Nitrogen Removal Process Research Articles

Transformation fate of bisphenol A in aerobic denitrifying cultures and its coercive mechanism on the nitrogen transformation pathway.

Bisphenol A (BPA) is a commonly used endocrine-disrupting chemical found in high levels in wastewater worldwide. Aerobic denitrification is a promising alternative to conventional nitrogen removal processes. However, the effects of BPA on this novel nitrogen removal process have rarely been reported. Herein, we investigated the removal and interaction effects of BPA (0, 0.1, 1, and 10 mg/L) in aerobic denitrifying cultures. Our experimental results demonstrated that the aerobic denitrification system could remove 66%-86% of BPA from wastewater. Fourier transform infrared spectroscopy revealed that polysaccharides and amides were the primary sites for adsorption. An increase in the type and number of intermolecular hydrogen bonds might enhance the ability of aerobic denitrifying cultures to adsorb BPA. Adsorption kinetics analysis demonstrated that inhomogeneous multilayer adsorption was the leading cause of BPA removal. Adsorbed BPA decreased the sedimentation, flocculation, and hydrophobicity of aerobic denitrifying cultures, triggering changes in the levels of proteins and polysaccharides in extracellular polymeric substances. As the influent BPA increased from 0 to 10 mg/L, the nitrate-nitrogen and total organic carbon in the reactor effluent increased from 0.4 ± 0.2 and 26 ± 7.9 mg/L to 18.8 ± 9.3 and 116.2 ± 55.6 mg/L, respectively. BPA (initial concentration range: 1-10 mg/L) significantly influenced the abundance of genes involved in the nitrogen transformation pathway, contributing to the increase in the abundance of gaseous NOx-transformed genes and altering the relative abundance of denitrifying bacteria, particularly Thauera. Correlation analyses revealed that Pseudomonas, Thauera, and AKYH767 are important for maintaining systemic nitrogen transformations and BPA adsorption.

Biotic factors shape the structure and dynamics of denitrifying communities within cyanobacterial aggregates.

Eutrophication caused by human activities has severely impacted freshwater ecosystems, leading to harmful cyanobacterial blooms that threaten water quality and ecosystem stability. During blooms, denitrification is a key process for nitrogen removal, which can occur both in the sediment and in the waterbody mediated by cyanobacterial aggregate (CA)-associated microorganisms. In this study, the structure, dynamics and assembly mechanisms of CA-associated nirK-, nirS-, and nosZ-encoding denitrifying communities were investigated in the eutrophic Lake Taihu across the bloom season. External environmental factors showed limited influence on denitrifying community structure, which were more strongly shaped by biotical factors. Network analysis revealed both monofunctional and multifunctional modules, with a small subset of linker OTUs playing a critical role in maintaining these multifunctional modules by mediating interactions among different functional communities. Biotic regulation was further demonstrated by coupling patterns among denitrifiers associated with specific microbial lineages, as well as niche partitioning driven by cyanobacterial composition. While the assembly of the total phycospheric communities was governed by stochastic processes, the denitrifying communities were predominantly shaped by homogeneous selection. These findings indicate biological processes, particularly synergistic relationships and autotroph-heterotroph interactions, were key drivers of the structure and dynamics of denitrifying communities within CAs. Moreover, the key taxa identified in this study may provide potential targets for regulating nitrogen cycling in the management of cyanobacterial blooms.

Double-edged effects and regulation mechanism of hydroxylamine in novel nitrogen removal processes: A comprehensive review

Double-edged effects and regulation mechanism of hydroxylamine in novel nitrogen removal processes: A comprehensive review

Microbially mediated iron redox processes for carbon and nitrogen removal from wastewater: Recent advances.

Iron is the most abundant redox-active metal on Earth. The microbially mediated iron redox processes, including dissimilatory iron reduction (DIR), ammonium oxidation coupled with Fe(III) reduction (Feammox), Fe(III) dependent anaerobic oxidation of methane (Fe(III)-AOM), nitrate-reducing Fe(II) oxidation (NDFO), and Fe(II) dependent dissimilatory nitrate reduction to ammonium (Fe(II)-DNRA), play important parts in carbon and nitrogen biogeochemical cycling globally. In this review, the reaction mechanisms, electron transfer pathways, functional microorganisms, and characteristics of these processes are summarized; the prospective applications for carbon and nitrogen removal from wastewater are reviewed and discussed; and the research gaps and future directions of these processes for the treatment of wastewater are also underlined. This review is expected to give new insights into the development of economic and environmentally friendly iron-based wastewater treatment procedures.

Gas-delivery membrane as an alternative aeration method to remove dissolved methane from anaerobically treated wastewater.

Dissolved methane is a hurdle for anaerobic wastewater treatment, which would be stripped into the atmosphere by conventional bubble aeration and increase the release of greenhouse gases into the environment. The high oxygen transfer efficiency and less turbulence in membrane aerated biofilm reactor (MABR) could prevent the stripping of dissolved methane. In this study, an MABR was established to remove dissolved methane aerobically in parallel to the nitrogen removal driven by the anammox process. The long-term results demonstrated that aerobic methane oxidation has a short start-up period, in which a high level (>90 %) of dissolved methane removal was achieved in 20 days. Meanwhile, the anammox-based nitrogen removal process reached a total nitrogen removal rate of ∼150 mg N/L/d (0.27 g N/m2/d). In situ batch tests confirmed the active bioreactions of ammonia-oxidizing bacteria, nitrite-oxidizing bacteria, anammox bacteria and aerobic methanotrophs, while 16S rRNA gene amplicon sequencing further validated their existence. Moreover, nitrite/nitrate-dependent anaerobic methane oxidation (n-DAMO) bacteria were enriched to a relative abundance of 2.5 % on Day 372, suggesting their potential role in removing nitrogen and dissolved methane in the MABR. This study provides an alternative technology for removing dissolved methane and nitrogen in parallel from anaerobically treated wastewater.

Effects of pristine and photoaged tire wear particles and their leachable additives on key nitrogen removal processes and nitrous oxide accumulation in estuarine sediments.

Despite growing attention to the environmental pollution caused by tire wear particles (TWPs), the effects of pristine and photoaged TWPs (P-TWPs and A-TWPs) and their TWP leachates (TWPLs; P-TWPL and A-TWPL) on key nitrogen removal processes in estuarine sediments remain unclear. This study explores the responses of the denitrification rate, anammox rate, and nitrous oxide (N2O) accumulation to P-TWP, A-TWP, P-TWPL, and A-TWPL exposure in estuarine sediments, and assesses the potential biotoxic substances present in TWPLs. P-TWPs reduced the denitrification rate by 17.1 ± 10.0 % and increased N2O accumulation by 28.1 ± 18.7 %. The A-TWPs not only reduced the denitrification rate by 31.3 ± 8.3 % and increased N2O accumulation by 43.1 ± 22.0 %, but also decreased the anammox rate by 22.1 ± 13.3 %. A-TWPs further inhibited the denitrification rate by reducing nitrate reductase activity and the abundance of its gene (narG), while simultaneously decreasing hydrazine synthase activity and the abundance of its gene (hzo), thereby slowing the anammox rate. N2O accumulation after exposure to TWPs and TWPLs was positively correlated with the activity ratio of N2O-producing and N2O-consuming enzymes. Zinc (Zn) release in A-TWPL was 48.5 ± 6.9 % higher than that in P-TWPL, which is a crucial reason for the higher biotoxicity produced by A-TWPs. In addition, the abundance of denitrifying and anammox bacteria closely linked to the Zn, manganese, and arsenic concentrations in the TWPLs. This study provides insights into assessing the environmental risks posed by TWPs to estuarine ecosystems.

Formation and Fate of Reactive Nitrogen during Biological Nitrogen Removal from Water: Important Yet Often Ignored Chemical Aspects of the Nitrogen Cycle.

Hydroxylamine, nitrous acid, and nitric oxide are obligate intermediates or side metabolites in different nitrogen-converting microorganisms. These compounds are unstable and susceptible to the formation of highly reactive nitrogen species, including nitrogen dioxide, dinitrogen trioxide, nitroxyl, and peroxynitrite. Due to the high reactivity and cytotoxicity, the buildup of reactive nitrogen can affect the interplay of microorganisms/microbial processes, stimulate the reactions with organic compounds like organic micropollutants (OMP) and act as the precursors of nitrous oxide (N2O). However, there is little understanding of the occurrence and significance of reactive nitrogen during biological nitrogen conversions in engineered water systems. In this review, we evaluate the formation and fate of reactive nitrogen produced by microorganisms involved in biological nitrogen removal (BNR) processes, i.e., nitritation/nitrification, denitratation/denitrification, anammox, and the combined processes. While the formation of reactive nitrogen intermediates is entirely controlled by microbial activities, the consumption can be either biological or purely chemical. Changes in environmental conditions, such as redox transition, pH, and substrate availability, can imbalance the production and consumption of these reactive intermediates, thus leading to the transient accumulation of species. Based on previous experimental evidence, environmental relevance of reactive nitrogen in BNR systems, particularly related to abiotic N2O production and OMP transformation, is demonstrated.

Exploring the Microbial Ecology of Nitrification Denitrification in Wastewater Treatment: Innovations and Future Directions

The nitrification-denitrification process serves as a critical mechanism in the biological removal of nitrogen from wastewater, a necessity for mitigating the environmental impact of nitrogen compounds on aquatic ecosystems. This complex process is orchestrated by diverse microbial communities, whose ecological interactions and functional capabilities are central to the efficiency and stability of nitrogen removal. In recent years, advances in molecular biology and microbial ecology have shed new light on the intricate dynamics within these microbial communities, revealing the roles of lesser known microbial taxa and the importance of microbial community structure and function in optimizing nitrification denitrification processes. This review provides a comprehensive examination of the microbial ecology involved in nitrification and denitrification, exploring the roles of key microbial players, including ammonia oxidizing bacteria (AOB), Nitrite oxidizing Bacteria (NOB), ammonia oxidizing archaea (AOA), and denitrifying bacteria. The review also delves into the environmental factors that influence microbial community dynamics, such as pH, temperature, oxygen availability, and the presence of inhibitory substances. By understanding these factors, the review highlights the challenges of maintaining stable and efficient nitrogen removal processes, particularly in the face of environmental fluctuations and operational disturbances, the review discusses recent innovations in wastewater treatment technologies that leverage microbial ecology to enhance process efficiency. These include bio-augmentation strategies, where specific microbial strains are introduced to bolster nitrification and denitrification, and advanced bioreactor designs, such as moving bed biofilm reactors (MBBRs) and membrane bioreactors (MBRs), which provide optimized environments for microbial growth and activity. The integration of anaerobic ammonium oxidation (anammox) processes, which offer significant energy savings and reduce greenhouse gas emissions, is also explored as a promising alternative to traditional nitrification pathways.

Insights into the influence of organic and salinity on the two-stage partial nitritation/anammox process in treating food waste digestate

ABSTRACT Food waste digestate (FWD), which contains significant levels of ammonium, organic matter, and salinity, can interfere with treatment performance of the anammox process. In this study, a two-stage partial nitritation/anammox (PN/A) process was established to investigate nitrogen removal and microbial response in treating FWD at a nitrogen loading rate (NLR) of 0.27 ± 0.02 gN/L/d. High concentrations of free ammonia (58 mg/L) and free nitrous acid (0.3 mg/L) facilitated the initiation of the partial nitritation (PN) process, achieving an average NO2 −/NH4 + ratio of 1.28 ± 0.08. For the anammox process, a nitrogen removal rate (NRR) of 0.72 ± 0.13 gN/L/d was achieved. Free ammonia (NH3) stripping, Anammox pathway, and denitrification pathway contributed 4.1 ± 0.3%, 5.1 ± 0.2%, and 84.0 ± 1.5% of the total nitrogen removal, respectively. Nitrosomonas, a salt-tolerant ammonia-oxidizing bacteria (AOB), was enriched to 1.0%, while Nitrospira, a nitrite-oxidizing bacteria (NOB), was effectively suppressed to 0.003%. The salt-tolerant anammox genera unclassified_f__Brocadiaceae (13.9%) and Candidatus_Kuenenia (4.8%) dominated the nitrogen removal pathway. The high enrichment of unclassified_f__Brocadiaceae ensured stable operation of the anammox process at 0.62 ± 0.11% salinity, even with a high initial FA inhibition concentration of 40 mg/L. Additionally, norank_f_A4b (1.34%) and norank_f_norank_o_SBR1031 (52.1%) facilitated the hydrolysis of refractory organic matter. Denitrifying bacteria, including Hyphomicrobium, Truepera, and unclassified_c__Alphaproteobacteria, played significant roles in nitrate removal, with a CODconsumed/NO3 − removed ratio of 2.7 ± 0.2. This study highlights the application of a two-stage PN/A process for rapid startup and effective nitrogen removal from FWD.

Making waves: Harnessing anammox bacteria coupled with dissimilatory nitrate reduction to ammonium for sustainable wastewater management.

Anaerobic ammonia oxidation (anammox) which converts nitrite and ammonium to dinitrogen gas is an energy-efficient nitrogen removal process. One of the bottlenecks for anammox application in wastewater treatment is the stable supply of nitrite for anammox bacteria. Dissimilatory nitrate reduction to ammonium (DNRA) is a process that converts nitrate to nitrite and then to ammonium. Significantly, it has been reported that some anammox bacteria can perform DNRA by reducing nitrate to nitrite and ammonium nitrogen with little low-molecular-weight organic acids such as volatile fatty acids. Here, we propose an innovative nitrogen removal process, i.e., nitrification and anammox coupled with partial DNRA (i.e., NPDA), and make a theoretical comparison with previously accepted partial nitrification and anammox (PNA) and partial denitrification and anammox (PdNA) for nitrogen removal. Under similar conditions of oxygen consumption, removal efficiency, external carbon source addition, and greenhouse gas emission, the novel NPDA process can better facilitate resource-effective and environment-friendly wastewater treatment. Thermodynamic analysis indicates that partial DNRA-anammox appears to be preferred, oxidizing per mole of NH4 +produces higher energy gain than that of conventional anammox alone. The carbon source limitation rather than nitrate limitation is the key to the realization of NPDA process. This perspective highlights the positive role of DNRA for sustainable wastewater management.

A green process for total nitrogen removal without extra energy consumption: Synergistic actions for wastewater treatment

A green process for total nitrogen removal without extra energy consumption: Synergistic actions for wastewater treatment

The novel Dirammox process for advanced nitrogen removal from high COD content and ammonium-rich wastewater

The novel Dirammox process for advanced nitrogen removal from high COD content and ammonium-rich wastewater

Research progress on solid-phase electron donors for the denitrification of wastewater: A review

Electron donor-mediated biological denitrification, the most extensively employed method for the treatment of nitrate nitrogen (NO3−-N), involves NO3−-N directly or indirectly by acquiring electrons provided by electron donors and converting them into N2. Currently, the most widely researched electron donors are gaseous, liquid, and solid forms. Owing to the difficulties in storing and transporting gaseous and liquid electron donors, and their potential for causing secondary pollution, solid-phase electron donors (SPEDs), which can be slowly utilized by microorganisms, have gradually gained attention. SPEDs not only serve as a carrier for microbial attachment, but most SPEDs are also low-cost and readily available, making them advantageous for practical applications. In this review, the different types of SPEDs are classified and their microbial utilization mechanisms in the biological denitrification process discussed based on their classification. Their denitrification performance, influencing factors, practical applications, and existing issues are summarized. This review provides a reference for future research on SPED and its applications. It also provides an outlook on SPED-mediated mixotrophic denitrification and SPED-coupled electrochemical technology for enhanced nitrogen removal processes, in view of this hot direction in SPED research.

Mechanically Induced Green Targeted Conversion of Ammonia Nitrogen to N2: Based on Cavitation Effects and ROS Oxidation.

Addressing the mounting challenge of ammonia nitrogen pollution in aquatic ecosystems necessitates the selective oxidation of ammonia nitrogen to nitrogen gas, a pivotal aspect of eco-friendly nitrogen removal processes. Ultrasound cavitation, renowned for its capacity to generate reactive oxygen species (ROS), has garnered considerable attention in environmental remediation. This study reveals a highly synergistic mechanism in ultrasound coupled stirring (US-ST), establishing optimal coupling conditions through sound field monitoring and quantification of ROS. In comparison to ultrasound treatment alone (US), the sound pressure amplitude significantly increased from ±18 to ±30 kPa in US-ST, markedly reducing the cavitation nucleation threshold and augmenting the steady-state concentration of hydroxyl radicals (HO•) by 13-fold. Further, with appropriate charge transfer conditions enabled by the acoustoelectric characteristics of the passive film on stirring paddles, the concentrations of superoxide (•O2-) and singlet oxygen (1O2) elevated to 9.54 × 10-10 M and 8.43 × 10-13 M, respectively. Under the regulation of 500 rpm stirring vortex, a maximum sonochemical efficiency of 6.5 × 10-5 mg J-1 was achieved. In the context of domestic wastewater, ammonia nitrogen degradation was achieved through the oxidation and thermal dissociation effects of US-ST. The concentration decreased from 27.5 to 3.4 mg/L after 2 h, with an impressive N2 selectivity of 96.8%. This study elucidates the targeted conversion mechanism of ammonia nitrogen in US-ST, introducing an emerging water treatment technology propelled by mechanical energy.

Single-stage versus two-stage partial nitritation - anammox reactor systems for deammoniafication under hypersaline conditions

The production of increasing amounts of high salinity wastewaters in our industrialized society has prioritized their treatment to prevent environmental pollution. The partial nitritation - anammox (PN/A) process for nitrogen removal has been little investigated for hypersaline wastewaters (salinity greater than 3%). In the investigation presented here, single-stage versus two-stage partial nitritation - anammox (PN/A) reactor systems for deammonification at 4% (40 g/kg) saline conditions were investigated and compared in completely mixed fixed bed reactors. In the two-stage system, the first stage reactor achieved a nitritation rate of 1.9 gN/L-reactor/d. Effluent from the partial nitritation reactor was then fed to the second two-stage anammox reactor and the maximal nitrogen removal of 0.8 g/L-reactor/d was achieved. The dominant microbial species for the ammonia oxidizing and anammox reactions in the nitritation (first) reactor and the second reactor were identified as Nitrosococcus oceani and Candidatus Scalindua wagneri, respectively, both obligate halophiles. In the single-stage reactor, deammonification rates reached 0.6 gN/L-reactor/d. Nitrosomonas marina and Candidatus Scalindua wagneri were the dominant AOB and anammox bacteria, respectively. Maintaining free ammonia (FA) concentrations above 1 mg/L was found to selectively inhibit nitrite oxidizing bacteria (NOB) and resulted in long term stable nitritation. At FA concentrations lower than 1 mg/L, nitrate began to appear after 20 days of reactor operation. Nitritation was recovered after increasing FA in the reactor to inhibitory concentrations. Overall N2O emissions were shown to be significantly lower in the single-stage PN/A reactor than the two stage PN/A reactor system.

Nitrogen removal capacity and influencing factors of inland wetland under human activities: Jiashan County, China

AbstractInland wetlands play a vital role in mitigating non‐point source nitrogen loads through denitrification and anammox processes. However, the impact of varying human activities within the same region on these nitrogen removal processes of inland wetlands remains unclear. This study investigated the differences in nitrogen removal rates of wetland sediments under various human activities in Jiashan County, China. 15N isotope tracing was used to determine denitrification and anammox rates. Denitrification was the primary nitrogen removal pathway, with an average contribution of 82.34%. Denitrification and anammox rates varied significantly under different human influences, with the highest rates in residential and agricultural areas. NH4+, chlorophyll a, NO3−, and pH were the main environmental factors. This study highlights the need for targeted wetland management and restoration strategies based on the type and intensity of human activities. Protecting and restoring these “natural purifiers” is crucial for improving regional water quality and maintaining the ecological functions of wetlands in rapidly urbanizing landscapes.

In Situ Enrichment of Anammox Bacteria from Pig Farm Anoxic Sludge Through Co-Cultivation with a Quorum-Sensing Functional Strain Pseudomonas aeruginosa

Anaerobic ammonium oxidation (anammox), as an efficient and low-carbon method for nitrogen removal from wastewater, faces the challenge of slow enrichment of functional bacteria. In this study, the enrichment of anammox bacteria Candidatus Brocadia was successfully accelerated by co-culturing with the quorum-sensing strain Pseudomonas aeruginosa and anoxic sludge from a pig farm. Experimental results showed that the R2, which had Pseudomonas aeruginosa added, exhibited chemical reaction ratios RS (NO2−-N consumption/NH4+-N consumption) and RP (NO3−-N production/NH4+-N consumption) closer to the theoretical values of the anammox reaction since Phase Ⅱ. Bacterial community analysis indicated that the abundance of Candidatus Brocadia in R2 reached 1.63% in cycle 20, significantly higher than the 0.45% in R1. More quorum-sensing signaling molecules, primarily C6-HSL, were detected in R2. C6-HSL was positively correlated with processes such as the secretion of anammox extracellular polymers (EPS) and the regulation of nitric oxide reductase (Nir), which may explain the reason behind the accelerated increase in the abundance of Candidatus Brocadia through co-culturing. Moreover, the metabolism of the dominant genus Paracoccus within the two groups of reactors also showed positive regulation by C6-HSL, with its abundance trend similar to that of Candidatus Brocadia, jointly completing the nitrogen removal process in the reactors. However, it is still unknown which genera secrete large amounts of C6-HSL after inoculation with Pseudomonas aeruginosa. This research provides a novel and low-cost method for the enrichment of anammox bacteria.

Novel strategies for enhancing energy metabolism and wastewater treatment in algae-bacteria symbiotic system through carbon dots-induced photogenerated electrons: The definitive role of accelerated electron transport

The nitrogen removal efficiency of the algae-bacteria symbiotic system (ABSS) remains challenging due to competition for light capture and insufficient electron donors in aquaculture wastewater. To overcome this bottleneck issue, we first identified and reported the introduction of photogenerated electrons into ABSS by coupling them with carbon dots (CDs). The energy metabolism (21.52 %-56.25 %) and pollutant degradation efficiency (89.36 %-96.42 %) of ABSS/CDs surpassed the biochemical limit observed in traditional ABSS. Notably, the addtion of CDs facilitated an opening of the plastoquinone-pool (PQ-pool) valve and accelerated electron transfer rates, enabling photogenerated electrons to selectively target light-harvesting complexs as activation sites, thereby mobilizing photosynthesis and respiratory system activity to enhance nicotinamide adenine dinucleotide reduced (NADH) and adenosine triphosphate (ATP) production while promoting tricarboxylic acid (TCA) and Calvin cycle processes. In simulated swine wastewater treatment, ABSS/CDs achieved higher removal rates for the chemical oxygen demand (COD), total nitrogen (TN) and ammonium-nitrogen (NH4+-N) (100 %, 99.23 % and 98.62 % respectively) compared to pure culture systems alone. Network-based analysis revealed that ABSS/CDs improved microbial community structure stability within the photobioreactor while demonstrating efficient coupling between fungi, nitrifying bacteria, and denitrifying bacteria for enhanced performance in nitrogen removal processes. The upregulation of nitrogen metabolism and PQ-pool genes maintains efficient electron transport rates and contributes to nitrogen removal. This study expands our understanding of the physiological aptitudes of ABSS and provides valuable insights into applying CDs photoelectrons for enhanced wastewater treatment.

Runoff variation alters estuarine sediment microbiome and nitrogen removal processes by affecting salinity

Runoff variations shape the dynamics of the estuarine environmental factors, profoundly influencing the nitrogen cycle in estuarine sediments. However, our understanding of how these changes regulate microbially-mediated nitrogen removal processes remains limited. In this study, the impacts of changes in environmental factors caused by normal and low runoffs on denitrification and anammox in sediments of the Liao River Estuary in China, were investigated, using continuous-flow experiments combined with 15N tracing techniques and molecular methods. Results indicated that denitrification was the main nitrogen removal process in estuarine sediments under both runoff conditions. Elevated salinity under low runoff condition increased the abundance of nitrifying bacteria (Nitrospina, Nitrosomonas and Nitrosomonadaceae), thereby promoting the coupled nitrification-denitrification nitrogen removal process. Furthermore, seawater intrusion under low runoff contributed to dilute nitrite concentrations, resulting in decreased denitrification rates in sediments. Overall, this study highlighted the impacts of runoff variations on biological nitrogen removal process through affecting environmental factors, gene abundance and microbial community in the estuary.

Revisit the role of hydroxylamine in sulfur-driven autotrophic denitrification

Through dedicated batch tests using the enriched sludge dominated by sulfur-oxidizing bacteria (SOB), the potential transformation of hydroxylamine (NH2OH) by SOB and the effects of NH2OH on the rate-limiting sequential reduction processes of sulfur-driven autotrophic denitrification (SDAD) were systematically explored in this study. The results indicated that NH2OH might be first converted to NO by SOB and then participate in the SDAD process, thus accelerating the utilization of S2- and contributing to the formation of N2O. Up to 3.5 mg-N/L NH2OH didn't affect the NO3- or NO2- reduction of SDAD, during which no significant changes were observed for the NH2OH concentration. Comparatively, even though NH2OH had no direct impact on the N2O reduction of SDAD, it could be consumed and therefore affect the depletion of N2O indirectly by regulating the toxic effect and electron supply of S2-. These findings provide novel implications for applying NH2OH to SDAD-based integrated processes for biological nitrogen removal.