Biochar-mediated DNRA pathway of anammox bacteria under varying COD/N ratios

Microbial and functional resilience of a flocculent sludge-based partial denitrification-anammox system against cold temperature

The anammox process offers low-carbon, cost-effective solution for municipal wastewater treatment but faces challenges in non-tropical regions due to temperature sensitivity. This study investigated the nitrogen removal performance, microbial dynamics, and functional gene responses of an integrated partial denitrification-anammox (PDA) system using flocculent sludge under gradual and sudden temperature declines. The PDA process maintained stable nitrogen removal under gradual cooling, with effluent total nitrogen (TN) <1.5 mg/L, supported by stable NO2--supply despite a decreased NO3--to-NO2- conversion rate. Anammox activity progressively declined with temperature dropped, resulting in higher TN-concentration (reactor-concentration at 2 h, <2 mg/L at >20°C, ∼19 mg/L at 15 °C and ∼26 mg/L at 10 °C). Notably, anammox bacteria (AnAOB) remained functional despite reduced abundance. However, sudden cooling caused irreversible damage to AnAOB (cell may rupture), leading to NH4+-accumulation and system failure. Overall, these findings provide important insights into extending anammox technology to non-tropical wastewater treatment, enhancing its sustainable application.

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

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

Enhanced denitrification using iron modified biochar under low carbon source condition: Modulating community assembly, allocating carbon metabolism and facilitating electron transfer

Biochar can modulate microbial community structure to enhance denitrification but the activity is still restricted by the availability of electron transfer (ETS) under metabolic imbalance conditions. Here we developed iron (Ⅲ) modified biochar (FeBC) to substantially mitigate this electron limitation, enhance ETS and accelerate denitrification reaction via intracellular metabolism and community interaction. The results demonstrated that FeBC could significantly improve the denitrification performance, the nitrate removal rate was significantly increased by 30 % at C/N ratio of 3 (W/W) with little nitrite and nitrous oxide accumulation, attributing to the enhanced activities of the ETS and denitrifying reductases and complex microbial interactions via increased abundance of microorganisms involved in carbon and nitrogen transformations. Biochemical quantification and electrochemical analysis, revealed that FeBC activated the acceleration of the ETS process. Comparative metagenome analyses indicated that upregulating key enzymes in the tricarboxylic acid cycle was the potential respiratory enzyme associated with FeBC-mediated ETS. NADH/NAD+ circulation stimulate the startup of carbon metabolism. This energy-linked mechanism could provide ATP for denitrification. This study revealed the functional roles of FeBC in mediating ETS and regulating the bacterial community to achieve enhanced denitrification.

Roles of waste iron scraps in anammox system treating sulfide-containing wastewater: Alleviating sulfide inhibition, promoting novel anammox bacteria enrichment, and enhancing nitrogen removal capacity

In this study, waste iron scraps (WIS) were exerted to alleviate sulfide inhibition on anammox bacteria and promote anammox nitrogen removal from sulfide-containing wastewater.Short-term batch experiments showed that WIS-addition led to the anammox bacteria activity increasing by 124.8 % at an initial sulfide concentration of 40 mgS/L. During the long-term experiments, the nitrogen removal rate (NRR) reached to 8.76 kg/(m3·d) in the WIS-added reactor, while the maximum NRR was only 3.77 ± 0.31 kg/(m3·d) in the non-WIS reactor. In contrast to anammox bacteria development in the non-WIS reactor, the relative abundance of Candidatus Kuenenia (1.4-3.7 %) declined significantly in the WIS-added reactor, but novel potential anammox bacteria Brocadiaceae_unclassified (60.1 %-78.6 %) were highly enriched. Overall, the experimental evidence suggested that WIS-addition not only mitigated the sulfide inhibition on anammox bacteria, but also promoted novel anammox bacteria proliferation. The findings of this work provide a promising solution for wide engineering applications of anammox treating sulfide-containing wastewater.

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

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

Screening, identification, and application of anaerobic ammonia oxidizing bacteria in activated sludge systems: A comprehensive review

Anaerobic ammonium oxidation (Anammox) has garnered significant attention due to its ability to eliminate the need for aeration and supplementary carbon sources in biological nitrogen removal process, relying on the capacity of anaerobic ammonium oxidizing bacteria (AnAOB) to directly convert ammonium and nitrite nitrogen into nitrogen gas. This review consolidates the latest advancements in AnAOB research, outlining the mechanisms and enzymatic processes of Anammox, and summarizing the molecular biological techniques used for studying AnAOB, such as 16s rRNA sequencing, qPCR, and metagenomic sequencing. Additionally, it also overviews the currently identified AnAOB species and their distinct metabolic traits, while consolidating strategies to improve their performance. It further delineates coupled processes that utilize Anammox technology, offering practical insights for process selection. Eventually, the review concludes by suggesting future research directions and highlighting critical areas for further investigation. This review serves as a theoretical reference for the enrichment and cultivation of AnAOB, environmental impact management, and the selection of effective treatment processes.

KOH-modified biochar enhances nitrogen metabolism of the chloroquine phosphate-disturbed anammox: Physical binding, EPS modulation and versatile metabolic hierarchy

Chloroquine phosphate (CQ) poses strong biotoxicity on anammox process, and thus detoxifying is essential for the stable operation of anammox in treating CQ-bearing wastewater. Biochar has been proven to simultaneously detoxify pollutant and modulate nitrogen cycle in anammox by its shelter effect and electron exchange capacity (EEC) ability. To further improve the ability of biochar to promote the nitrogen metabolism of anammox, a KOH modification strategy was used to tailor a high-EEC biochar in this work. KOH modified biochar can bind CQ for detoxification driven by hydrogen bond, π-π interaction, and electrostatic interaction. Meanwhile, the EEC of modified biochar increased by 70 % than that of pristine biochar, thus improving nitrogen removal efficiency by 55.6 % and 9.5 % than CQ and BC group, respectively. Besides, the microorganism regulated by modified biochar produced more α-helix configuration, improving EPS barrier ability to CQ and sludge granulation. Lastly, metagenomic analysis revealed that modified biochar can stimulate the Wood-Ljungdahl pathway, increased the relative abundance of CODH from 0.74 % in CQ to 1.00 % in modified BC group. It favored the proliferation of autotrophic microorganisms, especially increased the relative abundance of anammox bacteria by 86.8 % than CQ group. This work will shed the light on integrating high-EEC biochar into anammox to cope with the micropollutants stress.

Unveiling the mechanism underlying in-situ enhancement on anammox system by sulfide: Integration of biological and isotope analysis

The in-situ utilization of sulfide to remove the nitrate produced during the anaerobic ammonium oxidation (anammox) process can avoid prolonged sludge acclimatization, facilitating the rapid initiation of coupled nitrogen removal processes. However, the understanding of in-situ enhancement on anammox system by sulfide remains unclear. Herein, sulfide (Na2S) was introduced as an additional electron donor to remove the nitrate derived from the anammox under varying sulfide/nitrogen (S/N, S2--S/NO3--N, molar ratio) ratios (0.004-4.375). The underlying mechanisms were elucidated by molecular biology techniques including flow cytometry, quantitative polymerase chain reaction, and 16S rRNA amplicon sequencing, alongside isotope tracer analysis. Results revealed that anammox reactors, when operated with in-situ sulfide addition, exhibited a significant enhancement in total nitrogen removal efficiency (NRE) ranging from 11.5 %-41.7 % (achieved 96 %), with the optimal S/N ratios of 0.01-0.8. Isotope tracer analysis indicated the successful coupling of the anammox, sulfur autotrophic denitrification (SADN), and dissimilatory nitrate reduction to ammonium (DNRA) processes within the system, with their contributions to nitrogen removal being 46 %-50 %, 24 %-30 %, and 20 %-22 %, respectively. Moreover, a notable increase in the abundance of sulfur-oxidizing bacteria (SOB) (20 %-40 % increase) and DNRA bacteria (10 %-20 % increase) were observed. Effective collaboration was further supported by the sustained viability of microbial communities. It is speculated that the heightened presence of SOB and DNRA bacteria created a low toxicity environment by converting sulfide to biogenic sulfur, thereby promoting the well-being of anammox bacteria. However, the excessive dosage of sulfide (S/N = 1.8) intensified the DNRA process (contribution>35 %) and weakened the anammox process, leading to an increase in effluent NH4+-N concentration and a decline in NRE. This study confirms that the in-situ adding an appropriate amount of sulfide favors achieving complete nitrogen removal in anammox system, which provides a novel avenue to resolve the issue of the residual nitrate in anammox process.

Nitrate formation in anammox process: Mechanisms and operating conditions

Anaerobic ammonium oxidation (anammox) is an energy-efficient technology for wastewater nitrogen removal. However, the byproduct nitrate has hindered development and application of anammox process. Meanwhile, the knowledge of nitrate formation during anammox process is insufficient, which prohibits high nitrogen removal. This review firstly summaries and discusses valuable findings on nitrate formation, including molecular mechanism of nitrate production, microbial pathway of nitrate reduction and its net formation. Specially, influences of operating conditions on mechanisms and patterns of nitrate formation are analyzed. Then, based on nitrate formation mechanism, current strategies of nitrate removal from anammox process are reevaluated. Finally, the key knowledge gaps and further process development are presented. Overall, this review sheds light on the understanding of nitrate formation of anammox process, which would further facilitate and optimize the process design and operation for high performance nitrogen removal.

Sulfide addition accelerates anammox sludge granulation and promotes microbial cooperation

The granular anaerobic ammonium oxidation (anammox) system has attractive advantages in tolerance to environmental-stress and enhancement of nitrogen removal capacity. Sulfide addition can improve nitrogen removals in anammox systems via inducing sulfur denitrification, yet its function in the improvement of the property of anammox granular sludge remains unclear. Herein, we investigated the variations in the morphological and microbial properties of the anammox sludge response to different sulfide concentrations (Na2S: 10-100 mg/L) through a long-term experiment. By comparing the sludge diameter and heme c content, it comes that a relatively low sulfide (S/N [nitrate] molar ratio of 0.18-0.50) significantly promoted the average diameter and heme c concentration of sludge by 25-175 % and 75-95 %, respectively, compared to that of both without sulfide addition and a high sulfide addition (S/N > 0.85). This enhancement is primarily because a low amount of sulfide had stimulated the secretion of extracellular polymeric substance, induced slight biogenic sulfur accumulation as microbial nuclei, and facilitated the appropriate amount of filamentous bacteria proliferation. Microbial metabolism functions analyses revealed a robust granular anammox coupled with sulfur denitrification in the sulfide-mediated anammox reactor, and the assembled granules exhibited exceptional tolerance to environmental stress. Significantly, the anammox bacteria (Candidatus_Brocadia) dominating the granules displayed satisfactory anammox activity (21.8 ± 2.1 mg N/g VSS h), and their produced nitrate was efficiently removed by the sulfur-oxidizing bacteria (Thiobacillus) that predominantly occurred in the flocs. This collaboration ensured an efficient sulfide-mediated anammox granules system, achieving nitrogen removal efficiency exceeding 95 %. These results highlight the function of sulfide in improving the morphological property of anammox sludge as well as the creation of a favorable ecological niche for the functional microorganism, which is important to maintain the efficiency and robustness of the anammox process in treating wastewater.

Fimbriimonadales performed dissimilatory nitrate reduction to ammonium (DNRA) in an anammox reactor

Bacteria belonging to the order Fimbriimonadales are frequently detected in anammox reactors. However, the principal functions of these bacteria and their potential contribution to nitrogen removal remain unclear. In this study, we aimed to systematically validate the roles of Fimbriimonadales in an anammox reactor fed with synthetic wastewater. High-throughput 16S rRNA gene sequencing analysis revealed that heterotrophic denitrifying bacteria (HDB) were the most abundant bacterial group at the initial stage of reactor operation and the abundance of Fimbriimonadales members gradually increased to reach 38.8 % (day 196). At the end of reactor operation, Fimbriimonadales decreased to 0.9 % with an increase in anammox bacteria. Correlation analysis demonstrated nitrate competition between Fimbriimonadales and HDB during reactor operation. Based on the phylogenetic analysis, the Fimbriimonadales sequences acquired from the reactor were clustered into three distinct groups, which included the sequences obtained from other anammox reactors. Metagenome-assembled genome analysis of Fimbriimonadales allowed the identification of the genes narGHI and nrfAH, responsible for dissimilatory nitrate reduction to ammonium (DNRA), and nrt and nasA, responsible for nitrate and nitrite transport. In a simulation based on mass balance equations and quantified bacterial groups, the total nitrogen concentrations in the effluent were best predicted when Fimbriimonadales was assumed to perform DNRA (R2 = 0.70 and RMSE = 18.9). Moreover, mass balance analysis demonstrated the potential contribution of DNRA in enriching anammox bacteria and promoting nitrogen removal. These results prove that Fimbriimonadales compete with HDB for nitrate utilization through DNRA in the anammox reactor under non-exogenous carbon supply conditions. Overall, our findings suggest that the DNRA pathway in Fimbriimonadales could enhance anammox enrichment and nitrogen removal by providing substrates (nitrite and/or ammonium) for anammox bacteria.

Emerging biotechnological applications of anaerobic ammonium oxidation

Anaerobic ammonium oxidation (anammox) is an energy-efficient method for nitrogen removal that opens the possibility for energy-neutral wastewater treatment. Research on anammox over the past decade has primarily focused on its implementation in domestic wastewater treatment. However, emerging studies are now expanding its use to novel biotechnological applications and wastewater treatment processes. This review highlights recent advances in the anammox field that aim to overcome conventional bottlenecks, and explores novel and niche-specific applications of the anammox process. Despite the promising results and potential of these advances, challenges persist for their real-world implementation. This underscores the need for a transition from laboratory achievements to practical, scalable solutions for wastewater treatment which mark the next crucial phase in the evolution of anammox research.

Study on the mechanism of enhanced anaerobic ammonia oxidation performance by extracellular electron acceptor biochar

ABSTRACTAnaerobic ammonia oxidation process has the advantages of energy and cost reduction, therefore, it has been considered as one of the main alternatives to conventional biological denitrification process in recent years. Biochar has been applied in the anammox process for nitrogen removal efficiency. But, due to its extracellular electron transfer capacity and abundance of redox-specific functional groups, which served as extracellular electron acceptor to anaerobically oxidize NH4+ is still controversy. In this study, the anaerobic ammonia oxidation was investigated when biochar was used as electron acceptor in the wastewater. According to the optimal process variables determined in the batch tests, when the influent NH4+-N concentration in the anaerobic ammonia oxidation reaction was 30-50 mg/L and the biochar dosing was at 10 g/L, it showed some promotion in the long-term experiments. The anaerobic ammonia oxidation process with biochar as the electron acceptor reached more than 60% NH4+-N removal efficiency in the system, and the ΔNO3--N/ΔNH4+-N ratio reached 0.19 which tended to the theoretical value. After 20 days, the voltage in the system keeps fluctuating about 4 mV, indicated that the functional bacteria using biochar as the electron acceptor gradually dominated the system. In addition, the abundance of norank_f__norank_o__SBR1031 in the Chloroflexi phylum has increased significantly at 29.92%, while the abundance of the major genus Candidatus_Kuenenia in AnAOB has decreased slightly at 11.47%.

Recovery strategies and mechanisms of anammox reaction following inhibition by environmental factors: A review

Anaerobic ammonium oxidation (anammox) is a promising biological method for treating nitrogen-rich, low-carbon wastewater. However, the application of anammox technology in actual engineering is easily limited by environmental factors. Considerable progress has been investigated in recent years in anammox restoration strategies, significantly addressing the challenge of poor reaction performance following inhibition. This review systematically outlines the strategies employed to recover anammox performance following inhibition by conventional environmental factors and emerging pollutants. Additionally, comprehensive summaries of strategies aimed at promoting anammox activity and enhancing nitrogen removal performance provide valuable insights into the current research landscape in this field. The review contributes to a comprehensive understanding of restoration strategies of anammox-based technologies.

Mechanisms underlying the interactions and adaptability of nitrogen removal microorganisms in freshwater sediments

Microorganisms in eutrophic water play a vital role in nitrogen (N) removal, which contributes significantly to the nutrient cycling and sustainability of eutrophic ecosystems. However, the mechanisms underlying the interactions and adaptation strategies of the N removal microorganisms in eutrophic ecosystems remain unclear. We thus analyzed field sediments collected from a eutrophic freshwater ecosystem, enriched the N removal microorganisms, examined their function and adaptability through amplicon, metagenome and metatranscriptome sequencing. We found that the N removal activities could be affected through potential competition and inhibition among microbial metabolic pathways. High-diversity microbial communities generally increased the abundance and expression of N removal functional genes. Further enrichment experiments showed that the enrichment of N removal microorganisms led to a development of simplified but more stable microbial communities, characterized by similar evolutionary patterns among N removal microorganisms, tighter interactions, and increased adaptability. Notably, the sustained provision of NH4+ and NO2- during the enrichment could potentially strengthen the interconnections among denitrification, anaerobic ammonium oxidation (anammox) and dissimilatory nitrate reduction to ammonium (DNRA) processes. Moreover, the identification of shared metabolic traits among denitrification, anammox and DNRA implies important cooperative associations and adaptability of N removal microorganisms. Our findings highlight the microbial interactions affect the adaptive strategies of key microbial taxa involved in N removal.

Refractory dissolved organic matters in sludge leachate trigger the combination of anammox and denitratation for advanced nitrogen removal

The cost-effective treatment of sludge leachate (SL) with high nitrogen content and refractory dissolved organic matter (rDOM) has drawn increasing attention. This study employed, for the first time, a rDOM triggered denitratation-anammox continuous-flow process to treat landfill SL. Moreover, the mechanisms of exploiting rDOM from SL as an inner carbon source for denitratation were systematically analyzed. The results demonstrated outstanding nitrogen and rDOM removal performance without any external carbon source supplement. In this study, effluent concentrations of 4.27 ± 0.45 mgTIN/L and 5.58 ± 1.64 mgTN/L were achieved, coupled with an impressive COD removal rate of 65.17 % ± 1.71 %. The abundance of bacteria belonging to the Anaerolineaceae genus, which were identified as rDOM degradation bacteria, increased from 18.23 % to 35.62 %. As a result, various types of rDOM were utilized to different extents, with proteins being the most notable, except for lignins. Metagenomic analysis revealed a preference for directing electrons towards NO3--N reductase rather than NO2--N reductase, indicating the coupling of denitratation bacteria and anammox bacteria (Candidatus Brocadia). Overall, this study introduced a novel synergy platform for advanced nitrogen removal in treating SL using its inner carbon source. This approach is characterized by low energy consumption and operational costs, coupled with commendable efficiency.

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.

Salt stimulates sulfide-driven autotrophic denitrification: Microbial network and metagenomics analyses

Sulfur autotrophic denitrification (SADN) is a promising biological wastewater treatment technology for nitrogen removal, and its performance highly relies on the collective activities of the microbial community. However, the effect of salt (a prevailing characteristic of some nitrogen-containing industrial wastewaters) on the microbial community of SADN is still unclear. In this study, the response of the sulfide-SADN process to different salinities (i.e., 1.5 % salinity, 0.5 % salinity, and without salinity) as well as the involved microbial mechanisms were investigated by molecular ecological network and metagenomics analyses. Results showed that the satisfactory nitrogen removal efficiency (>97 %) was achieved in the sulfide-SADN process (S/N molar ratio of 0.88) with 1.5 % salinity. In salinity scenarios, the genus Thiobacillus significantly proliferated and was detected as the dominant sulfur-oxidizing bacteria in the sulfide-SADN system, occupying a relative abundance of 29.4 %. Network analysis further elucidated that 1.5 % salinity had enabled the microbial community to form a more densely clustered network, which intensified the interactions between microorganisms and effectively improved the nitrogen removal performance of the sulfide-SADN. Metagenomics sequencing revealed that the abundance of functional genes encoding for key enzymes involved in SADN, dissimilatory nitrate reduction to ammonium, and nitrification was up-regulated in the 1.5 % salinity scenario compared to that without salinity, stimulating the occurrence of multiple nitrogen transformation pathways. These multi-paths contributed to a robust SADN process (i.e., nitrogen removal efficiency >97 %, effluent nitrogen <2.5 mg N/L). This study deepens our understanding of the effect of salt on the SADN system at the community and functional level, and favors to advance the application of this sustainable bioprocess in saline wastewater treatment.

Sustainable biochar application in anammox process: Unveiling novel pathways for enhanced nitrogen removal and efficient start-up at low temperature

This study explored the use of biochar to accelerate the establishment of anaerobic ammonium oxidation (anammox) reactors operating at 15 ± 1℃. Incorporating 10 g/L bamboo charcoal in S1 accelerated the start-up of anammox in 87 days, which was significantly shorter than 103 days in S0 (without biochar). After 140 days, S1 exhibited a 10.9 % increase in nitrogen removal efficiency due to a 28.9 % elevation in extracellular polymeric substances, bolstering anammox bacterial resilience. Predominant anammox bacteria (Cadidatus Brocadia and Cadidatus Jettenia) showed relative abundances of 3.19 % and 0.38 % in S1, respectively, which were significantly higher than 0.40 % and 0.05 % in S0. Biochar provides favorable habitats for the enrichment of anammox bacteria and accelerates the establishment of anammox at low temperatures. This finding holds promise for enhancing the efficiency of anammox in cold climates and advancing sustainable wastewater nitrogen removal.

Review of the mechanisms involved in dissimilatory nitrate reduction to ammonium and the efficacies of these mechanisms in the environment

Dissimilatory nitrate reduction to ammonium (DNRA) is currently of great interest because it is an important method for recovering nitrogen from wastewater and offers many advantages, over other methods. A full understanding of DNRA requires the mechanisms, pathways, and functional microorganisms involved to be identified. The roles these pathways play and the effectiveness of DNRA in the environment are not well understood. The objectives of this review are to describe our current understanding of the molecular mechanisms and pathways involved in DNRA from the substrate transfer perspective and to summarize the effects of DNRA in the environment. First, the mechanisms and pathways involved in DNRA are described in detail. Second, our understanding of DNRA by actinomycetes is reviewed and gaps in our understanding are identified. Finally, the effects of DNRA in the environment are assessed. This review will help in the development of future research into DNRA to promote the use of DNRA to treat wastewater and recover nitrogen.

NH2-MIL-101(Fe)-mediated photo-Fenton reaction enhanced simultaneous removal of nitrogen and refractory organics in anammox process through interfacial electron transfer

In this study, H2O2 (0.1 ‰) and NH2-MIL-101(Fe)-driven (150 mg/L) photo-Fenton-coupled anammox were proposed to simultaneously improve the removal efficiency of nitrogen and humic acid. Long-term experiments showed that the total nitrogen removal efficiency was increased by the photo-Fenton reaction to 91.9 ± 1.5 % by altering the bioavailability of refractory organics. Correspondingly, the total organic carbon removal efficiency was significantly increased. Microbial community analyses indicated that Candidatus_Brocadia maintained high activity during photo-Fenton reaction and was the most abundant genus in the reactor. Dissimilatory nitrate reduction to ammonium process and denitrification process were enhanced, resulting in reduced NO3--N production. The establishment of electron transfer between microorganisms and NH2-MIL-101 (Fe) improved the charge separation efficiency of the quantum dots and increased the intracellular adenosine triphosphate content of anammox bacteria. These results indicated that photo-Fenton-anammox process promoted the removal of nitrogen and refractory organics in one reactor which had good economic value and application prospects.

Formation of autotrophic nitrogen removal granular sludge driven by the dual-partition airlift internal circulation: Insights from performance assessment, community succession, and metabolic mechanism

Granular sludge has been recognized as an effective method for the application and industrialization of the anammox-based process due to its good biomass retention capacity and environmental tolerance. In this study, a one-stage autotrophic nitrogen removal (ANR) dual-partition system with airlift internal circulation was implemented for 320 days. A high nitrogen removal efficiency of 84.6% was obtained, while the nitrogen removal rate reached 1.28 g-N/L/d. ANR granular sludge dominated by Nitrosomonas and Candidatus Brocadia was successfully cultivated. Results showed that activity and abundance of functional flora first increased with granulation process, but eventually declined slightly when particle size exceeded the optimal range. Total anammox activity was observed to be significantly correlated with protein content (R2 = 0.9623) and nitrogen removal performance (R2 = 0.8796). Correlation network revealed that AnAOB had complex interactions with other bacteria, both synergy for nitrogen removal and competition for substrate. Changes in abundances of genes encoding the Carbohydrate Metabolism, Energy Metabolism, and Membrane Transport suggested energy production and material transfer were possibly blocked with further sludge granulation. Formation of ANR granular sludge promoted the interactions and metabolism of functional microorganisms, and the complex nitrogen metabolic pathways improved the performance stability. These results validated the feasibility of granule formation in the airlift dual-partition system and revealed the response of the ANR system to sludge granulation.

Nitrate removal by anammox bacteria utilizing photoexcited electrons via inward extracellular electron transfer channel

Dissimilatory nitrate reduction to ammonium (DNRA) has been found to occur in some anammox bacteria species, and the DNRA metabolites (nitrite and ammonium) can further be removed to nitrogen from water. However, the activation of DNRA pathway of anammox bacteria is usually limited by the access to electron donors. Herein, we constructed a photosensitized hybrid system combining anammox bacteria (Candidatus Kuenenia stuttgartiensis and Candidatus Brocadia anammoxidans) with CdS nanoparticles semiconductor for energy-efficient NO3- removal. Such photosensitized anammox-CdS hybrid systems achieved NO3- removal with an average efficiency of 88% (the maximum of 91%) and a N2 selectivity of 72%, only with photoexcited electrons as donors. The DNRA-anammox metabolism of anammox bacteria was proved to responsible for NO3- removal via inward extracellular electron transfer channel. The greatly up-regulated genes encoding c-type cytochrome proteins (5 or 11 hemes) in the outer membrane, c-type cytochrome protein (4 hemes) and electron transport protein RnfA-E in the inner membrane, ferredoxin (2Fe-2S) in the cytoplasm and c-type cytochrome bc1 in anammoxosome membrane were supposed to play key roles in the inward extracellular electron transfer pathway. This work provides a novel insight into the design of the biotic-abiotic hybrid photosynthetic systems, and opens a new strategy for light-driven NO3- removal from the perspective of light energy input.

Innovative determination of the specific anammox activity for anammox sludge from continuous flow reactors: A comparison between continuous flow test and batch test

A novel method for measuring specific anammox activity (SAA) was proposed based on continuous flow tests to accurately determine the SAA of anammox sludge from continuous flow reactors, resolving the challenges of inaccurate SAA assessment caused by substrate shock to anammox bacteria. Results showed SAA of expanded granular sludge bed sludge via batch tests (0.101 ± 0.018 g-N·g-VSS-1·d-1) was lower than continuous flow tests (0.206 ± 0.010 g-N·g-VSS-1·d-1) (p < 0.05), highlighting the impact of substrate shock. Conversely, SAA of sequencing batch reactor sludge assessed via batch tests (0.878 ± 0.008 g-N·g-VSS-1·d-1) was higher than continuous flow tests (0.809 ± 0.005 g-N·g-VSS-1·d-1) (p < 0.01), attributed to endogenous denitrification. The advantages of continuous flow tests over batch tests included milder feeding way, stricter anaerobic conditions, and minimal sampling impact on system. Our study contributes to more accurate measurements of SAA of anammox sludge from continuous flow reactors, favoring long-term robust operation of anammox reactors.

Analyzing the nitrogen removal performance and cold adaptation mechanism of immobilized cold-acclimation ANAMMOX granules at low temperatures

To improve the treatment performance of anaerobic ammonium oxidation (ANAMMOX) processes at low temperatures, the immobilized cold-acclimation ANAMMOX granules (R3) were prepared and their low-temperature nitrogen removal ability as well as the cold adaptation mechanism were analyzed. The results indicated that the total inorganic nitrogen (TIN) removal efficiency of R3 was significantly higher than that of R2 (cold-acclimation granules without immobilization) and R1 (common granules), especially at 11 ± 2 and 7 ± 2°C (68% and 54%). These were attributed to the remarkable biomass retention capacity of R3, high up to 4.3-4.9 mg/gVSS even at 5-18°C. Besides, higher protein (PN) content of tightly bound extracellular polymeric substances (TB-EPS) also facilitated microbial aggregation in R3. Meanwhile, R3 granules retained higher ANAMMOX activity and heme c content at 5-25°C. The original dominant ANAMMOX genus (Candidatus Kuenenia) in R3 kept higher abundance (49%-57%) at 23 ± 2 and 16 ± 2°C, whereas Candidatus Brocadia became the dominant ANAMMOX genus (25%-32%) in R3 at 11 ± 2 and 7 ± 2°C. Notably, different ANAMMOX genera in R3 may adapt to cold environment by regulating the expression of cold-stress proteins (CspA, CspB, PpiD, and UspA). PRACTITIONER POINTS: Immobilized cold-acclimation ANAMMOX granules showed higher nitrogen removal efficiency at 23°C → 5°C. Immobilization method effectively retained biomass (Candidatus Kuenenia and Candidatus Brocadia). Immobilization facilitated TB-EPS release and biological aggregation in cold-acclimation granules. Expression of cold-stress proteins in immobilized cold-acclimation granules was more active.

Nitrous oxide emissions in novel wastewater treatment processes: A comprehensive review

The proliferation of novel wastewater treatment processes has marked recent years, becoming particularly pertinent in light of the strive for carbon neutrality. One area of growing attention within this context is nitrous oxide (N2O) production and emission. This review provides a comprehensive overview of recent research progress on N2O emissions associated with novel wastewater treatment processes, including Anammox, Partial Nitrification, Partial Denitrification, Comammox, Denitrifying Phosphorus Removal, Sulfur-driven Autotrophic Denitrification and n-DAMO. The advantages and challenges of these processes are thoroughly examined, and various mitigation strategies are proposed. An interesting angle that delve into is the potential of endogenous denitrification to act as an N2O sink. Furthermore, the review discusses the potential applications and rationale for novel Anammox-based processes to reduce N2O emissions. The aim is to inform future technology research in this area. Overall, this review aims to shed light on these emerging technologies while encouraging further research and development.

Anammox Coupled with Photocatalyst for Enhanced Nitrogen Removal and the Activated Aerobic Respiration of Anammox Bacteria Based on cbb3-Type Cytochrome c Oxidase

This study introduced photogenerated electrons into the anammox system by coupling them to a g-C3N4 nanoparticle photocatalyst. A high nitrogen removal efficiency (94.25%) was achieved, exceeding the biochemical limit of 89% imposed by anammox stoichiometry. Photogenerated electrons boosted anammox metabolic activity by empowering key enzymes (NIR, HZS, and WLP-related proteins) and triggered rapid algal enrichment by enhancing the algal Calvin cycle, thus developing multiple anammox-algae synergistic nitrogen removal processes. Remarkably, the homologous expression of cbb3-type cytochrome c oxidase (CcO) in anammox bacteria was discovered and reported in this study for the first time. This conferred aerobic respiration capability to anammox bacteria and rendered them the principal oxygen consumer under 7.9-19.8 mg/L dissolved oxygen, originating from algal photosynthesis. Additionally, photogenerated electrons selectively targeted the cb1 complex and cbb3-type CcO as activation sites while mobilizing the RegA/B regulatory system to activate the expression of cbb3-type CcO. Furthermore, cbb3-type CcO blocked oxidative stress in anammox by depleting intracellular oxygen, a substrate for reactive oxygen species synthesis. This optimized the environmental sensitivity of anammox bacteria and maintained their high metabolic activity. This study expands our understanding of the physiological aptitudes of anammox bacteria and provides valuable insights into applying solar energy for enhanced wastewater treatment.

Sludge granulation in PN/A enhances nitrogen removal from mainstream anaerobically pretreated wastewater

Treating anaerobically pretreated wastewater using partial nitritation/anammox (PN/A) process faces severe challenges because of the complex syntrophic and competitive relationship among various bacteria. Results of this study suggested a continuous low dissolved oxygen (DO) concentration failed to sustain NH4+ removal (<80 %), whereas moderate DO concentrations with high aerobic periods suppressed anammox reaction. Through implementing a moderate DO concentration with low aerobic periods (MDO-LA), NH4+ and total nitrogen removal efficiency reached 91.5 ± 5.5 % and 71.3 ± 2.8 % respectively. The specific activities of ammonium-oxidizing bacteria (AOB) and anaerobic ammonium-oxidizing bacteria (AnAOB) reached 0.942 ± 0.030 and 0.277 ± 0.010 g nitrogen per gram mixed liquor volatile suspended solids, respectively, mainly because MDO-LA favored Thiothrix (filamentous bacteria) wash-out and promoted Nitrosomonas growth. Moreover, sludge granules covered by a thin exterior rim with abundant AOB were formed, favoring Ca. Brocadia growth (5.4 % to 13.2 %) and mass transfer between AOB and AnAOB, which consequently increased the expression of genes coding hydroxylamine oxidase and hydrazine synthase. Overall, achievements in this study provide a promising operating strategy for PN/A treating anaerobically pretreated wastewater.

Effects of biochar-amended soils as intermediate covers on the physical, mechanical and biochemical behaviour of municipal solid wastes

The effects of biochar-amended soils as landfill covers have been extensively studied in terms of liquid and gas permeability. However, the influences of biochar-amended soils on the performance of municipal solid wastes (MSWs) in bioreactor landfills have not been well understood. This paper investigates the potential application of biochar-amended soils as final and intermediate covers in landfills. The MSWs with biochar-amended soils as final and intermediate covers were recirculated with mature leachate in laboratory-scale bioreactors. The pH, chemical oxygen demand, ammonia and volatile fatty acids (VFAs) concentrations of leachates, mass reduction rates, settlement, methane, and total gas generations of MSWs were investigated. The results indicate that biochar-amended soils as intermediate landfill covers can provide pH-buffer capacity, increase the pH of leachate and decrease the accumulation of VFAs in the early stage of decomposition. The concentration of ammonia in the leachate with biochar-amended soils as intermediate cover is lower than that with natural soils. The application of biochar-amended soils as intermediate and/or final covers increases the biocompression ratios and settlement of MSWs. The application of biochar-amended soils as final cover slightly decreases the methane generation potential (L0). Biochar-amended soils as intermediate covers increase L0 by 10%, and biochar-amended soils as both intermediate and final covers enhance L0 by 25%. The increase in the ammonia removal, settlement, and methane yield indicates the viability of biochar-amended soils as intermediate landfill covers. Further studies can focus on the long-term behaviour of MSWs with soil covers with different biochar amendment rates and particle sizes.

Electron transfer mechanism of intracellular carbon-dependent DNRA inside anammox bacteria

Generally, anaerobic ammonium oxidation (anammox) converts nitrite (NO2-) and ammonium (NH4+) to nitrogen gas (N2) but generates some nitrate (NO3-) (equivalent to 11% of inlet total nitrogen (TN)). Although it reported that anammox bacteria could degrade NO3- via dissimilatory nitrate reduction to ammonium (DNRA) pathway using the intracellular carbon as the electron donor, it is still unclear the specific electron transfer mechanism in this intracellular carbon-dependent DNRA inside anammox bacteria, and whether the sole anammox bacteria could achieve higher TN removal efficiency more than the theoretical maximum of 89%. In this study, transcriptome analysis and metabolic inhibitor experiments demonstrated that NADH generated from the decomposition of the intracellular carbon (glycogen) supplied electrons for the NO3-conversion; the electrons were transferred from NADH to nitrate reductase (Nar) and nitrite reductase forming ammonium (NrfA) from ubiquinone (UQ) and complex III, respectively. Combining the intracellular carbon-dependent DNRA with normal anammox process, an average TN removal efficiency of 95% was achieved by the sole anammox bacteria in a sequencing batch reactor. Fluorescent in situ hybridization (FISH) images and real-time fluorescence quantitative PCR (qPCR) results illustrated anammox bacteria could survive and proliferate in the SBR. Our work improved the understanding of the electron transfer mechanism inside anammox bacteria, and further exploit its potential in nitrogen pollutants removal.

Efficiency and mechanism of the degradation of ciprofloxacin by the oxidation of peroxymonosulfate under the catalysis of a Fe3O4/N co-doped sludge biochar

A novel and recyclable composite material, Fe₃O₄/N co-doped sludge biochar (FNBC), was developed from original sludge biochar (BC) and found to have excellent stability and superior catalytic capacity during the ciprofloxacin (CIP) degradation under the action of peroxymonosulfate (PMS). In the FNBC/PMS system, an approximately complete removal of CIP was achieved within 60 min under the condition of 1.0 g/L FNBC, 3.0 mM PMS, and 20 mg/L CIP, which was about 2.08 times of that in BC/PMS system (48.01%). Besides, FNBC/PMS system could effectively remove CIP under the influence of wide pH (2.0-10.0) or inorganic ions compared with BC/PMS system. Moreover, it was found that there were radical produced under the effect of Fe element, defects, functional groups, pyridinic N and pyrrolic N and non-radical caused by graphitic N, carbon atoms next to the iron atoms and better adsorption capacity in the FNBC/PMS system. It was observed that the contribution of hydroxyl radical (•OH), sulfate radical (SO₄^•-) and singlet oxygen (¹O₂), which were the main reactive oxygen species, during the CIP degradation, were 75.80%, 11.49% and 10.26%, respectively. Furthermore, total organic carbon (TOC) variation was analyzed and the degradation pathway of CIP was speculated. The application of this material could combine the recycling of sludge with the effective degradation of refractory organic pollutant, providing an environmentally friendly and economic method.

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.

Sustainable nitrogen removal in anammox-mediated systems: Microbial metabolic pathways, operational conditions and mathematical modelling

Anammox-mediated systems have attracted considerable attention as alternative cost-effective technologies for sustainable nitrogen (N) removal from wastewater. This review comprehensively highlights the importance of understanding microbial metabolism in anammox-mediated systems under crucial operation parameters, indicating the potentially wide applications for the sustainable treatment of N-containing wastewater. The partial nitrification-anammox (PN-A), simultaneous PN-A and denitrification (SNAD) processes have demonstrated sustainable N removal from sidestream wastewater. The partial denitrification-anammox (PD-A) and denitrifying anaerobic methane oxidation-anammox (DAMO-A) processes have advanced sustainable N removal efficiency in mainstream wastewater treatment. Moreover, N2O production/emission hotspots are extensively discussed in anammox-based processes and are related to the dominant ammonia-oxidizing bacteria (AOB) and denitrifying heterotrophs. In contrast, N2O is not produced in the metabolism pathways of AnAOB and DAMO-archaea; Moreover, the actual contribution of N2O production by dissimilatory nitrate reduction to ammonium (DNRA) and DAMO-bacteria in their species remains uncertain. Thus, PD-A and DAMO-A processes would achieve reduction in greenhouse gas production, as well as energy consumption for the reliability of N removal efficiencies. In addition to reaction mechanisms, this review covers the mathematical models for simultaneous anammox, partial nitrification and/or denitrification (i.e., PN-A, PD-A, and SNAD). Promising NO3- reduction technologies by endogenous PD, sulfur-driven autotrophic denitrification, and DNRA by anammox are also discussed. In summary, this review provides a better understanding of sustainable N removal in anammox-mediated systems, thereby encouraging future investigation and exploration of the sustainable N bio-treatment from wastewater.

Deciphering the fate of osmotic stress priming on enhanced microorganism acclimation for purified terephthalic acid wastewater treatment with high salinity and organic load

Osmotic stress priming (OSP) was an effective management strategy for improving microbial acclimation to salt stress. In this study, the interaction between pollutants and microbiota, and microbial osmoregulation were investigated triggered by OSP (alternately increasing salinity and organic loading). Results showed that OSP significantly improved COD removal from 31.53 % to 67.99 % and mitigated the terephthalate inhibition produced by toluate, decreasing from 1908.08 mg/L to 837.16 mg/L compared with direct priming. Due to an increase in salinity, Pelotomaculum and Mesotoga were enriched to facilitate terephthalate degradation and syntrophic acetate oxidation (SAO). And organic load promoted acetate formation through syntrophic metabolism of Syntrophorhabdus/Pelotomaculum and SAO-dependent hydrogenotrophic methanogenesis. K⁺ absorbing, proline and trehalose synthesis participated in osmoregulation at 0.5 % salinity, while only ectoine alleviated intracellular osmolarity under 1.0 % salinity with OLR of 0.44 kg COD /m³. This study provided in-depth insight for microbial acclimation process of anaerobic priming of saline wastewater.

Electricity-Driven Microbial Metabolism of Carbon and Nitrogen: A Waste-to-Resource Solution

Electricity-driven microbial metabolism relies on the extracellular electron transfer (EET) process between microbes and electrodes and provides promise for resource recovery from wastewater and industrial discharges. Over the past decades, tremendous efforts have been dedicated to designing electrocatalysts and microbes, as well as hybrid systems to push this approach toward industrial adoption. This paper summarizes these advances in order to facilitate a better understanding of electricity-driven microbial metabolism as a sustainable waste-to-resource solution. Quantitative comparisons of microbial electrosynthesis and abiotic electrosynthesis are made, and the strategy of electrocatalyst-assisted microbial electrosynthesis is critically discussed. Nitrogen recovery processes including microbial electrochemical N₂ fixation, electrocatalytic N₂ reduction, dissimilatory nitrate reduction to ammonium (DNRA), and abiotic electrochemical nitrate reduction to ammonia (Abio-NRA) are systematically reviewed. Furthermore, the synchronous metabolism of carbon and nitrogen using hybrid inorganic-biological systems is discussed, including advanced physicochemical, microbial, and electrochemical characterizations involved in this field. Finally, perspectives for future trends are presented. The paper provides valuable insights on the potential contribution of electricity-driven microbial valorization of waste carbon and nitrogen toward a green and sustainable society.

Use of iron-loaded biochar to alleviate anammox performance inhibition under PFOA stress conditions: Integrated analysis of sludge characteristics and metagenomics

The negative effects of perfluorooctanoic acid (PFOA) on biological nitrogen removal performance in wastewater treatment plants, are receiving increasing attention due to the widespread reporting of this issue. In this study, pomelo peel iron-loaded biochar (Fe-PBC) was added to an anammox bioreactor to alleviate the negative effects of PFOA. Results showed that the addition of Fe-PBC increased the ammonia and nitrite removal efficiencies from 77.7 ± 9.6 % and 79.5 ± 5.6 % to 94.45 ± 5.1 % and 95.9 ± 5.0 %, respectively. In addition, Fe-PBC promoted the removal of PFOA from wastewater, increasing the PFOA removal efficiency from 5.2 % to 29.2 ± 4.3 % from 100 to 200 days. The introduction of iron-loaded biochar into the anammox bioreactor increased the CO ratio by 13.64 % by 150 days. In addition, a CO fitting peak was detected in the Fe-PBC, indicating that the Fe-PBC was loaded with microorganisms. Microbial community analysis showed a decrease in the relative abundances of Proteobacteria and Nitrospirae from 31 % and 3.4 % to 16.8 % and 0.9 %, respectively, while the relative abundance of Planctomycetes increased from 26.8 % to 44.1 %. Metagenomic analysis found that the functional genes hzsB and hdh increased from 98,666 ± 11,400 and 3190 ± 460 to 119,333 ± 15,534 and 138,650 ± 11,233 copy numbers/MLSS. The increase in anammox biomass may be attributed to the presence of iron, an essential element for the synthesis of key anammox enzyme. Furthermore, iron was also associated with the enhanced extracellular electron transfer in the anammox system induced by Fe-PBC.

Exploring and comparing the impacts of low temperature to endogenous and exogenous partial denitrification: The nitrite supply, transcription mechanism, and microbial dynamics

Nitrite supply was pretty significant to exogenous or endogenous partial denitrification (ExPD or EdPD) for their combination with anammox in removing nitrogen. This study investigated how temperature impacted the nitrite supply of ExPD and EdPD, through long-term experiments in two 10 L sequencing batch reactors (SBRs) and 12 batch temperature tests, with sodium acetate as organic. It was demonstrated that low temperature (5-15 °C) favored higher nitrite transformation rate (NTR) for two systems (1.1-1.3 and 1.1-1.2 times higher separately), and ExPD owned higher nitrite-supply ability than EdPD (32.8 % higher NTR). Moreover, quantitative reverse transcription PCR and 16srDNA sequencing were conducted, exploring the inherent mechanism and microbial dynamics. Results presented that more inhibition to transcription and translation of nirSK genes than narG in low temperature induced higher NTR. Besides, compared with ExPD, less microbial dynamics and granule size reduction occurred to EdPD, which was more capable of adapting to low temperature.

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.

Introduction of acid-neutralizing layer to facilitate the stabilization of municipal solid waste landfill

Rapid stabilization is important for landfill operation and beneficial for treatment capacity recovery, biogas production, and pollution control. Acidification of municipal solid waste (MSW) landfill hinders the degradation of MSW. In this study, a leachate-recirculated landfill bioreactor with acid-neutralizing layer (reactor BL) and a control landfill bioreactor without the acid-neutralizing layer (CL) were operated for 509 days. The pH of the landfill was increased by the acid-neutralizing layer. The landfill gas production volume increased by 18.3 % in reactor BL compared with CL during the study period, and the CH4 concentration was also increased. A greater MSW mass reduction was observed in reactor BL than in CL. Microbial community analysis demonstrated that the presence of the acid-neutralizing layer promoted the abundance of methanogens. Based on these observations, it is believed that application of the acid-neutralizing layer accelerated the stabilization by mitigating the acidification of landfill, which promote the abundance of methanogens and enhance the MSW degradation. These results help to understand the influencing mechanism of acid-neutralizing layer on the landfill stabilization, and provide a new approach for the practical landfill to achieve fast stabilization.

Small biochar addition enhanced anammox granular sludge system for practical wastewater treatment: Performance and microbial community

Anaerobic ammonium oxidation (Anammox) granular sludge (AnGS) has poor strength and is prone to disintegration under complex environmental conditions, especially in the presence of complex organic carbon, which renders the Anammox process instable. Herein, with a mixture of landfill leachate and domestic sewage as wastewater, the effect on the properties of AnGS with two small particle size (0.1-0.2 mm) biochars (coconut and peach biochars) addition were investigated at different COD concentrations (150 mg·L^-1, 200 mg·L^-1, and 250 mg·L^-1), as well as at different BOD/TN (B/N) (0.3 and 0.5). Results showed that the nitrogen removal efficiencies decreased from 89 % to 72 % as the COD concentration increased by 100 mg·L^-1, while peach biochar reactor had better nitrogen removal performance. Excessive organic carbon supply inhibits AnAOB proliferation and B/N had the most significant effect on AnAOB (p < 0.05). The Polymerase Chain Reaction (PCR) indicated peach biochar reactor get higher activity of anammox-related functional genes (hzsA, hdh).

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.

Achieving PN through the selective recovery of AOB activity in inactivated nitrifying bacteria: Combined aerobic starvation and FA

A novel strategy combining aerobic starvation and free ammonia (FA) was proposed to achieve partial nitrification (PN). The impact of the combined strategy on nitrifying bacteria was explored in a 200-day experiment. The effluent concentration of ammonia was below the detection limits (0.1 mg/L), and the effluent concentration of nitrite and nitrate was 68.12 mg/L and 3.46 mg/L without adding carbon source to the artificial wastewater. The nitrite accumulation rate (NAR) was maintained at 90.15% even when the dissolved oxygen (DO) concentration was 1.50 mg/L. Further analysis showed that PN was achieved by selectively restoring the activity of ammonia-oxidizing bacteria (AOB) in nitrifying bacteria that had lost their activity after starvation. The specific ammonia oxidation rate (SAOR) was 46.25 mg N/g MLVSS/h, and the specific nitrate product rate (SNPR) was only 0.73 mg N/g MLVSS/h in the stable operation stage. The increase in AOB abundance (from 2.79% to 7.13%) and the decrease in nitrite-oxidizing bacteria (NOB) abundance (from 8.75% to 1.44%) explained this phenomenon. Finally, the analyses on the secretion of extracellular polymer substance (EPS), strategies to resist harsh environments, and physical properties of sludge explored the potential mechanism and provided references for applying the combined strategy.