Simultaneous Partial Nitrification, Anammox And Denitrification Research Articles

Responses of suspended sludge and biofilm in a SNAD system under C/N elevation: Microbial activity, nitrogen conversion flux and molecular ecological network

The simultaneous partial nitrification, anammox and denitrification (SNAD) process had received widespread attention as an advanced wastewater treatment process. In this study, the SNAD mainstream nitrogen removal process with the incorporation of polyurethane sponge packing under different C/N conditions was investigated. Results showed that the highest nitrogen removal efficiency of the system was achieved at the C/N of 2.0, while the high C/N (3.5) significantly deteriorate the nitrogen removal efficiency. Meanwhile, high C/N (3.5) significantly inhibited the activity and abundance of anammox bacteria (mainly Candidatus_Kuenenia), resulting in the decreased contribution of anammox (from 63.14 % to 48.09 %). The significant divergence of microbial interactions in the suspended sludge and biofilm was observed with increasing C/N. Compared with suspended sludge, biofilm facilitated higher abundance and activity of anammox bacteria, and the molecular ecological network of biofilm displayed better stability and more efficient mass transfer efficiency between microorganisms. The C/N of 3.5 simplified the subnetworks of Chloroflexi and Proteobacteria but increased the positive interactions between Planctomycetota and other microbes. Anammox bacteria were found as keystone species only in biofilm system. This study provided a theoretical basis and technical guidance for the application of SNAD process in municipal wastewater treatment.

Microbial activity along the depth of biofilm in the simultaneous partial nitrification, anammox and denitrification (SNAD) system

ABSTRACT Simultaneous partial nitrification, anammox and denitrification (SNAD) is a sustainable and cost-effective technology for nitrogen removal from low-strength wastewater. However, knowledge of the biofilm microenvironment of the SNAD system is currently unsatisfactory. The purpose of this study was to evaluate organic carbon effects on the microenvironment and microbial growth in the SNAD biofilm system. Microelectrodes were used to investigate microbial activity in-depth within biofilms. ORP distribution of the SNAD system was positively related to anammox activity(R 2 = 0.9), and had some influence on microbial community structure. The synergistic effect of anammox bacteria and denitrifiers could be achieved when the abundance ratio of anammox bacteria to denitrifying bacteria is greater than 1.2.

Start-up of the anaerobic hydrolysis acidification (ANHA)- simultaneous partial nitrification, anammox and denitrification (SNAD)/enhanced biological phosphorus removal (EBPR) process for simultaneous nitrogen and phosphorus removal for domestic sewage treatment

The simultaneous partial nitrification, anammox and denitrification (SNAD) process has been widely used in domestic sewage biological denitrification technology because of its high efficiency and low consumption. However, the simultaneous removal of another important pollution element, phosphorus, has been difficult, and its C/N ratio limitation of the influent is strict. The start-up of the anaerobic hydrolysis acidification (ANHA)- simultaneous partial nitrification, anammox and denitrification (SNAD)/enhanced biological phosphorus removal (EBPR) coupling process achieves the treatment of urban sewage for carbon, nitrogen and phosphorus removal. Under optimal conditions, the final total nitrogen and total phosphorus removal rates reached 91.59% and 89.10%, respectively. High-throughput sequencing technology showed that the ANHA reactor was mainly Lactococcus. At the same time, the main bacteria in the SNAD/EBPR process were anammox bacteria (AnAOB, Candidatus_Kuenenia, Candidatus_Brocadia) primarily existing in biofilms, while the ammonium oxidizing bacteria (AOB, Nitrosomonas), denitrifying polyphosphate-accumulating organisms (DPAOs, Pseudomonas, Flavobacterium, Bdellovibrio) and Denitrifying bacteria (DNB, Thauera, Denitratisoma, Rhodobacteraceae).were mainly found in the suspended sludge. These conclusions provide valuable information for the full-scale treatment of domestic sewage.

Weak magnetic field stimulated nitrite oxidizing bacteria and destroyed the SNAD process

Simultaneous partial nitrification, anammox and denitrification (SNAD) process was successfully constructed in a fixed bed reactor. The ammonium and TN removal efficiencies exceeded 90% under the collaboration of ammonium oxidizing bacteria (AOB), anammox and denitrifying bacteria (DNB). However, the well cooperation was interrupted after introducing the weak MF of 3.5 mT, which was reflected directly by the increased nitrite oxidizing bacteria (NOB) activity and decreased AOB, anammox and DNB activities calculated via mass balance. Moreover, drastic microbial community structure changes induced by weak MF were observed. The dominant phylum changed from Proteobacteria (25.6%) to Deferribacteres (35.2%). The weak MF suppressed the growth of AOB (genus Nitrosomanas), anammox (genus Candidatus Brocadia and clone Anammox) and DNB (genus Denitratisoma) while promoted the enrichment of NOB (genus Nitrospira). Moreover, the weakened NH3-NH2OH, NO-N2O and N2O-N2 pathways might explain the deterioration of SNAD process caused by weak MF.

Optimization of nitrogen and carbon removal with simultaneous partial nitrification, anammox and denitrification in membrane bioreactor.

In this study, a membrane bioreactor (MBR) was used to achieve both nitrogen and carbon removal by a simultaneous partial nitrification, anammox and denitrification (SNAD) process. During the entire experiment, the intermittent aeration (non-aerobic time : aeration time, min min−1) cycle was controlled by a time-controlled switch, and the aeration rate was controlled by a gas flowmeter, and the optimal operating parameters as determined by response surface methodology (RSM) were a C/N value of 1.16, a DO value of 0.84 mg l−1 and an aerobic time (Tae) of 15.75 min. Under these conditions, the SNAD process achieved efficient and stable nitrogen and carbon removal; the total inorganic nitrogen removal efficiency and chemical oxygen demand removal efficiency were 92.31% and 95.67%, respectively. With the formation of granular sludge, the membrane fouling rate decreased significantly from 35.0 Pa h−1 at SNAD start-up to 19.9 Pa h−1 during stable operation. Fluorescence in situ hybrid analyses confirmed the structural characteristics and the relative ratio of aerobic ammonia-oxidizing bacteria, anaerobic ammonia-oxidizing bacteria and denitrifying bacteria in the SNAD system.

Start-up Characteristics of SNAD Process and Functional Abundance Succession of Volcanic Rock Biological Aerated Filter

A SNAD(simultaneous partial nitrification,ANAMMOX,and denitrification) process initiated in a submerged biological aerated filter (SBAF) was started up by seeding nitrification sludge and an ANAMMOX filter to investigate the operating characteristics and the succession of functional bacteria. The results showed that when the autotrophic nitrogen removal and denitrification were operated stably for 67 days at an initial COD concentration of 60 mg·L-1, the maximum nitrogen removal efficiency, the COD removal rate, and the nitrogen removal rate were 92.0%, 82.9%, and 2.3 kg·(m3·d)-1, respectively. Moreover, the total nitrogen removal rate of the SNAD process in this study was 12.6% higher than that of CANON process. The results of quantitative PCR showed that the abundance of AOB slightly increased, while the abundance of ANAMMOX was one order higher than that before the start-up of SNAD. In comparison, the abundances of denitrifiers and NOB remained at a relatively low level (107 copies·g-1). Taken together, these results suggested that the SBAF process with volcanic filter was beneficial for the accumulation of ANAMMOX and AOB.

Nitrogen removal in recirculating aquaculture water with high dissolved oxygen conditions using the simultaneous partial nitrification, anammox and denitrification system

The efficient removal of nitrogen pollutants in the aquaculture systems is still a challenge due to the low concentration of organic carbon and high concentration of dissolved oxygen (DO) in the wastewater. The simultaneous partial nitrification, anammox and denitrification (SNAD) bioreactor was firstly used for the treatment of aquaculture wastewater in recirculating aquaculture system. The bioreactor operated for 180 days without adding extra organic carbon. After 60-day operation, the bioreactor reached the stable stage with the average concentration of ammonia/nitrate/nitrite/COD in the effluent with 0.26/0.75/0.47/0.27 mg/L. The Pseudoxanthomonas was the dominant genus in the biofilm samples. The typical nitrogen functional bacteria and genes for nitrification, anammox and denitrification were detected with different abundance in different procedures along the bioreactor. Network analysis revealed the significant correlations between nitrogen functional bacteria and genes. The SNAD bioreactor achieved the effective removal for nitrogen and COD under high DO conditions in recirculating aquaculture system.

An improved start-up strategy for mainstream anammox process through inoculating ordinary nitrification sludge and a small amount of anammox sludge

The difficulties in enriching anammox bacteria and maintaining stable partial nitrification during start-up phase limit the application of mainstream anammox process. In this study, the feasibility of starting up simultaneous partial nitrification, anammox and denitrification (SNAD) reactor treating municipal wastewater by inoculating ordinary nitrification sludge (96.2%) and a small amount of anammox sludge (3.8%) was investigated. A sequencing batch reactor with intermittent aeration was used for the SNAD process. The SNAD reactor was started up in 75 days with a nitrogen removal efficiency of 85.4% at ambient temperature. The nitrogen removal performance maintained stable despite the fluctuating inflow. Anammox bacterial activity exponentially increased although nitrite oxidizing bacteria (NOB) activity in seeding sludge was high. The enhanced ammonium oxidizing bacterial activity and partial denitrification provided sufficient nitrite for anammox bacteria. Moreover, NOB was inhibited by intermittent aeration, anammox bacteria had competitive advantage on nitrite. The improved particle size and settleability of activated sludge also favored the anammox bacterial enrichment. This study provided an improved and easily-implemented start-up strategy for mainstream anammox. The seeding sludge was easily obtained and the operation strategy was simple. These findings were meaningful to the engineering application of mainstream anammox.

Simultaneous partial nitrification, anammox and denitrification (SNAD) process for nitrogen and refractory organic compounds removal from mature landfill leachate: Performance and metagenome-based microbial ecology

In this study, a simultaneous partial nitrification, Anammox and denitrification (SNAD) bioreactor was constructed for mature landfill leachate treatment, which exhibited favorable NH4+-N (98.9–99.9%), TN (90.7–94.9%) and bio-refractory organic compounds (46.2–67.7%) removal efficiencies. Stoichiometric analysis demonstrated that the synergy of ammonium-oxidizing bacteria and Anammox bacteria dominated TN removal (96.1–97.2%). NO3−-N produced in Anammox could be further reduced through (partial) denitrification and dissimilatory nitrate reduction to ammonium (DNRA). The results highlighted that humic-like and their intermediates might serve as the electron donor for these (partial) denitrifiers and DNRA bacteria to remove NO3−-N, and could be effectively removed from mature landfill leachate in SNAD bioreactor. Metagenomic characterization further demonstrated that phyla Chloroflexi, Chlorobi and genera Nitrosomonas, Ignavibacterium and Aminiphilus might be responsible for such humic-like degradation. Overall, this work offers new insights into the metagenome-based bioinformatic roles for the previously understudied microorganisms in SNAD bioreactor for mature landfill leachate treatment.

Enhanced nitrogen removal of the simultaneous partial nitrification, anammox and denitrification (SNAD) biofilm reactor for treating mainstream wastewater under low dissolved oxygen (DO) concentration

The simultaneous partial nitrification, anammox and denitrification (SNAD) process for treating mainstream wastewater was investigated under different intermittent aeration modes. By controlling the aeration time of 20, 60 and 180 min during the intermittent modes, the oxygen concentration remained 3.50, 1.45 and 0.70 mg·L−1. Correspondingly, the reactor achieved the nitrogen removal rate of 0.17, 0.29 and 0.30 kg N·m−3·d−1. Meanwhile, the average total inorganic nitrogen (TIN) removal efficiency reached 93.4%, 87.5% and 92.7%. The effluent NO3−-N concentration was very low. High-throughput sequencing analysis indicated that the proportion of nitrite oxidization bacteria (NOB), anammox bacteria and denitrification bacteria was 0.15%, 0.33% and 8.78%. Candidatus Anammoxoglobus was the abundant anammox bacteria genus. Further study on the unclassified sequences revealed the possibility of the high relative abundance of Nitrosomonas-related genus and Candidatus Kuenenia-related genus on the SNAD biofilm.

Biological Nitrogen Removal Process in a Microbubble-aerated Biofilm Reactor Treating Low C/N Wastewater

The microbubble-aerated biofilm reactor as a new treatment process combines microbubble aeration technology with aerobic biological treatment. A microbubble aerated biofilm reactor was used in this study to treat low C/N ratio wastewater at a low air/water ratio. The process and performance of biological nitrogen removal were investigated, and the functional bacterial populations for nitrogen removal in the biofilm were analyzed. The results showed that the biological nitrogen removal process was converted from simultaneous nitrification-denitrification to simultaneous partial nitrification, ANAMMOX and denitrification (SNAD) processes when DO concentration was controlled by an air/water ratio of lower than 1:2 and the influent C/N ratio was reduced. As a result, the efficient biological nitrogen removal performance was achieved when treating low C/N ratio wastewater. When the DO concentration was lower than 1.0 mg·L-1 and the influent C/N ratio was 1:2.8, the SNAD process became dominant for biological nitrogen removal. In this case, the average total nitrogen (TN) removal efficiency was 76.3%, and the average TN loading rate removed was 1.42 kg·(m3·d)-1. In addition, it was estimated that 86.0% of TN removal was attributed to the ANAMMOX process. The relative abundances of ammonia-oxidizing bacteria populations and ANAMMOX bacteria populations in the biofilm increased gradually, while the relative abundances of nitrite-oxidizing bacteria populations and denitrifying bacteria populations decreased gradually, with a decrease in influent C/N ratio. The variation of functional bacterial populations for nitrogen removal was consistent with the conversion of nitrogen removal process to SNAD process.

Achieving efficient nitrogen removal and nutrient recovery from wastewater in a combining simultaneous partial nitrification, anammox and denitrification (SNAD) process with a photobioreactor (PBR) for biomass production and generated dissolved oxygen (DO) recycling

This study presents a new way to achieve energy neutral wastewater treatment based on a combined nitrification, anammox, and denitrification (SNAD) process and photobioreactor (PBR) configuration with external recycling instead of aeration, and without an additional carbon source, using fixed-film-activated sludge technology (IFAS). The SNAD-PBR process achieved total nitrogen (TN) and phosphorus removal efficiencies of 90 and 100%, respectively. In addition, dissolved oxygen (DO) was controlled in the range 0.4–1.2 mg/L by the introduction of an external recycling system. The presence of microalgae to serve as a carbon source in the SNAD reactor enabled the denitrifiers to survive. When the reflux ratio was 1:3, the lower COD/N protected the activity of the anammox bacteria, not suppressed by the heterotrophic denitrifiers. Microbial community analysis by Illumina MiSeq sequencing revealed that the new environment was more suitable for Candidatus Brocadia when a reflux system was introduced.

Nitrogen removal by simultaneous partial nitrification, anammox and denitrification (SNAD) in a structured-bed reactor treating animal feed processing wastewater: Inhibitory effects and bacterial community

The aim of this study was to investigate the post-treatment of UASB effluent by treating animal feed production wastewater using simultaneous partial nitrification, anammox and denitrification (SNAD) in a structured-bed reactor subjected to low aeration and recirculation. The average nitrogen loads applied were 0.307, 0.249 and 0.149 kgN m−3d−1, correlated to COD/N ratios of 0.28, 0.41 and 0.26 (Phases 1, 2 and 3, respectively). The best mean values for removal efficiencies of total-N and COD were obtained in Phase 1 with 48 ± 24% and 63 ± 20%, respectively, reaching a maximum total-N removal efficiency of 79%. The anammox process was the main pathway of nitrogen removal, as pointed out in the nitrogen mass balance. High free ammonia (FA) concentrations in Phases 2 and 3, associated to the limitation of oxygen supply, caused inhibition of nitrite oxidizing bacteria (NOB) activity, leading to NO2 accumulation, having an impact on the denitrifying activity. At the end of the operational period, sequencing analysis detected sequences related to heterotrophic denitrifiers (22.5%), anammox bacteria, Candidatus Anammoximicrobium (2%) and ammonia oxidizing bacteria (AOB) belonging to Nitrosomonadales and Nitrosomonas (0.6%). These results demonstrated that nitritation, denitrification and anammox were likely in the processes involved in nitrogen removal in this reactor.

Start-up of a full-scale SNAD-MBBR process for treating sludge digester liquor

A simultaneous partial nitrification, anammox and denitrification (SNAD) process was started up successfully to treat sludge digester liquor in a full-scale moving bed biofilm reactor (MBBR). This reactor treated up to 530 kg N/d with a nitrogen removal efficiency of 70%. During the start-up period, efficient and stable partial nitrification was attained by controlling the concentration of DO (0.3–0.5 mg/L), free ammonia (0.6–8.2 mg/L) and free nitrous acid (0.03–1.20 mg/L). The full-scale anammox reactor (832 m3) took about 240 days to start up successfully after inoculation with nitrifying sludge and a small amount of anammox sludge. The preformation of anammox biofilm in the pilot-scale MBBR played a vital role in the fast start-up of the full-scale anammox MBBR. An average nitrogen removal rate of 0.41 kg N/m3/d was achieved. In the SNAD reactor, high-throughput sequencing techniques revealed that the anammox bacteria in the biofilm accounted for a higher proportion than in the suspended sludge, indicating that the carriers effectively protected the anammox from washout. This study provides valuable experience for treatment of wastewater with low carbon-to-nitrogen ratio.

Pathway governing nitrogen removal in artificially aerated constructed wetlands: Impact of aeration mode and influent chemical oxygen demand to nitrogen ratios

This study aimed at assessing the influence of aeration mode and influent COD/N ratio on nitrogen removal in constructed wetlands (CWs). The results showed that a simultaneous partial nitrification, anammox and denitrification (SNAD) process was established in the intermittent aerated V1. While nitrogen removal pathway gradually changed from partial nitrification-denitrification to complete nitrification-denitrification along with reducing COD/N ratio in the continuous limited aerated V2. Effective inhibition of NOBs under intermittent aeration conditions, good retention of anammox bacteria biomass and much faster depletion of COD prior to substantial NH4+-N conversion jointly led to the successful achievement of stable SNDA process with elevated influent COD/N ratios in V1. Furthermore, the presence of SNAD ensured a robust ammonium (84–92%) and TN (80–91%) removal efficiency in V1 under varying COD loading rates. In contrast, the TN removal efficiency decreased rapidly along with the reducing influent COD/N ratios in V2.

Robustness and microbial consortia succession of simultaneous partial nitrification, ANAMMOX and denitrification (SNAD) process for mature landfill leachate treatment under low temperature

The purpose of this research was to assess the feasibility of simultaneous partial nitrification, ANAMMOX and denitrification (SNAD) for mature landfill leachate treatment under low temperature. To this end, bench-scale bioreactors receiving ammonium-rich wastewater were adapted to progressively reduced temperature after 370-day operation (Period I), and subsequently acclimatized to raw leachate for 86 days (Period II). Maximum nitrogen removal rates (NRR) of 218.9, 211.9, 201.1 and 146.9 g N m−3 d−1 were achieved at 30 °C, 20 °C, 15 °C and 10 °C respectively during Period I, mainly ascribing to partial nitritation/ANAMMOX. Furthermore, the SNAD bioreactor turned out to be feasible for prolonged treatment of mature landfill leachate under 15 °C (Period II), with simultaneous degradation of nitrogen (207.9 g N m−3 d−1) and bio-refractory organics (94.9 g COD m−3 d−1) respectively. High-throughput sequencing implied that better denitrogenation performance during Period II might be ascribed to reappearance of denitrifying bacteria (9.88%) and complete suppression of nitrite oxidation bacteria (NOB), although ammonium oxidation bacteria (AOB) and ANAMMOX bacteria (AMX) decreased by 6.05% and 4.56% respectively. Furthermore, the enriched bio-refractory organics degradation bacteria (24.38%) were conductive to the performance robustness. This research presents perspective for the application of efficient treatment of mature landfill leachate under low temperature.

Fast start-up of anammox process with mixed activated sludge and settling option

ABSTRACTIn this study, successful start-up of the anaerobic ammonium oxidation (anammox) process in a sequencing batch reactor (SBR) was achieved by seeding mixed activated sludge which included aerobic sludge, anaerobic sludge, simultaneous partial nitrification, anammox and denitrification (SNAD) sludge, and anammox sludge with low activity at a 2200:2100:5:2 volume ratio. On day 15, the effective anammox activity was attained in SBR, with the specific total nitrogen removal rate (SRR) of 0.214 gNg−1 VSSd−1. The total nitrogen removal rate (NRR) increased to 230 gNm−3 d−1 by gradually reducing the setting time to 10 min. With the nitrogen loading rate (NLR) up to 506 gNm−3 d−1, the total NRR of the SBR reached 433 gNm−3 d−1 during stationary phase. Candidatus Brocadia was detected as predominant functional microbes in the anammox SBR. The results demonstrated the feasibility of seeding mixed activated sludge to start-up an anammox SBR by settling option.

Long-term operation of oxygen-limiting membrane bioreactor (MBR) for the development of simultaneous partial nitrification, anammox and denitrification (SNAD) process

ABSTRACTIn this study, an oxygen-limiting membrane bioreactor (MBR) with recirculation of biogas for relieving membrane fouling was successfully operated to realize the simultaneous partial nitrification, anammox and denitrification (SNAD) process. The MBR operation was considered effective in the long-term test with total nitrogen (TN) and chemical oxygen demand (COD) removal efficiencies of 94.86% and 98.91%, respectively. Membrane fouling was significantly alleviated due to the recirculation of biogas and the membrane had been cleaned four times with a normal filtration period of 52 days. The co-existence of ammonia-oxidizing bacteria (AOB), anammox and denitrifying bacteria in MBR was confirmed by scanning electron microscopy (SEM) and fluorescence in situ hybridizations (FISH) analysis. Furthermore, AOB were found close to the granule surface, while denitrifying bacteria and anammox were in the deeper layer of granules. Potential in excellent TN and COD removal, operational stability and sustainability, as well as in alleviating membrane fouling is expected by using this oxygen-limiting MBR.

Evaluation of mainstream nitrogen removal by simultaneous partial nitrification, anammox and denitrification (SNAD) process in a granule-based reactor

The mainstream anaerobic ammonium oxidation (anammox) has attracted extensive attention recently, particularly due to its potential of transforming current wastewater treatment plants from energy consuming to energy neutral or positive. However, the presence of biodegradable chemical oxygen demanding (COD, 20–80mgCODL−1) in the mainstream anammox reactor stimulates the growth of heterotrophic bacteria, which would compete for oxygen with ammonia-oxidizing bacteria (AOB) and for nitrite with anammox bacteria, thus interfering with the autotrophic nitrogen removal process. In the present work, with consideration of granule size distribution, a one-dimensional model describing the mainstream simultaneous partial nitrification, anammox and denitrification (SNAD) in a granule-based reactor was established, calibrated and validated, based on the long-term experimental results. Through applying the verified model, simulation studies were conducted and the results showed that the effluent total nitrogen concentration of <5mgNL−1 could be achieved at C/N ratio of 0.2–0.6, DO concentration of 0.2–0.4mgL−1 and granule radius of 300–600μm. The combined effects indicated that the SNAD process with TN removal efficiency >90% was obtained at C/N ratio and DO concentration of 0.2–1.0 and 0.2–0.4mgO2L−1 respectively. Finally, the various granule size distribution patterns were simulated, which confirmed that the size distribution needed to be incorporated in the model to accurately describe the granular anammox system, considering a model based on a uniform granule size does not reflect the real situations. These results provide guides to optimize the operation of mainstream granular autotrophic nitrogen removal process.

Efficient simultaneous partial nitrification, anammox and denitrification (SNAD) system equipped with a real-time dissolved oxygen (DO) intelligent control system and microbial community shifts of different substrate concentrations

Simultaneous partial nitrification, anammox and denitrification (SNAD) process was studied in a sequencing batch biofilm reactor (SBBR) fed with synthetic wastewater in a range of 2200 mgN/L ∼ 50 mgN/L. Important was an external real-time precision dissolved oxygen (DO) intelligent control system that consisted of feed forward control system and feedback control system. This DO control system permitted close control of oxygen supply according to influent concentration, effluent quality and other environmental factors in the reactor. In this study the operation was divided into six phases according to influent nitrogen applied. SNAD system was successfully set up after adding COD into a CANON system. And the presence of COD enabled the survival of denitrifiers, and made Thauera and Pseudomonas predominant as functional denitrifiers in this system. Denaturing gradient gel electrophoresis (DGGE), fluorescence in situ hybridization (FISH) and 16S rRNA amplicon pyrosequencing were used to analyze the microbial variations of different substrate concentrations. Results indicated that the relative population of ammonia oxidizing bacteria (AOB) members decreased when influent ammonia concentration decreased from 2200 mg/L to 50 mg/L, while no dramatic drop of the percent of anammox bacteria was seen. And Nitrosomonas europaea was the predominant AOB in SNAD system treating sewage, while Candidatus Brocadia was the dominant anammox bacteria.