Microbial physiology and interactions in anammox systems with the intermittent addition of organic carbons

Integrating network and in-silico simulation insights into the ecological interactions shaped by carbon sources in partial denitrification and anammox system

The underlying ecological mechanism of microbial communities shaped by carbon source in partial denitrification and anammox (PDA) systems remains poorly understood, despite the potential of multiple carbon sources to support the partial denitrification process. Herein, the integrated network and in-silico simulation methods were used to evaluate the considerable impact of carbon sources on the dynamics of ecological interactions. The fluctuation of carbon source (from acetate to glucose and ethanol) significantly destabilized the performance of PDA system (total nitrogen removal efficiency decreased from 96.8% to 69.1%). Glucose simultaneously altered the composition of denitrifying bacteria, resulting in a significant enrichment of the genus Elstera (from 0% to 12.7%). By contrast, genus Thauera re-dominated for partial denitrification with ethanol as carbon source. Importantly, heterotrophic bacteria (e.g., genus Calditrichia) gradually enriched by utilizing ethanol. The presence of acetate in phase IV further enhanced the competitive advantage of heterotrophic bacteria over denitrifying bacteria, thereby resulting in the deteriorated performance of the PDA system. The in-silico simulation of co-culture further revealed that the overgrowth of auxotrophic species Calditrichia utilized amounts of nutrients and limited other functional bacteria. Additionally, the whole co-occurrence network indicated that positive interactions likely improved the adaptability of anammox bacteria under the unsteady conditions. This study provides profound insights into the ecological interactions shaped by carbon sources in PDA systems and underscores the necessity of comprehensive review of the external carbon source to ensure optimal performance.

Polyphenolic compounds mitigate the oxidative damage of anammox sludge under long-term light irradiation

Continuous high-intensity light exposure can inhibit anaerobic ammonium oxidation (anammox) bacteria activity, though the specific impacts on anammox reactor performance remain unclear. This study investigates the effects of long-term light stress on anammox sludge reactors and explores the use of tea polyphenols as an engineering interventions to mitigate photo oxidation damage. The results showed that the nitrogen removal efficiency (NRE) of the reactor rapidly deteriorated to 41.4 % under 10,000 lx light conditions. However, reactors supplemented with 1 mg·L-1 and 5 mg·L-1 tea polyphenols sustained NREs of 75.2 % and 82.5 %, respectively. The addition of tea polyphenols alleviated oxidative stress by scavenging reactive oxygen species such as ·OH and H2O2, and by enhancing the activities of antioxidant enzymes including total superoxide dismutase and glutathione peroxidase. Candidatus Kuenenia was negatively impacted by light, while unclassified_f__Brocadiaceae thrived under light stress. These findings provide insights for the development of stable nitrogen removal systems under light exposure.

Mixotrophic anammox bacteria outcompete dissimilatory nitrate reduction and denitrifying bacteria in propionate-containing wastewater

Organic carbon can influence nitrogen removal during the anaerobic ammonia oxidation (anammox) process. Propionate, a common organic compound in pretreated wastewater, its impacts on mixotrophic anammox bacteria and the underlying mechanisms have not been fully elucidated. This study investigated the core metabolism and shift in behavior patterns of mixotrophic Candidatus Brocadia sapporoensis (AMXB) under long-term propionate exposure. Genome-resolved metagenomic analysis revealed that AMXB could convert nitrate generated by anammox bacteria to ammonium via the DNRA pathway, leveraging propionate as an electron donor. This recycled ammonium was then used to sustain the anammox process, thereby enhancing nitrogen removal efficiency. Notably, AMXB grew more efficiently than DNRA and denitrifying bacteria due to its more energy-efficient propionate metabolic pathway. This finding suggests that AMXB, as a mixotrophic anammox bacterium, has a competitive advantage in nitrogen metabolism in low C/N wastewater, contributing to efficient nitrogen removal.

Deciphering key microbes and their interactions within anaerobic ammonia oxidation systems

The stability of anaerobic ammonium oxidation (anammox) performance is inseparably linked to the dynamic equilibrium of microbial interactions. However, understanding of the key microbes within anammox systems remains limited. Through the analysis of 186 16S rRNA datasets combined with various ecological analysis methods, this study identified key microbes in the anammox process. Interactions between Candidatus_Kuenenia and other key microbes are the most significant with greater tolerance to differing water quality, while Candidatus_Jettenia have higher habitat specificity. Under adverse conditions, anammox bacteria can reduce the impact of unfavorable environments by enhancing interactions with certain microbes. This study comprehensively reviews the main functions of key microbes in the anammox system and their interactions, and summarizes several common interaction mechanisms, providing new insights for understanding the startup and stable operation of the anammox process.

Increased anaerobic conditions promote the denitrifying nitrogen removal potential and limit anammox substrate acquisition within paddy irrigation and drainage units

Microbial nitrogen (N) removal is crucial for purifying surface water quality in paddy irrigation and drainage units (IDUs). However, the spatiotemporal microbial N removal potential characteristics within these IDUs and the effects of changing anaerobic conditions on this potential remain insufficiently studied. In this study, we investigated the microbial N removal potential of conventional rice-wheat rotation and anaerobically enhanced rice-crayfish rotation IDUs using field measurements, isotope tracing techniques, and quantitative PCR. Our findings reveal that paddy fields were identified as hotspots for anammox activity, contributing to 76.0 %-97.4 % of the total anammox N removal potential in the IDU, while denitrification processes in ditches accounted for 43.5 %-77.4 % of the IDU's denitrification potential. During the rice transplanting period, the anammox N removal potential peaked, representing 35.8 % and 71.8 % of the total anammox N removal potential of the paddy fields in rice-wheat and rice-crayfish IDUs, respectively. An increase in anaerobic conditions diminished the anammox N removal potential while amplifying denitrification capabilities. The N removal potential in paddy fields decreased with increasing depth, contrasting with the relative stability in ditches. Spatiotemporal fluctuations in N removal potentials within these units are influenced by Fe2+ concentration, carbon and N content, WFPS, and pH levels. This study provides a scientific basis for improving nitrogen removal and water quality treatment in IDUs.

The effects of graphene on low-temperature anammox process: The insights from short-term tests and long-term operation

Effluent quality deterioration caused by seasonal low temperature is a great challenge to the application of anammox technology. Here, the effects of different graphene materials on anammox process were investigated under both optimal temperature and low-temperature. The batch tests showed that at 30 °C, 300 mg/L of reduced graphene oxide‑sodium alginate gel (RGOSA) had the most significant promoting effect, reaching nitrogen removal efficiency (NRE) and nitrogen removal rate (NRR) of 95 % and 8.88 mgN/L/d, respectively. The changes of EPS secretion patterns and increasing of key enzymes activity might contribute to the enhanced anammox activity. During the long-term operation of anammox reactor, the NRE and NRR of the reactor decreased when the temperature dropped to 15 °C, showing an NRE of 50 %-57 % with the addition of 200 mg/L of reduced graphene oxide (RGO) and 40 %-45 % with the addition of 20 mg/L of RGO. Furthermore, specific anammox activity (SAA) of the RGO200 reactor at 15 °C increased by 57.1 % compared to the UASB reactor without graphene addition. Additionally, 16S rRNA and metagenomic analysis results revealed anammox bacteria Ca. Kuenenia was the dominant bacteria. Moreover, the RGO can significantly increase the relative abundance of N-converting functional genes. This study demonstrates the graphene materials can help anammox process adapting to low temperatures, providing a possible solution for the application of anammox technology.

Life history strategies determine response to SRT driven crash in anammox bioreactors

Anaerobic ammonium oxidation (anammox) is a biological process often applied in wastewater treatment plants for nitrogen removal from highly concentrated side-stream effluents from anaerobic digesters. However, they are vulnerable to process instability prompted by operational shocks and microbial community imbalances, resulting in lengthy recovery times. These issues are further compounded by a lack of understanding of how sustained press disturbances influence the microbial ecology of the system. Here we investigate the response and recovery of an anammox membrane bioreactor to a solids retention time (SRT)-induced reactor crash using 16S rRNA gene and shotgun metagenomic sequencing. We observed a strong selection of bacterial groups based on reproduction strategies, with the Orders Rhodospirillales and Sphingobacteriales increasing from 1.0 % and 11.9 % prior to the crash to 31.9 % and 18.1 % during the crash respectively. The Orders Brocadiales and Anaerolineales decreased from 17.3 % and 28.3 % to 7.3 % and 1.4 % over the same time period, respectively. Metagenomic and metatranscriptomic analyses revealed differential crash responses in metabolically distinct groups of bacteria, with increased expression of genes for extracellular carbohydrate active enzymes, peptidases and membrane transporters. Following the crash, the reactor recovered to its prior state of nitrogen removal performance and pathway analysis demonstrated increased expression of genes related to exopolysaccharide biosynthesis and quorum sensing during the reactor recovery period. This study highlights the effects of reactor perturbations on microbial community dynamics in anammox bioreactors and provides insight into potential recovery mechanisms from severe disturbance.

Characterization of EPS subfractions from a mixed culture predominated by partial-denitrification functional bacteria

Extracellular polymeric substances (EPS) play a crucial role in the aggregation of partial denitrification (PD) consortia, as EPS is closely linked to bioreactor performance. However, the structural and compositional properties of EPS from PD consortia have not yet been investigated. In this study, photometric measurements indicated that PD consortia contained significantly more EPS (168.81 ± 2.10 mg/g VSS) compared to conventional activated sludge (79.79 mg/g VSS). The EPS of PD consortia exhibited a significant predominance of proteins over polysaccharides, with a protein/polysaccharide ratio of 1.43 ± 0.10. FTIR analysis revealed that the EPS of PD consortia contained fewer hydrophilic functional groups, particularly carboxyl and carbonyl groups, indicating a high aggregation potential. The content comparison of EPS and functional groups across three stratified EPS subfractions from PD consortia consistently followed the sequence: TB-EPS > LB-EPS > S-EPS. XPS results corroborated the FTIR findings and the protein/polysaccharide ratio determined by photometric measurements, all of which suggested that the EPS of PD consortia exhibited a higher abundance of hydrophobic functional groups. However, the higher α-helix/(β-sheet + random coil) ratio (0.99) suggested that the proteins in PD consortia had a compact structure, making inner hydrophobic groups difficult to expose. This compact protein structure could limit aggregation among bacterial cells, indicating the need for process optimization to enhance sludge aggregation in PD-related processes. Overall, understanding the aggregation characteristics of PD consortia could improve the application of PD-based processes.

Chemomixoautotrophy and stress adaptation of anammox bacteria: A review

Anaerobic ammonium oxidizing (anammox) bacteria, which were first discovered nearly three decades ago, are crucial for treating ammonium-containing wastewater. Studies have reported on the biochemical nitrogen conversion process and the physiological, phylogenic, and ecological features of anammox bacteria. For a long time, anammox bacteria were assumed to have a lithoautotrophic lifestyle. However, recent studies have suggested the functional versatility of anammox bacteria. Genome-based analysis and experiments with enrichment cultures have demonstrated the association of the metabolic activities of anammox bacteria with different stress conditions, revealing the importance of utilizing specific organic substances, including organoautotrophy, for growth and adaptation to stress conditions. Our understanding regarding the utilization and metabolism of organic substances and their associations with anammox reactions in anammox bacteria is growing but still incomplete. In this review, we summarize the effect of the utilization of organic substances by anammox bacteria under environmental stress conditions, emphasizing their potential organoautotrophic activity and metabolic flexibility. Although most anammox bacteria may utilize specific organic substances, Ca. Brocadia exhibited the highest level of mixoautotrophic activity. The environmental factors that substantially affect the organoautotrophic activities of anammox bacteria were also examined. This review provides a new perspective on the organoautotrophic capacity of anammox bacteria.

Towards low carbon demand and highly efficient nutrient removal: Establishing denitrifying phosphorus removal in anaerobic/anoxic/oxic + nitrification system

A two-sludge anaerobic/anoxic/oxic + nitrification system with simultaneous nitrogen and phosphorus removal was studied for enhanced low-strength wastewater treatment. After 158 days of operation, excellent NH4+-N, chemical oxygen demand (COD) and PO43--P removal (99.0 %, 90.0 % and 92.0 %, respectively) were attained under a low carbon/nitrogen ratio of 5, resulting in effluent NH4+-N, COD and PO43--P concentrations of 0.3, 30.0 and 0.5 mg/L, respectively. The results demonstrate that the anaerobic/anoxic/oxic sequencing batch reactor (A2-SBR) and nitrification sequencing batch reactor (N-SBR) had favorable denitrifying phosphorus removal and nitrification performance, respectively. High-throughput sequencing results indicate that the phosphate-accumulating organisms Dechloromonas (1.1 %) and Tetrasphaera (1.2 %) were enriched in the A2-SBR, while the ammonia-oxidizing bacteria Nitrosomonas (7.8 %) and the nitrite-oxidizing bacteria Nitrospira (18.1 %) showed excellent accumulation in the N-SBR. Further analysis via functional prediction revealed that denitrification is the primary pathway of nitrogen metabolism throughout the system. Overall, the system achieved low carbon and high efficiency nutrient removal.

Insight into microbial adaptability in continuous flow anaerobic ammonium oxidation process for low-strength sewage treatment

Organic matter concentration is a critical factor influencing the adaptability of anaerobic ammonium oxidation (anammox) bacteria to low-strength sewage treatment. To address this challenge and achieve stable anammox activity, a micro-aeration partial nitrification-anammox process was developed for continuous-flow municipal sewage treatment. Under limited ammonium conditions, the effective utilization of organics in denitrification promoted the stable accumulation of nitrite and enhanced anammox activity. This, in turn, led to enhanced nitrogen removal efficiency, reaching approximately 87.7%. During the start-up phase, the protein content of extracellular polymeric substances (EPS) increased. This enhanced EPS intensified the inhibitory effect of denitrifying bacteria (DNB) on nitrite-oxidizing bacteria through competition for nitrite, thereby facilitating the proliferation of anammox bacteria (AnAOB). Additionally, several types of DNB capable of utilizing slowly biodegradable organics contributed to the adaptability of AnAOB. These findings provide valuable insights for ensuring efficient anammox performance and robust nitrogen removal in the treatment of low-strength sewage.

Synergistic interactions between anammox and dissimilatory nitrate reducing bacteria sustains reactor performance across variable nitrogen loading ratios

Anaerobic ammonium oxidizing (anammox) bacteria are utilized for high efficiency nitrogen removal from nitrogen-laden sidestreams in wastewater treatment plants. The anammox bacteria form a variety of competitive and mutualistic interactions with heterotrophic bacteria that often employ denitrification or dissimilatory nitrate reduction to ammonium (DNRA) for energy generation. These interactions can be heavily influenced by the influent ratio of ammonium to nitrite, NH4+:NO2-, where deviations from the widely acknowledged stoichiometric ratio (1:1.32) have been demonstrated to have deleterious effects on anammox efficiency. Thus, it is important to understand how variable NH4+:NO2- ratios impact the microbial ecology of anammox reactors. We observed the response of the microbial community in a lab scale anammox membrane bioreactor (MBR) to changes in the influent NH4+:NO2- ratio using both 16S rRNA gene and shotgun metagenomic sequencing. Ammonium removal efficiency decreased from 99.77 ± 0.04% when the ratio was 1:1.32 (prior to day 89) to 90.85 ± 0.29% when the ratio was decreased to 1:1.1 (day 89-202) and 90.14 ± 0.09% when the ratio was changed to 1:1.13 (day 169-200). Over this same timespan, the overall nitrogen removal efficiency (NRE) remained relatively unchanged (85.26 ± 0.01% from day 0-89, compared to 85.49 ± 0.01% from day 89-169, and 83.04 ± 0.01% from day 169-200). When the ratio was slightly increased to 1:1.17-1:1.2 (day 202-253), the ammonium removal efficiency increased to 97.28 ± 0.45% and the NRE increased to 88.21 ± 0.01%. Analysis of 16 S rRNA gene sequences demonstrated increased relative abundance of taxa belonging to Bacteroidetes, Chloroflexi, and Ignavibacteriae over the course of the experiment. The relative abundance of Planctomycetes, the phylum to which anammox bacteria belong, decreased from 77.19% at the beginning of the experiment to 12.24% by the end of the experiment. Analysis of metagenome assembled genomes (MAGs) indicated increased abundance of bacteria with nrfAH genes used for DNRA after the introduction of lower influent NH4+:NO2- ratios. The high relative abundance of DNRA bacteria coinciding with sustained bioreactor performance indicates a mutualistic relationship between the anammox and DNRA bacteria. Understanding these interactions could support more robust bioreactor operation at variable nitrogen loading ratios.

Research advances in application of mainstream anammox processes: Roles of quorum sensing and microbial metabolism

Anaerobic ammonium oxidation (anammox) is a low-carbon biological nitrogen removal process, that has been widely applied to treat high-strength wastewater. However, the practical application of mainstream anammox treatment is limited due to the slow growth rate of anammox bacteria (AnAOB). Therefore, it is important to provide a comprehensive summary of the potential impacts and regulatory strategies for system stability. This article systematically reviewed the effects of environmental fluctuations on anammox systems, summarizing the bacterial metabolisms and the relationship between metabolite and microbial functional effects. To address the shortcoming of mainstream anammox process, molecular strategies based on quorum sensing (QS) were proposed. Sludge granulation, gel encapsulation and carrier-based biofilm technologies were adopted to enhance the QS function in microbial aggregation and reduction of biomass loss. Furthermore, this article discussed the application and progress of anammox-coupled processes. Valuable insights were provided for the stable operation and development of mainstream anammox process from the perspectives of QS and microbial metabolism.

Double-edged sword effects of dissimilatory nitrate reduction to ammonium (DNRA) bacteria on anammox bacteria performance in an MBR reactor

Dissimilatory nitrate reduction to ammonium (DNRA) bacteria imposing double-edged sword effects on anammox bacteria were investigated in an anammox-membrane bioreactor (MBR) experiencing an induced crash-recovery event. During the experiment, the anammox-MBR was loaded with NH₄⁺-N:NO₂^--N ratios (RatioNH₄⁺-N: NO₂^--N) of 1.20-1.60. Initially, the anammox-MBR removed over 95% of 100 mg/L NH₄⁺-N and 132 mg/L NO₂^--N (RatioNH₄⁺-N: NO₂^--N = 0.76, the well accepted stoichiometric RatioNH₄⁺-N: NO₂^--N for anammox) in the influent (Stage 0). Then, we induced a system crash-recovery event via nitrite shock loadings to better understand responses from different guilds of bacteria in anammox-MBR, loaded with 1.60 RatioNH₄⁺-N: NO₂^--N with 100 mg/L NO₂^--N in the influent (Stage 1). Interestingly, the nitrogen removal by anammox bacteria was maintained for about 20 days before starting to decrease significantly. In Stage 2, we further increased influent nitrite concentration to 120 mg/L (1.33 RatioNH₄⁺-N: NO₂^--N) to simulate a high nitrite toxicity scenario for a short period of time. As expected, nitrogen removal efficiency dropped to only 16.8%. After the induced system crash, anammox-MBR performance recovered steadily to 93.2% nitrogen removal with a 1.25 RatioNH₄⁺-N:NO₂^--N and a low nitrite influent concentration of 80 mg/L NO₂^--N. Metagenomics analysis revealed that a probable causality of the decreasing nitrogen removal efficiency in Stage 1 was the overgrowth of DNRA-capable bacteria. The results showed that the members within the Ignavibacteriales order (21.7%) out competed anammox bacteria (17.0%) in the anammox-MBR with elevated nitrite concentrations in the effluent. High NO₂^--N loading (120 mg N/L) further caused the predominant Candidatus Kuenenia spp. were replaced by Candidatus Brocadia spp. Therefore, it was evident that DNRA bacteria posed negative effects on anammox with 1.60 RatioNH₄⁺-N: NO₂^--N. Also, when 120 mg/L NO₂^--N fed to anammox-MBR (RatioNH₄⁺-N: NO₂^--N = 1.33), canonical denitrification became the primary nitrogen sink with both DNRA and anammox activities decreased. They probably fed on lysed microbial cells of anammox and DNRA. In Stage 3, a low RatioNH₄⁺-N: NO₂^--N (1.25) with 80 mg/L NO₂^--N was used to rescue the system, which effectively promoted DNRA-capable bacteria growth. Although anammox bacteria's abundance was only 7.7% during this stage, they could be responsible for about 90% of the total nitrogen removal during this stage.

Achieving stable anammox process and revealing shift of bacteria during the start-up in landfill leachate treatment

Partial nitrification-anammox (PN/A) is an energy-efficient technology for nitrogen removal in landfill leachate treatment. Numerous studies have reported successful implementation of the PN/A process and its stable operation under laboratory conditions. One of the primary challenges in PN/A engineering applications is the mass of the seed sludge required for start-up. This study examined the PN/A using a sequence batch reactor (SBR) inoculating a common mixture to treat landfill leachate. After 70 days of operation, the system successfully realized a one-stage PN/A process and maintained a stable ammonium NH 4 + $$ \left({NH}_4^{+}\right) $$ removal efficiency of 97.65% ± 1%, where the effluent of NH 4 + $$ {NH}_4^{+} $$ and nitrate ( NO 3 - $$ {NO}_3^{-} $$ ) were less than 4 ± 1.5 mg L^-1 and 10 mg L^-1 . In addition, the relative abundances of Ca. Kuenenia and Ca. Brocadia, which are typical anaerobic ammonia-oxidizing bacteria (AnAOB), increased from 0.08% to 3.99% (70 days) and 0.01% to 0.45%, respectively. The relative abundances of ammonia-oxidizing bacteria (AOB) Nitrosomonas and Nitrosospira increased from 0.9% to 2.89% and 0.007% to 0.1% (70 days), respectively. Both AnAOB and AOB are important niches of the system. PRACTITIONER POINTS: The research realized PN/A rapidly by inoculating common mixture sludge. The experiment successfully enriched AnAOB from 0.09% to 3.89% within 70 days. The article revealing the ecological roles of AOB and AnAOB in the landfill leachate treatment.

A critical review on microbial ecology in the novel biological nitrogen removal process: Dynamic balance of complex functional microbes for nitrogen removal

The novel biological nitrogen removal process has been extensively studied for its high nitrogen removal efficiency, energy efficiency, and greenness. A successful novel biological nitrogen removal process has a stable microecological equilibrium and benign interactions between the various functional bacteria. However, changes in the external environment can easily disrupt the dynamic balance of the microecology and affect the activity of functional bacteria in the novel biological nitrogen removal process. Therefore, this review focuses on the microecology in existing the novel biological nitrogen removal process, including the growth characteristics of functional microorganisms and their interactions, together with the effects of different influencing factors on the evolution of microbial communities. This provides ideas for achieving a stable dynamic balance of the microecology in a novel biological nitrogen removal process. Furthermore, to investigate deeply the mechanisms of microbial interactions in novel biological nitrogen removal process, this review also focuses on the influence of quorum sensing (QS) systems on nitrogen removal microbes, regulated by which bacteria secrete acyl homoserine lactones (AHLs) as signaling molecules to regulate microbial ecology in the novel biological nitrogen removal process. However, the mechanisms of action of AHLs on the regulation of functional bacteria have not been fully determined and the composition of QS system circuits requires further investigation. Meanwhile, it is necessary to further apply molecular analysis techniques and the theory of systems ecology in the future to enhance the exploration of microbial species and ecological niches, providing a deeper scientific basis for the development of a novel biological nitrogen removal process.

Role of quorum sensing-based regulation in development of anaerobic ammonium oxidation process

Shortage of anaerobic ammonium oxidation (anammox) sludge greatly limits the extensive full-scale application of anammox-based processes. Although numerous start-up strategies have been proposed, the interaction among microbial consortia and corresponding mechanism during the process development remain unknown. In this study, three reactors were established based on different seed sludges. After 27 days, the anammox process inoculated with anammox granules and activated sludge (1:5) was firstly achieved, and the highest nitrogen removal rate was 1.17 kg N m^-3 d^-1. Correspondingly, the anammox activity and abundances of related functional genes increased. Notably, the dominant anammox bacteria shifted from Candidatus Kuenenia to Candidatus Brocadia. Metagenomic analysis indicated that quorum sensing-based regulation mainly contributed to the proliferation and accumulation of anammox bacteria. This work provides an insight into the quorum sensing (QS)-regulated microbial interactions in the anammox and activated sludge consortia during the process development.

The organics-mediated microbial dynamics and mixotrophic metabolisms in anammox consortia under micro-aerobic conditions

The engineering applications of mainstream anaerobic ammonium oxidation (anammox) have raised increasing attention due to its energy-efficient, however, the organics-mediated microbial dynamics and mixotrophic metabolisms in anammox consortia under micro-aerobic conditions are still elusive. Here, the response of the anammox process to sodium acetate and glucose at a C/N ratio ranging from 0 to 0.5 was investigated under micro-aerobic conditions, respectively. Results showed that the additional glucose could promote the nitrogen removal efficiency (NRE) and nitrogen removal rate (NRR) of anammox processes at a low C/N ratio (0.3), representing 84.00% and 0.53 N kg·m^-3·d^-1. The introduced organics could regulate the diversity of the microbial community and simplify the microbial relationship in anammox consortia. Anammox could not benefit from the introduced sodium acetate, while glucose could effectively enhance the anammox activity and microbial interactions in anammox consortia. Glucose might also stimulate the mixotrophic mechanism of Ca. Kuenenia, further promotes the proliferation of anammox sludge under micro-aerobic conditions. This study reveals that glucose could positively mediate microbial interactions and mixotrophic metabolism in anammox consortia under micro-aerobic conditions, which raises a new horizon for the proliferation of anammox sludge for mainstream engineering applications.

Effects mechanism of bio-carrier filling rate on rotating biofilms and the reactor performance optimization method

Benefited from the massive filling bio-carriers, the packed cage rotating biological contactors (RBCs) have better performance and application potentiality in wastewater treatment. Investigating the effects mechanism of bio-carrier filling rate is crucial for such reactors management. In this study, the pollutants removal performance, biofilms physical characteristics, and microbial communities of the biofilms under a series of bio-carrier filling rates were analyzed. The results shown, the pollutant removal rate and amount were quite different under different filling rates, and biofilms structure and microbial composition were the main factors affecting the pollutants removal performance. With the increasing filling rates, the biofilms were more mass increased (dry weight from 0.066 to 0.148 g/per carrier), thicker (from 340.30 to 850.84 μm) and lower dense (from 0.068 to 0.060 g/cm³). The microbial community composition of those biofilms was also quite different at the genus level. The effects mechanism of bio-carrier filling rate can be summarized: the filling rates affect the physical and biological characteristics of biofilms, which will further affect the microenvironment and microbial distribution in biofilms, and then determines the pollutant metabolic rate and metabolic pathway. This study will contribute to design better bio-carrier filling rate according to different wastewater treatment scenario, and promote the performance optimization of packed cage RBCs.

The mixed/mixotrophic nitrogen removal for the effective and sustainable treatment of wastewater: From treatment process to microbial mechanism

Biological nitrogen removal (BNR) is one of the most important environmental concerns in the field of wastewater treatment. The conventional BNR process based on heterotrophic nitrogen removal (HeNR) is suffering from several limitations, including external carbon source dependence, excessive sludge production, and greenhouse gas emissions. Through the mediation of autotrophic nitrogen removal (AuNR), mixed/mixotrophic nitrogen removal (MixNR) offers a viable solution to the optimization of the BNR process. Here, the recent advance and characteristics of MixNR process guided by sulfur-driven autotrophic denitrification (SDAD) and anammox are summarized in this review. Additionally, we discuss the functional microorganisms in different MixNR systems, shedding light on metabolic mechanisms and microbial interactions. The significance of MixNR for carbon reduction in the BNR process has also been noted. The knowledge gaps and the future research directions that may facilitate the practical application of the MixNR process are highlighted. Overall, the prospect of the MixNR process is attractive, and this review will provide guidance for the future implementation of MixNR process as well as deciphering the microbially metabolic mechanisms.

Distribution of extracellular amino acids and their potential functions in microbial cross-feeding in anaerobic digestion systems

Anaerobic digestion is a prevalent bioenergy production process relying on a complex network of symbiotic interactions, where the nutrient based cross-feeding is an essential microbial mechanism. Here, the cross-feeding function was assessed by analyzing extracellular polymeric substances-associated amino acids in microbial aggregates collected from 14 lab-scale anaerobic digesters, as well as deciphering their genetically biosynthetic potential by syntrophic bacteria and methanogens. The total concentration of essential amino acids ranged from 1.2 mg/g VSS to 174.0 mg/g VSS. The percentages of glutamic acid (8.5 ∼ 37.6%), lysine (2.7 ∼ 22.6%), alanine (5.6 ∼ 13.2%), and valine (3.0 ∼ 10.4%) to the total amount of detected amino acids were the highest in most samples. Through metagenomics analysis, several investigated syntrophs (i.e., Smithella, Syntrophobacter, Syntrophomonas, and Mesotoga) and methanogens (i.e., Methanothrix and Methanosarcina) were auxotrophies, but the genetic ability of syntrophs and methanogens to synthesize some essential amino acids could be complementary, implying potential cross-feeding partnership.

Synergistic ammonia and nitrate removal in a novel pyrite-driven autotrophic denitrification biofilter

Pyrite is one kind of cost-effective electron donors for nitrate denitrification. In this study, a pyrite-driven autotrophic denitrification biofilter was applied for simultaneous removal of NH₄⁺ and NO₃^- over the 150-day. The influent NH₄⁺/NO₃^- ratio (0.3-1.7) had less effect on system performance, while for the hydraulic retention times (HRTs, 24-3 h), the removal percentage of both > 90% and removal loading rates of 52.8 and 59.4 mg N/(L·d) for NH₄⁺ and NO₃^- removal were obtained at the HRT of 6 h. The 16S rRNA genes analysis showed that Ferritrophicum, Thiobacillus, Candidatus_Brocadia, and unidentified_Nitrospiraceae were predominant. Analyses of nitrogen and sulfur metabolism showed that ammonia was removed by complete nitrification, nitrate was reduced to N₂, and sulfide was oxidized to sulfate. Dynamics of pollutants within the reactor and microbial activity showed nitrification/Anammox and pyrite-driven autotrophic denitrification were responsible for the synergistic removal of NH₄⁺/NO₃^- in this system.

Active assimilators of soluble microbial products produced by wastewater anammox bacteria and their roles revealed by DNA-SIP coupled to metagenomics

Heterotrophic bacteria grow on influent organics or soluble microbial products (SMP) in wastewater anammox processes, playing key roles in facilitating microbial aggregation and reducing excess nitrate. The overgrowth of heterotrophs represents one of the major causes of anammox process failure, while the metabolic functions of coexisting heterotrophs and their roles in anammox process remain vague. This study aimed at revealing metabolic interactions between AnAOB and active SMP assimilators by integrating ¹³C DNA-stable isotope probing, metabolomic and metagenomic approaches. Glycine, aspartate, and glutamate with low biosynthetic energy cost were the major SMP components produced by AnAOB (net yield: 44.8, 10.4, 8.1 mg·g NH₄⁺-N^-1). Glycine was likely synthesized by AnAOB via the reductive glycine pathway which is oxygen-tolerant, supporting heterotrophic growth. Fermentative Chloroflexi bacterium OLB13, denitrifying Gemmatimonadaceae and Burkholderiaceae bacterium JOSHI-001 were active SMP assimilators, which were prevalent in globally distributed wastewater anammox reactors as core taxa. They likely formed a mutualistic relationship with auxotrophic Ca. Kuenenia by providing necessities such as methionine, folate, 4'-phosphopantetheine, and molybdopterin cofactor, and receiving vitamin B₁₂ for methionine synthesis. For the first time, the identify and metabolic features of SMP assimilators in wastewater anammox communities were revealed. Supplying necessities secreted by heterotrophs could be helpful to the endeavor of AnAOB enrichment. Practically, maintaining active but not overgrown SMP assimilators is critical to efficient and stable operation of wastewater anammox processes.

Biochar enhances partial denitrification/anammox by sustaining high rates of nitrate to nitrite reduction

To examine the short-term effects of biochar on the partial-denitrification anammox (PD/A) process, the adsorption kinetics, nitrogen degradation, electron transfer properties, and microbial community succession of wastewater treatment systems with and without biochar added were monitored and characterized. The results showed that biochar increased nitrate reduction rates, which enhanced total nitrogen (TN) removal of the system by about 10%. The findings attributed improved TN removal to biochar's influence in accelerating electron transfer rather than its adsorption properties. Analysis of the nitrogen transfer pathway showed that when sufficient substrate was available, the anammox and denitrification reactions simultaneously removed nitrogen. When the nitrite supply was insufficient, the anammox reaction outcompeted the denitrification reaction for regenerated nitrite. Integrated microbial community and functional protein analyses indicated that biochar addition increased the abundance of Ca. Kuenenia and Pseudomonas. Meanwhile, biochar modulates denitrifying cellular metabolism by inducing protein changes.

Enhanced nitrogen removal by partial nitrification-anammox process with a novel high-frequency micro-aeration (HFMA) mode: Metabolic interactions among functional bacteria

A novel high-frequency micro-aeration (HFMA) mode with aeration frequency of 15 times/h and DO concentration lower than 0.5 mg/L was proposed. Advanced partial nitrification-anammox (PN-A) performance was achieved in a two-stage sequencing batch reactor-integrated fixed-film activated sludge reactor with the HFMA mode. When treating wastewater with carbon/nitrogen ratio of 3, the abundance of NO₂^--N oxidation related genes decreased, and the genes carried out NO₃^--N reduction and carbon source consumption were up-regulated. These variations in microbial metabolism brought more NO₂^--N substrate for the subsequent anammox process, and consumed part of the accumulated organic matter and NO₃^--N. Thus, the HFMA conditions eventually promoted the expression of anammox bacteria with NH₂OH as an intermediate metabolite and the substance exchange activity of anammox bacteria. The changes in microorganisms lead to increase in the nitrite accumulation rate, nitrogen removal efficiency and abundance of anammox bacteria (16.34%, 18.71% and 5.92%, respectively).

Applications of Up-Flow Anaerobic Sludge Blanket (UASB) and Characteristics of Its Microbial Community: A Review of Bibliometric Trend and Recent Findings

The interest in research on up-flow anaerobic sludge blanket (UASB) reactors is growing. The meta-analysis of bibliometric data highlighted the growing interest in four diverse topics: (i) energy recovery production; (ii) combination with other treatments; (iii) the study of processes for the removal of specific pollutants and, (iv) characterization of microbial community and granular sludge composition. In particular, the papers published in the first 6 months of 2021 on this process were selected and critically reviewed to highlight and discuss the results, the gaps in the literature and possible ideas for future research. Although the state of research on UASB is to be considered advanced, there are still several points that will be developed in future research such as the consolidation of the results obtained on a semi-industrial or real scale, the use of real matrices instead of synthetic ones and a more in-depth study of the effect of substances such as antibiotics on the microbiota and microbiome of UASB granular biomass. To date, few and conflicting data about the environmental footprint of UASB are available and therefore other studies on this topic are strongly suggested.

Organic carbon bioavailability: Is it a good driver to choose the best biological nitrogen removal process?

Organic carbon can affect the biological nitrogen removal process since the Anammox, heterotrophic and denitrifying bacteria have different affinities and feedback in relation to carbon/nitrogen ratio. Therefore, we reviewed the wastewater carbon concentration, its biodegradability and bioavailability to choose the appropriate nitrogen removal process between conventional (nitrification-denitrification) and Anammox-based process (i.e. integrated with the partial nitritation, nitritation, simultaneous partial nitrification and denitrification or partial-denitrification). This review will cover: (i) strategies to choose the best nitrogen removal route according to the wastewater characteristics in relation to the organic matter bioavailability and biodegradability; (ii) strategies to efficiently remove nitrogen and the remaining carbon from effluent in anammox-based process and its operating cost; (iii) an economic analysis to determine the operational costs of two-units Anammox-based process when compared with the commonly applied one-unit Anammox system (partial-nitritation-Anammox). On this review, a list of alternatives are summarized and explained for different nitrogen and biodegradable organic carbon concentrations, which are the main factors to determine the best treatment process, based on operational and economic terms. In summary, it depends on the wastewater carbon biodegradability, which implies in the wastewater treatment cost. Thus, to apply the conventional nitrification/denitrification process a COD_b/N ratio higher than 3.5 is required to achieve full nitrogen removal efficiency. For an economic point of view, according to the analysis the minimum COD_b/gN for successful nitrogen removal by nitrification/denitrification is 5.8 g. If ratios lower than 3.5 are applied, for successfully higher nitrogen removal rates and the economic feasibility of the treatment, Anammox-based routes can be applied to the wastewater treatment plant.

One-stage partial nitrification and anammox process in a sequencing batch biofilm reactor: Start-up, nitrogen removal performance and bacterial community dynamics in response to temperature

A one-stage partial nitrification and anammox (PN/A) process was started up and operated under varying temperatures in a lab-scale sequencing batch biofilm reactor. The start‑up phase took 110 days with an intermittent aeration strategy, and the removal efficiencies of ammonia‑nitrogen and total nitrogen were found to be 92.22% and 76.07%, respectively. The total nitrogen removal efficiency (NRE) increased by 9.49% when temperature decreased from 30 °C to 25 °C, but declined by 83.84% from 25 °C to 20 °C. The PN process was inhibited and subsequently limited the nitrogen removal performance at 20 °C. When temperature returned to 28 °C, the NRE recovered to 67.27%, but it was still lower than the value before the decrease in temperature (79.40%). Microbial community analysis showed that the predominant ammonia oxidation bacteria and anammox bacteria were Nitrosomonas and Candidatus Kuenenia, respectively. Nitrosomonas grew, while the relative abundance of Candidatus Kuenenia increased as temperature decreased and vice versa.

Mining Synergistic Microbial Interactions: A Roadmap on How to Integrate Multi-Omics Data

Mining interspecies interactions remain a challenge due to the complex nature of microbial communities and the need for computational power to handle big data. Our meta-analysis indicates that genetic potential alone does not resolve all issues involving mining of microbial interactions. Nevertheless, it can be used as the starting point to infer synergistic interspecies interactions and to limit the search space (i.e., number of species and metabolic reactions) to a manageable size. A reduced search space decreases the number of additional experiments necessary to validate the inferred putative interactions. As validation experiments, we examine how multi-omics and state of the art imaging techniques may further improve our understanding of species interactions’ role in ecosystem processes. Finally, we analyze pros and cons from the current methods to infer microbial interactions from genetic potential and propose a new theoretical framework based on: (i) genomic information of key members of a community; (ii) information of ecosystem processes involved with a specific hypothesis or research question; (iii) the ability to identify putative species’ contributions to ecosystem processes of interest; and, (iv) validation of putative microbial interactions through integration of other data sources.