Reducing N2O Emission from a Domestic-Strength Nitrifying Culture by Free Nitrous Acid-Based Sludge Treatment

How to Provide Nitrite Robustly for Anaerobic Ammonium Oxidation in Mainstream Nitrogen Removal

Innovation in decarbonizing wastewater treatment is urgent in response to global climate change. The practical implementation of anaerobic ammonium oxidation (anammox) treating domestic wastewater is the key to reconciling carbon-neutral management of wastewater treatment with sustainable development. Nitrite availability is the prerequisite of the anammox reaction, but how to achieve robust nitrite supply and accumulation for mainstream systems remains elusive. This work presents a state-of-the-art review on the recent advances in nitrite supply for mainstream anammox, paying special attention to available pathways (forward-going (from ammonium to nitrite) and backward-going (from nitrate to nitrite)), key controlling strategies, and physiological and ecological characteristics of functional microorganisms involved in nitrite supply. First, we comprehensively assessed the mainstream nitrite-oxidizing bacteria control methods, outlining that these technologies are transitioning to technologies possessing multiple selective pressures (such as intermittent aeration and membrane-aerated biological reactor), integrating side stream treatment (such as free ammonia/free nitrous acid suppression in recirculated sludge treatment), and maintaining high activity of ammonia-oxidizing bacteria and anammox bacteria for competing oxygen and nitrite with nitrite-oxidizing bacteria. We then highlight emerging strategies of nitrite supply, including the nitrite production driven by novel ammonia-oxidizing microbes (ammonia-oxidizing archaea and complete ammonia oxidation bacteria) and nitrate reduction pathways (partial denitrification and nitrate-dependent anaerobic methane oxidation). The resources requirement of different mainstream nitrite supply pathways is analyzed, and a hybrid nitrite supply pathway by combining partial nitrification and nitrate reduction is encouraged. Moreover, data-driven modeling of a mainstream nitrite supply process as well as proactive microbiome management is proposed in the hope of achieving mainstream nitrite supply in practical application. Finally, the existing challenges and further perspectives are highlighted, i.e., investigation of nitrite-supplying bacteria, the scaling-up of hybrid nitrite supply technologies from laboratory to practical implementation under real conditions, and the data-driven management for the stable performance of mainstream nitrite supply. The fundamental insights in this review aim to inspire and advance our understanding about how to provide nitrite robustly for mainstream anammox and shed light on important obstacles warranting further settlement.

Effect and microbial mechanism of pharmaceutical and personal care product exposure on partial nitrification process and nitrous oxide emission

This study focused on the short- and long-term exposure of pharmaceutical and personal care products (PPCPs) to the partial nitrification process and nitrous oxide emission. The corresponding microbial mechanisms were also explored. The results revealed a concentration-dose effect on the partial nitrification process. Moreover, the PPCP concentration of ≥2 μg/L featured inhibitory effects on the process. The solo effect of PPCP on the partial nitrification process was analyzed through microcosmic experiments, and the results revealed significant variations in PN. A dose-effect relationship existed between the PPCP concentration and N2O emission intensity. After exposure to PPCPs, the N2O emission released during the partial nitrification process was significantly reduced. Different PPCPs featured various effects in mitigating N2O emissions. Low PPCP concentrations led to a reduction in the richness and diversity of microbes, but their community structure remained significantly unchanged. High PPCP concentrations (≥5 μg/L) resulted in increased species richness and diversity, but their microbial community composition was significantly affected. The function prediction and nitrogen metabolic pathway analysis indicated that PPCP exposure led to the inhibition of the ammonia oxidation process. However, all genes encoding denitrification enzymes were upregulated. The microorganisms in the microbial community featured modular structural properties and wide synergistic relationships between genera. This study provides valuable insights into the effect of PPCP exposure on the particle nitrification process and corresponding changes in the microbial community.

Comparison of typical nitrite oxidizing bacteria suppression strategies and the effect on nitrous oxide emissions in a biofilm reactor

In mainstream partial nitritation/anammox (PN/A), suppression of nitrite oxidizing bacteria (NOB) and mitigation of N2O emissions are two essential operational goals. The N2O emissions linked to three typical NOB suppression strategies were tested in a covered rotating biological contactor (RBC) biofilm system at 21 °C: (i) low dissolved oxygen (DO) concentrations, and treatments with (ii) free ammonia (FA), and (iii) free nitrous acids (FNA). Low emerged DO levels effectively minimized NOB activity and decreased N2O emissions, but NOB adaptation appeared after 200 days of operation. Further NOB suppression was successfully achieved by periodic (3 h per week) treatments with FA (29.3 ± 2.6 mg NH3-N L-1) or FNA (3.1 ± 0.3 mg HNO2-N L-1). FA treatment, however, promoted N2O emissions, while FNA did not affect these. Hence, biofilm PN/A should be operated at relatively low DO levels with periodic FNA treatment to maximize nitrogen removal efficiency while avoiding high greenhouse gas emissions.

Decoupling the simultaneous effects of NO2-, pH and free nitrous acid on N2O and NO production from enriched nitrifying activated sludge

In the pursuit of energy and carbon neutrality, nitrogen removal technologies have been developed featuring nitrite (NO2-) accumulation. However, high NO2- accumulations are often associated with stimulated greenhouse gas (i.e., nitrous oxide, N2O) emissions. Furthermore, the coexistence of free nitrous acid (FNA) formed by NO2- and proton (pH) makes the consequence of NO2- accumulation on N2O emissions complicated. The concurrent three factors, NO2-, pH and FNA may play different roles on N2O and nitric oxide (NO) emissions simultaneously, which has not been systematically studied. This study aims to decouple the effects of NO2- (0-200 mg N/L), pH (6.5-8) and FNA (0-0.15 mg N/L) on the N2O and NO production rates and the production pathways by ammonia oxidizing bacteria (AOB), with the use of a series of precisely executed batch tests and isotope site-preference analysis. Results suggested the dominant factors affecting the N2O production rate were NO2- and FNA concentrations, while pH alone played a relatively insignificant role. The most influential factor shifted from NO2- to FNA as FNA concentrations increased from 0 to 0.15 mg N/L. At concentrations below 0.0045 mg HNO2-N/L, nitrite rather than FNA played a significant role stimulating N2O production at elevated nitrite concentrations. The inhibition effect of FNA emerged with further increase of FNA between 0.0045-0.015 mg HNO2-N/L, weakening the promoting effect of increased nitrite. While at concentrations above 0.015 mg HNO2-N/L, FNA inhibited N2O production especially from nitrifier denitrification pathway with the level of inhibition linearly correlated with the FNA concentration. pH and the nitrite concentration regulated the production pathways, with elevated pH promoting the nitrifier nitrification pathway, while elevated NO2- concentrations promoting the nitrifier denitrification pathway. In contrast to N2O, NO emission was less susceptible to FNA at concentrations up to 0.015 mg N/L but was stimulated by increasing NO2- concentrations. This study, for the first time, distinguished the effects of pH, NO2- and FNA on N2O and NO production, thereby providing support to the design and operation of novel nitrogen removal systems with NO2- accumulation.

Unveiling the roles of biofilm in reducing N2O emission in a nitrifying integrated fixed-film activated sludge (IFAS) system

Biofilm process such as integrated fixed-film activated sludge (IFAS) system has been preliminarily found to produce less nitrous oxide (N2O) than suspended sludge system. However, the N2O emission behaviors and underlying N2O mitigation mechanism in such hybrid system remain unclear. This study therefore aims to fully unveil the roles of biofilm in reducing N2O emission in a nitrifying IFAS system with the aid of some advanced technologies such as N2O microsensor and site-preference analysis. It was found that ammonia oxidation occurred mostly in the sludge flocs (˃ 86%) and biofilm could reduce N2O emission by 43.77% in a typical operating cycle. Biofilm not only reduced nitrite accumulation in nitrification process, inhibiting N2O production via nitrifier denitrification pathway, but also served as a N2O sink, promoting the reduction of N2O via endogenous denitrification. As a result, N2O emissions from the IFAS system were 50%-83% lower than those from the solo sludge flocs. Further, more N2O emission was reduced in the presence of biofilm with decreasing the dissolved oxygen level in the range of 0.5-3.0 mg O2/L. Microbial community and key enzyme analyses revealed that biofilm had relatively high microbial diversity and unique enzyme composition, providing a reasonable explanation for the changed contributions of different N2O production pathways and reduced N2O emission.

Insights into N2O turnovers under polyethylene terephthalate microplastics stress in mainstream biological nitrogen removal process

The ubiquitous microplastics in wastewater have raised growing concerns due to their unintended effects on microbial activities. However, whether and how microplastics affect nitrous oxide (N₂O) (a potent greenhouse gas) turnovers in mainstream biological nitrogen removal (BNR) process remain unclear. This work therefore aimed to fill such knowledge gap by conducting both long-term and batch tests. After over 100 days of feeding with wastewater containing polyethylene terephthalate (PET) microplastics (0-500 μg/L), the long-term results showed that both production and reduction of N₂O during denitrification were reduced, as well as the N₂O production during nitrification. Accordingly, 60% reduction in N₂O accumulation and 70% reduction in N₂O production were observed in the denitrification and nitrification batch tests, respectively. Nevertheless, the long-term N₂O emission factors under PET microplastics stress were comparable to that in the control reactor, mainly because PET microplastics led to more nitrite accumulation in anoxic period. With the aid of online N₂O sensors and site-preference analysis, it was demonstrated that the heterotrophic bacteria pathway and ammonia oxidizing bacteria denitrification pathway for N₂O production were negatively affected by PET microplastics, whereas a clear increase in the contribution of hydroxylamine pathway (+ 22.9%) was observed. Further investigation revealed that PET microplastics even at environmental level (i.e. 10 μg/L) significantly reshaped the BNR sludge characteristics (e.g. much larger particle size) and microbial communities (e.g. Thauera, Rhodobacte and Nitrospira) as well as the nitrogen metabolism pathways, which were chiefly responsible for the changes of N₂O turnovers and N₂O production pathways under the PET microplastics stress.

Changes in the Species and Functional Composition of Activated Sludge Communities Revealed Mechanisms of Partial Nitrification Established by Ultrasonication

To achieve energy-efficient shortcut nitrogen removal of wastewater in the future, selective elimination of nitrite-oxidizing bacteria (NOB) while enriching ammonia-oxidizing microorganisms is a crucial step. However, the underlying mechanisms of partial nitrification are still not well understood, especially the newly discovered ultrasound-based partial nitrification. To elucidate this issue, in this study two bioreactors were set up, with one established partial nitrification by ultrasonication while the other didn't. During the operation of both reactors, the taxonomic and functional composition of the microbial community were investigated through metagenomics analysis. The result showed that during ultrasonic partial nitrification, ammonia-oxidizing archaea (AOA), Nitrososphaerales, was enriched more than ammonia-oxidizing bacteria (AOB), Nitrosomonas. The enrichment of microorganisms in the community increased the abundance of genes involved in microbial energy generation from lipid and carbohydrates. On the other hand, the abundance of NOB, Nitrospira and Nitrolancea, and Comammox Nitrospira decreased. Selective inhibition of NOB was highly correlated with genes involved in signal transduction enzymes, such as encoding histidine kinase and serine/threonine kinase. These findings provided deep insight into partial nitrification and contributed to the development of shortcut nitrification in wastewater treatment plants.

A novel stable nitritation process: Treating sludge by alternating free nitrous acid/heat shock

The feasibility of stable nitritation of sludge alternately treated by free nitrous acid (FNA) and heat shock in a sequencing batch reactor (SBR) was investigated in this study. The linear regression method was used to determine the optimal treatment conditions. Results revealed that an FNA concentration of 2.20 mg HNO₂-N/L, exposure time of 24 h, and treatment ratio of 20% could inhibit nitrite oxidizing bacteria (NOB) activity to the greatest extent while maintaining the maximum ammonium oxidizing bacteria (AOB) activity; after heat shock at 60 °C for 20 min, NOB were inhibited while AOB still had certain activity. In the long-term continuous-flow experiment, the single FNA or heat shock treatments easily allowed adapt NOB to affect the stability of nitritation. The alternating FNA/heat shock treatment can achieve long-term stability of nitritation. Microbial community analysis revealed that the alternating FNA/heat shock treatment could inhibit NOB while maintaining high AOB abundance.

Heterotrophic nitrification and aerobic denitrification process: Promising but a long way to go in the wastewater treatment

The traditional biological nitrogen removal (BNR) follows the conventional scheme of sequential nitrification and denitrification. In recent years, novel processes such as anaerobic ammonia oxidation (anammox), complete oxidation of ammonia to nitrate in one organism (comammox), heterotrophic nitrification and aerobic denitrification (HN-AD), and dissimilatory nitrate reduction to ammonium (DNRA) are gaining tremendous attention after the discovery of metabolically versatile bacteria. Among them, HN-AD offers several advantages because individual bacteria could achieve one-stage nitrogen removal under aerobic conditions in the presence of organic carbon. In this review, besides classical BNR processes, we summarized the existing literature on HN-AD bacteria which have been isolated from diverse habitats. A particular focus was given on the diversity and physiology of HN-AD bacteria, influences of physiological and biochemical factors on their growth, nitrogen removal performances, as well as limitations and strategies in unraveling HN-AD metabolic pathways. We also presented case studies of HN-AD application in wastewater treatment facilities, pointed out forthcoming challenges of HN-AD in these systems, and presented modulation strategies for HN-AD application in engineering. This review may help improve the existing design of wastewater treatment plants by harnessing HN-AD bacteria for effective nitrogen removal.

Recent advances in partial denitrification-anaerobic ammonium oxidation process for mainstream municipal wastewater treatment

To solve the bottleneck of the unstable accumulation of nitrite in the partial nitrification (PN)-anammox (AMX) in municipal wastewater treatment, a novel process called partial denitrification (PD)-AMX has been developed. PD-AMX, which is known for cost-efficiency and environmental friendliness, has currently exhibited a promising potential for the removal of biological nitrogen from municipal wastewater and has attracted much research interest regarding its process mechanisms, as well as its practical applications. Here, we review the recent advances in the PD process and its coupling to the anammox process, including the development, basic principles, main characteristics, and critical process parameters of the stable operation of the PD-AMX process. We also explore the microbial community and its characteristics in the system and summarize the knowledge of the dominant bacteria to clarify the key factors affecting PD-AMX. Then, we introduce the engineering feasibility and economic feasibility as well as the potential challenges of the process. The induction and implementation of partial denitrification and maintenance of mainstream anammox are critical issues to be urgently solved. Meanwhile, the implementation of a full mainstream anammox application remains burdensome, while the mechanism of partial denitrification coupled to anammox needs to be further studied. Additionally, stable operation performance and process control¹ methods need to be optimized or developed for the PD-AMX system for better engineering practice. This review can help to accelerate the research and application of the PD-AMX process for municipal wastewater treatment.

Enhancing nitrogen removal from anaerobically-digested swine wastewater through integration of Myriophyllum aquaticum and free nitrous acid-based technology in a constructed wetland

Despite of low operation costs and convenient maintenance, the application of natural systems for swine wastewater treatment has been limited by large construction area and unsatisfactory effluent quality. Introducing ammonium high uptake aquatic plants and shifting nitrogen removal pathway from nitrate to nitrite in constructed wetlands (CWs) has been regarded as promising approach to promote their performances. This study aimed to establish nitrite pathway and enhance N removal via free nitrous acid (FNA)-sediment treatment and Myriophyllum aquaticum vegetation in the CWs treating anaerobically digested swine wastewater. Nitrite pathway was successfully and stably achieved in the M. aquaticum CW with FNA-treated sediment. The overall removal efficiencies of ammonium nitrogen and total nitrogen were 42.3 ± 10.2% and 37.7 ± 9.3% in the planted CWs with FNA-treated sediment, which were 76.3% and 65.4% higher than those in the conventional oxidation pond system, respectively. Microbial community analysis (qPCR and metagenomics) suggested that the nitrite pathway established through FNA-sediment treatment was based on the inactivation of nitrite oxidizing bacteria (lower nxrA gene abundance) and the reduction of relative abundances of NOB (especially Nitrobacter and Nitrospira). During the denitrification processes, the integration of M. aquaticum vegetation with FNA-sediment treatment can lower the nitrate reduction by decreasing narG gene abundances and decreasing the relative abundances of napA affiliated bacteria (especially Bradyrhizobium), while strengthening reduction of nitrite and nitrous oxide by increasing nirK and nosZ gene abundances and enriching the corresponding affiliated microbial taxa, Mycobacterium and Bacillus, respectively. Our findings suggest that applying FNA-based technology in CW systems is technically and economically feasible, which holds promise for upgrading current CW systems treating swine wastewater to meet future water quality requirements.

Enhancing bio-cathodic nitrate removal through anode-cathode polarity inversion together with regulating the anode electroactivity

Bio-cathodic nitrate removal uses autotrophic nitrate-reducing bacteria as catalysts to realize the nitrate removal process and has been considered as a cost-effective way to remove nitrate contamination. However, the present bio-cathodic nitrate removal process has problems with long start-up time and low performance, which are urgently required to improve for its application. In this study, we investigated an anode-cathode polarity inversion method for rapidly cultivating high-performance nitrate-reducing bio-cathode by regulating bio-anodic bio-oxidation electroactivities under different external resistances and explored at the first time the correlation between the oxidation performance and the reduction performance of one mixed-bacteria bioelectrode. A high bio-electrochemical nitrate removal rate of 2.74 ± 0.03 gNO₃^--N m^-2 d^-1 was obtained at the bioelectrode with high bio-anodic bio-oxidation electroactivity, which was 4.0 times that of 0.69 ± 0.03 gNO₃^--N m^-2 d^-1 at the bioelectrode with low bio-oxidation electroactivity, and which was 1.3-7.9 times that of reported (0.35-2.04 gNO₃^--N m^-2 d^-1). 16S rRNA gene sequences and bacterial biomass analysis showed higher bio-cathodic nitrate removal came from higher bacterial biomass of electrogenic bacteria and nitrate-reducing bacteria. A good linear correlation between the bio-cathodic nitrate removal performance and the reversed bio-anodic bio-oxidation electroactivity was presented and likely implied that electrogenic biofilm had either action as autotrophic nitrate reduction or promotion to the development of autotrophic nitrate removal system. This study provided a novel strategy not only to rapidly cultivate high-performance bio-cathode but also to possibly develop the bio-cathode with specific functions for substance synthesis and pollutant detection.

Recent advances in nitrous oxide production and mitigation in wastewater treatment

Nitrous oxide (N₂O) emitted from wastewater treatment plants has caused widespread concern. Over the past decade, people have made tremendous efforts to discover the microorganisms responsible for N₂O production, elucidate metabolic pathways, establish production models and formulate mitigation strategies. The ultimate goal of all these efforts is to shed new light on how N₂O is produced and how to reduce it, and one of the best ways is to find key opportunities by integrating the information obtained. This review article critically evaluates the knowledge gained in the field within a decade, especially in N₂O production microbiology, biochemistry, models and mitigation strategies, with a focus on denitrification. Previous research has greatly deepened the understanding of the N₂O generation mechanism, but further efforts are still needed due to the lack of standardized methodology for establishing N₂O mitigation strategies in full-scale systems. One of the challenges seems to be to convert the denitrification process from a net N₂O source into an effective sink, which is recommended as a key opportunity to reduce N₂O production in this review.

Return-Sludge Treatment with Endogenous Free Nitrous Acid Limits Nitrate Production and N2O Emission for Mainstream Partial Nitritation/Anammox

Nitrite oxidizing bacteria (NOB) and nitrous oxide (N₂O) hinder the development of mainstream partial nitritation/anammox. To overcome these, endogenous free ammonia (FA) and free nitrous acid (FNA), which can be produced in the sidestream, were used for return-sludge treatment for two integrated-film activated sludge reactors containing biomass in flocs and on carriers. The repeated exposure of biomass from one reactor to FA shocks had a limited impact on NOB suppression but inhibited anammox bacteria (AnAOB). In the other reactor, repeated FNA shocks to the separated flocs failed to limit the system's nitrate production since NOB activity was still high on the biofilms attached to the unexposed carriers. In contrast, the repeated FNA treatment of flocs and carriers favored aerobic ammonium-oxidizing bacteria (AerAOB) over NOB activity with AnAOB negligibly affected. It was further revealed that return-sludge treatment with higher FNA levels led to lower N₂O emissions under similar effluent nitrite concentrations. On this basis, weekly 4 h FNA shocks of 2.0 mg of HNO₂-N/L were identified as an optimal and realistic treatment, which not only enabled nitrogen removal efficiencies of ∼65% at nitrogen removal rates of ∼130 mg of N/L/d (20 °C) but also yielded the lowest cost and carbon footprint.

Improving wastewater management using free nitrous acid (FNA)

Free nitrous acid (FNA), the protonated form of nitrite, has historically been an unwanted substance in wastewater systems due to its inhibition on a wide range of microorganisms. However, in recent years, advanced understanding of FNA inhibitory and biocidal effects on microorganisms has led to the development of a series of FNA-based applications that improve wastewater management practices. FNA has been used in sewer systems to control sewer corrosion and odor; in wastewater treatment to achieve carbon and energy efficient nitrogen removal; in sludge management to improve the sludge reduction and energy recovery; in membrane systems to address membrane fouling; and in wastewater algae systems to facilitate algae harvesting. This paper aims to comprehensively and critically review the current status of FNA-based applications in improving wastewater management. The underlying mechanisms of FNA inhibitory and biocidal effects are also reviewed and discussed. Knowledge gaps and current limitations of the FNA-based applications are identified; and perspectives on the development of FNA-based applications are discussed. We conclude that the FNA-based technologies have great potential for enhancing the performance of wastewater systems; however, further development and demonstration at larger scales are still required for their wider applications.

Anammox-based processes: How far have we come and what work remains? A review by bibliometric analysis

Nitrogen contamination remains a severe environmental problem and a major threat to sustainable development worldwide. A systematic analysis of the literature indicates that the partial nitritation-anammox (PN/AMX) process is still actively studied as a viable option for energy-efficient and feasible technology for the sustainable treatment of N- rich wastewaters, since its initial discovery in 1990. Notably, the mainstream PN/AMX process application remains the most challenging bottleneck in AMX technology and fascinates the world's attention in AMX studies. This paper discusses the recent trends and developments of PN/AMX research and analyzes the results of recent years of research on the PN/AMX from lab-to full-scale applications. The findings would deeply improve our understanding of the major challenges under mainstream conditions and next-stage research on the PN/AMX process. A great deal of efforts has been made in the process engineering, PN/AMX bacteria populations, predictive modeling, and the full-scale implementations during the past 22 years. A series of new and excellent experimental findings at lab, pilot and full-scale levels including good nitrogen removal performance even under low temperature (15-10 °C) around the world were achieved. To date, pilot- and full-scale PN/AMX have been successfully used to treat different types of industrial sewage, including black wastewater, sludge digester liquids, landfill leachate, monosodium glutamate wastewater, etc. Supplementing the qualitative analysis, this review also provides a quantitative bibliometrics study and evaluates global perspectives on PN/AMX research published during the past 22 years. Finally, general trends in the development of PN/AMX research are summarized with the aim of conveying potential future trajectories. The current review offers a valuable orientation and global overview for scientists, engineers, readers and decision makers presently focusing on PN/AMX processes.

Co-metabolic substrates enhanced biological nitrogen removal from cellulosic ethanol biorefinery wastewater using aerobic granular sludges

The effect of different co-metabolic substrates (glucose, acetic acid and ethanol) on aerobic granular sludge treating cellulose ethanol wastewater was investigated using sequencing batch reactors. The efficiencies of the three substrates in removing chemical oxygen demand were respectively 18.87%, 28.88% and 27.99%, all of which were remarkably higher than without co-metabolic substrates, indicating that co-metabolic substrates can promote the degradation of the refractory substances. With acetic acid as the co-metabolic substrate, the removal amount of ammonium nitrogen and nitrate nitrogen was greater than glucose and ethanol used. The nitrogen removals by the three co-metabolic substrates were 53.18%, 72.15%, 69.36%, respectively, which were 1.4, 1.8, and 1.6 times the removal without co-metabolic substrates. Fluorescence in situ hybridization results showed that the proportion of ammonium-oxidizing bacteria in the granular sludge was greater than that of nitrite-oxidizing bacteria after adding co-metabolic substrates, and the order was acetic acid > ethanol > glucose.

Nitrate addition improves hydrogen production from acidic fermentation of waste activated sludge

In this work, a low-cost alternative method (i.e., adding nitrate into WAS) to significantly enhance hydrogen production was reported. Experimental results showed that with an increase of nitrate addition from 0 to 362 mg/L, the maximal hydrogen production from acidic (pH 5.5) fermentation of WAS obviously increased from 12.6 ± 0.5 to 19.3 ± 0.9 mL per gram volatile suspended solids (VSS). The mechanism investigations illustrated more substrates were provided for subsequent hydrogen production. Although the nitrate added inhibited all the biological processes, its inhibition to the hydrogen consumption processes was much severer than that to the hydrogen production processes. The enzyme analyses on the long-term semi-continuous fermenters showed that the nitrate addition slightly inhibited the relative activities of protease, butyrate kinase, acetate kinase, CoA-transferase, and [FeFe] hydrogenase but largely suppressed the relative activities of coenzyme F₄₂₀, carbon monoxide dehydrogenase, and adenylyl sulfate reductase.

Achieving efficient nitrogen removal from real sewage via nitrite pathway in a continuous nitrogen removal process by combining free nitrous acid sludge treatment and DO control

The incomplete denitrification due to insufficient carbon resource in the wastewater treatment plants (WWTPs) resulted in low nitrogen removal efficiency, which has become a widespread problem in China and all around the world. Reducing the requirement of carbon source by manipulating the nitrogen removal pathway from conventional nitrification-denitrification to partial nitrification-denitrification is considered as an efficient solution. In this article, the feasibility of combining free nitrous acid (FNA) sludge treatment and DO control to achieve partial nitrification-denitrification in a continuous flow system (aerobic-anoxic-oxic process) using real sewage was assessed. The nitrite pathway was rapidly established in the experimental reactor within 23 days by simultaneously lowering DO concentration in aerobic zone to 0.5 mg/L and treating 30% of the activated sludge per day from the reactor in the FNA sludge treatment unit with FNA concentration of 1.2 mg N/L and exposure time of 18 h. The nitrite oxidizing bacteria (NOB) were efficiently washed out and the partial nitrification process could maintain stable in the experimental reactor even after cease of FNA treatment and increase of DO concentrations in the main stream to 1.5 mg/L, with an average nitrite accumulation rate of above 78%. In contrast, the nitrite accumulation rate was just around 58% during low DO concentrations phase and declined quickly to below 1% after the DO concentrations were increased to 1.5 mg/L in the control reactor which only utilized single strategy of DO control to achieve nitrite pathway. Moreover, a better sludge settleability and nitrogen removal performance could also be realized in the experimental reactor. The results of nitrifying bacteria activities and quantities detection demonstrated that although NOB activities in both reactors were effectively inhibited, a certain amount of NOB (6.26 × 10⁶ copies/g MLSS) were remained in the control reactor and multiplied rapidly as the DO concentration increased, which might break down the partial nitrification. Furthermore, the quantity results of nitrogen cycling related functional genes showed that the increment of the ratio of nitrate reduced bacteria to total bacteria was 0.35% larger than that of nitric oxide bacteria in the control reactor, while those two ratios increased similarly by 1.11% and 1.12% in the experimental reactor, respectively, which might be one potential cause of reduction in N₂O emission of nitrite pathway achieved by FNA-based technologies.

Free nitrous acid pretreatment of sludge to achieve nitritation: The effect of sludge concentration

This study investigated the effect of sludge concentration (expressed by mixed liquor volatile suspended solids, MLVSS) on free nitrous acid (FNA) pretreatment strategy to achieve nitritation. Results showed when FNA was 0.308 mgHNO₂-N/L, nitrite oxidizing bacteria (NOB) activity increased by 70.2% as MLVSS increased from 8.4 to 16.8 g/L. Nitrite accumulation ceased as MLVSS increased to 12.6 g/L, indicating that FNA inhibition of NOB gradually weakened with increasing MLVSS. When FNA was higher than 0.770 mgHNO₂-N/L, NOB activity was completely inhibited and the effect of MLVSS on FNA inhibition was negligible, with nitrite accumulation potential (NAP) varying from 88.1% to 90.0%. Mechanism investigation demonstrated flocs sizes distinctly declined, with more extracellular polymeric substances (EPS) released to resist FNA inactivation. Linear fitting showed NAP increased with FNA/MLVSS increment. Therefore, MLVSS affected FNA pretreatment performance, with FNA/MLVSS proposed as a more valuable criterion in FNA pretreatment strategy development, than the solely FNA.

Free nitrous acid-based nitrifying sludge treatment in a two-sludge system obtains high polyhydroxyalkanoates accumulation and satisfied biological nutrients removal

A novel strategy to achieve substantial polyhydroxyalkanoates (PHA) accumulation in waste activated sludge (WAS) was developed, which was conducted in a two-sludge system consisted of an anaerobic/anoxic/oxic reactor (AAO-SBR) and a nitrifying reactor (N-SBR), where the nitrifying-sludge was treated by free nitrous acid (FNA). Initially, 0.98 ± 0.09 and 1.46 ± 0.10 mmol-c/g VSS of PHA were respectively determined in the control-SBR and AAO-SBR. When 1/16 of nitrifying sludge was daily treated with 1.49 mg N/L FNA for 24 h, ∼46.5% of nitrite was accumulated in the N-SBR, ∼2.43 ± 0.12 mmol-c/g VSS of PHA was accumulated in WAS in AAO-SBR without deteriorating nutrient removal. However, nutrient removal of control-SBR was completely collapsed after implementing the same FNA treatment. Further investigations revealed that the activity and abundance of nitrite oxidizing bacteria (NOB) was decreased significantly after FNA treatment. Finally, sludge with high PHA level to generate more methane was confirmed.

Enhancing the digestion of waste activated sludge through nitrite addition: insight on mechanism through profiles of extracellular polymeric substances (EPS) and microbial communities

Extracellular nitrite has been used to improve the digestion of waste activated sludge (WAS). However, the underlying mechanism remains largely unknown. In this study, WAS was treated with 0.2 gNO₂^--N/gVSS for 7 days and its performance was compared to that of aerobic and anaerobic treatments. The addition of nitrite had a distinct effect on the reduction of VSS/SS and the accumulation of soluble organics compared to the control reactors. As evident by the variations of extracellular polymeric substances (EPS), nitrite addition had a positive effect on decreasing protein. In particular, the decrease of protein mainly occurred in tightly bound EPS (TB-EPS), which caused sludge disintegration and enhanced sludge reduction. Additionally, the decrease of microbial diversity with nitrite addition was significant compared to the control reactors, accompany with a decrease of live/dead cells ratio and an increase of supernatant DNA concentration. This suggests that nitrite could cause cell death and lysis, resulting in sludge degradation. Thus, nitrite addition enhanced sludge treatment through the combined effect of TB-EPS disintegration and cell lysis. These findings will be useful for the optimization of sludge treatment process based on nitrite addition.

Impact of free nitrous acid shock and dissolved oxygen limitation on nitritation maintenance and nitrous oxide emission in a membrane bioreactor

This study investigated the initiation and maintenance of nitritation in a membrane bioreactor (MBR) with long solids retention time (SRT) of 43.8 days. Nitritation was initiated within 65 days in the MBR via dissolved oxygen (DO) limitation (<0.5 mg/L). However, nitrite oxidizing bacteria (NOB) (Nitrospira and Nitrobacter) acclimated to the low DO environment and proliferated from day 81, leading to nitrate accumulation. Thereafter, the combined strategy of DO limitation and in-situ generated free nitrous acid (FNA) shock successfully restored and maintained stable nitritation for >70 days. Quantitative polymerase chain reaction (qPCR) results showed that cell abundances of Nitrospira and Nitrobacter decreased by between 50.0 to 68.9% and 60.6 to 96.4%, respectively following the FNA shocks. The maximum ammonium loading rate achieved was 1.81 kg N/(m³ day) with ammonium removal ratio and nitrite accumulation ratio of over 0.97 and 0.96, respectively. Average emission rate of N₂O from the MBR was 2.1 ± 0.72% of ammonium removed. FNA shock on day 195 reduced the N₂O emission by 13.6%. The strategy developed in this study verified that spiked FNA shock together with DO limitation can be used for maintaining nitritation in MBRs with long SRTs. This method can potentially allow for maintaining nitritation at relatively low capital and operating expenditure when treating high concentration ammonium wastewater.

Overcoming Nitrite Oxidizing Bacteria Adaptation through Alternating Sludge Treatment with Free Nitrous Acid and Free Ammonia

Stable suppression of nitrite oxidizing bacteria (NOB) is one of the major bottlenecks for achieving mainstream nitrite shunt or partial nitritation/anammox (PN/A). It is increasingly experienced that NOB could develop resistance to suppressions over an extended time, leading to failure of nitrite shunt or PN/A. This study reports and demonstrates the first effective strategy to overcome NOB adaptation through alternating sludge treatment with free nitrous acid (FNA) and free ammonia (FA). During over 650 days of reactor operation, NOB adaptation to both FNA and FA was observed, but the adaptation was successfully overcome by deploying the alternate treatment strategy. Microbial community analysis showed Nitrospira and Nitrobacter, the key NOB populations in the reactor, have the ability to adapt to FNA and FA, respectively, but do not adapt to the alternation. Stable nitrite shunt with nitrite accumulation ratio over 95% and excellent nitrogen removal were maintained for the last 10 months with only one alternation applied. N₂O emission increased initially as the attainment of nitrite shunt but exhibited a declining trend during the study. By using on-site-produced nitrite and ammonium, the proposed strategy is feasible and sustainable. This study brings the mainstream nitrite shunt and PN/A one step closer to wide applications.

Approaches and processes for ammonia removal from side-streams of municipal effluent treatment plants

The main objective of this review article is to provide a comprehensive view on various conventional and emerging side-stream ammonia removal treatment options for municipal wastewater treatment plants (WWTPs). Optimization of wastewater treatment facilities from an energy and emissions stand-point necessitates consideration of the impact of the various internal side-streams. Side-streams from anaerobic sludge digesters in particular have the potential to be a significant ammonium load to the mainstream treatment process. However, the literature suggests that managing side-streams through their treatment in the mainstream process is not the most energy efficient approach, nor does it allow for practical recovery of nutrients. Furthermore, as effluent criteria become more stringent in some jurisdictions and sludge hydrolysis pre-treatment for digesters more common, an understanding of treatment options for ammonia in digester supernatant becomes more important. Given these considerations, a variety of side-stream treatment processes described in the literature are reviewed.

Free ammonia-based sludge treatment reduces sludge production in the wastewater treatment process

Excessive sludge production is one of the major challenges for biological wastewater treatment plants. This paper reports a new strategy to enhance sludge reduction from the wastewater treatment process. In this strategy, 1/5 of the sludge is withdrawn from the mainstream reactor into a side-stream unit for sludge treatment with 16 mg/L free ammonia (FA) for 24-40 h. The FA-treated sludge mixture is then returned to the mainstream reactor. To demonstrate this concept, two reactors treating synthetic domestic wastewater were operated, with one serving as the experimental reactor and the other as the control. Experimental results showed that the experimental reactor exhibited 20% lower in sludge production than the control. FA treatment effectively disintegrated a portion of extracellular or intracellular substances of sludge cells in the FA treatment unit and lowered the observed sludge yields in the mainstream reactor, which were the main reasons for the sludge reduction. Although FA treatment decreased the activities of nitrifiers, denitrifiers, and polyphosphate accumulating organisms in the FA treatment unit, this strategy did not negatively affect the reactor performance and sludge properties of the experimental reactor such as sludge settleability, organic removal, nitrogen removal and phosphorus removal. Further investigation showed that the organics released from the FA treatment process could be used by PAOs and denitrifiers for carbon sources.

Mechanisms of Persistence of the Ammonia-Oxidizing Bacteria Nitrosomonas to the Biocide Free Nitrous Acid

Free nitrous acid (FNA) exerts a broad range of antimicrobial effects on bacteria, although susceptibility varies considerably among microorganisms. Among nitrifiers found in activated sludge of wastewater treatment processes (WWTPs), nitrite-oxidizing bacteria (NOB) are more susceptible to FNA compared to ammonia-oxidizing bacteria (AOB). This selective inhibition of NOB over AOB in WWTPs bypasses nitrate production and improves the efficiency and costs of the nitrogen removal process in both the activated sludge and anaerobic ammonium oxidation (Anammox) system. However, the molecular mechanisms governing this atypical tolerance of AOB to FNA have yet to be understood. Herein we investigate the varying effects of the antimicrobial FNA on activated sludge containing AOB and NOB using an integrated metagenomics and label-free quantitative sequential windowed acquisition of all theoretical fragment ion mass spectra (SWATH-MS) metaproteomic approach. The Nitrosomonas genus of AOB, on exposure to FNA, maintains internal homeostasis by upregulating a number of known oxidative stress enzymes, such as pteridine reductase and dihydrolipoyl dehydrogenase. Denitrifying enzymes were upregulated on exposure to FNA, suggesting the detoxification of nitrite to nitric oxide. Interestingly, proteins involved in stress response mechanisms, such as DNA and protein repair enzymes, phage prevention proteins, and iron transport proteins, were upregulated on exposure to FNA. In addition enzymes involved in energy generation were also upregulated on exposure to FNA. The total proteins specifically derived from the NOB genus Nitrobacter was low and, as such, did not allow for the elucidation of the response mechanism to FNA exposure. These findings give us an understanding of the adaptive mechanisms of tolerance within the AOB Nitrosomonas to the biocidal agent FNA.

Performance and mechanism of free nitrous acid on the solubilization of waste activated sludge

Free nitrous acid (FNA) is a promising chemical reagent for excess sludge reduction. The distinctive properties of FNA treatment on waste activated sludge (WAS) disposal have previously been demonstrated, however, the cellular response, permeabilization, and disruption caused by low-concentration FNA and the direct cell solubilization of WAS using concentrated FNA should be better understood. In this study, the parameters that influence the sludge solubilization efficiency were optimized over a wide range of FNA concentrations. The sludge solubilization efficiency was found to be superior when the sludge was exposed to FNA (when the dosage of NaNO2 was 0.12 g g-1 TSS and the pH was 3.0, FNA = 20.94 mg L-1) for 10 h at 25 °C, and the TSS removal and COD dissolution efficiencies were found to be prominent at 38% and 7%, respectively. In the FNA treatment of WAS, some FNA-tolerable cells increased the K+, Ca2+, and H+ effluxes under low concentrations of FNA, and finally achieved ion homeostasis based on the results using a scanning ion-selective electrode measurement technique. This could cause the cells in WAS to maintain cytoactivity and integrity under a low-concentration FNA treatment. Furthermore, flow cytometry was used to assess the permeabilization and disruption of sludge cells toward a concentration gradient of FNA. Flow cytometry results indicated that cells in sludge flocs were disrupted within 30 minutes when the FNA concentration was above 8 mg L-1.

Quantification of N2O and NO emissions from a small-scale pond-ditch circulation system for rural polluted water treatment

The pond-ditch circulation system (PDCS) is an efficient and economical solution for the restoration of degraded rural water environments. However, little is known about nitrous oxide (N₂O) and nitric oxide (NO) emissions in the microbial removal process of nitrogen in PDCSs, and their contribution to nitrogen removal. The aim of this study was to quantify N₂O and NO emissions from the PDCS, evaluate their capacities, and elucidate the key environmental factors controlling them. The results showed that N₂O and NO fluxes were in the ranges 1.1-2055.1μgNm^-2h^-1 and 0.1-6.8μgNm^-2h^-1 for the PDCS, respectively. Meanwhile, the N₂O and NO fluxes from the two ponds in the PDCS were significantly higher than those in the static system. Moreover, the amount of N₂O and NO emissions in the PDCS accounted for 0.17-4.32% of the total nitrogen (TN) removal. According to the partial least squares (PLS) approach and Pearson's correlation coefficients, nitrate nitrogen in water (W-NO₃^--N), dissolved oxygen in water (W-DO), dissolved oxygen in sediment (DO), pH in water (W-pH), pH in sediment (pH), total kjeldahl nitrogen (TKN), and soil organic carbon (SOC) significantly affected the N₂O flux (p<0.05), whereas W-NO₃^--N, DO, and nitrite nitrogen in sediment (NO₂^--N) significantly affected the NO emission (p<0.05).

Low temperature effects on nitrification and nitrifier community structure in V-ASP for decentralized wastewater treatment and its improvement by bio-augmentation

The vegetation-activated sludge process (V-ASP) has been proved to be an environment-friendly decentralized wastewater treatment system with extra esthetic function and less footprint. However, the effects of low temperature on the treatment performance of V-ASP and related improvement methods are rarely investigated, up to now. In this work, the effect of low temperature on nitrification in V-ASP was comprehensively investigated from overall nitrification performance, substrate utilization kinetics, functional enzymatic activities, and microbial community structure shift by comparison with conventional ASP. Bio-augmentation methods in terms of single-time nitrifier-enriched biomass dosage were employed to improve nitrification efficiency in bench- and full-scale systems. The experiment results demonstrated that the NH₄⁺-N removal efficiency in V-ASP system decreased when the operational temperature decreased from 30 to 15 °C, and the decreasing extent was rather smaller compared to ASP, as well as ammonium and nitrite oxidation rates and enzymatic activities, which indicated the V-ASP system possesses high resistance to low temperature. With direct dosage of 1.6 mg nitrifier/gSS sludge, the nitrification efficiency in V-ASP was enhanced dramatically from below 50% to above 90%, implying that bio-augmentation was effective for V-ASP whose enzymatic activities and microbial communities were both also improved. The feasibility and effectiveness of bio-augmentation was further confirmed in a full-scale V-ASP system after a long-term experiment which is instructive for the practical application.

Multi-point monitoring of nitrous oxide emissions in three full-scale conventional activated sludge tanks in Europe

The large global warming potential of nitrous oxide (N₂O) is currently of general concern for the water industry, especially in view of a new regulatory framework concerning the carbon footprint of water resource recovery facilities (WRRFs). N₂O can be generated through different biological pathways and from different treatment steps of a WRRF. The use of generic emission factors (EF) for quantifying the emissions of WRRFs is discouraged. This is due to the number of different factors that can affect how much, when and where N₂O is emitted from WRRFs. The spatial and temporal variability of three WRRFs in Europe using comparable technologies is presented. An economically feasible and user-friendly method for accounting for the contribution of anoxic zones via direct gas emission measurements was proven. The investigation provided new insights into the contribution from the anoxic zones versus the aerobic zones of biological WRRF tanks and proved the unsuitability of the use of a single EF for the three WRRFs. Dedicated campaigns for N₂O emissions assessment are to be advised. However, similarities in the EF magnitude can be found considering treatment strategy and influent water composition.

Stable long-term operation of an upflow anammox sludge bed reactor at mainstream conditions

The efforts for implementing the anammox process at mainstream conditions with high nitrogen removal rates have gained much attention in the race for achieving an energy-positive urban wastewater treatment plant. Here, the successful and stable long-term operation of an Upflow Anammox Sludge Bed (UAnSB) reactor treating a low-strength synthetic influent amended with ammonium and nitrite for 420 days, and a nitrite-amended pre-treated real urban wastewater for 110 days at temperatures as low as 11 °C is presented. The short and long-term effects of temperature on anammox activity were assessed when the synthetic influent was treated, and the UAnSB reactor was demonstrated to be a robust reactor to confront low temperatures, typically found at mainstream conditions. In fact, a nitrogen loading rate as high as 1.8 ± 0.1 g N L^-1 d^-1 with 82 ± 4% of nitrogen removal was achieved at 11 °C treating the low-strength synthetic influent. Furthermore, the effect of treating a nitrite-amended pre-treated real urban wastewater at 11 °C at long-term in the UAnSB reactor was evaluated, and a stable operation was achieved with a high average nitrogen removal rate (1.2 ± 0.5 g N L^-1 d^-1). The relative abundance of anammox bacteria was maintained higher than 70% according to fluorescence in situ hybridization during the whole operation, being Candidatus Brocadia anammoxidans the predominant microbial species. The presence of heterotrophs in the sludge bed was surmised through heterotrophic batch tests, but anammox activity was demonstrated to be higher than heterotrophic activity, even when the synthetic influent was replaced by the nitrite-amended pre-treated real wastewater. The feasibility of operating an enriched anammox reactor at high nitrogen removal rate at long-term at mainstream conditions was demonstrated in this study.

Characteristics of N2O production and hydroxylamine variation in short-cut nitrification SBR process

In order to study the characteristics of nitrous oxide (N₂O) production and hydroxylamine (NH₂OH) variation under oxic conditions, concentrations of NH₂OH and N₂O were simultaneously monitored in a short-cut nitrification sequencing batch reactor (SBR) operated with different influent ammonia concentrations. In the short-cut nitrification process, N₂O production was increased with the increasing of ammonia concentration in influent. The maximum concentrations of dissolved N₂O-N in the reactor were 0.11 mg/L and 0.52 mg/L when ammonia concentrations in the influent were 50 mg/L and 70 mg/L respectively. Under the low and medium ammonia load phases, the concentrations of NH₂OH-N in the reactor were remained at a low level which fluctuated around 0.06 mg/L in a small range, and did not change with the variation of influent NH₄⁺-N concentration. Based on the determination results, the half-saturation of NH₂OH in the biochemical conversion process of NH₂OH to NO₂^--N was very small, and the value of 0.05 mg NH₂OH-N/L proposed in the published literature was accurate. NH₂OH is an important intermediate in the nitrification process, and the direct determination of NH₂OH in the nitrification process was beneficial for revealing the kinetic process of NH₂OH production and consumption as well as the effects of NH₂OH on N₂O production in the nitrification process.

Effect of ciprofloxacin on biological nitrogen and phosphorus removal from wastewater

In this work, both short-term and long-term experiments were therefore conducted to assess the effects of ciprofloxacin (0.2 and 2mg·L^-1) on wastewater nutrient removal. The results showed that both levels of ciprofloxacin had no acute and chronic adverse effects on the surface integrity and viability of activated sludge. Short-term exposure to all the ciprofloxacin levels induced negligible influences on wastewater nutrient removal. However, the prolonged exposure to ciprofloxacin decreased total phosphorus and nitrogen removal efficiencies from 96.8, 95.8% (control) to 91.7, 84.9% (0.2mg·L^-1) and 90.5%, 80.2% (2mg·L^-1), respectively. The mechanism study showed that ciprofloxacin exposure suppressed denitrification and phosphorus uptake processes. It was also found that ciprofloxacin affected the transformations of intracellular polyhydroxyalkanoates and glycogen in the oxic and anoxic stages. Moreover the activities of nitrite reductase and polyphosphate kinase were inhibited by the presence of ciprofloxacin. Further analysis with high-throughput sequencing revealed that compared with the control, the abundances of polyphosphate accumulating organisms, glycogen accumulating organisms and denitrifying bacteria in ciprofloxacin exposure reactors reduced, which were consistent with the decreased nutrient removal performance measured in these reactors.

Smart operation of nitritation/denitritation virtually abolishes nitrous oxide emission during treatment of co-digested pig slurry centrate

The implementation of nitritation/denitritation (Nit/DNit) as alternative to nitrification/denitrification (N/DN) is driven by operational cost savings, e.g. 1.0-1.8 EUR/ton slurry treated. However, as for any biological nitrogen removal process, Nit/DNit can emit the potent greenhouse gas nitrous oxide (N₂O). Challenges remain in understanding formation mechanisms and in mitigating the emissions, particularly at a low ratio of organic carbon consumption to nitrogen removal (COD_rem/N_rem). In this study, the centrate (centrifuge supernatant) from anaerobic co-digestion of pig slurry was treated in a sequencing batch reactor. The process removed approximately 100% of ammonium a satisfactory nitrogen loading rate (0.4 g N/L/d), with minimum nitrite and nitrate in the effluent. Substantial N₂O emission (around 17% of the ammonium nitrogen loading) was observed at the baseline operational condition (dissolved oxygen, DO, levels averaged at 0.85 mg O₂/L; COD_rem/N_rem of 2.8) with ∼68% of the total emission contributed by nitritation. Emissions increased with higher nitrite accumulation and lower organic carbon to nitrogen ratio. Yet, higher DO levels (∼2.2 mg O₂/L) lowered the aerobic N₂O emission and weakened the dependency on nitrite concentration, suggesting a shift in N₂O production pathway. The most effective N₂O mitigation strategy combined intermittent patterns of aeration, anoxic feeding and anoxic carbon dosage, decreasing emission by over 99% (down to ∼0.12% of the ammonium nitrogen loading). Without anaerobic digestion, mitigated Nit/DNit decreases the operational carbon footprint with about 80% compared to N/DN. With anaerobic digestion included, about 4 times more carbon is sequestered. In conclusion, the low COD_rem/N_rem feature of Nit/DNit no longer offsets its environmental sustainability provided the process is smartly operated.

Bacterial community and molecular ecological network in response to Cr2O3 nanoparticles in activated sludge system

The potential environmental risks of chromium oxide nanoparticles (Cr₂O₃ NPs) have caused great concerns. However, their possible impacts on activated sludge process are very limited. In this study, we carried out long-term exposure experiments to evaluate the influence of Cr₂O₃ NPs on wastewater nutrient removal, bacterial community and molecular ecological network (MEN) in the sequencing batch reactor (SBR). It was found that 1 mg/L Cr₂O₃ NPs increased the effluent concentrations of NO₃^--N and TP from 6.5 to 15.4 mg/L, and 0.6-2.7 mg/L, indicating the serious deterioration of denitrification and phosphorus removal. Cr₂O₃ NPs significantly decreased the bacterial richness in terms of the number of different OTUs (626 OTUs in Cr₂O₃ samples and 728 OTUs in controls). Detrended correspondence analysis (DCA) showed that the overall taxonomic structure of bacterial community was altered at Cr₂O₃ NPs in activated sludge systems. Further analysis revealed that three genera related to denitrification (Desulfovibrio, Pseudomonas and Hyphomicrobium) and two genera related to phosphorus removal (Accumulibacter and Rhodobacter) decreased significantly, which was consistent with the observed influences of Cr₂O₃ NPs on denitrification and phosphorus removal. MEN analysis showed that the overall architecture of the network under Cr₂O₃ NPs was substantially alerted. β-Proteobacteria, playing an important role in nutrients removal, had less complex interactions in the presence of Cr₂O₃ NPs, which may be associated with the deterioration of denitrification and phosphorus removal. This study provides insights into our understanding of shifts in the bacteria community and their MEN under Cr₂O₃ NPs in activated sludge systems.

Free nitrous acid-based nitrifying sludge treatment in a two-sludge system enhances nutrient removal from low-carbon wastewater

A new method to enhance nutrient removal from low carbon-wastewater was developed. The method consists of a two-sludge system (i.e., an anaerobic-anoxic-oxic reactor coupled to a nitrifying reactor (N-SBR)) and a nitrifying-sludge treatment unit using free nitrous acid (FNA). Initially, 65.1±2.9% in total nitrogen removal and 69.6±3.4% in phosphate removal were obtained without nitrite accumulation. When 1/16 of the nitrifying sludge was daily treated with FNA at 1.1mgN/L for 24h, ∼28.5% of nitrite was accumulated in the N-SBR, and total nitrogen and phosphate removal increased to 72.4±3.2% and 76.7±2.9%, respectively. About 67.8% of nitrite was accumulated at 1.9mgN/L FNA, resulting in 82.9±3.8% in total nitrogen removal and 87.9±3.5% in phosphate removal. Fluorescence in-situ hybridization analysis showed that FNA treatment reduced the abundance of nitrite oxidizing bacteria (NOB), especially that of Nitrospira sp.

Quantifying nitrous oxide production pathways in wastewater treatment systems using isotope technology - A critical review

Nitrous oxide (N₂O) is an important greenhouse gas and an ozone-depleting substance which can be emitted from wastewater treatment systems (WWTS) causing significant environmental impacts. Understanding the N₂O production pathways and their contribution to total emissions is the key to effective mitigation. Isotope technology is a promising method that has been applied to WWTS for quantifying the N₂O production pathways. Within the scope of WWTS, this article reviews the current status of different isotope approaches, including both natural abundance and labelled isotope approaches, to N₂O production pathways quantification. It identifies the limitations and potential problems with these approaches, as well as improvement opportunities. We conclude that, while the capabilities of isotope technology have been largely recognized, the quantification of N₂O production pathways with isotope technology in WWTS require further improvement, particularly in relation to its accuracy and reliability.

Effects of influent COD/N ratios on nitrous oxide emission in a sequencing biofilm batch reactor for simultaneous nitrogen and phosphorus removal

The characteristics of N₂O emissions from an anaerobic/aerobic/anoxic (A/O/A) sequencing biofilm batch reactor (SBBR) were investigated under different influent COD/nitrogen (C/N) ratios (from 1-4). Results indicated that the C/N ratios affected the quantity of polyhydroxybutyrate (PHB) and residual organic substances after the anaerobic period, resulting in the largest N₂O emission during aerobic period occurred at a C/N of 2. Moreover, during the anoxic PHB-driven denitrification period, the rapid decline in the dissolved N₂O concentration indicated that the nitrite inhibition threshold for N₂O reduction increased with the increased C/N ratios, which means the higher influent C/N ratios could lower the inhibition of nitrite on N₂O reduction. Finally, more PHB and residual organic substances were provided to denitrification at a high C/N ratio, resulting in less total N₂O emission was achieved at a high C/N ratio in the A/O/A SBBR.

Is denitrifying anaerobic methane oxidation-centered technologies a solution for the sustainable operation of wastewater treatment Plants?

With the world's increasing energy crisis, society is growingly considered that the operation of wastewater treatment plants (WWTPs) should be shifted in sustainable paradigms with low energy input, or energy-neutral, or even energy output. There is a lack of critical thinking on whether and how new paradigms can be implemented in WWTPs based on the conventional process. The denitrifying anaerobic methane oxidation (DAMO) process, which uses methane and nitrate (or nitrite) as electron donor and acceptor, respectively, has recently been discovered. Based on critical analyses of this process, DAMO-centered technologies can be considered as a solution for sustainable operation of WWTPs. In this review, a possible strategy with DAMO-centered technologies was outlined and illustrated how this applies for the existing WWTPs energy-saving and newly designed WWTPs energy-neutral (or even energy-producing) towards sustainable operations.

The metabolic impact of extracellular nitrite on aerobic metabolism of Paracoccus denitrificans

Nitrite, in equilibrium with free nitrous acid (FNA), can inhibit both aerobic and anaerobic growth of microbial communities through bactericidal activities that have considerable potential for control of microbial growth in a range of water systems. There has been much focus on the effect of nitrite/FNA on anaerobic metabolism and so, to enhance understanding of the metabolic impact of nitrite/FNA on aerobic metabolism, a study was undertaken with a model denitrifying bacterium Paracoccus denitrificans PD1222. Extracellular nitrite inhibits aerobic growth of P. denitrificans in a pH-dependent manner that is likely to be a result of both nitrite and free nitrous acid (pK_a = 3.25) and subsequent reactive nitrogen oxides generated from the intracellular passage of FNA into P. denitrificans. Increased expression of a gene encoding a flavohemoglobin protein (Fhp) (Pden_1689) was observed in response to extracellular nitrite. Construction and analysis of a deletion mutant established Fhp to be involved in endowing nitrite/FNA resistance at high extracellular nitrite concentrations. Global transcriptional analysis confirmed nitrite-dependent expression of fhp and indicated that P. denitrificans expressed a number of stress response systems associated with protein, DNA and lipid repair. It is therefore suggested that nitrite causes a pH-dependent stress response that is due to the production of associated reactive nitrogen species, such as nitric oxide from the internalisation of FNA.

Comparison of heterotrophic and autotrophic denitrification processes for treating nitrate-contaminated surface water

The goal of this study was to compare the nitrogen removal rate, effluent algal growth potential (AGP), nitrous oxide (N₂O) emissions and global warming potential (GWP) between two laboratory-scale bioreactors: the autotrophic denitrification biofilter (ADBF) and heterotrophic denitrification biofilter (HDBF) for treating nitrate-contaminated surface water. The comparative study of nitrogen removal rate between ADBF and HDBF was conducted by a long-term experiment, and the comparative study of the effluent AGP, N₂O emissions and GWP between ADBF and HDBF were carried out by the corresponding batch tests. The results show that the heterotrophic and autotrophic denitrification rates were close to each other. Besides, the AGP of the ADBF effluent was 2.08 times lower than that of the HDBF effluent, while the N₂O concentration in off-gas emitted from HDBF was 6-8 times higher than that from ADBF. The higher N₂O-N emission rate of HDBF was mainly responsible for the higher GWP of HDBF than that of ADBF. Furthermore, with a novel light-weight filtration media (NLWFM) for filtration, the autotrophic denitrification (ADN) process combined with biofilter process would be the optimal denitrification process for nitrogen removal from nitrate-contaminated surface water. The study also provided a systematic method for evaluation of biological nitrogen removal (BNR) process.

Complete Nitrogen Removal from Synthetic Anaerobic Sludge Digestion Liquor through Integrating Anammox and Denitrifying Anaerobic Methane Oxidation in a Membrane Biofilm Reactor

Partial nitritation and Anammox processes are increasingly used for nitrogen removal from anaerobic sludge digestion liquor. However, their nitrogen removal efficiency is often limited due to the production of nitrate by the Anammox reaction and the sensitivity to the nitrite to ammonium ratio. This work develops and demonstrates an innovative process that achieves complete nitrogen removal from partially nitrified anaerobic sludge digestion liquor through the use of a membrane biofilm reactor (MBfR), with methane supplied through hollow fiber membranes. When steady state with a hydraulic retention time (HRT) of 1 day was reached, the process achieved complete nitrite and ammonium removal at rates of 560 mg N/L/d and 470 mg N/L/d, respectively, without any nitrate accumulation. The process is relatively insensitive to the nitrite to ammonium ratio, achieving complete nitrogen removal when their ratio in influent varied in the range of 1.125-1.32. Pyrosequencing and fluorescence in situ hybridization analysis revealed that denitrifying anaerobic methane oxidation (DAMO) archaea, Anammox bacteria and DAMO bacteria jointly dominated the microbial community. Mass balance analysis showed that nitrate produced by Anammox (122.2 mg N/L/d) was entirely converted to nitrite by DAMO archaea, while nitrite in the feed and produced by DAMO archaea was jointly removed by Anammox (90%) and DAMO bacteria (10%). The nitrogen removal rate of over 1 kg N/m³/d is comparable to the practical rates reported for side-stream nitrogen removal processes.

The behavior of melamine in biological wastewater treatment system

Melamine (MA) is a significant raw material for industry and home furnishing, and an intermediate for pharmacy. However it is also a hazardous material when being added to food as a protein substitute due to the high nitrogen content. In this study, the behavior of MA in activated sludge was investigated. Experiments showed that MA was removed during biological wastewater treatment process, and the removal was mainly achieved by activated sludge adsorption instead of biodegradation. Low levels of MA (0.001-0.10mg/L) in wastewater had negligible influence on the performance of activated sludge, but high levels of MA deteriorated biological nitrogen and phosphorus removal. The presence of MA (1.00 and 5.00mg/L) decreased total nitrogen removal efficiency from 94.15% to 79.47% and 68.04%, respectively. The corresponding concentration of effluent phosphorus increased from 0.11 to 1.45 and 2.06mg/L, respectively. It was also observed that MA inhibited the enzyme activities of nitrite oxidoreductase, nitrate reductase, nitrite reductase and exopolyphosphatase, which were closely relevant to nitrogen and phosphorus removal. Further investigation showed that the presence of high MA concentrations promoted the consumption and synthesis of glycogen, thereby providing the advantage for the growth of glycogen accumulating organisms.