Achieving high-level nitrogen removal in mainstream by coupling anammox with denitrifying anaerobic methane oxidation in a membrane biofilm reactor

Anammox-based technologies: A review of recent advances, mechanism, and bottlenecks

The removal of nitrogen via the ANAMMOX process is a promising green wastewater treatment technology, with numerous benefits. The incessant studies on the ANAMMOX process over the years due to its long start-up and high operational cost has positively influenced its technological advancement, even though at a rather slow pace. At the moment, relatively new ANAMMOX technologies are being developed with the goal of treating low carbon wastewater at low temperatures, tackling nitrite and nitrate accumulation and methane utilization from digestates while also recovering resources (phosphorus) in a sustainable manner. This review compares and contrasts the handful of ANAMMOX -based processes developed thus far with plausible solutions for addressing their respective bottlenecks hindering full-scale implementation. Ultimately, future prospects for advancing understanding of mechanisms and engineering application of ANAMMOX process are posited. As a whole, technological advances in process design and patents have greatly contributed to better understanding of the ANAMMOX process, which has greatly aided in the optimization and industrialization of the ANAMMOX process. This review is intended to provide researchers with an overview of the present state of research and technological development of the ANAMMOX process, thus serving as a guide for realizing energy autarkic future practical applications.

Deciphering the formation of granules by n-DAMO and Anammox microorganisms

Nitrate/nitrite-dependent anaerobic methane oxidation (n-DAMO) process is a promising wastewater treatment technology, but the slow microbial growth rate greatly hinders its practical application. Although high-level nitrogen removal and excellent biomass accumulation have been achieved in n-DAMO granule process, the formation mechanism of n-DAMO granules remains unresolved. To elucidate the role of functional microbes in granulation, this study attempted to cultivate granules dominated by n-DAMO microorganisms and granules coupling n-DAMO with anaerobic ammonium oxidation (Anammox). After long-term operation, dense granules were developed in the two systems where both n-DAMO archaea and n-DAMO bacteria were enriched, whereas granulation did not occur in the other system dominated by n-DAMO bacteria. Extracellular polymeric substances (EPS) measurement indicated the critical role of EPS production in the granulation of n-DAMO process. Metagenomic and metatranscriptomic analyses revealed that n-DAMO archaea and Anammox bacteria were active in EPS biosynthesis, while n-DAMO bacteria were inactive. Consequently, more EPS were produced in the systems containing n-DAMO archaea and Anammox bacteria, leading to the successful development of n-DAMO granules. Furthermore, EPS biosynthesis in n-DAMO systems is potentially regulated by acyl-homoserine lactones and c-di-GMP. These findings not only provide new insights into the mechanism of granule formation in n-DAMO systems, but also hint at potential strategies for management of the granule-based n-DAMO process.

Characterization of the redox-active extracellular polymeric substances in an anaerobic methanotrophic consortium

Anaerobic oxidation of methane (AOM) is a microbial process of importance in the global carbon cycle. AOM is predominantly mediated by anaerobic methanotrophic archaea (ANME), the physiology of which is still poorly understood. Here we present a new addition to the current physiological understanding of ANME by examining, for the first time, the biochemical and redox-active properties of the extracellular polymeric substances (EPS) of an ANME enrichment culture. Using a 'Candidatus Methanoperedens nitroreducens'-dominated methanotrophic consortium as the representative, we found it can produce an EPS matrix featuring a high protein-to-polysaccharide ratio of ∼8. Characterization of EPS using FTIR revealed the dominance of protein-associated amide I and amide II bands in the EPS. XPS characterization revealed the functional group of C-(O/N) from proteins accounted for 63.7% of total carbon. Heme-reactive staining and spectroscopic characterization confirmed the distribution of c-type cytochromes in this protein-dominated EPS, which potentially enabled its electroactive characteristic. Redox-active c-type cytochromes in EPS mediated the EET of 'Ca. M. nitroreducens' for the reduction of Ag+ to metallic Ag, which was confirmed by both ex-situ experiments with extracted soluble EPS and in-situ experiments with pristine EPS matrix surrounding cells. The formation of nanoparticles in the EPS matrix during in-situ extracellular Ag + reduction resulted in a relatively lower intracellular Ag distribution fraction, beneficial for alleviating the Ag toxicity to cells. The results of this study provide the first biochemical information on EPS of anaerobic methanotrophic consortia and a new insight into its physiological role in AOM process.

Advanced nitrogen removal of sulfur-driven autotrophic denitrification from landfill leachate after partial nitrification and denitrification pretreatment

Sulfur-driven autotrophic denitrification (S0dAD) was employed to remove residual nitrogen from the biological effluent of landfill leachate after partial nitrification and denitrification pretreatment. The performance of S0dAD were assessed with various NOx--N (NO2--N and NO3--N) loadings over a 185-day operational period. The results demonstrated that a notable NOx--N removal efficiency of 97.8 ± 2.0% was achieved under nitrogen removal rates of 0.12 ± 0.02 kg N/(m3· d), leading to total nitrogen concentrations of 8.6 ± 3.8 mg/L in the effluent. Batch experiments revealed competitive utilization of nitrogenous electron acceptors, with NO2--N demonstrating 2-4 times higher denitrification rates than NO3--N under coexistence conditions. Genus-level microbial community identified that Thiobacillus and Sulfurovum was highly enriched with as key denitrifying bacteria in the S0dAD system. These findings provide insights for advanced nitrogen removal coupling S0dAD with partial nitrification and denitrification process for landfill leachate treatment.

Deep insights into the biofilm formation mechanism and nitrogen-transformation network in a nitrate-dependent anaerobic methane oxidation biofilm

Nitrate/nitrite-dependent anaerobic methane oxidation (n-DAMO) process offers a promising solution for simultaneously achieving methane emissions reduction and efficient nitrogen removal in wastewater treatment. Although nitrogen removal at a practical rate has been achieved by n-DAMO biofilm process, the mechanisms of biofilm formation and nitrogen transformation remain to be elucidated. In this study, n-DAMO biofilms were successfully developed in the membrane aerated moving bed biofilm reactor (MAMBBR) and removed nitrate at a rate of 159 mg NO3--N L-1 d-1. The obvious increase in the content of extracellular polymeric substances (EPS) indicated that EPS production was important for biofilm development. n-DAMO microorganisms dominated the microbial community, and n-DAMO bacteria were the most abundant microorganisms. However, the expression of biosynthesis genes for proteins and polysaccharides encoded by n-DAMO archaea was significantly more active compared to other microorganisms, suggesting the central role of n-DAMO archaea in EPS production and biofilm formation. In addition to nitrate reduction, n-DAMO archaea were revealed to actively express dissimilatory nitrate reduction to ammonium and nitrogen fixation. The produced ammonium was putatively converted to dinitrogen gas through the joint function of n-DAMO archaea and n-DAMO bacteria. This study revealed the biofilm formation mechanism and nitrogen-transformation network in n-DAMO biofilm systems, shedding new light on promoting the application of n-DAMO process.

Impact of hydrogen sulfide on anammox and nitrate/nitrite-dependent anaerobic methane oxidation coupled technologies

The coupling between anammox and nitrate/nitrite-dependent anaerobic methane oxidation (n-DAMO) has been considered a sustainable technology for nitrogen removal from sidestream wastewater and can be implemented in both membrane biofilm reactor (MBfR) and granular bioreactor. However, the potential influence of the accompanying hydrogen sulfide (H2S) in the anaerobic digestion (AD)-related methane-containing mixture on anammox/n-DAMO remains unknown. To fill this gap, this work first constructed a model incorporating the C/N/S-related bioprocesses and evaluated/calibrated/validated the model using experimental data. The model was then used to explore the impact of H2S on the MBfR and granular bioreactor designed to perform anammox/n-DAMO at practical levels (i.e., 0∼5% (v/v) and 0∼40 g/S m3, respectively). The simulation results indicated that H2S in inflow gas did not significantly affect the total nitrogen (TN) removal of the MBfR under all operational conditions studied in this work, thus lifting the concern about applying AD-produced biogas to power up anammox/n-DAMO in the MBfR. However, the presence of H2S in the influent would either compromise the treatment performance of the granular bioreactor at a relatively high influent NH4+-N/NO2--N ratio (e.g., >1.0) or lead to increased energy demand associated with TN removal at a relatively low influent NH4+-N/NO2--N ratio (e.g., <0.7). Such a negative effect of the influent H2S could not be attenuated by regulating the hydraulic residence time and should therefore be avoided when applying the granular bioreactor to perform anammox/n-DAMO in practice.

Response mechanism of a highly efficient partial nitritation-anammox (PN/A) process under antibiotic stress: Extracellular polymers, microbial community, and functional genes

The Partial nitritation-Anammox (PN/A) process can be restricted when treating high ammonia nitrogen wastewater containing antibiotics. This study aims to explore the response mechanism of the PN/A process under antibiotic stress. Results showed the PN/A process achieved a nitrogen removal rate higher than 1.01 ± 0.03 kg N/m3/d under long-term sulfamethazine stress. The increase of extracellular polymers from 22.52 to 43.96 mg/g VSS was conducive to resisting antibiotic inhibitory. The increase of Denitratisoma and SM1A02 abundance as well as functional genes nirS and nirK indicated denitrifiers should play an important role in the stability of the PN/A system under sulfamethazine stress. In addition, antibiotic-resistant genes (ARGs) sul1 and intI1 significantly increased by 8.78 and 5.12 times of the initial values to maintain the resistance of PN/A process to sulfamethazine stress. This study uncovers the response mechanism of the PN/A process under antibiotic stress, offering a scientific basis and guidance for further application in the future.

Multiple pathways of vanadate reduction and denitrification mediated by denitrifying bacterium Acidovorax sp. strain BoFeN1

Contamination of aquifers by a combination of vanadate [V(V)] and nitrate (NO3-) is widespread nowadays. Although bioremediation of V(V)- and nitrate-contaminated environments is possible, only a limited number of functional species have been identified to date. The present study demonstrates the effectiveness of V(V) reduction and denitrification by a denitrifying bacterium Acidovorax sp. strain BoFeN1. The V(V) removal efficiency was 76.5 ± 5.41 % during 120 h incubation, with complete removal of NO3- within 48 h. Inhibitor experiments confirmed the involvement of electron transport substances and denitrifying enzymes in the bioreduction of V(V) and NO3-. Cyt c and riboflavin were important for extracellular V(V) reduction, with quinone and EPS more significant for NO3- removal. Intracellular reductive compounds including glutathione and NADH directly reduce V(V) and NO3-. Reverse transcription quantitative PCR confirmed the important roles of nirK and napA genes in regulating V(V) reduction and denitrification. Bioaugmentation by strain BoFeN1 increased V(V) and NO3- removal efficiency by 55.3 % ± 2.78 % and 42.1 % ± 1.04 % for samples from a contaminated aquifer. This study proposes new microbial resources for the bioremediation of V(V) and NO3-contaminated aquifers, and contributes to our understanding of coupled vanadium, nitrogen, and carbon biogeochemical processes.

Coupling Partial Nitritation, Anammox and n-DAMO in a membrane aerated biofilm reactor for simultaneous dissolved methane and nitrogen removal

Anaerobic technologies with downstream autotrophic nitrogen removal have been proposed to enhance bioenergy recovery and transform a wastewater treatment plant from an energy consumer to an energy exporter. However, approximately 20-50 % of the produced methane is dissolved in the anaerobically treated effluent and is easily stripped into the atmosphere in the downstream aerobic process, contributing to the release of greenhouse gas emissions. This study aims to develop a solution to beneficially utilize dissolved methane to support high-level nitrogen removal from anaerobically treated mainstream wastewater. A novel technology, integrating Partial Nitritation, Anammox and Methane-dependent nitrite/nitrate reduction (i.e. PNAM) was demonstrated in a membrane-aerated biofilm reactor (MABR). With the feeding of ∼50 mg NH4+-N/L and ∼20 mg/L dissolved methane at a hydraulic retention time of 15 h, around 90 % of nitrogen and ∼100 % of dissolved methane can be removed together in the MABR. Microbial community characterization revealed that ammonia-oxidizing bacteria (AOB), nitrite-oxidizing bacteria (NOB), anammox bacteria, nitrite/nitrate-dependent anaerobic methane oxidation microorganisms (n-DAMO bacteria and archaea) and aerobic methanotrophs co-existed in the established biofilm. Batch tests confirmed the active microbial pathways and showed that AOB, anammox bacteria and n-DAMO microbes were jointly responsible for the nitrogen removal, and dissolved methane was mainly removed by the n-DAMO process, with aerobic methane oxidation making a minor contribution. In addition, the established system was robust against dynamic changes in influent composition. The study provides a promising technology for the simultaneous removal of dissolved methane and nitrogen from domestic wastewater, which can support the transformation of wastewater treatment from an energy- and carbon-intensive process, to one that is energy- and carbon-neutral.

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

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

Enhancement of electron transfer via magnetite in nitrite-dependent anaerobic methane oxidation system

Nitrite-dependent anaerobic methane oxidation (n-DAMO) is a novel denitrification process that simultaneously further removes and utilizes methane from anaerobic effluent from wastewater treatment plants. However, the metabolic activity of n-DAMO bacteria is relative low for practical application. In this study, conductive magnetite was added into lab-scale sequencing batch reactor inoculated with n-DAMO bacteria to study the influence on n-DAMO process. With magnetite amendment, the nitrogen removal rate could reach 34.9 mg N·L-1d-1, nearly 2.5 times more than that of control group. Magnetite significantly facilitated the interspecies electron transfer and built electrically connected community with high capacitance. Enzymatic activities of electron transport chain were significantly elevated. Functional gene expression and enzyme activities associated with nitrogen and methane metabolism had been highly up-regulated. These results not only propose a useful strategy in n-DAMO application but also provide insights into the stimulating mechanism of magnetite in n-DAMO process.

Microbial stratification protects denitrifying anaerobic methane oxidation archaea and bacteria from external oxygen shock in membrane biofilm reactor

Different gradients of dissolved oxygen (DO) regulate the microbial community and nitrogen removal pathways of denitrifying anaerobic methane oxidation (DAMO) and anaerobic ammonium oxidation (Anammox) coupled process in a batch biofilm reactor. Under completely anaerobic condition, approximately 72 mg NO3--N/L was removed at a daily rate of 6.55 mg N/L, whereas a peak accumulation of 95 mg NO3--N/L was observed during DO reached 0.5 mg/L. There is a decrease in the abundance of Candidatus Methylomirabilis (24.1%), Candidatus Methanoperedens (23.3%), and Candidatus Kuenenia (22.6%) to below 5% when DO levels reached 0.2 mg/L. Moreover, key genes associated with the reverse methanogenesis (mcrA) and anaerobic ammonium oxidase (hzo) decreased. These findings indicate that during oxygen shock, methanotrophs and denitrifiers replace Anammox bacteria on the outer sphere of the biofilm, whereas DAMO bacteria and archaea are protected from external oxygen shock due to the microbial stratification of biofilm.

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.

Prospect of denitrifying anaerobic methane oxidation (DAMO) application on wastewater treatment and biogas recycling utilization

Old-fashioned wastewater treatments for nitrogen depend on heterotrophic denitrification process. It would utilize extra organic carbon source as electron donors when the C/N of domestic wastewater was too low to ensure heterotrophic denitrification process. It would lead to non-compliance with carbon reduction targets and impose an economic burden on wastewater treatment. Denitrifying anaerobic methane oxidation (DAMO), which could utilize methane serving as electron donors to replace traditional organic carbon (methanol or sodium acetate), supplies a novel approach for wastewater treatment. As the primary component of biogas, methane is an inexpensive carbon source. With anaerobic digestion becoming increasingly popular for sludge reduction in wastewater treatment plants (WWTPs), efficient biogas utilization through DAMO can offer an environmentally friendly option for in-situ biogas recycling. Here, we reviewed the metabolic principle and relevant research for DAMO and biogas recycling utilization, outlining the prospect of employing DAMO for wastewater treatment and biogas recycling utilization in WWTPs. The application of DAMO provides a new focal point for enhancing efficiency and sustainability in WWTPs.

Revisiting the Engineering Roadmap of Nitrate/Nitrite-Dependent Anaerobic Methane Oxidation

Nitrate/nitrite-dependent anaerobic oxidation of methane (n-DAMO) is a recently discovered process, which provides a sustainable perspective for simultaneous nitrogen removal and greenhouse gas emission (GHG) mitigation by using methane as an electron donor for denitrification. However, the engineering roadmap of the n-DAMO process is still unclear. This work constitutes a state-of-the-art review on the classical and most recently discovered metabolic mechanisms of the n-DAMO process. The versatile combinations of the n-DAMO process with nitrification, nitritation, and partial nitritation for nitrogen removal are also clearly presented and discussed. Additionally, the recent advances in bioreactor development are systematically reviewed and evaluated comprehensively in terms of methane supply, biomass retention, membrane requirement, startup time, reactor performance, and limitations. The key issues including enrichment and operation strategy for the scaling up of n-DAMO-based processes are also critically addressed. Moreover, the challenges inherent to implementing the n-DAMO process in practical applications, including application scenario recognition, GHG emission mitigation, and operation under realistic conditions, are highlighted. Finally, prospects as well as opportunities for future research are proposed. Overall, this review provides a roadmap for potential applications and further development of the n-DAMO process in the field of wastewater treatment.

Model-based assessment of mainstream nitrate/nitrite-dependent anaerobic methane oxidation and Anammox process in granular sludge at low temperature

The process of nitrate/nitrite-dependent anaerobic methane oxidation (n-DAMO) coupled with anaerobic ammonium oxidation (Anammox) is one of groundbreaking discoveries for nitrogen removal and methane emission reduction from wastewater simultaneously. Yet its treatment of mainstream wastewater at low temperature is still a major challenge. In this work, a one-dimensional granular sludge model incorporating Arrhenius conversion for temperature effects was constructed to depict the relationships among n-DAMO microorganisms and Anammox. The model framework was successfully evaluated with 380 days measurement data from a membrane granular sludge reactor (MGSR) operated at temperature of 20-10 °C and fed with ammonium and nitrite. The model could satisfactorily predict the kinetics of nitrogen removal rates, effluent nitrogen concentrations and biomass fractions in MGSR at varying temperatures. Despite the decrease in microbial activity of functional microorganisms, the coupled n-DAMO and Anammox process based on granule system in mainstream wastewater treatment achieved a TN removal efficiency of about 98 % and a stable nitrogen removal rate of 0.55 g L-1 d-1. The model developed is expected to facilitate fundamentally understanding the underlying mechanisms of the coupled process and provide proposals for its practical engineering application in wastewater treatment plants.

Metagenomics insight into the long-term effect of ferrous ions on the mainstream anammox system

Anaerobic ammonium oxidation (anammox) bacteria have a high requirement for iron for their growth and metabolism. However, it remains unclear whether iron supplementation can sustain the stability of mainstream anammox systems at varying temperatures. Here, we investigated the long-term effects of Fe2+ on the mainstream anammox systems. Our findings revealed that the nitrogen removal efficiency (NRE) of the anammox system supplemented with 5 mg/L Fe2+ decreased from 76.5 ± 0.76% at 35 °C to 39.0 ± 9.9% at 25 °C. Notably, higher dosages of Fe2+ (15 mg/L and 30 mg/L) inhibited the anammox system, resulting in NREs of 15.9 ± 8.1% and 2.5 ± 1.1% at 25 °C, respectively. The results of microbial communities and function profiles suggested that the high Fe2+ dosage seriously affected the iron assimilation and utilization in the mainstream anammox system. This was evident from the decreased abundance of genes associated with Fe(II) transport and uptake, which in turn hindered the biosynthesis of intracellular iron-cofactors, resulting in decrease in the absolute abundance of Candidatus Brocadia, a key anammox bacterium, as well as a decline in NRE. Furthermore, our results showed that the anammox process was more susceptible to iron supplementation at 25 °C compared to 35 °C, which may be due to the oxidative stress reactions induced by combined lowered temperature and a high Fe2+ dosage. Overall, these findings offer a deeper understanding of the effect of iron in mainstream anammox systems, which can contribute to improved stability maintenance and effectiveness of anammox processes.

Evidence for Nitrous Oxide Emissions by Nitrite-Dependent Anaerobic Methane Oxidizing Bacteria

Nitrite-dependent anaerobic methane oxidizing (n-DAMO) bacteria generally convert nitrite to dinitrogen and bypass the nitrous oxide (N2O) formation step. However, N2O is often detected in n-DAMO bacteria dominated cultures and it remains an open question as to the microbial origin of N2O in these enrichments. Using a stable nitrite consuming microbial community enriched for n-DAMO bacteria, we demonstrated that N2O production was coupled to methane oxidation and the higher initial nitrite concentrations led to increased quantities of N2O being formed. Moreover, continuous exposure of the enrichment culture to about 5 mg of N L-1 nitrite resulted in constant N2O being produced (12.5% of nitrite was reduced to N2O). Metatranscriptomic analyses revealed that nitrite reductase (nirS) and nitric oxide reductase (norZ) transcripts from n-DAMO bacteria increased in response to nitrite exposure. No other bacteria significantly expressed nor genes under these conditions, suggesting n-DAMO bacteria are responsible for N2O being produced. In a 35-day bioreactor experiment, N2O produced by the n-DAMO bacteria accumulated when nitrite was in excess; this was found to be up to 3.2% of the nitrogen that resulted from nitrite removal. Together, these results suggested that excess nitrite is an important driver of N2O production by n-DAMO bacteria. To this end, proper monitoring and control of nitrite levels in wastewater treatment plants would be effective strategies for mitigating N2O emissions to the atmosphere.

Deep insights into the population shift of n-DAMO and Anammox in granular sludge: From sidestream to mainstream

Granular sludge combined n-DAMO and Anammox (n-D/A) is an energy-efficient biotechnique for the simultaneous removal of nitrogen and dissolved methane from wastewater. However, the lack of knowledge so far about the metabolic interactions between n-DAMO and Anammox in response to operation condition in granular sludge restrains the development of this biotechnology. To address this gap, three independent membrane granular sludge reactors (MGSRs) were designed to carry out the granule-based n-D/A process under different conditions. We provided the first deep insights into the metabolic interactions between n-DAMO and Anammox in granular sludge via combined metagenomic and metatranscriptomic analyses. Our study unveiled a clear population shift of n-DAMO community from Candidatus Methanoperedens to Candidatus Methylomirabilis from sidestream to mainstream. Candidatus Methanoperedens with relative abundance of 25.2% played the major role in nitrate reduction and methane oxidation under sidestream condition, indicated by the high expression activities of mcrA and narG. Candidatus Methylomirabilis dominated the microbial community under mainstream condition with relative abundance of 32.1%, supported by the high expression activities of pmoA and hao. Furthermore, a transition of Anammox population from Candidatus Kuenenia to Candidatus Brocadia was also observed from sidestream to mainstream. Candidatus Kuenenia and Candidatus Brocadia jointly contributed to the primary anaerobic ammonium oxidation suggested by the high expression value of hdh and hzs. Candidatus Methylomirabilis was speculated to perform ammonium oxidation mediated by pMMO under mainstream condition. These findings might help to reveal the microbial interactions and ecological niches of n-DAMO and Anammox microorganisms, shedding light on the optimization and management of the granule-based n-D/A system.

Simultaneous dissolved methane and nitrogen removal from low-strength wastewater using anaerobic granule-based sequencing batch reactor

Anaerobic treatment of mainstream wastewater has been proposed as a promising solution to enhance bioenergy recovery for wastewater treatment plants (WWTPs). However, the limited organics for downstream nitrogen removal and emissions of dissolved methane into the atmosphere are two major barriers to the broad application of anaerobic wastewater treatment. This study aims to develop a novel technology to overcome these two challenges by achieving simultaneous removal of dissolved methane and nitrogen, and unravel the microbial competitions underpinning the process from the microbial and kinetic perspectives. To this end, a laboratory granule-based sequencing batch reactor (GSBR) coupling anammox and nitrite/nitrate-dependent anaerobic methane oxidation (n-DAMO) microorganisms was developed to treat wastewater mimicking effluent from mainstream anaerobic treatment. The GSBR achieved high-level nitrogen and dissolved methane removal rates (> 250 mg N/L/d and > 65 mg CH4/L/d) and efficiencies (> 99% total nitrogen removal and > 90% total methane removal) during the long-term demonstration. The availability of different electron acceptors (nitrite or nitrate) imposed significant effects on the removal of ammonium and dissolved methane, as well as on the microbial communities, and the abundance and expression of functional genes. The analysis of apparent microbial kinetics showed that anammox bacteria had a higher nitrite affinity than n-DAMO bacteria, while n-DAMO bacteria had a higher methane affinity than n-DAMO archaea. These kinetics underpin the observation that nitrite is a preferred electron acceptor for removing ammonium and dissolved methane than nitrate. The findings not only extend the applications of novel n-DAMO microorganisms in nitrogen and dissolved methane removal, but also provide insights into microbial cooperation and competition in granular systems.

Microbial Niche Differentiation during Nitrite-Dependent Anaerobic Methane Oxidation

Nitrite-dependent anaerobic methane oxidation (n-DAMO) has been demonstrated to play important roles in the global methane and nitrogen cycle. However, despite diverse n-DAMO bacteria widely detected in environments, little is known about their physiology for microbial niche differentiation. Here, we show the microbial niche differentiation of n-DAMO bacteria through long-term reactor operations combining genome-centered omics and kinetic analysis. With the same inoculum dominated by both species "Candidatus Methylomirabilis oxyfera" and "Candidatus Methylomirabilis sinica", n-DAMO bacterial population was shifted to "Ca. M. oxyfera" in a reactor fed with low-strength nitrite, but shifted to "Ca. M. sinica" with high-strength nitrite. Metatranscriptomic analysis showed that "Ca. M. oxyfera" harbored more complete function in cell chemotaxis, flagellar assembly, and two-component system for better uptake of nitrite, while "Ca. M. sinica" had a more active ion transport and stress response system, and more redundant function in nitrite reduction to mitigate nitrite inhibition. Importantly, the half-saturation constant of nitrite (0.057 mM vs 0.334 mM NO₂^-) and inhibition thresholds (0.932 mM vs 2.450 mM NO₂^-) for "Ca. M. oxyfera" vs "Ca. M. sinica", respectively, were highly consistent with genomic results. Integrating these findings demonstrated biochemical characteristics, especially the kinetics of nitrite affinity and inhibition determine niche differentiation of n-DAMO bacteria.

Microbial Stratification Affects Conversions of Nitrogen and Methane in Biofilms Coupling Anammox and n-DAMO Processes

A methane-based membrane biofilm reactor (MBfR) has a suitable configuration to incorporate anammox and nitrite/nitrate-dependent anaerobic methane oxidation (n-DAMO) processes because of its high gas-transfer efficiency and efficient biomass retention. In this study, the spatial distribution of microorganisms along with the biofilm depth in methane-based MBfRs was experimentally revealed, showing the dominance of anammox bacteria, n-DAMO bacteria, and n-DAMO archaea in the outer layer, middle layer, and inner layer of biofilms, respectively. The long-term and short-term experimental investigations in conjunction with mathematical modeling collectively revealed that microorganisms living in the outer layer of biofilms tend to use substrates from wastewater, while microorganisms inhabiting the inner layer of biofilms tend to use substrates originating from biofilm substratum. Specifically, anammox bacteria dominating the biofilm surface preferentially removed the nitrite provided from wastewater, while n-DAMO bacteria mostly utilized the nitrite generated from n-DAMO archaea as these two methane-related populations spatially clustered together inside the biofilm. Likewise, the methane supplied from the membrane was mostly consumed by n-DAMO archaea, while the dissolved methane in wastewater would be primarily utilized by n-DAMO bacteria. This study offers novel insights into the impacts of microbial stratification in biofilm systems, not only expanding the fundamental understanding of biofilms and microbial interactions therein but also providing a rationale for the potential applications of methane-based MBfRs in sewage treatment.

Reactivated biofilm coupling n-DAMO with anammox achieved high-rate nitrogen removal in membrane aerated moving bed biofilm reactor

As a promising technology, the combination of nitrate/nitrite-dependent anaerobic methane oxidation (n-DAMO) with Anammox offers a solution to achieve effective and sustainable wastewater treatment. However, this sustainable process faces challenges to accumulate sufficient biomass for reaching practical nitrogen removal performance. This study developed an innovative membrane aerated moving bed biofilm reactor (MAMBBR), which supported sufficient methane supply and excellent biofilm attachment, for cultivating biofilms coupling n-DAMO with Anammox. Biofilms were developed rapidly on the polyurethane foam with the supply of ammonium and nitrate, achieving the bioreactor performance of 275 g N m^-3 d^-1 within 102 days. After the preservation at -20 °C for 8 months, the biofilm was successfully reactivated and achieved 315 g N m^-3 d^-1 after 188 days. After reactivation, MAMBBR was applied to treat synthetic sidestream wastewater. Up to 99.9% of total nitrogen was removed with the bioreactor performance of 4.0 kg N m^-3 d^-1. Microbial community analysis and mass balance calculation demonstrated that n-DAMO microorganisms and Anammox bacteria collectively contributed to nitrogen removal in MAMBBR. The MAMBBR developed in this study provides an ideal system of integrating n-DAMO with Anammox for sustainable wastewater treatment.

Strategy of nitrate removal in anaerobic ammonia oxidation-dependent processes

The anaerobic ammonium oxidation (anammox), a microbial process that is considered as a low-cost and high efficient wastewater treatment, has received extensive attention with an attractive application prospect. The anammox process reduces nitrite (NO₂^-) to nitrogen gas (N₂) with ammonium (NH₄⁺) as the electron donor. However, some nitrate (NO₃^-) equivalent to 11% of total nitrogen (TN) is generated in this process, which limits the development of anammox. To overcome this problem, many efforts have been made in this regard, mainly combining with other biological treatment methods (denitrification, denitrifying anaerobic methane oxidation, etc.), introducing the substance into anammox process, etc. Herein, we summarized a detailed review of previous researches on the removal of NO₃^- in the anammox-dependent processes. It is hoped that this review could serve as valuable guidance in future research and practical applications of anammox.

Instrumental role of bioreactors in nitrate/nitrite-dependent anaerobic methane oxidation-based biotechnologies for wastewater treatment: A review

Recently, the nitrate/nitrite-dependent anaerobic methane oxidation (n-DAMO) processes have become a research hotspot in the field of wastewater treatment. The n-DAMO processes could not only mitigate direct and indirect carbon emissions from wastewater treatment plants but also strengthen biological nitrogen removal. However, the applications of n-DAMO-based biotechnologies face practical difficulties mainly caused by the distinctive properties of n-DAMO microorganisms and the limited/availability of methane with poor solubility. In this sense, the choice of bioreactors will play important roles that influence the growth and functioning of n-DAMO microorganisms, thus enabling dedicated development of the n-DAMO processes and efficient applications of n-DAMO-based biotechnologies. Therefore, this paper aims to discuss the three commonly-applied types of bioreactors, covering the individual working principle and state-of-the-art removal performance of nitrogen as well as dissolved methane observed when adopted for n-DAMO-based biotechnologies. With noted limitations for each bioreactor type, several key perspectives were proposed which hopefully would inspire future investigation and practical applications of the n-DAMO processes.

Coupling methanotrophic denitrification to anammox in a moving bed biofilm reactor for nitrogen removal under hypoxic conditions

Simultaneous removal of ammonium and nitrate was achieved in a methane-fed moving bed biofilm reactor (MBBR). In the reactor, methanotrophic microorganisms oxidized methane under hypoxic conditions likely to methanol, hence providing an electron donor to denitrifiers to reduce nitrate to nitrite that then allowed anaerobic ammonium oxidizing bacteria (Anammox) to remove excess ammonium as N₂. The ammonium and nitrate removal rates reached 72.09 ± 5.81 mgNH₄⁺-N/L/d and 62.61 ± 4.17 mgNO₃^--N/L/d when the MBBR was operated in continuous mode. Nitrate removal by the methane-fed mixed consortia was confirmed in a batch test revealing a CH₄/NO₃^- molar removal ratio of 1.15. The functional populations were unveiled by FISH analysis and 16S rRNA gene sequencing, which showed that the biofilm was dominated by Anammox bacteria (Candidatus Kuenenia) and diverse taxa associated with the capacity for denitrification: aerobic methanotrophs (Methylobacter, Methylomonas, and unclassified Methylococcaceae), methylotrophic denitrifiers (Opitutaceae and Methylophilaceae), and other heterotrophic denitrifiers (Ignavibacteriaceae, Anaerolineaceae, Comamonadaceae, Rhodocyclaceae and Thauera). Neither DAMO archaea nor DAMO bacteria were found in the sequencing analysis, indicating that more unknown community members possess the metabolic capacity of methanotrophic denitrification.

Application of an integrated loach-plant-substrate-microbes non-aerated saturated vertical flow constructed wetlands: Mechanisms of pollutants removal and greenhouse gases reduction

This study established an integrated loach-plant-substrate-microbes non-aerated saturated vertical flow constructed wetlands (VFCWs) to enhance pollutants removal efficiencies and reduce greenhouse gas emissions simultaneously. The results of the VFCWs experiment indicated that the removal efficiencies of chemical oxygen demand, total phosphorous, and total nitrogen in loach systems were significantly higher than those of non-loach systems, achieving 59.16%, 35.98%, and 40.96%, respectively. The CH₄ and N₂O emission fluxes were also significantly reduced in the integrated system, resulting in lower global warming potential (GWP) and GWP per unit of pollutants removal. Loaches promoted the transportation of oxygen, facilitated the re-contact and utilization of sediments, reduced CH₄ emission, and enhanced nitrogen conversion and phosphorus accumulation. Increased bioavailable carbon and nitrate-nitrogen in the integrated system improved the abundance of denitrifying bacteria, which supported complete denitrification, reducing N₂O emissions with high pollutant removal.

Effect of aeration modes on nitrogen removal and N2O emission in the partial nitrification and denitrification process for landfill leachate treatment

The anoxic/multi-aerobic process is widely applied for treating landfill leachate with low carbon to nitrogen ratio. In this study, the effect of two aeration modes in the aerobic phase, i.e. decreasing dissolved oxygen (DO) and increasing DO, on nitrogen removal and N₂O emission in the process were systematically compared. The results demonstrate that the aerobic phase with increasing DO mode has a positive effect on improved total nitrogen removal (78 %) under the COD/N ratio as low as 3.45 and minimized N₂O emission. DO concentration higher than 1.5 mg/L in the aerobic phase reduced nitrogen removal and led to a significant high N₂O emission in the process. Complete nitrite denitrification in the anoxic phase correlated with minimized N₂O emission. Under efficient nitrogen removal stage, N₂O emission factor was 2.4 ± 1.0 % of the total incoming nitrogen. Microbial analysis revealed that increasing DO mode increased the abundance of ammonia oxidizing bacteria and denitrifiers.

Sulfammox forwarding thiosulfate-driven denitrification and anammox process for nitrogen removal

The coupled process of thiosulfate-driven denitrification (NO₃^-→NO₂^-) and Anammox (TDDA) was a promising process for the treatment of wastewater containing NH₄⁺-N and NO₃^--N. However, the high concentration of SO₄^2- production limited its application, which needs to be alleviated by an economical and effective way to promote the application of TDDA process. In this study, TDDA process was started in a relatively short time by stepwise replacing nitrite with nitrate and operated continuously for 146 days. Results presented that the average total nitrogen removal efficiency of 82.18% can be acquired at a high loading rate of 1.98 kg N/(m³·d) with maximum nitrogen removal efficiency up to 87.04%. It was observed that the increase of S/N ratio improved the denitrification efficiency and slightly inhibit the Anammox process. Batch tests showed that Sulfammox process appeared in TDDA process under certain conditions, further contributing 2.59% nitrogen removal and 10.46% sulfur removal (14.42 mg/L NH₄⁺-N and 37.68 mg/L SO₄^2--S were removed). This finding was mainly attributed to the reduction of sulfate in TDDA system to elemental S⁰ or HS^-, which subsequently was used as an electron donor to realize the recycling of sulfate (SO₄^2--S) pollutants and promote the sulfur-nitrogen (S-N) cycle. High-throughput analysis displayed that Anammox bacteria (Candidatus_Kuenenia), Sulfur-oxidizing bacteria (Thiobacillus) with relatively high abundance of 5.37%, 7.74%, respectively, guaranteeing the excellent nitrogen and sulfate removal performance in the reactor. The enrichment of phyla Chloroflexi (31.79%), Proteobacteria (31.82%), class Ignavibacteriales (10.55%), genus Planctomycetes (13.57%) further verified the exitence of Sulfammox process in the TDDA reactor. This study provides a new perspective for the practical application of TDDA in terms of reducing the production of high concentration SO₄^2- and saving operational cost and strengthening deeply nitrogen removal.

Nitrite reduction using a membrane biofilm reactor (MBfR) in a hypoxic environment with dilute methane under low pressures

Methane-based membrane biofilm reactors (MBfRs) can be an effective solution for nitrogen control in wastewater, but there is limited information on nitrite reduction for dilute wastewater (e.g., municipal wastewater) in hypoxic MBfRs. This study assessed the impacts of dilute (20 %), low-pressure methane (0.35-2.41 kPa) applied to MBfRs at hydraulic retention times (HRTs) of 2-12 h on nitrite removals, dissolved methane concentrations, and the resulting changes in the microbial community. High nitrite flux along with rapid and virtually complete (>99 %) nitrite removals were observed at methane pressures of 1.03-2.41 kPa at HRTs above 4 h, despite the use of diluted methane gas for the MBfR. The lowest methane pressure (0.35 kPa) was also able to achieve up to 98 % nitrite removals but required HRTs of up to 12 h. All scenarios had low dissolved methane concentrations (<10 mg/L), indicating that dilute methane at low supply pressures can effectively remove nitrite while meeting dissolved methane guidelines in treated effluent. Methylococcus genus was the key bacterium in MBfR biofilm grown at different HRTs and methane pressures, along with Methylocystis and other heterotrophic denitrifiers (Terrimonas and Hyphomicrobium). This study indicates that methane-based denitrification MBfRs can be a valuable tool to meet nitrogen limits for dilute wastewater coupled to partial nitrification, while limiting the release of methane to the environment.

Enhancement of denitrifying anaerobic methane oxidation via applied electric potential

In this study, single-chamber three-electrode electrochemical sequencing batch reactor (ESBR) was set up to investigate the impact of applying potential on denitrifying anaerobic methane oxidation (DAMO) process. When the applied potential was +0.8 V, the conversion rate of nitrite to nitrogen was superior to those of other potentials. With the optimal potential of +0.8 V for 60 days, the nitrite removal rate of ESBR could reach 3.34 ± 0.28 mg N/L/d, which was 4.5 times more than that of the non-current control (0.74 ± 0.16 mg N/L/d). The DAMO functional bacteria Candidatus Methylomirabilis exhibited noticeable enrichment under applying potential, and its functional gene of pmoA was significantly expressed. Through untargeted LC-MS metabolomics analysis, applied potential was shown to affect the regulation of prior metabolites including spermidine, spermine and glycerophosphocholine that were related to the metabolic pathways of glycerophospholipid metabolism and arginine and proline metabolism, which had positive effects on DAMO process. These results show that applying electric potential could be a useful strategy in DAMO process used for methane and nitrogen removal.

Simultaneous high p-nitrophenol concentration and nitrogen removal by two-stage membrane biofilm reactor

P-nitrophenol (PNP) is highly toxic and difficult to degrade, causing great harm to the ecological environment and human health. A two-stage bench-scale membrane biofilm reactor (MBfR) was constructed to treat wastewater containing high concentration of PNP and the generated nitrogen without external organic carbon sources. The two reactors were supplied with oxygen and methane, respectively. O₂-MBfR was used for the degradation of PNP and the improvement of wastewater biodegradability. CH₄-MBfR was used for the total nitrogen (TN) removal treatment from O₂-MBfR effluent. In this experiment, the performance of the two-stage MBfR process was evaluated and optimized by adjusting operational parameters (aeration pressure, HRT, and pH). Under the optimal operation parameters, the removal efficiencies of PNP (100 mg/L) and TN attained 89.70% and 69.24%, respectively, and the removal loads were 0.930 g·m^-2·d^-1 and 241.42 mg·m^-2·d^-1, respectively. The reactor was able to accommodate the concentrations of PNP up to 200-400 mg/L, and the reactor reached maximum efficiency throughout the process when the concentration of PNP in the wastewater was 250 mg/L. The removal rates of PNP and TN reached 95.0% and 69.48%, respectively, and the removal loads were 2.37 g·m^-2·d^-1 and 96.22 mg·m^-2·d^-1, respectively. This research provides a better solution for multi-MBfR to treat toxic industrial wastewater containing phenol, nitrophenol, and further TN removal, which would not release any air pollutants into the atmosphere.

Rapid enrichment of denitrifying methanotrophs in a series hollow-fiber membrane biofilm reactor

Denitrifying anaerobic methane oxidation (DAMO) process uses methane as electron donor to reduce nitrate/nitrite to dinitrogen, which is a potentially efficient, low-cost and clean biological nitrogen removal technology. However, slow microbial growth rate severely limits the application of this promising process. In this study, a series hollow-fiber membrane biofilm reactor (HfMBR) was operated for 90 days to achieve rapid enrichment of these denitrifying methanotrophs. Finally, the highest relative abundance of denitrifying methanotrophic archaea and bacteria (DAMO archaea and bacteria) reached 47.5% and 11.3%, respectively. And the average abundance of DAMO archaea and bacteria increased 92.9 and 136.6 times respectively during the 90-day enrichment. High growth rate of DAMO archaea with a doubling time of 11.6 days was achieved in the second HfMBR according to quantitative PCR results. The results implied that dissolved oxygen would inhibit the growth of DAMO archaea, but the series HfMBR could effectively counteract this unfavorable factor. This work provided theoretical guidance for the rapid enrichment of denitrifying methanotrophs and contributed to the application of methane-dependent denitrification process.

Influences of longitudinal gradients on methane-driven membrane biofilm reactor for complete nitrogen removal: A model-based investigation

Integrating anammox with denitrifying anaerobic methane oxidation (DAMO) in the membrane biofilm reactor (MBfR) is a promising technology capable of achieving complete nitrogen removal from wastewater. However, it remains unknown whether reactor configurations featuring longitudinal gradients parallel to the membrane surface would affect the performance of the CH₄-driven MBfR. To this end, this work aims to study the impacts of longitudinal heterogeneity potentially present in the gas and liquid phases on a representative CH₄-driven MBfR performing anammox/DAMO by applying the reported modified compartmental modeling approach. Through comparing the modeling results of different reactor configurations, this work not only offered important guidance for better design, operation and monitoring of the CH₄-driven MBfR, but also revealed important implications for prospective related modeling research. The total nitrogen removal efficiency of the MBfR at non-excessive CH₄ supply (e.g., surface loading of ≤0.064 g-COD m^-2 d^-1 in this work) was found to be insensitive to both longitudinal gradients in the liquid and gas phases. Comparatively, the longitudinal gradient in the liquid phase led to distinct longitudinal biomass stratification and therefore played an influential role in the effective CH₄ utilization efficiency, which was also related to the extent of reactor compartmentation considered in modeling. When supplied with non-excessive CH₄, the MBfR is recommended to be designed/operated with both the biofilm reactor and the membrane lumen as plug flow reactors (PFRs) with co-current flow of wastewater and CH₄, which could mitigate dissolved CH₄ discharge in the effluent. For the reactor configurations with the biofilm reactor designed/operated as a PFR, multi-spot sampling in the longitudinal direction is needed to obtain a correct representation of the microbial composition of the MBfR.

Advanced organic recovery from municipal wastewater with an enhanced magnetic separation (EMS) system: Pilot-scale verification

The up-concentration process has been demonstrated as an attractive approach to carbon-neutral wastewater treatment. Innovation in the separation processes can help eliminate the current heavy dependence on gravity, and credible pilot-scale verification is crucial for application promotion. We hereby proposed a pilot-scale enhanced magnetic separation (EMS) system as an up-concentration step to maximize energy recovery from municipal wastewater. The design of EMS was based on the hypothesis that magnetic-driven separation could be a breakthrough in separation speed, and adsorption could further enhance the separation efficiency by capturing soluble substances. Jar tests confirmed the feasibility of activated carbon adsorption, which could also roughen the surface of aggregates. Further, over one-year operation of a 300 m³/d EMS equipment provided optimum operation strategies and evidence of system effectiveness. More than 80% of particulate organics and 60% of soluble organics were removed within 10 min at an energy consumption of only 0.036 kWh/m³. The characteristics of sludge were clarified in terms of organic concentration, extracellular polymeric substances composition, and micro-community analysis. The anaerobic experiments further demonstrated the potential value of the concentrated products. Surprisingly, the developed EMS system exhibited significant advantages in time consumption and space occupation, with competitive operating cost and energy consumption. Overall, the results of this study posed the EMS process for up-concentration as a potential approach to organics recovery from municipal wastewater.

Challenges, solutions and prospects of mainstream anammox-based process for municipal wastewater treatment

Anaerobic ammonia oxidation (anammox) process has a promising application prospect for the mainstream deammonification of municipal wastewater due to its high efficiency and low energy consumption. In this paper, challenges and solutions of mainstream anammox-based process are summarized by analyzing the literature of recent ten years. Slow growth rate of anammox bacteria is a main challenge for mainstream anammox-based process, and enhancement of bacteria retention has been recognized to be necessary. Compared with directly increasing sludge retention time (SRT) with membrane bioreactors or sequencing batch reactors, culturing anammox bacteria in the form of biofilm or granule sludge is more promising for its feasibility of eliminating nitrite oxidizing bacteria (NOB). Besides, adding external electron donors or conductive materials and enriching the concentration of ammonia with absorption materials have also been proved helpful to improve the activity of anammox bacteria. Other challenges include the elimination of NOB and achieving ideal ratio of NH₄⁺ and NO₂^-. To solve these problems and achieve stable partial nitrification, composite control strategies based on low SRT and limited aeration are needed based on the special characteristics of ammonia oxidizing bacteria (AOB) and NOB. When treating actual wastewater, interference of low temperature and components in the influent is another problem. Relatively high activity of anammox bacteria has been realized after artificial acclimation at low temperature and the mechanism was also preliminary explored. Different pre-treatment sections have been designed to reduce the concentration of COD and S^2- from the influent. As for the nitrate produced by the anammox reaction, coupling processes are useful to reduce the concentration of nitrate in the effluent. In brief, suitable reactor and coupling process should be selected according to the temperature, influent quality and discharge targets of different regions. The future prospects of the mainstream anammox-based process are also put forward.

Candida tropicalis prompted effectively simultaneous removal of carbon, nitrogen and phosphorus in activated sludge reactor: Microbial community succession and functional characteristics

A new Candida tropicalis that simultaneously remove nitrogen and phosphorus, and degrade organic matters was isolated. Three continuous stirred tank reactors inoculated with C. tropicalis, activated sludge, and their co-existing system in aerobic condition were operated for 150 days. Results demonstrated that the inoculation of C. tropicalis in the co-existing system remarkably improved the carbon, nitrogen, and phosphorus removal efficiencies. The co-existing system had increased carbon, nitrogen, and phosphorus removal efficiencies (92%, 73%, and 63%, respectively); decreased biomass (reduced from 1200 mg/L to 500 mg/L); and C. tropicalis as the dominant strain. The relative abundance of traditional nitrogen- and phosphorus-removing microorganisms, such as Mycobacterium, Flavonifactor, and Devsia, increased in the co-existing system. Metagenomic analysis showed that the presence of the PCYT2, EPT1, and phnPP genes and more complexed metabolism pathways in the co-existing system might be responsible for the more activated metabolism process.

Biological Nitrogen Removal Database: A Manually Curated Data Resource

Biological nitrogen removal (BNR) technologies are the most effective approaches for the remediation of environmental nitrogen pollutants from wastewater treatment plants (WWTPs). Presently, research is going on to elucidate the structure and function of BNR microbial communities and optimizing BNR treatment systems to enhance nitrogen removal efficiency. The literature on BNR microbial communities and experimental datasets is not unified across various repositories, while a uniform resource for the collection, annotation, and structuring of these BNR datasets is still unavailable. Herein, we present the Biological Nitrogen Removal Database (BNRdb), an integrated resource containing various manually curated BNR-related data. At present, BNRdb contains 23,308 microbial strains, 46 gene families, 24 enzymes, 18 reactions, 301 BNR treatment datasets, 860 BNR-associated next-generation sequencing datasets, and 6 common BNR bioreactor systems. BNRdb provides a user-friendly interface enabling interactive data browsing. To our knowledge, BNRdb is the first BNR data resource that systematically integrates BNR data from archaeal, bacterial, and fungal communities. We believe that BNRdb will contribute to a better understanding of BNR process and nitrogen bioremediation research.

A mechanistic model for denitrifying anaerobic methane oxidation coupled to dissimilatory nitrate reduction to ammonium

Nitrate/nitrite-dependent anaerobic methane oxidation (n-DAMO) is an important process linking nitrogen and carbon cycle. It is recently demonstrated that n-DAMO archaea are able to couple n-DAMO to dissimilatory nitrate reduction to ammonium (DNRA). In this work, a mathematical model is developed to describe DNRA by n-DAMO archaea for the first time. The anabolic and catabolic processes of n-DAMO archaea, n-DAMO bacteria and anaerobic ammonium oxidation (Anammox) bacteria are involved. The different impacts of exogenous and endogenous nitrite on DNRA and n-DAMO microbes are considered. The developed model is calibrated and validated using experimental data collected from a sequencing batch reactor (SBR) and a counter-diffusion membrane biofilm bioreactor (MBfR). The model outputs fit well with the profiles of nitrogen (N) dynamics and biomass changes in both reactors, demonstrating its good predictive ability. The developed model is further used to simulate the counter-diffusion MBfR incorporating n-DAMO and Anammox process to treat sidestream wastewater. The simulated distribution profiles of N removal/production rates by different microbes along biofilm depth reveal that DNRA by n-DAMO archaea plays an important role in N transformation of the integrated n-DAMO and Anammox process. It is further suggested that the counter-diffusion MBfR under the investigated conditions should be operated at proper hydraulic retention times (HRTs) (i.e. 6h and 8h) with exogenous NO₂^- in the range of 0-10 mg N/L or at HRTs >3h with the absence of exogenous NO₂^- in order to achieve dischargeable effluent.

Sulfidation forwarding high-strength Anammox process using nitrate as electron acceptor via thiosulfate-driven nitrate denitratation

A single up-flow thiosulfate-driven nitrate denitratation coupled with the sulfurized Anammox (TDSA) with the core-shell structure (S⁰@ Anammox granules) provided a chemical/energy-saving way for the removal of high-content ammonium with nitrate as electron acceptor. Approximately 83.66% total nitrogen removal efficiency (TNRE) could be achieved by the sulfurized Anammox encrusted by S⁰/S_n^2- at a high loading rate (2.6 kg-N/(m³·d)) via resisting high concentration of free ammonia (FA) (22.35 mg/L), mainly through S₂O₃^2-, S⁰/S_n^2- -driven partial denitrification-Anammox (PDN-Anammox) process. Moreover, S⁰/S_n^2--PDN-Anammox was largely restricted when intermittently aerated, but still resulting in 74.47% TNRE due to the partial nitrification-Anammox (PN-Anammox). The sequencing analysis revealed that Anammox bacterium (Candidatus_Kuenenia) and sulfur-oxidizing bacterium (Thiobacillus) coexisted, in which Anammox process occurred mainly via NO instead of NH₂OH. This study provided a new perspective for high concentration nitrogen wastewater removal in engineering applications.

Simultaneous partial nitritation, anammox, and denitrification process for the treatment of simulated municipal sewage in a single-stage biofilter reactor

This study provides a feasible scheme for the treatment of municipal sewage through simultaneous partial nitritation, anammox, and denitrification (SNAD) process, which was realized in a single-stage biofilter reactor (BFR). First, the BFR was started up to enrich the anaerobic ammonium-oxidizing bacteria (AnAOB) in the upper part of the reactor through the operation mode of the top influent and bottom effluent. Then, the BFR was inoculated with activated sludge and aerated continuously at the bottom to realize the coupling of SNAD, which was accompanied by a two-point influent from the bottom and top effluent. Results indicated that the high removal efficiency of NH₄⁺-N (93.40%), total nitrogen (TN, 89.95%), and soluble chemical oxygen demand (SCOD, 92.68%) were achieved with an air-water ratio of 4.29 and hydraulic retention time (HRT) of 6 h. During the SNAD steady phase for the treatment of simulated municipal sewage with a soluble chemical organic demand to nitrogen (C/N) ratio of 2.31, low concentrations of NH₄⁺-N (4.13 mg/L), TN (6.44 mg/L), and SCOD (11.29 mg/L) were attained in the effluent. High-throughput sequencing analysis indicated that the relative abundance of Nitrosomonas, Candidatus Brocadia, and Denitratisoma were 0.77%, 0.43%, and 4.07% in the biofilm at the 0-12.5 cm zone, respectively, suggesting successful implementation of the SNAD process.

Mainstream Nitrogen and Dissolved Methane Removal through Coupling n-DAMO with Anammox in Granular Sludge at Low Temperature

Mainstream anaerobic wastewater treatment has received increasing attention for the recovery of methane-rich biogas from biodegradable organics, but subsequent mainstream nitrogen and dissolved methane removal at low temperatures remains a critical challenge in practical applications. In this study, granular sludge coupling n-DAMO with Anammox was employed for mainstream nitrogen removal, and the dissolved methane removal potential of granular sludge at low temperatures was investigated. A stable nitrogen removal rate (0.94 kg N m^-3 d^-1 at 20 °C) was achieved with a high-level effluent quality (<3.0 mg TN L^-1) in a lab-scale membrane granular sludge reactor (MGSR). With decreasing temperature, the nitrogen removal rate dropped to 0.55 kg N m^-3 d^-1 at 10 °C, while the effluent concentration remained <1.0 mg TN L^-1. The granular sludge with an average diameter of 1.8 mm proved to retain sufficient biomass (27 g VSS L^-1), which enabled n-DAMO and Anammox activity at a hydraulic retention time as low as 2.16 h even at 10 °C. 16S rRNA gene sequencing and scanning electron microscopy revealed a stable community composition and compact structure of granular sludge during long-term operation. Energy recovery could be maximized by recovering most of the dissolved methane in mainstream anaerobic effluent, as only a small amount of dissolved methane was capable of supporting denitrifying methanotrophs in granular sludge, which enabled high-level nitrogen removal.

Emerging onsite electron donors for advanced nitrogen removal from anammox effluent of leachate treatment: A review and future applications

Partial nitrification-anammox process is promising in leachate treatment, but the 11% residue nitrate limits the total nitrogen removal efficiency. Denitrification or partial denitrification and anammox are both practical polishing processes of anammox effluent, requiring extra electron donors. Fortunately, there are organic matter, sulfide and methane in leachate or produced by leachate treatment, which can serve as onsite electron donors. In this review, the mechanisms and processes using these three kinds of electron donors for residue nitrate reduction in anammox effluent of leachate are systematically summarized and discussed. It can be concluded that, biodegradable organic matter is an effective electron donor, sulfide is a promising electron donor, methane is a potential electron donor. Two possible applications in future based on anammox treatment of fresh and mature leachate using sulfide and methane as onsite electron donors are proposed. Through sulfide reutilization, energy-saving with about 14% of aeration reduction can be achieved.

Novel biotechnologies for nitrogen removal and their coupling with gas emissions abatement in wastewater treatment facilities

Wastewaters contaminated with nitrogenous pollutants, derived from anthropogenic activities, have exacerbated our ecosystems sparking environmental problems, such as eutrophication and acidification of water reservoirs, emission of greenhouse gases, death of aquatic organisms, among others. Wastewater treatment facilities (WWTF) combining nitrification and denitrification, and lately partial nitrification coupled to anaerobic ammonium oxidation (anammox), have traditionally been applied for the removal of nitrogen from wastewaters. The present work provides a comprehensive review of the recent biotechnologies developed in which nitrogen-removing processes are relevant for the treatment of both wastewaters and gas emissions. These novel processes include the anammox process with alternative electron acceptors, such as sulfate (sulfammox), ferric iron (feammox), and anodes in microbial electrolysis cells (anodic anammox). New technologies that couple nitrate/nitrite reduction with the oxidation of methane, H₂S, volatile methyl siloxanes, and other volatile organic compounds are also described. The potential of these processes for (i) minimizing greenhouse gas emissions from WWTF, (ii) biogas purification, and (iii) air pollution control is critically discussed considering the factors that might trigger N₂O release during nitrate/nitrite reduction. Moreover, this review provides a discussion on the main challenges to tackle towards the consolidation of these novel biotechnologies.

Insights into complete nitrate removal in one-stage nitritation-anammox by coupling heterotrophic denitrification

Nitritation-anammox has been considered to be the most promising process for nitrogen (N) removal from wastewater. However, the anammox reaction still produces an amount of nitrate, which cannot be removed further. This study hypothesizes that heterotrophic denitrification can be an appealing option to remove the residual nitrate in the one-stage nitritation-anammox process. Through monitoring N-removal performance and microbial community succession of a laboratory microaerobic reactor, the effect of four different levels of oxygen supply on nitrate removal was investigated. The reactor was continuously fed with real manure-free piggery wastewater containing ~240 mg NH₄⁺-N/L and chemical oxygen demand (COD)/total nitrogen (TN) ratio of less than 1 for 180 days. With a high influent loading rate of 0.7 kg N/(m³·d), efficient total nitrogen removal (>80 %) was achieved during stable operation of dissolved oxygen (DO) concentrations between 0.3 and 0.6 mg O₂/L, indicating N-removal via the nitritation-anammox pathway in the low-carbon wastewater treatment. At the same time, the effluent nitrate reduced with decreased oxygen supply and completely depleted at DO of 0.3 ± 0.1 mg O₂/L. In addition to oxygen, preventing ammonia nitrogen from falling to very low levels (<10 mg/L) could be also useful for the complete nitrate removal and stable nitritation-anammox. 16S rRNA gene-based analyses confirmed a complex microbial community including nitrifiers, denitrifiers and anammox bacteria in the biomass of the reactor. Collectively, this study provides new insights into high-level N-removal of a nitritation-anammox process by complete nitrate depletion.

Efficient nitrogen removal from mainstream wastewater through coupling Partial Nitritation, Anammox and Methane-dependent nitrite/nitrate reduction (PNAM)

The application of partial nitritation and anammox to remove nitrogen from mainstream wastewater is of great interest because of the potential to reduce energy cost and carbon dosage. However, this process confronts a dilemma of relatively high effluent nitrogen concentration (>10 mg N/L), owning to the unwanted prevalence of nitrite-oxidizing bacteria (NOB) and the intrinsic nitrate production by anammox bacteria. Here, a novel technology, named the one-stage PNAM, that integrates Partial Nitritation, Anammox and Methane-dependent nitrite/nitrate reduction reactions, was developed in a single membrane biofilm reactor (MBfR). With feeding of synthetic mainstream wastewater containing ∼50 mg NH₄⁺-N/L at a hydraulic retention time of 12 h, more than 95% nitrogen was removed in the established one-stage PNAM process at a practically useful rate of 0.1 kg N/m³/d. Microbial community characterization and in-situ batch tests revealed a sophisticated microbial structure consisting of ammonia-oxidizing bacteria (AOB), anammox bacteria, nitrite/nitrate-dependent anaerobic methane oxidation (n-DAMO) bacteria and archaea, and a small fraction of NOB and aerobic methanotrophs. The role of methane in removing nitrate was confirmed by switching on/off the methane supply, which relaxed the requirement for NOB suppression. In addition, the established system was relatively robust against temperature variations, evidenced by a total nitrogen removal efficiency above 80% at temperature as low as 14 ℃. The results provide a promising alternative for efficient nitrogen removal from domestic wastewater using methane as the sole carbon source.

Denitrifying anaerobic methane oxidation (DAMO) cultures: Factors affecting their enrichment, performance and integration with anammox bacteria

The recently discovered process, denitrifying anaerobic methane oxidation (DAMO), links the carbon and nitrogen biogeochemical cycles via coupling the anaerobic oxidation of methane to denitrification. The DAMO process, in this respect, has the potential to mitigate the greenhouse effect through the assimilation of dissolved methane. Denitrification via methane oxidation rather than organic matter, provides a new perspective to performing this once thought to be well established process. The two main species responsible for this process are "Candidatus Methylomirabilis oxyfera (M. oxyfera), and "Candidatus Methanoperedens nitroreducens" (M. nitroreducens). M. oxyfera is responsible of reducing nitrite while M. nitroreducens reduces nitrate to nitrite. These two microorganisms, despite their different pathways, were found to exist together in nature through a syntrophic relationship. Their co-existence with anaerobic ammonium oxidation (Anammox) bacteria was also revealed in the last decade. Anammox bacteria are chemolithoautotrophs, converting ammonium and nitrite to N₂ and nitrate. They are responsible for the release of more than 50% of oceanic N₂, hence play an important role in the global nitrogen cycle. Factors leading to the enrichment of DAMO cultures and their cultivation with Anammox cultures are of significance for improved nitrogen removal systems with decreased greenhouse effect, and even for further full-scale applications. This study, therefore, aims to present an updated review of the DAMO process, by focusing on the factors that might have a significant role in enrichment of DAMO microorganisms and their co-existence with Anammox bacteria. Factors such as temperature, pH, inoculum and feed type, trace metals and reactor configuration are among the ones discussed in detail. Factors, which have not been investigated, are also elucidated to provide a better understanding of the process and set research goals that will aid in the development of DAMO-centered wastewater treatment alternatives.

Continuous anaerobic oxidation of methane: Impact of semi-continuous liquid operation and nitrate load on N2O production and microbial community

This work proves the feasibility of employing regular secondary activated sludge for the enrichment of a microbial community able to perform the anaerobic oxidation of methane coupled to nitrate reduction (N-AOM). After 96 days of activated sludge enrichment, a clear N-AOM activity was observed in the resulting microbial community. The methane removal potential of the enriched N-AOM culture was then studied in a stirred tank reactor (STR) operated in continuous mode for methane supply and semi-continuous mode for the liquid phase. The effect of applying nitrate loads of ∼22, 44, 66, and 88 g NO₃^- m^-3 h^-1 on (i) STR methane and nitrate removal performance, (ii) N₂O emission, and (iii) microbial composition was investigated. Methane elimination capacities from 21 ± 13.3 to 55 ± 12 g CH₄ m^-3 h^-1 were recorded, coupled to nitrate removal rates ranging from 6 ± 3.2 to 43 ± 14.9 g NO₃^- m^-3 h^-1. N₂O production was not detected under the three nitrate loading rates applied for the assessment of potential N₂O emission in the continuous N-AOM process (i.e. ∼22-66 g NO^-3 m^-3 h^-1). The lack of N₂O emissions during the process was attributed to the N₂O reducing capacity of the bacterial taxa identified and the rigorous control of dissolved O₂ and pH implemented (dissolved O₂ values ≤ 0.07 g m^-3 and pH of 7.6 ± 0.4). Microbial characterization showed that the N-AOM process was performed in absence of putative N-AOM archaea and bacteria (ANME-2d, M. oxyfera). Instead, microbial activity was driven by methane-oxidizing bacteria and denitrifying bacteria (Bacteroidetes, α-, and γ-proteobacteria).

Methanotrophic denitrification in wastewater treatment: microbial aspects and engineering strategies

Anaerobic technologies are consolidated for sewage treatment and are the core processes for mining marketable products from waste streams. However, anaerobic effluents are supersaturated with methane, which represents a liability regarding greenhouse gas emissions. Meanwhile, anaerobic technologies are not capable of nitrogen removal, which is required to ensure environmental protection. Methane oxidation and denitrification processes can be combined to address both issues concurrently. Aerobic methane oxidizers can release intermediate organic compounds that can be used by conventional denitrifiers as electron donors. Alternatively, anoxic methanotrophic species combine methane oxidation with either nitrate or nitrite reduction in the same metabolism. Engineered systems need to overcome the long doubling times and low NO_x consumption rates of anoxic methanotrophic microorganisms. Another commonly reported bottleneck of methanotrophic denitrification relates to gas-liquid mass transfer limitations. Although anaerobic effluents are supersaturated with methane, experimental setups usually rely on methane supply in a gaseous mode. Hence, possibilities for the application of methane-oxidation coupled to denitrification in full scale might be overlooked. Moreover, syntrophic relationships among methane oxidizers, denitrifiers, nitrifiers, and other microorganisms (such as anammox) are not well understood. Integrating mixed populations with various metabolic abilities could allow for more robust methane-driven wastewater denitrification systems. This review presents an overview of the metabolic capabilities of methane oxidation and denitrification and discusses technological aspects that allow for the application of methanotrophic denitrification at larger scales.