surface, which was essential to offer the required niche for the selective enrichment of anammox bacteria (genus Candidatus Kuenenia) in the inner layer. And the present of various heterotrophic organisms with general functions, known as fermentation and denitrification, could not be overlooked. In addition, it was believed that an adequate excess of ammonium in the bulk liquid played a key role in maintaining process stability, by suppressing the growth of nitrite-oxidizing bacteria through dual-substrate competitions.

Nitrogen removal performance and loading capacity of a novel single-stage nitritation-anammox system with syntrophic micro-granules

The operation performance of a novel micro-granule based syntrophic system of nitritation and anammox was studied by controlling the oxygen concentration and maintaining a constant temperature of 25°C. With the oxygen concentration of around 0.11 (<0.15)mg/L, the single-stage nitritation-anammox system was startup successfully at a nitrogen loading rate (NLR) of 1.5kgN/m³/d. The reactor was successfully operated at volumetric N loadings ranging from 0.5 to 2.5kgN/m³/d with a high nitrogen removal of 82%. The microbial community was composed by ammonia oxidizing bacteria (AOB) and anammox bacteria forming micro-granules with an average diameter of 0.8mm and good settleability. Results from pyrosequencing analysis revealed that Ca. Kuenenia and Nitrosomonas were selected and enriched in the community over the startup period, and these were identified as the dominant anammox bacteria and AOB species, respectively.

Anammox Process

Application of anammox based processes is nowadays an efficient way to remove nitrogen from wastewaters, being good alternative to the conventional nitrification-denitrification process. This chapter reviews the possible configurations to apply the anammox process, being special attention to the previous partial nitritation, necessary to obtain the adequate substrates for anammox bacteria. Furthermore a description of the main technologies developed and patented by different companies was performed, with focus on the advantages and bottlenecks of them. These technologies are classified in the chapter based on the type of biomass: suspended, granular and biofilm. Also a review is presented for the industrial applications (food industry, agricultural wastes, landfill leachates, electronic industry, etc.), taking into account full scale experiences and laboratory results, as well as microbiology aspects respect to the anammox bacteria genera involved. Finally the possibility to couple nitrogen removal, by anammox, with phosphorus recovery, by struvite precipitation, is also evaluated.

Technical and economical optimization of a full-scale poultry manure treatment process: total ammonia nitrogen balance

A full-scale process for the treatment of 80 tons per day of poultry manure was designed and optimized. A total ammonia nitrogen (TAN) balance was performed at steady state, considering the stoichiometry and the kinetic data from the anaerobic digestion and the anaerobic ammonia oxidation. The equipment, reactor design, investment costs, and operational costs were considered. The volume and cost objective functions optimized the process in terms of three variables: the water recycle ratio, the protein conversion during AD, and the TAN conversion in the process. The processes were compared with and without water recycle; savings of 70% and 43% in the annual fresh water consumption and the heating costs, respectively, were achieved. The optimal process complies with the Chilean environmental legislation limit of 0.05 g total nitrogen/L.

Effects of dissolved oxygen on microbial community of single-stage autotrophic nitrogen removal system treating simulating mature landfill leachate

The performance of four identical sequencing biofilm batch reactors (SBBR) for autotrophic nitrogen removal was investigated with 2000mg/L ammonia-containing mature landfill leachate at 30°C. The main objective of this study was to evaluate the effects of dissolved oxygen (DO) on the performance and microbial community of single-stage nitrogen removal using anammox and partial nitritation (SNAP) system. At an applied load of 0.5kgNm(-3)d(-1), average total nitrogen removal efficiency (TNRE) above 90% was long-term achieved with an optimal DO concentration of 2.7mg/L. The microelectrode-measured profiles showed the microenvironments inside the biofilms. 16S ribosomal Ribonucleic Acid (rRNA) amplicon pyrosequencing and denaturing gradient gel electrophoresis (DGGE) were used to analyze the microbial variations of different DO concentrations and different positions inside one reactor.

Biodegradation of tetramethylammonium hydroxide (TMAH) in completely autotrophic nitrogen removal over nitrite (CANON) process

This study conducted a completely autotrophic nitrogen removal over nitrite (CANON) process in a continuous anoxic upflow bioreactor to treat synthetic wastewater with TMAH (tetramethylammonium hydroxide) ranging from 200 to 1000mg/L. The intermediates were analyzed for understanding the metabolic pathway of TMAH biodegradation in CANON process. In addition, (15)N-labeled TMAH was used as the substrate in a batch anoxic bioreactor to confirm that TMAH was converted to nitrogen gas in CANON process. The results indicated that TMAH was almost completely biodegraded in CANON system at different influent TMAH concentrations of 200, 500, and 1000mg/L. The average removal efficiencies of total nitrogen were higher than 90% during the experiments. Trimethylamine (TMA) and methylamine (MA) were found to be the main biodegradation intermediates of TMAH in CANON process. The production of nitrogen gas with (15)N-labeled during the batch anaerobic bioreactor indicated that CANON process successfully converted TMAH into nitrogen gas.

Effect of dissolved oxygen on elemental sulfur generation in sulfide and nitrate removal process: characterization, pathway, and microbial community analysis

Microaerobic bioreactor treatment for enriched sulfide and nitrate has been demonstrated as an effective strategy to improve the efficiencies of elemental sulfur (S(0)) generation, sulfide oxidation, and nitrate reduction. However, there is little detailed information for the effect and mechanism of dissolved oxygen (DO) on the variations of microbial community in sulfur generation, sulfide oxidation, and nitrate reduction systems. Polymerase chain reaction denaturing gradient gel electrophoresis (PCR-DGGE) was employed to evaluate the variations of microbial community structures in a sulfide oxidation and nitrate reduction reactor under different DO conditions (DO 0-0.7 mg · L(-1)). Experimental results revealed that the activity of sulfide-oxidizing bacteria (SOB) and nitrate-reducing bacteria (NRB) could be greatly stimulated in 0.1-0.3 mg-DO · L(-1). However, when the DO concentration was further elevated to more than 0.5 mg · L(-1), the abundance of NRB was markedly decreased, while the heterotrophic microorganisms, especially carbon degradation species, were enriched. The reaction pathways for sulfide and nitrate removal under microaerobic conditions were also deduced by combining batch experiments with functional species analysis. It was likely that the oxidation of sulfide to sulfur could be performed by both aerobic heterotrophic SOB and sulfur-based autotrophic denitrification bacteria with oxygen and nitrate as terminal electron acceptor, respectively. The nitrate could be reduced to nitrite by both autotrophic and heterotrophic denitrification, and then the generated nitrite could be completely converted to nitrogen gas via heterotrophic denitrification. This study provides new insights into the impacts of microaerobic conditions on the microbial community functional structures of sulfide-oxidizing, nitrate-reducing, and sulfur-producing bioreactors, which revealing the potential linkage between functional microbial communities and reactor performance.