Nitrogen Removal System Research Articles

Effect of Alkaline Sludge Fermentation Products on the Nitrification Process and Performance

The aim of the present study was to investigate the effect of alkaline sludge fermentation products as a carbon source on the nitrification process and performance. During the operation of a biological nitrogen removal (BNR) system with sludge fermentation mixture as the carbon source, the activities of ammonia-oxidizing bacteria (AOB) and nitrite-oxidizing bacteria (NOB) were inhibited at the beginning. After 16 days, the activity of AOB began to recover rapidly, but the activity of NOB was still inhibited. The specific nitrate production rate (SNaPR, N/VSS) decreased from 0.1791 g·(g·d)-1 to 0.0078 g·(g·d)-1. At the same time, the nitrite accumulation rate increased from 8.12% to 91.42% and remained stable. The sludge fermentation mixture was separated into sludge fermentation liquid and sludge fermentation sediment. The changes in nitrification activity by adding different types of fermentation products were investigated. The results showed that the activity of NOB decreased in the experimental group fed with the sludge fermentation mixture and the fermentation liquid. The SNaPR decreased from an initial 0.1793 g·(g·d)-1 to 0.1510 g·(g·d)-1 and 0.1617 g·(g·d)-1, respectively. In the experimental group fed with fermentation sediment, the activity of NOB increased. SNaPR rose from 0.1793 g·(g·d)-1 to 0.1864 g·(g·d)-1. Therefore, the activity of the NOB can be inhibited when the sludge fermentation mixture and the fermentation liquid are used as a carbon source in the nitrification process. In addition, the short-range nitrification process can be realized, which is beneficial to accelerating the reaction speed and saving investment in this type of carbon source.

Free ammonia shock treatment eliminates nitrite-oxidizing bacterial activity for mainstream biofilm nitritation process

Nitritation (NH4+ → NO2−) is a critical step to provide nitrite for the followed anammox in a two-stage nitrogen removal system. In the mainstream line of wastewater treatment plants, the nitritation has not been applied to date, because of the significant difficulties in the suppression of nitrite-oxidizing bacteria (NOB), especially in biofilm systems. This study aims to systematically assess the effect of free ammonia (FA) shock treatment on the mainstream biofilm nitritation process through an integration of laboratory reactor operation, microbial community analysis, incubation tests, and kinetic model evaluation. In a laboratory nitrifying moving bed biofilm reactor (MBBR) fed with domestic-strength synthetic wastewater, it was shown that with the exposure of carrier-biofilms containing ammonia-oxidizing bacteria (AOB) Nitrosomonas and NOB Nitrospira to a high-level of FA (1068 mg NH3-N/L) over two days, a much higher residual AOB level was retained in the biofilm in comparison to NOB. The higher residual AOB on biofilm led to much faster recovery of AOB over NOB after the shock treatment, when normal operation resumed with the dissolved oxygen (DO) controlled at around 0.2 mg/L. The faster recovery of AOB than NOB subsequently gave rise to a stable, high nitrite accumulation ratio (nearly 100%) over a long period (two months). Collectively, these results suggest that FA shock treatment in conjunction with limited DO control is effective in eliminating NOB for mainstream biofilm nitritation process. The chemical cost would be marginal given the intermittent nature of the FA shock strategy and the readily available ammonium in the anaerobic sludge digestion liquor.

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.

Application of nitrification and denitrification processes in a direct water reuse system for pacific white shrimp farmed in biofloc system

The aim of the present study was to propose a low-cost nitrogen removal system through the nitrification / denitrification process in order to maintain the water quality required for the Pacific white shrimp superintensive cultivation in closed systems without water renewal. The increase in productivity consequently causes the accumulation of organic matter and nitrogenous compounds, especially ammonia nitrogen and nitrite, which in high concentrations can be lethal to aquatic organisms. In addition, the accumulation of solids in the system provides conditions for the emergence of opportunistic pathogens, microalgae booms, and increases the producer's cost of inputs to maintain the equilibrium physicochemical relationships required for shrimp farming. The experimental productive cycle lasted 36 days using Litopenaeus vannamei shrimps with 7.1 g ± 0.56 g and density of 350 shrimps m−³. The nitrogen removal efficiency observed during the study period was 71.3 ± 5.3 %, and the shrimp had a survival of 92.9 % and a final weight of 13.1 ± 1.4 g. Thus, we established a system (ammonia and nitrite), capable of managing solids without interaction with the sea, ensuring high biosecurity against exogenous diseases in marine shrimps farms.

Effects of COD/TN ratio on nitrogen removal efficiency, microbial community for high saline wastewater treatment based on heterotrophic nitrification-aerobic denitrification process

High salinity and high organic concentration impose negative impacts on autotrophic nitrification, which hinders efficient nitrogen removal. To achieve efficient nitrogen removal, high saline wastewater nitrogen removal systems with different COD/TN ratios based on heterotrophic nitrification-aerobic denitrification (HN-AD) process were established in this study. Results demonstrated that the COD/TN ratio had significant effects on nitrogen removal efficiency, microbial community structures and metabolic pathways. The optimal COD/TN ratio was 25 for nitrogen removal, with NH4+-N and TN removal rates of 11.86 mg·L−1·h−1 and 11.50 mg·L−1·h−1, respectively (3.65 and 3.31 times higher than those COD/TN ratio of 10). 16S rRNA high-throughput sequencing revealed that HN-AD functional bacteria (Pseudomonas, Bacillus, Paracoccus) with highest abundance (7.61%) played a key role in high saline wastewater treatment. And Halomonas, Nitrincola and Oceanimonas participated in the denitrification process. Moreover, the abundance of genes related to nitrogen removal was the highest (1.90%) revealed by functional genes prediction.

Sequencing batch airlift reactor system for simultaneous removal of nitrogen and organic carbon from synthetic tannery wastewater

Sequencing batch airlift reactor system for simultaneous removal of nitrogen and organic carbon from synthetic tannery wastewater

산기식 막분리 생물막 (MABR) 공정을 이용한 저에너지 고효율 질소제거 시스템 현황 및 향후 개발 전망

산기식 막분리 생물막 (MABR) 공정을 이용한 저에너지 고효율 질소제거 시스템 현황 및 향후 개발 전망

Enhanced nitrogen removal from low C/N wastewater using biodegradable and inert carriers: Performance and microbial shift

This work aims at investigating the enhancing effect of biodegradable and inert carriers on nitrogen removal from low C/N wastewater and revealing temporal changes in community succession. Natural corncobs and commercial fibers were used as biodegradable and inert carriers, respectively. Results showed that the TN removal efficiency was enhanced by 24% and 8.98% using biodegradable and inert carriers, respectively. For corncob carriers, denitrifiers achieved an obvious enrichment and reached a peak on the 30th day. On contrast, inert carriers were more favorable for the enrichment of nitirifiers. Additionally, the dominant denitrifying genus in the corncob system had changed to Dechloromonas, while it remained as Thauera at inert carriers. Finally, the potential coupling pattern of corncob conversion with denitrification in the corncob system was proposed based on the relevant functional enzymes. This work promotes a comprehensive understanding about carrier-enhanced nitrogen removal systems.

Metatranscriptomic analysis of adaptive response of anammox bacteria Candidatus ‘Kuenenia stuttgartiensis’ to Zn(II) exposure

Zn(II) is frequently detected in biological nitrogen removal systems treating high-strength wastewater (e.g., landfill leachate), yet the cellular defense strategies of anammox bacteria against Zn(II) cytotoxicity is largely unknown. To uncover survival mechanisms under Zn(II) stress, responses of enriched anammox bacteria Candidatus 'Kuenenia stuttgartiensis' under exposure of various levels of Zn (II) were investigated through metatranscriptomic sequencing. Although increasing Zn(II) levels (50, 100 and 150mg/L) resulted in decreasing anammox activities (86.1±0.8%, 66.1±1.4% and 43.9±1.5% of the control, respectively), the viable cells in anammox sludge remained stable. Candidatus 'K. stuttgartiensis' possesses a complex network of regulatory systems to confer cells with the ability against Zn(II) toxicity, including functions related to substrate degradation, Zn(II) efflux, chelation, DNA repair, protein degradation, protein synthesis and signal transduction processes. Particularly, in order to maintain Zn(II) homeostasis, Candidatus 'K. stuttgartiensis' upregulated genes encoding RND efflux family (czcA, czcB, czcC, kustd1923 and kuste2279) for exporting Zn(II) actively. These heavy metal exporting genes could act as "sentinel genes" to detect the initial stage of Zn(II) inhibition on anammox bacteria, which might be beneficial to develop a diagnostic approach to predict the risk of operational failure when Zn(II) shock occurs.

Operation adjustments of an electrochemically coupled system for total nitrogen removal and the associated mechanism.

A coupled system consisting of sequencing batch reactor and microbial fuel cell (SBR-MFC) was designed to buffer pH drift and purify wastewater. The addition of nitrifying sludge and the adjustment of hydraulic retention time (HRT) were performed to achieve better removal of total nitrogen (TN). When anaerobic/aerobic/anoxic phases in one cycle were 6/4/2h, the removal efficiency of ammonium was 99.0±1.3%, whereas denitrification was insufficient and the overall removal efficiency of TN was only 29.1±5.8%. When the phases were adjusted to 6/2/4h, the removal efficiencies of ammonium were 100.0±0.0% in both closed and open circuits, and the overall removal efficiencies of TN were 91.4±0.2% and 71.7±4.2%, respectively, improved by 20% in MFC mode; the maximum voltage (200Ω) maintained at 0.1V. Ammonium-oxidizing bacteria (AOB) and Nitrite-oxidizing bacteria (NOB) in the sludge carried out nitrification. The main denitrification pathways in anoxic phase involved polyhydroxyalkanoate (PHA) denitrification by denitrifying glycogen accumulating organisms (GAOs) and electrochemical denitrification by electrochemical active bacteria (EAB). Few polyphosphate accumulating organisms (PAOs) were present, which accounted for poor P removal.

Response of FANIR system to starvation stress: “Dormancy”

Anaerobic ammonium oxidation (Anammox) process has been successfully applied in the nitrogen removal from high-strength wastewaters. However, little information is available for its treatment of low-strength wastewaters. In this study, a Famine Anammox NItrogen Removal (FANIR) system was developed to investigate the effect of long-term substrate starvation at the low nitrogen concentration (the influent total nitrogen was set at ∼1 mg/L). The results showed that the response of FANIR system to the starvation stress took on two phases: the functional decline phase (0–54 day) and the functional stabilization phase (62–116 day). Over the two phases, the Nitrogen Removal Rate (NRR) of anammox reactor firstly dropped sharply; and then came to a constant level. The activity and settleability of Anammox Granular Sludge (AnGS) firstly deteriorated seriously, and then stayed in a stable range. The relative abundance of Anaerobic Ammonium Oxidation Bacteria (AnAOB) firstly decreased markedly, and then approached a steady state with the change of dominant genus from Candidatus Brocadia to Candidatus Kuenenia. The abundance of 16S rRNA gene and hzs gene of AnAOB and their transcription level firstly declined largely as well, and then became stable with the 16S rRNA gene, hzs gene, 16S rRNA and hzs-mRNA of AnAOB at 23.9%, 9.1%, 1.2% and 1.0% of the initial value, respectively. To our delight, the behavior of FANIR system in the functional stabilization phase was proved indeed consistent with the feature for AnAOB to enter the dormancy state. These findings are helpful to understand the physiology of AnAOB over the starvation stress and to promote the extension of anammox process to the treatment of low-strength wastewaters.

A Pilot-Scale Hybrid Adsorption–Biological Treatment System for Nitrogen Removal in Onsite Wastewater Treatment

AbstractOnsite wastewater treatment systems (OWTS) are significant nonpoint sources of nutrients to surface and groundwater worldwide. Advanced OWTS are challenged by highly transient nutrient load...

Responses of nitrogen transformation processes and N2O emissions in biological nitrogen removal system to short-term ZnO nanoparticle stress

Although the adverse effects of ZnO nanoparticles (ZnO NPs) on biological nitrogen removal (BNR) processes have widely been reported, the impacts of ZnO NPs on the whole nitrogen transformation processes, especially the production of nitrous oxide (N2O), a typical greenhouse gas in a BNR system have rarely been systematically studied yet. In this study, the performances of both the nitrification and denitrification processes were investigated and the N2O emission was simultaneously monitored in a sequencing batch reactor (SBR) when exposed to 1, 25 or 50 mg/L ZnO NPs for one cycle. The dose-dependent ZnO NP depression effects were generally observed on denitrification processes, total nitrogen (TN) removal efficiency and N2O emissions but not nitrification process. Meanwhile, the N2O emission was positively correlated with NO2−-N accumulation in the oxic stage. Further investigation showed that the expressions of nitrate (NO3−) reduction associated narG gene were down-regulated with the increase of NP stress, and the transcript ratios of NO2−/NO reduction gene to N2O reduction one (nirK/nosZ and norB/nosZ) decreased. The released Zn2+ from ZnO NPs took an important role in the inhibition of denitrification processes. ZnO NPs addition also induced the dose-dependent variations in the superoxide dismutase (SOD) and catalase (CAT) activities, which probably contributed to the suppression of the excess reactive oxygen species (ROS) generations to mitigate nanotoxicity. The excessive secretion of protein (PN) in tightly bound EPS (TB-EPS) when ZnO NPs were no <25 mg/L further supported the system's potential self-regulation mechanism for nanotoxicity resistance. CapsuleThe effects of ZnO NPs on the whole nitrogen transformation processes in a biological nitrogen removal sequencing batch reactor, including the N2O emissions were investigated. The system's potential self-regulation mechanism for nanotoxicity resistance was addressed.

Achieving Stable Partial Nitritation in an Acidic Nitrifying Bioreactor.

Partial nitritation providing a suitable effluent for subsequent anammox is a critical step in a two-stage autotrophic nitrogen removal system. This study demonstrates an innovative approach for attaining partial nitritation in an acidic bioreactor operating at a slightly low pH (i.e., 5-6). This approach is based on our hypothesis in this study that acid-tolerant ammonia-oxidizing bacteria (AOB) can produce nitrite and protons to self-sustain free nitrous acid (FNA, NO2- + H+ ↔ HNO2) at a parts per million level, as an inhibitor of nitrite-oxidizing bacteria (NOB). With influent nitrogen of about 200 mg/L and operating conditions of high dissolved oxygen, long sludge retention time, and moderate temperature, a lab-scale acidic bioreactor with FNA up to 2 mg of HNO2-N/L successfully established stable nitrite accumulation in the effluent for 200 days, with an average ratio [NO2-/(NO2- + NO3-)] exceeding 95%. A 16S rRNA amplicon sequencing analysis showed that Nitrosospira was the dominant AOB in the biomass of the bioreactor, while Nitrosomonas and Nitrospira, two typical nitrifying genera in neutral wastewater treatment, both disappeared after the startup of partial nitritation. Kinetic characterization revealed that Nitrosospira had a substrate affinity of 11.4-16.5 mg of total ammonia (NH4+ + NH3)/L. It also revealed that less than 3.5 mg of HNO2-N/L FNA did not inhibit AOB activity significantly. Acidic operation is economically attractive because it can be achieved via acidophilic ammonia oxidation without adding chemical acid. However, hazardous gas, nitric oxide (NO), should be removed from gas produced by acidic nitrifying bioreactors.

Nitrous Oxide and Methane Dynamics in Woodchip Bioreactors: Effects of Water Level Fluctuations on Partitioning into Trapped Gas Phases.

Woodchip bioreactors (WBRs) are low-cost, passive systems for nonpoint source nitrogen removal at terrestrial-aquatic interfaces. The greenhouse gases nitrous oxide (N2O) and methane (CH4) can be produced within WBRs, and efforts to reduce N2O and CH4 emissions from WBR systems require improved understanding of the biogeochemical and physical-chemical mechanisms regulating their production, transport, and release. This study evaluates the impact of trapped gas-filled void volumes as sinks of dissolved gases from water and as sources of episodic fluxes when water levels fall. Dissolved gas tracer experiments in a laboratory bioreactor were used to parameterize nonequilibrium advection-dispersion-gas transfer models and quantify trapping of gas-filled voids as a function of antecedent hydrological conditions. Experiments following a water-level rise revealed that up to 24% of the WBR pore volume was occupied by trapped gas phases, which were primarily located in pore spaces inside woodchips. This finding was confirmed with X-ray-computed microtomography. N2O (3.3-10%) and CH4 (4.3-14%) injected into the reactor following a water table rise partitioned into gas-filled voids and were released when water tables fell. In the case of N2O, partitioning into trapped gas phases makes N2O unavailable for enzymatic reduction, potentially enhancing N2O fluxes under fluctuating water levels.

Long solids retention times and attached growth phase favor prevalence of comammox bacteria in nitrogen removal systems

The discovery of the complete ammonia oxidizing (comammox) bacteria overturns the traditional two-organism nitrification paradigm which largely underpins the design and operation of nitrogen removal during wastewater treatment. Quantifying the abundance, diversity, and activity of comammox bacteria in wastewater treatment systems is important for ensuring a clear understanding of the nitrogen biotransformations responsible for ammonia removal. To this end, we conducted a yearlong survey of 14 full-scale nitrogen removal systems including mainstream conventional and simultaneous nitrification-denitrification and side-stream partial nitrification-anammox systems with varying process configurations. Metagenomics and genome-resolved metagenomics identified comammox bacteria in mainstream conventional and simultaneous nitrification-denitrification systems, with no evidence for their presence in side-stream partial nitrification-anammox systems. Further, comammox bacterial diversity was restricted to clade A and these clade A comammox bacteria were detected in systems with long solids retention times (>10 days) and/or in the attached growth phase. Using a newly designed qPCR assay targeting the amoB gene of clade A comammox bacteria in combination with quantitation of other canonical nitrifiers, we show that long solids retention time is the key process parameter associated with the prevalence and abundance of comammox bacteria. The increase in comammox bacterial abundance was not associated with concomitant decrease in the abundance of canonical nitrifiers; however, systems with comammox bacteria showed significantly better and temporally stable ammonia removal compared to systems where they were not detected. Finally, in contrast to recent studies, we do not find any significant association of comammox bacterial prevalence and abundance with dissolved oxygen concentrations in this study.

The effects of influent and operational conditions on nitrogen removal in an upflow microaerobic sludge blanket system: A model-based evaluation

Recently, upflow microaerobic sludge blanket (UMSB) system has been developed to remove ammonium and organic matter simultaneously. This study aims to establish influent and operational conditions promoting anammox-based nitrogen removal process in the UMSB reactor by using a modified Activated Sludge Model. Experiments were performed on a laboratory-scale UMSB reactor treated piggery wastewater for over two years. With the experimentally determined model parameters, the established model well simulated the UMSB reactor performance. The maximum anammox growth rate was calibrated to be 0.41 d−1 at 35 °C. Further simulations showed that UMSB reactor operated with high influent organics or nitrogen loading rates at temperature above 15 °C can achieve efficient nitrogen removal (>70%). The nitrogen loading over 0.6 kg N/(m3·d)) significantly favors anammox activity. UMSB could also be a promising system for nitrogen removal from low-strength ammonium wastewater with fluctuated COD influence. These results provide support to UMSB design and operational optimization.

Metagenomic analysis of quorum sensing systems in activated sludge and membrane biofilm of a full-scale membrane bioreactor

Biofouling hampers the long-lasting operation of membrane bioreactor (MBR) and quorum sensing (QS) can be applied to control biofouling. In this study, N-acyl homoserine lactones (AHLs), QS/quorum quenching (QQ) functional genes, and microbial communities in activated sludge and membrane biofilm from a full-scale MBR were analyzed. Types and concentrations of AHLs in activated sludge and membrane biofilm were remarkably different. The major AHLs detected in membrane biofilm were N-decanoyl-L-homoserine lactone, N-dodecanoyl-L-homoserine lactone and N-3-oxo-decanoyl-L-homoserine lactone, while N-butyryl-L-homoserine lactone and N-octanoyl-L-homoserine lactone were dominant in activated sludge. The concentration of AHLs in membrane biofilm (1439.4 ng/g VSS) was 1.1 folder higher than that in activated sludge (676.7 ng/g VSS). All QS systems, AHLs, AI-2, DSF, PQS, AIP and c-di-GMP, were enhanced in membrane biofilm, with more types and higher abundances of genes and microorganisms. The QQ system was suppressed in membrane biofilm. Additionally, Nitrospira, Nitrosomonas, Candidatus Accumulibacter and Dechloromonas, the dominant microorganisms for nitrogen removal in membrane biofilm, also harbored diverse QS/QQ genes. The microorganisms responsible for both nitrogen removal and QS/QQ accounted for 14.1% in total bacteria of membrane biofilm (while 7.8% in activated sludge). The nitrogen removal system might contribute to the biofouling formation. This study provided useful information for biofouling control by regulating QS and QQ in full-scale MBRs.

An A2O-MBR system for simultaneous nitrogen and phosphorus removal from brewery wastewater

Anaerobic/Anoxic/Oxic – Membrane BioReactor (A2O-MBR) system was used to enhance simultaneous removal of nitrogen and phosphorus from brewery wastewater. The A2O unit containing microorganisms with short solids retention time (SRT) was employed mainly for removal of organic matter and phosphorus together with denitrification. The MBR containing microorganisms with long SRT was employed mainly for nitrification of NH4+-N and recirculation of NO3--N. The model of A2O-MBR system made from polyacrylic with the capacity of 49.5 liters was operated with hydraulic retention times decreased from 24, 18 to 12 hours corresponding to organic loading rates increased from 0.50, 0.75 to 1.00 kg COD/m3.day. The results showed that the model not only treated organic matter well but also nearly completely removed both nitrogen and phosphorus. For all three loading rates, chemical oxygen demand (COD) concentration decreased significantly in the anaerobic and anoxic compartments of the A2O unit, indicating that most of organic matter was utilized in the anaerobic and anoxic compartments for phosphorus release and denitrification, respectively. Nitrification in the MBR was almost perfectly completed, with average NH4 +-N removal efficiencies of over 98%. Denitrification in the anoxic compartment happened as much as possible. Demands for the development of PAOs, which were responsible for enhanced biological phosphorus removal (EBPR) processes, could be provided. For loading rate of 0.75 kg COD/m3.day, treatment efficiencies of COD, NH4+-N, total nitrogen (TN) and total phosphorus (TP) of the model were the highest as 95.4, 99.2, 86.7 and 84.6%, respectively. Output values of these parameters were within the limits of Vietnam National Technical Regulation on Industrial Wastewater (QCVN 40:2011/BTNMT), column A. The model of A2O-MBR system was capable of achieving effluents with very low nitrogen and phosphorus concentrations from brewery wastewater.&#x0D;

Cooperation between partial-nitrification, complete ammonia oxidation (comammox), and anaerobic ammonia oxidation (anammox) in sludge digestion liquid for nitrogen removal

The challenge of sludge digester liquor treatment is its high ammonium nitrogen (NH4+-N) concentration. Early reports found that complete ammonia oxidation (comammox) was not present and anaerobic ammonia oxidation (anammox) was difficult to achieve in most sludge digester liquor treatments. In this study, NH4+-N removal by cooperation between partial-nitrification, comammox, and anammox processes was achieved in a sequencing batch reactor (SBR) for sludge digester liquor treatment. The results showed that 2100–2200 mg/L of NH4+-N was removed in the SBR with 98.82% removal efficiency. In addition, 55.11% of NH4+-N was converted to nitrite nitrogen (NO2−-N) by partial-nitrification, 25.43% of NH4+-N was converted to nitrate nitrogen (NO3−-N) by comammox, and 18.28% of NH4+-N was removed by anammox. During the operation, in the SBR, the relative abundance of the dominant ammonia-oxidizing bacteria (Chitinophagaceae) was 18.89%, that of the dominant anammox bacteria (Candidatus Kuenenia) was 0.10%, and that of the dominant comammox bacteria (Nitrospira) was 0.20%. Therefore, the high nitrogen removal efficiency in this system was considered the result of the combination of the three processes. These results showed that comammox and anammox could play very important roles in nitrogen transformation and energy-saving in nitrogen removal systems.