Comparative Metagenomic and Metatranscriptomic Analyses Reveal the Functional Species and Metabolic Characteristics of an Enriched Denitratation Community

New insights into N2O emission and electron competition under different chemical oxygen demand to nitrogen ratios in a biofilm system

Nitrous oxide (N2O) is a greenhouse gas that could accumulate during the heterotrophic denitrification process. In this study, the effects of different chemical oxygen demand to nitrogen ratio (COD/N) on N2O production and electron competition was investigated. The electron competition was intensified with the decrease of electron supply, and Nos had the best electron competition ability. The model simulation results indicated that the degradation of NOx-Ns was a combination of diffusion and biological degradation. As reaction proceeding, N2O could accumulate inside biofilm. A thinner biofilm and a longer hydraulic retention time (HRT) might be an effective way to control N2O emission. The application of mathematical model is an opportunity to gain deep understanding of substrate degradation and electron competition inside biofilm.

Enhanced nitrate reduction in hypotrophic waters with integrated photocatalysis and biodegradation

Addressing nitrate contamination in water bodies is a critical environmental challenge, and Intimately Coupling Photocatalysis and Biodegradation (ICPB) presents a promising solution. However, there is still debate about the effectiveness of ICPB in reducing nitrate under hypotrophic conditions. Further research is needed to understand its microbial metabolic mechanism and the functional changes in bacterial structure. Here we explored microbial metabolic mechanisms and changes in bacterial structure in ICPB reactors integrating a meticulously screened TiO2/g-C3N4 photocatalyst with biofilm. We achieved a 26.3% increase in nitrate reduction using 12.2% less organic carbon compared to traditional biodegradation methods. Metagenomic analysis of the microbial communities in ICPB reactors revealed evolving metabolic pathways conducive to nitrate reduction. This research not only elucidates the photocatalytic mechanism behind nitrate reduction in hypotrophic conditions but also provides genomic insights that pave the way for alternative approaches in water remediation technologies.

Machine Learning-Assisted Optimization of Mixed Carbon Source Compositions for High-Performance Denitrification

Appropriate mixed carbon sources have great potential to enhance denitrification efficiency and reduce operational costs in municipal wastewater treatment plants (WWTPs). However, traditional methods struggle to efficiently select the optimal mixture due to the variety of compositions. Herein, we developed a machine learning-assisted high-throughput method enabling WWTPs to rapidly identify and optimize mixed carbon sources. Taking a local WWTP as an example, a mixed carbon source denitrification data set was established via a high-throughput method and employed to train a machine learning model. The composition of carbon sources and the types of inoculated sludge served as input variables. The XGBoost algorithm was employed to predict the total nitrogen removal rate and microbial growth, thereby aiding in the assessment of the denitrification potential. The predicted carbon sources exhibited an enhanced denitrification potential over single carbon sources in both kinetic experiments and long-term reactor operations. Model feature analysis shows that the cumulative effect and interaction among individual carbon sources in a mixture significantly enhance the overall denitrification potential. Metagenomic analysis reveals that the mixed carbon sources increased the diversity and complexity of denitrifying bacterial ecological networks in WWTPs. This work offers an efficient method for WWTPs to optimize mixed carbon source compositions and provides new insights into the mechanism behind enhanced denitrification under a supply of multiple carbon sources.

Molecular insight into the allocation of organic carbon to heterotrophic bacteria: Carbon metabolism and the involvement in nitrogen and phosphorus removal

Carbon metabolism and nutrient removal are crucial for biological wastewater treatment. This study focuses on analyzing carbon allocation and utilization by heterotrophic bacteria in response to increasing COD concentration in the influent. The study also assesses the effect of denitrification and biological phosphorus removal, particularly in combination with anaerobic ammonia oxidation (anammox). The experiment was conducted in a SBR operating under anaerobic/anoxic/oxic conditions. As COD concentration in the influent increased from 100 to 275 mg/L, intracellular COD accounted for 95.72 % of the COD removed. By regulating the NO3- concentration in the anoxic stage from 10 to 30 mg/L, the nitrite accumulation rate reached 69.46 %, which could serve as an electron acceptor for anammox. Most genes related to the tricarboxylic acid (TCA) cycle declined, while the genes involved in the glyoxylate cycle, gluconeogenesis, PHA synthesis increased. This suggests that glycogen accumulation and carbon storage, rather than direct carbon oxidation, was the dominant pathway for carbon metabolism. However, the genes responsible for the reduction of NO2--N (nirK) and NO (nosB) decreased, contributing to NO2- accumulation. The study also employed metagenomic analysis to reveal microbial interactions. The enrichment of specific bacterial species, including Dechloromonas sp. (D2.bin.10), Ca. Competibacteraceae bacterium (D9.bin.8), Ca. Desulfobacillus denitrificans (D6.bin.17), and Ignavibacteriae bacterium (D3.bin.9), played a collaborative role in facilitating nutrient removal and promoting the combination with anammox.

Efficient degradation of venlafaxine using intimately coupled high-active crystal facets exposed TiO2 and biodegradation system: Kinetic studies, biofilm stress behavior and transformation mechanism

Intimately coupled photocatalysis and biodegradation (ICPB) system is a potential wastewater treatment technology, of which TiO2-based ICPB system has been widely studied. There are many ways to improve the degradation efficiency of the ICPB process, but no crystal facet engineering method has been reported yet. In this work, a new ICPB system coated with NaF-TiO2 exposing high energy facets was designed to degrade biorecalcitrant psychotropic drug - venlafaxine (VNF). Initially, the TiO2 crystal surface was modified with NaF, resulting in the formation of NaF-TiO2 with a 14.4% increase in the exposure ratio of (001). The contribution rate of ·OH was increased by 9.5%, and the contribution rate of h+ was increased by 33.2%. Next, NaF-TiO2 was loaded onto the surface of the sponge carrier, and then the ICPB system was constructed after about 15 days of biofilm formation. After the ICPB system was acclimated with VNF, the removal rate of COD decreased significantly (the lowest was 62.7%), but that of ammonia nitrogen remained at 50.5 ± 6.0% and the extracellular polymeric substance (EPS) secretion increased by 84.1 mg/g VSS. According to the high throughput results, at the phylum level, Proteobacteria and Chloroflexi together maintain the nitrogen removal capability and structural stability of the ICPB system. The relative abundance of Bacteroidota was significantly increased by 14.2%, suggesting that there may be some correlation between Bacteroidota and certain metabolites of the anti-depressant active ingredients. At the genus level, the Thauera (3.1%∼11.5%) is the major bacterial group that secretes EPS, protecting biofilm against external influences. Most of the changes in microorganisms are consistent with the decontamination properties and macroscopic appearance of EPS in the ICPB system. Finally, the degradation efficiency of ICPB system for VNF was investigated (92.7 ± 3.8%) and it was mostly through hydroxylation and demethylation pathways, with more small molecular products detected, providing the basis for biological assimilation of VNF. Collectively, the NaF-TiO2 based ICPB system would be lucrative for the future degradation of venlafaxine.

Distinct microbial characteristics of the robust single-stage coupling system during the conversion from anammox-denitritation to anammox-denitratation patterns

Simultaneous anammox-denitrification is effectively operated in two types, i.e., the anammox-denitritation (SAD pattern) and the anammox-denitratation (PDA pattern). The nitrate derived from inevitable nitrite oxidization likely determines the practical operational pattern of the coupling system, while little information is available regarding the microbial characteristics during the pattern conversion. Here, the single-stage bioreactor coupling anammox with denitrification was operated under conditions with a changed ratio of influent nitrite and nitrate. Results showed that the bioreactor exhibited a robust performance during the conversion from SAD to PDA patterns, corresponding with the total nitrogen removal efficiency ranging from 89.5% to 92.4%. Distinct community structures were observed in two patterns, while functional bacteria including the genera Denitratisoma, Thauera, Candidatus Brocadia, and Ca. Jettenia steadily co-existed. Meanwhile, the high transcription of hydrazine synthase genes demonstrated a stable anammox process, while the up-regulated transcription of nitrite and nitrous oxide reductase genes indicated that the complete denitrification process was enhanced for total nitrogen removal during the PDA pattern. Ecologically, stochastic processes dominantly governed the community assembly in two patterns. The PDA pattern improved the interconnectivity of communities, especially for the cooperative behaviors between dominant denitrifying bacteria and low-abundant species. These findings deepen our understanding of the microbial mechanism underlying the different patterns of the coupling system and potentially expand its engineering application.

Nitrite accumulation in activated sludge through cyclic anaerobic exposure with acetate

Achieving nitrite accumulation still remains challenging for efficient short-cut biological nitrogen removal in municipal wastewater treatment. To tackle the problem of insufficient carbon in incoming wastewater for biological nutrient removal, a return activated sludge (RAS) fermentation method has been proposed and demonstrated to enable producing supplemental volatile fatty acids (VFAs) and enhance biological phosphorus removal via sludge cycling between mainstream and a sidestream anaerobic reactor. However, the impacts of long anaerobic exposure with acetate on nitrifying bacteria, known as the aerobic chemoautotrophic microorganisms, remains unexplored. In this study, the activated sludge underwent a cyclic anaerobic treatment with the addition of acetate (Ac), the effects on nitrification rate, abundance and microdiversity of nitrifying communities were comprehensively assessed. Firstly, batch activity tests proved the direct addition of high acetate (above 1000 mg/L) could cause inhibition on the nitrification rate, moreover, the inhibitory effect was stronger on nitrite-oxidizing bacteria (NOB) activity than that of ammonia-oxidizing bacteria (AOB). Then, a sequencing batch reactor (SBR) was applied to test the nitrogen conversion performance for low-strength ammonium wastewater. Nitrite accumulation could be achieved via the cyclic anaerobic exposure with 1000-5000 mg Ac/L. The maximum effluent concentration of nitrite was 40.8 ± 3.5 mg N/L with nitrite accumulation ratio (NAR) of 67.6 ± 3.5%. The decrease in NOB activity (72.7%) was greater than AOB of 42.4%, promoting nitrite accumulation via nitritation process. Furthermore, the cyclic anaerobic exposure with acetate can largely reshape the nitrifying communities. As the dominant AOB and NOB, the abundance of Nitrosomonas and Nitrospira were both decreased with species-level microdiversity in the nitrifying communities. However, the heterotrophic microorganism, Thauera, were found to be highly enriched (from 0 to 17.3%), which may act as the potential nitrite producer as proved by the increased nitrate reduction gene abundance. This study can provide new insights into achieving mainstream nitrite accumulation by involving sidestream RAS fermentation towards efficient wastewater treatment management.

Metagenomics insights into high-rate nitrogen removal from municipal wastewater by integrated nitrification, partial denitrification and Anammox at an extremely short hydraulic retention time

To achieve high-rate nitrogen removal in municipal wastewater treatment through anaerobic ammonia oxidation (Anammox), the nitrification, partial denitrification, and Anammox processes were integrated by a step-feed strategy. An exceptional nitrogen removal load of 0.224 kg N/(m3·d) was achieved by gradient-reducing the hydraulic retention time (HRT) to 5 h. Metagenomic analysis demonstrated that Nitrosospira could express all genes encoding ammonia oxidation under low nitrogen and dissolved oxygen conditions (less than 0.5 mg/L), enabling complete nitrification. With the short of HRT, the relative abundance of Thauera increased from 2.8 % to 6.4 %. Frequent substrate exchanges at such extremely short HRT facilitated enhanced synergistic interactions among Nitrosospira, Thauera, and Candidatus Brocadia. These findings provide a comprehensive understanding of the utilization of Anammox combined processes for high-speed nitrogen removal in municipal wastewater treatment and the microbial interactions involved.

Multi-omics analysis revealed the selective enrichment of partial denitrifying bacteria for the stable coupling of partial-denitrification and anammox process under the influence of low strength magnetic field

The microbial consortium involving anaerobic ammonium oxidation (anammox) and partial denitrification (PD), known as PD-anammox, is an emerging energy-efficient and lower carbon nitrogen removal process from wastewater. However, maintaining a stable PD process by locking nitrate reduction until nitrite was challenging. This study established the first stable connection of anammox with constant nitrite generation by PD bacteria under a low-strength (1.3 mT) magnetic field (MF). When the nitrogen loading rate was 1.81 kg-N/m3/d, the nitrogen removal efficiency of the control reactor (R1) was 75%, lower than that of the experimental reactor (R2), which was 85%. The expression of Thauera and Zoogloea, potential PD bacteria was substantially lower in R1 (5.75% and 1.21%, respectively) than in R2 (10.25 and 6.61%, respectively), according to a meta-transcriptomic analysis. At the same time, the mRNA expression of anammox genera Candidatus Brocadia and Candidatus Kuenenia was 33.53% and 3.83% in R1 and 22.86% and 1.87% in R2. Moreover, carbon and nitrogen metabolism pathways were more abundant under the influence of low-strength MF. The selective enrichment of PD bacteria can be attributed to the increased expression of carbon metabolic pathways like the citrate cycle, glycolysis/gluconeogenesis, and pyruvate metabolism. Interestingly, the control reactor was dominated by a hydroxylamine-dependent anammox process while a low-strength MF-enhanced nitric-oxide-dependent anammox process. For successful anammox-centered nitrogen removal from wastewater, this study demonstrated that low-strength MF is a convenient and applicable technique to lock the nitrate reduction until nitrite.

Evaluation of sludge anaerobic fermentation driving partial denitrification capability: In view of kinetics and metagenomic mechanisms

Partial denitrification (PD) provides a promising approach of efficient and stable nitrite (NO₂^--N) generation for annamox. In this study, the feasibility of short-term sludge anaerobic fermentation driving PD was evaluated. It was found that a higher NO₂^--N accumulation in nitrate (NO₃^--N) reduction was obtained with the 5-days fermented sludge compared to 8 and 15-days fermentation. Moreover, compared to acetate as carbon source, sludge fermentation products (SFPs) induced the higher NO₂^--N production with nitrate-to-nitrite transformation ratio (NTR) nearly 100 %. Denitrification activity of fermented sludge were obviously improved with SFPs as electron donor. Metagenomic analysis revealed that Thauera was the dominant bacteria, which was assumed to be the key contributor to PD performance by harboring the highest narGHI and napAB but much lower nirS and nirK. Under the conditions of SFPs and fermented sludge, Thauera was speculated to have higher resistance than other denitrifiers attributed to versatile carbon metabolic capabilities utilizing SFPs with the significantly improved genes for metabolism of complex organic carbon via glycolysis after anaerobic fermentation. A novel integration of sludge fermentation driving PD and anammox for mainstream wastewater treatment and sidestream polishing was proposed to offer a promising application with reduced commercial carbon source consumption and waste sludge reduction.

Regulation of denitrification performance and microbial topology by lights: Insight into wavelength effects towards microbiota

The low efficiency of conventional complete denitrification, as well as the unstable nitrite supply for partial-denitrification coupled anammox (PD/A) restrict the efficient removal of nitrogen from industrial wastewaters. Herein, we proposed an optical strategy to bidirectionally regulate denitrification by introducing lights at different wavelengths, and the underlying mechanisms were elucidated accordingly. It turned out that yellow light at wavelength of 590 nm accelerated denitrification by 35.4%, while blue light delayed denitrification with stable nitrite accumulation above 86.9% and high nitrate removal (99.8%). Microbial physiology and viability further supported the positive effects of yellow light on microbial activity. Additionally, despite the sluggish denitrification aroused by blue light, negligible cellular damage was observed. Antioxidant capability divergence, microbial community shifting and metabolic flux redirection contributed to the wavelength-dependent effects. Halomonas and Pseudomonas were identified as high-credit taxonomic biomarkers of yellow and blue light. As revealed by metabolomics, pantothenate and CoA biosynthesis, glutamate metabolism and alkaloid biosynthesis presented high impact values. Co-analysis of metabolomics and metagenomics based on microbial topology further distinguished pivotal metabolic pathways and genes. Oxidative phosphorylation contributed to the divergent denitrification performance through electron transfer chains, whereas glutamate and glutathione metabolism contributed to oxidative stress alleviation and mediated the metabolic flux between peroxisome and nitrogen metabolism. This study shed a light on the application of optical strategy to regulate denitrification performance and achieve either complete denitrification or PD/A.

Overlooked pathways of endogenous simultaneous nitrification and denitrification in anaerobic/aerobic/anoxic sequencing batch reactors with organic supplementation

The anaerobic/aerobic/anoxic (A/O/A) process is a promising biotechnology to intensify denitrification in low carbon/nitrogen (C/N) wastewater treatment, but the neglected typical rate-limiting step-nitrification-would hinder its wider application. Heterotrophic nitrification driven by intracellular carbon (PHAs) could enhance nitrification and achieve endogenous simultaneous nitrification and denitrification (ESND) in the A/O/A process, but its feasibility remains unexamined. Here we established four A/O/A-SBRs at different C/N ratios (3, 7.5, 12, and 16.5) to address the above-mentioned knowledge gaps. The results showed that organic supplementation promoted both nitrification and denitrification (performance and relevant enzymatic activities) until organic overdose (C/N = 16.5) exacerbated niche competitions from other non-functional heterotrophs. qPCR and batch tests indicated that high C/N ratios inhibited autotrophic nitrifiers, and heterotrophic nitrifiers (HNB) dominated in the enhanced nitrification. Given the high HNB contribution (43.7%) and low COD variation (< 10 mg L^-1) in the SND (76.4%) of CN12, we proposed a potential SND pathway based on heterotrophic nitrification and denitrification driven by PHAs and verified it with batch tests. Microbial and functional analyses suggested that CN12 favored the intracellular carbon transformation and harbored the minimum autotrophic nitrifiers, supporting the dominance of ESND in the enhanced SND. Our findings expand the understanding of the relationships between intracellular carbon transformation and SND and provide a novel nitrogen removal pathway for the practical application of the A/O/A process.

Effect of ferric iron (Fe(Ш)) on heterotrophic solid-phase denitrification: Denitrification performance and metabolic pathway

The effect of ferric iron (Fe(Ш)) on the performance of heterotrophic solid-phase denitrification (SPD) using biodegradable polymer composite as the electron donor was investigated. The results of continuous batch experiments showed that the addition of over 10 mg/L Fe(Ш) significantly inhibited nitrate removal and led to the accumulation of nitrite. The addition of Fe(Ш) reduced the microbial community diversity and shifted the community dominated by complete denitrifiers (e.g. Thauera) to that dominated by incomplete denitrifiers (e.g. Thermomonas, Stenotrophomonas and Sphingomonas). The predicted analysis of microbial function by PICRUSt2 indicated that the relative abundance of denitrifying genes, including napA/B, nirS and nosZ, were remarkably reduced in the Fe(Ш) groups comparing with the control group. In addition, Fe(Ш) inhibited the genes related to the generation of electron carriers, NADH and FADH₂, in TCA cycle and glycolysis processes, which could result in a lower carbon utilization efficiency for microbial denitrification.

pH-Dependent Hydrogenotrophic Denitratation Based on Self-Alkalization

Producing stable nitrite is a necessity for anaerobic ammonium oxidation (anammox) but remains a huge challenge. Here, we describe the design and operation of a hydrogenotrophic denitratation system that stably reduced >90% nitrate to nitrite under self-alkaline conditions of pH up to 10.80. Manually lowering the pH to a range of 9.00-10.00 dramatically decreased the nitrate-to-nitrite transformation ratio to <20%, showing a significant role of high pH in denitratation. Metagenomics combined with metatranscriptomics indicated that six microorganisms, including a Thauera member, dominated the community and encoded the various genes responsible for hydrogen oxidation and the complete denitrification process. During denitratation at high pH, transcription of periplasmic genes napA, nirS, and nirK, whose products perform nitrate and nitrite reduction, decreased sharply compared to that under neutral conditions, while narG, encoding a membrane-associated nitrate reductase, remained transcriptionally active, as were genes involved in intracellular proton homeostasis. Together with no reduction in only nitrite-amended samples, these results disproved the electron competition between reductions of nitrate and nitrite but highlighted a lack of protons outside cells constraining biological nitrite reduction. Overall, our study presents a stably efficient strategy for nitrite production and provides a major advance in the understanding of denitratation.

Denitrification for acidic wastewater treatment: Long-term performance, microbial communities, and nitrous oxide emissions

Acidic nitrogenous wastewater often requires alkali pretreatment before biological treatment, which results in increased system complexity and operating costs. The demonstration of denitrification under acidic conditions would provide a theoretical basis for the direct treatment of such wastewater. In this study, the denitrification performance, microbial community, and nitrous oxide (N₂O) emissions under acidic conditions were investigated using a sequencing batch reactor. When the influent pH decreased from 5.5 to 4.5, the sequencing batch reactor removed 99.8 ± 0.2% of the nitrate and 92.5 ± 1.6% (n = 171) of the chemical oxygen demand, and the production efficiency of N₂O increased significantly to 11.45%. This was 2.6-fold higher than that observed at pH 5.5. The long-term denitrification treatment of acidic wastewater (pH 4.5) led to the formation of granular sludge, and Thauera, Allorhizobium-Neorhizobium-Parararhizobium-Rhizobium, and Diaphorobacter became the dominant microbes with a collective abundance of 81.3%. More importantly, only 0.25% of the nitrate was denitrified as N₂O, and the batch test revealed that the emissions of N₂O decreased with the increase in sludge size. These results indicate that denitrifying granular sludge formed under acidic conditions and denitrifying bacteria capable of N₂O reduction proliferated, which both resulted in a significant reduction in the release of N₂O.

Enhanced nitritation/denitritation and potential mechanism in an electrochemically assisted sequencing batch biofilm reactor treating sludge digester liquor with extremely low C/N ratios

Nitritation/denitritation is a promising strategy to treat sludge digester liquor but would be unstable and inefficient at extremely low C/N ratios. Here, a novel electrochemically assisted sequencing batch biofilm reactor (E-SBBR) was established to treat synthetic/real sludge digester liquor with decreasing C/N ratios. The results showed that the E-SBBR achieved stable nitritation and appreciable TN removal (>70 %) even at C/N < 0.5. The high-strength free ammonium (FA) (91.1-132.8 mg NH₃-N/L) and long inhibition time (>9h) magnified by electrolysis promoted the robustness of nitritation through efficient nitrite-oxidizing bacteria elimination. Meanwhile, mass balance denoted that heterotrophic denitritation dominated in the enhanced TN removal and relied on carbon supplementation from cell apoptosis/lysis stimulated by electrolysis and high-strength FA, further supported by the recovery of heterotrophic denitrifiers, fermentation bacteria, and relevant functional genes at extremely low C/N ratios. This study provides a novel nitrogen removal approach for the sludge digester liquor treatment.

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.

A new insight to explore toxic Cd(II) affecting denitrification: Reaction kinetic, electron behavior and microbial community

Denitrification process is a crucial step in nitrogen removal and is more vulnerable to external shocks due to the fact that anoxic process is always located before aerobic process in conventional sewage treatment. This study aims to elaborate the nitrogen conversion characteristics by investigating denitrification kinetics, electron behavior and microbial community under Cd(II) shock. Reaction kinetics showed that 10 mg/L of Cd(II) accelerated nitrate reduction rate by 52.29% but 80 mg/L of Cd(II) severely decelerated it by 95.41% with the accumulation of nitrite. High concentration of COD (C/N = 10.4) in the system caused by Cd(II) disrupting the integrity of cell membrane (lactate dehydrogenase increased by 328.7%) was proved to induce occurrence of Dissimilatory Nitrate Reduction to Ammonia (DNRA). The electron transport system activity (ETSA), electron consumption and electron distribution were combined to reveal the electron behavior regulated by Cd(II). The electron ratio of nitrate reductase to nitrite reductase increased from 1.48 (control) to 3.91 and 3.52 (40 and 80 mg/L of Cd(II)) indicated the electrons allocating tendency and further explained the nitrite accumulation. High concentration of Cd(II) also decreased ETSA by weakening the physiological activities of flavin adenine dinucleotide, flavin mononucleotide and cytochrome c or hindered the microbes to secrete these electron carriers. Furthermore, Cd(II) inhibited dominant bacteria genera containing napA gene (Azospirillum and Thauera) and nirS gene (unclassified_c_Betaproteobacteria). Enterobacteriaceae family was found to dominate the DNRA process.

Reinforcement of denitrification in a biofilm electrode reactor with immobilized polypyrrole/anthraquinone-2,6-disulfonate composite cathode

In biofilm electrode reactors (BER), good nitrate removal performance can be achieved through cooperation of heterotrophic and hydrogen autotrophic denitrification under low carbon/nitrogen conditions. In this study, we proposed a more multifunctional composite cathode, which combine immobilized anthraquinone-2,6-disulphonic disodium salt (AQDS) with polypyrrole (PPy) by electrochemical polymerization-doping method. The nitrate removal performance in BER with PPy/AQDS composite cathode was obviously improved, the nitrate removal rate (4.96 mg/L·h) was almost 2.0 times higher than the control BER system, and relatively stabled nitrate removal efficiency (≥90.0%) was also achieved even as the COD/N of 2.50. Compared with the bare graphite felt, PPy/AQDS coating cathode showed much better electrocatalytic activities, which was more advantageous for in situ production of H₂ to support hydrogen autotrophic denitrification process. The PPy-bound AQDS could also act as electron intermediaries, which is beneficial to greatly promote indirect electron process between the denitrifiers and nitrate. Moreover, the PPy/AQDS composite layer formed many particles for improving the specific surface area and bio-attachment site for bacterial attachment, which was conducive for the proliferation of microorganisms and denitrification efficiency. The ratio of biofilm and electrode of PPy/AQDS biocathode was 0.32 ± 0.08, which was 2.46 times than bare electrode (0.13 ± 0.06). Furthermore, enrichment of specific denitrifiers and enhancement of denitrifying enzyme activity was obtained using PPy/AQDS treated electrode, the much higher relative abundance of Thauera of PPy/AQDS biocathode was 1.58 times to the application of bare graphite felt.

Linking Genome-Centric Metagenomics to Kinetic Analysis Reveals the Regulation Mechanism of Hydroxylamine in Nitrite Accumulation of Biological Denitrification

Given hydroxylamine accumulation in various nitrification systems and its potential mechanism in regulating the subsequent denitrification process were unraveled in this study. Hydroxylamine (>0.5 mgN/L) immediately induced nitrite accumulation of activated sludge by inhibiting the activities of nitrite reductases and their electron transport modules (Complex III and cytochrome c). Moreover, long-term exposure to 0.5-2.5 mgN/L hydroxylamine accelerated the functional transformation from denitrification to denitratation under low C/N conditions. However, genome-centric metagenomics indicated that a genotypic complete rather than truncated denitrifier Thauera aminoaromatica TJ127 was enriched and mainly responsible for acetate storage and nitrate reduction of the denitratation community. Interestingly, its enrichment resulted in nitrite production and reduction sequentially but reduced nitrate only to nitrite under carbon-limited conditions (C/N ≤ 3.0). Thus, it showed higher tolerance to hydroxylamine than the concurrent phenotype denitrifiers in activated sludge. Moreover, due to its higher anoxic storage capability in the feast phase, this enrichment became highly specialized by decreasing the feast/famine ratio, and thus a satisfactory denitratation performance was still maintained without hydroxylamine. These results suggested that the transient release of hydroxylamine from nitrification may interfere with subsequent denitrification metabolism, but its continuous accumulation is beneficial for achieving denitratation, which could steadily provide nitrite for mainstream anammox.

Synergistic Inorganic Carbon and Denitrification Genes Contributed to Nitrite Accumulation in a Hydrogen-Based Membrane Biofilm Reactor

Partial denitrification, the termination of NO3−-N reduction at nitrite (NO2−-N), has received growing interest for treating wastewaters with high ammonium concentrations, because it can be coupled to anammox for total-nitrogen removal. NO2− accumulation in the hydrogen (H2)-based membrane biofilm reactor (MBfR) has rarely been studied, and the mechanisms behind its accumulation have not been defined. This study aimed at achieving the partial denitrification with H2-based autotrophic reducing bacteria in a MBfR. Results showed that by increasing the NO3− loading, increasing the pH, and decreasing the inorganic-carbon concentration, a nitrite transformation rate higher than 68% was achieved. Community analysis indicated that Thauera and Azoarcus became the dominant genera when partial denitrification was occurring. Functional genes abundances proved that partial denitrification to accumulate NO2− was correlated to increases of gene for the form I RuBisCo enzyme (cbbL). This study confirmed the feasibility of autotrophic partial denitrification formed in the MBfR, and revealed the inorganic carbon mechanism in MBfR denitrification.

Insights into the associations of copper and zinc with nitrogen metabolism during manure composting with shrimp shell powder

The application of shrimp shell powder (SSP) in manure composting can promote the maturation of compost and reduce the associated environmental risk. This study investigated the response of adding SSP at different levels (CK: 0, L: 5%, M: 10%, and H: 15%) on heavy metal resistance genes (MRGs), nitrogen functional genes, enzymes, and microorganisms. SSP inhibited nitrification and denitrification via decreasing the abundances of functional genes and key enzymes related to Cu, Zn, and MRGs. The nitrate reductase and nitrous-oxide reductase in the denitrification pathway were lower under H. Phylogenetic trees indicated that Burkholderiales sp. had strong relationships with OTU396 and OTU333, with important roles in the nitrogen cycle and plant growth. Redundancy analysis and structural equation modeling showed the complex response between heavy metal and nitrogen that bio-Cu and bio-Zn had positive significantly relationships with nirK-type and amoA-type bacteria, and amoA-type bacteria might be hotspot of cueO.

Varied interspecies interactions between anammox and denitrifying bacteria enhanced nitrogen removal in a single-stage simultaneous anammox and denitrification system

The simultaneous anammox and denitrification (SAD) system has received growing interest for the enhanced nitrogen removal, while the ecological traits of microbial community including spatial distribution characteristics, assembly processes and interspecies interactions have not been fully unraveled. The present study applied metagenomics and ecological analysis methods to gain the ecological traits of microbial communities in the SAD system across different organic substrate loadings. Results showed that organic matter significantly affected the bioreactor performance, and the optimal total nitrogen removal efficiency reached 93.4 ± 0.7% under the COD concentrations of 180 ± 18.2 mg/L. Functional organisms including Candidatus Brocadia (3.9%), Denitratisoma (1.6%), Dokdonella (4.4%) and Thauera (4.6%) obviously enriched under the optimal organic loading conditions. Moreover, microbial communities were significantly governed by deterministic process under high organic concentrations, and the denitrifying organisms displayed important ecological roles in the communities. Although anammox bacteria obviously enriched at the middle of bioreactor, it possessed the highest expression activities at both bottom and middle sites. Denitrifying bacteria that enriched at the bottom sites strongly achieved nitrate reduction and provided nitrite for anammox bacteria, while these organisms trended to compete nitrite with anammox bacteria at the middle site. These findings highlight the importance of microbial ecology in the SAD systems, which may expand our understanding of the synergistic patterns between anammox and denitrifying bacteria.

Denitrification performance and mechanism of denitrification biofilm reactor based on carbon-nitrate counter-diffusional

This study researched denitrification performance and mechanism of denitrification biofilm reactor with different HRTs and carbon sources dosages. Experimental group (EG) had better nitrate and COD removal performance than control group (CG) with different HRTs or carbon doses, and the maximum nitrate-to-nitrite transformation ratio (NTR) of them reached 7.91 ± 1.60% and 17.50 ± 1.92%, respectively. Because organic carbon sources were added to the carrier's interior in EG, forming high local concentrations in biofilms and counter-diffusional with nitrate. By contrast, carbon sources and nitrate were provided from the aqueous phase in CG. Thus, the EG system has more active regions of the biofilm than CG. In addition, EG had higher proportions of microorganisms and enzymes related to denitrification and carbon metabolism. The most dominant phylum, genus, and species were Proteobacteria, Thaurea, and Thauera_sp._27, respectively. The transcript of acetyl-CoA synthetase (K01895) and denitrification (M00529) was mainly originated from unclassified_g__Pseudomonas and unclassified_g__Thauera, respectively.

Hydroxylamine metabolism in mainstream denitrifying ammonium oxidation (DEAMOX) process: Achieving fast start-up and robust operation with bio-augmentation assistance under ambient temperature

Nitrogen removal from mainstream wastewater via DEnitrifying AMmonium OXidation (DEAMOX) is often challenged by undulated actual temperature and high loading rate. Here, we discovered NH₂OH addition (HA) and bio-augmentation (BA) tactics on start-up and operation performance of DEAMOXs (R1 and R2) under ambient temperature (11.3-31.7 °C). Over 340-day operation suggested that R2 received 10 mg/L HA and 1:25 BA ratio (v/v, anammox/partial denitrification sludge) achieved desirable nitrogen removal efficiency (NRE) of 97.22% after 145-day, while R1 under higher BA ratio of 1:12.5 without HA obtained lower NRE (90.86%) after 184-day. Batch tests revealed that nitrate-nitrite transformation ratio reached 98.64% at low COD/NO₃^--N of 2.6 with HA. Significantly, compared with R2, R1 recovered quickly with satisfactory effluent total nitrogen of 4.21 mg/L despite nitrogen loading rate greater than 0.15 kg N/m³/d and temperature decreased to 14.6 °C. The abundant narG represented high nitrate reduction potential, hzsA and hdh were extensively detected as the symbolisation of anammox metabolism. Thauera, Denitratisoma and unclassified f Comamonadaceae dominated nitrite accumulation. Ca. Brocadia as the dominant anammox bacteria, and its population maintained stable against low temperature and load shocks by NH₂OH intensification. Overall, this study offers an opportunity for the wide-applications of DEAMOX treating mainstream wastewater.

A novel simultaneous partial nitritation, denitratation and anammox (SPNDA) process in sequencing batch reactor for advanced nitrogen removal from ammonium and nitrate wastewater

This study presented a novel simultaneous partial nitritation (PN), denitratation and anammox (SPNDA) process for treating ammonium and nitrate wastewater. Results indicated that SPNDA could achieve a great total nitrogen (TN) removal of 97.6 ± 0.5%, leading to effluent TN concentration of only 3.4 mg/L. Mass balance indicated that nitrogen removal rates via anammox, simultaneous nitrification and denitrification were 96.7% and 3.3%, respectively. Extended aerobic duration (12 h) and low dissolved oxygen (DO) concentration (0.15 mg/L) could improve ammonia-oxidizing bacteria (AOB) activity and maintain PN stability. The stable suppression of nitrite-oxidizing bacteria activity was attributed to the low DO (0.15 mg/L) and high free ammonia (3.63 mg/L) in SPND. Besides, the nitrogen conversion mechanisms for SPNDA were revealed based on a typical operational cycle. Microbial analysis showed that AOB (Nitrosomonas) and partial denitrifying bacteria (Thauera and Denitratisoma) coexisted with anammox bacteria (Candidatus Brocadia and Candidatus Anammoxoglobus) in the mixotrophic bio-community.

The metabolic patterns of the complete nitrates removal in the biofilm denitrification systems supported by polymer and water-soluble carbon sources as the electron donors

In this study, two denitrification bio-filters adopted polycaprolactone (PCL) and sodium acetate (NaAc) as polymer and water-soluble carbon sources respectively. With the increasing influent nitrate concentrations, NaAc bio-filter always had shorter HRT to achieve complete nitrate removal. Furthermore, the optimal volumetric denitrification rate in NaAc bio-filter was 0.728 g N/(L·d), which was higher than 0.561 g N/(L·d) in PCL bio-filter. For nitrates removal, the costs of bio-filters supported by NaAc and PCL were 24.93 and 120.25 CNY/kg N respectively. Although Proteobacteria in PCL bio-filter was abundant with 83.98%, NaAc bio-filter had better denitrification performance, due to the appropriate ratio of nitrate removal microorganisms and organic matters degradation organisms. The total abundance value of the denitrification genera is NaAc (16.06%) < PCL (41.19%). However, PCL bio-filter had poor denitrification performance, due to the lower adequacy of PCL depolymerization enzymes and the low expression of the key genes for denitrification.

Metagenomics reveals functional species and microbial mechanisms of an enriched thiosulfate-driven denitratation consortia

In this study, thiosulfate-driven denitratation (TDD) system was successfully established under optimal S/N molar ratio of 1.00, with nitrite accumulation efficiency (NAE) of 82.24 ± 17.09%. This work highlighted that thiosulfate significantly preferred the reduction of nitrate than nitrite. However, after the depletion of thiosulfate, the in-situ formed intermediate product element sulfur (S⁰) served as the main electron donor, and significantly favored the reduction of nitrite than nitrate, which constrained nitrite accumulation and nitrate removal. In addition, metagenomic sequencing revealed that the functional denitratation species might be Thiobacillus_sp._65-29, but the occurrence of Nir-annotated species would decrease nitrite accumulation. Under S/N ratio of 1.00, the decreased abundant Nir-annotated species (e.g., Thiobacillus_sp.), as well as the down-regulated quorum sensing interactions between Nar- and Nir-annotated species were key microbial metabolisms of high NAE in the TDD system. Overall, this work provides new sight into the metagenome-base functional species and metabolic potential of thiosulfate-driven denitratation.

Achieving and control of partial denitrification in anoxic-oxic process of real municipal wastewater treatment plant

Partial denitrification is an alternative process to provide stable nitrite for anammox. In this study, based on full-scale and lab-scale experiments, achieving and control of partial denitrification and the microbial mechanism were studied for 17 months in municipal wastewater treatment plant (MWWTP). Using glucose (GLC) as sole carbon source, partial denitrification was successfully achieved with nitrite accumulation percentage (NAP) higher than 90%; whereas, using sodium acetate (NaAc) as sole carbon source, nitrite accumulation was effectively controlled with economic and efficient carbon usage. Candidatus Competibacter and Thaurea were the dominant communities for partial denitrification. Denitrifying glycogen accumulating organisms (DGAOs), Thauera, denitrifying phosphorus accumulating organisms (DPAOs), GAOs, PAOs and denitrifiers coexisted in MWWTP, resulting in COD specific removal rate (CODSRR) of 883.10 ~ 1188.92mgN/gMLVSS/h during partial denitrification. Through adjustment of Anoxic-Oxic (A/O) operation to anoxic operation, the growth of GAOs and PAOs could be limited.

Diversified metabolism makes novel Thauera strain highly competitive in low carbon wastewater treatment

Thauera, as one of the core members of wastewater biological treatment systems, plays an important role in the process of nitrogen and phosphorus removal from low-carbon source sewage. However, there is a lack of systematic understanding of Thauera's metabolic pathway and genomics. Here we report on the newly isolated Thauera sp. RT1901, which is capable of denitrification using variety carbon sources including aromatic compounds. By comparing the denitrification processes under the conditions of insufficient, adequate and surplus carbon sources, it was found that strain RT1901 could simultaneously use soluble microbial products (SMP) and extracellular polymeric substances (EPS) as electron donors for denitrification. Strain RT1901 was also found to be a denitrifying phosphate accumulating bacterium, able to use nitrate, nitrite, or oxygen as electron acceptors during poly-β-hydroxybutyrate (PHB) catabolism. The annotated genome was used to reconstruct the complete nitrogen and phosphorus metabolism pathways of RT1901. In the process of denitrifying phosphorus accumulation, glycolysis was the only pathway for glycogen metabolism, and the glyoxylic acid cycle replaced the tricarboxylic acid cycle (TCA) to supplement the reduced energy. In addition, the abundance of conventional phosphorus accumulating bacteria decreased significantly and the removal rates of total nitrogen (TN) and chemical oxygen demand (COD) increased after the addition of RT1901 in the low carbon/nitrogen (C/N) ratio of anaerobic aerobic anoxic-sequencing batch reactor (AOA-SBR). This research indicated that the diverse metabolic capabilities of Thauera made it more competitive than other bacteria in the wastewater treatment system.

Keystone Species and Niche Differentiation Promote Microbial N, P, and COD Removal in Pilot Scale Constructed Wetlands Treating Domestic Sewage

The microbial characteristics related to nitrogen (N), phosphorus (P), and chemical oxygen demand (COD) removal were investigated in three pilot scale constructed wetlands (CWs). Compared to horizontal subsurface flow (HSSF) and surface flow (SF) CWs, the aerobic vertical flow (VF) CW enriched more functional bacteria carrying genes for nitrification (nxrA, amoA), denitrification (nosZ), dephosphorization (phoD), and methane oxidation (mmoX), while the removal of COD, total P, and total N increased by 33.28%, 255.28%, and 299.06%, respectively. The co-occurrence network of functional bacteria in the HSSF CW was complex, with equivalent bacterial cooperation and competition. Both the VF and SF CWs exhibited a simple functional topological structure. The VF CW reduced functional redundancy by forming niche differentiation, which filtered out keystone species that were closely related to each other, thus achieving effective sewage purification. Alternatively, bacterial niche overlap protected a single function in the SF CW. Compared with the construction type, temperature, and plants had less effect on nutrient removal in the CWs from this subtropical region. Partial least-squares path modeling (PLS-PM) suggests that high dissolved oxygen and oxidation-reduction potential promoted a diverse bacterial community and that the nonkeystone bacteria reduced external stress for functional bacteria, thereby indirectly promoting nutrient removal.

Development of a novel denitrifying phosphorus removal and partial denitrification anammox (DPR + PDA) process for advanced nitrogen and phosphorus removal from domestic and nitrate wastewaters

A novel energy-efficient DPR + PDA (denitrifying phosphorus removal and partial denitrification anammox) process for enhanced nitrogen and phosphorus removal was developed in the combined ABR-CSTR reactor. After 220 days operation, excellent total inorganic nitrogen (TIN) and phosphorus removal (97.57% and 95.66%, respectively) were obtained under external C/NO₃^--N of 0.7, with the effluent TIN and PO₄^3--P concentrations of 3.51 mg/L and 0.28 mg/L, respectively. At the steady period, DPR contributed major TN removal (58.65%), while PDA mediated an increasingly considerable impact and finally achieved 37.07%, in which anammox accounted for a significant percentage. Batch tests demonstrated that efficient PD with nitrate-to-nitrite transformation ratio of 97.67% supplying stable nitrite for anammox, and phosphorus was mainly removed using nitrate as electron acceptor via DPR with the ideal phosphorus release/uptake rate (7.73/22.17 mgP/gVSS/h). Accumulibacter (6.24%) dominated high phosphorus removal performance, while Thauera (8.26%) and Candidatus Brocadia (2.57%) represented the superior nitrogen removal performance.