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

Quality decline of prepared dishes stored at 4 °C: Microbial regulation of nitrite and biogenic amine formation

Prepared dishes have high water content and complex nutritional composition, making them highly susceptible to microbial contamination and oxidative spoilage during storage. To elucidate the effects of microorganisms on the quality of prepared dishes and the formation of contaminants, the dishes "stewed pork with cabbage" were stored at 4 °C for 0, 1, 3, 5, and 7 days. Techniques such as high-performance liquid chromatography (HPLC), ion chromatography (IC), and 16S rRNA sequencing were used to explore changes in quality of the prepared dishes. The results showed that the total volatile basic nitrogen (TVB-N) and total viable count (TVC) consistently increased at similar rates during storage. The thiobarbituric acid reactive substances (TBARS) value reached 1 mg MDA/kg after 7 days of storage. Nitrite content and nitrate reductase activity rose as nitrate content decreased with longer storage. The contents of four free amino acids (FAAs) (Glu, His, Phe, Lys) and biogenic amines (BAs) (Putrescine, Histamine, Phenethylamine, Cadaverine) showed opposite trends. The abundance of the dominant genus, including Brochothrix, Acinetobacter and Weissella was significantly (P < 0.05) changed after the 3rd day of storage. Brochothrix, Acinetobacter, Pseudomonas, and Weissella promoted the decarboxylation of FAAs to form (BAs). Acinetobacter and Brochothrix accelerated the conversion of nitrate to nitrite. This study elucidates degradation of the quality of prepared dishes and the effect of five genera (Brochothrix, Acinetobacter, Pseudomonas, Weissella and Brochothrix) on the formation of nitrite and BAs, providing important theoretical support for enhancing nutritional quality and preserving the freshness during storage of prepared dishes.

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.

Light-regulated dentification and dissimilatory nitrate reduction by nano-bio electric syntrophic consortium

Microbial-oriented nitrogen recycling is a vital strategy for nitrogen pollution control in the treatment of low C/N wastewater. However, the deficient electron donors in water body limits the reactive nitrogen recovery. Herein, we design a nano-bio electric syntrophic consortium for light-regulated dentification and dissimilatory nitrate reduction to ammonium (DNRA) without organic carbon sources input. Using Fe0 coupons as the sole electron donor, the extracellular electron uptake rate of a model denitrifier (Pseudomonas aeruginosa PAO1) is enhanced by coculturing it with an electroactive bioregulator, Shewanella oneidensis MR-1 (MR-1), thereby achieving an average nitrate removal rate of 63.8 ± 0.1 mg N/d/L with ammonium recovery efficiency of 27.1 ± 0.2 % under illumination. Notably, in situ self-assembled FeS nanoparticles via a bottom-up Fe0 biocorrosion approach are observed on the outer membrane and periplasmic space of MR-1. Under illumination, native MtrCOmcA-CymA protein complex and FeS nanoparticles act in electron conduits to facilitate transmembrane photoelectron uptake of MR-1 for microbial DNRA process. Biochemical and transcriptomic analyses reveal that the NADH generation, chemotaxis moving and energy-taxis of MR-1 hybrids strength the driving force for microbial DNRA process. Overall, we demonstrate that the constructed FeS-assisted coculture, as an emerging model of electric syntrophy, could support the solar-triggered nitrogen metabolism from non-phototrophic microbes. Given that Fe0 biocorrosion is a facile route to MR-1 growth, this nano-bio system also affords a promising pathway for low C/N wastewater treatment and reactive nitrogen recovery via DNRA process.

Nitrogen Recovery through Dissimilatory Nitrate Reduction to Ammonium: Impact of Environmental Factors

The application of the bacterial dissimilatory nitrate reduction to ammonium (DNRA) process for treating nitrate-rich wastewater offers an environmentally friendly and resource-efficient strategy with significant potential for ammonium nitrogen recovery. This study investigates the impact of carbon sources, C/N ratios, pH, and temperature on the DNRA efficiency of Pseudomonas sp. strain LZ-1 (strain LZ-1). The results revealed that sodium citrate is the most favorable carbon source among sodium formate, sodium acetate, sodium propionate, and sodium citrate for enhancing DNRA in strain LZ-1. Ammonia production by strain LZ-1 peaks at a C/N of 8 within the range of 3 to 20, increasing before and decreasing thereafter. Furthermore, neutral to alkaline conditions (pH 7-10) are favorable for the DNRA process, with an optimal initial pH of 9. Temperature studies indicate a similar trend of initial increase followed by a decline in DNRA efficiency as temperatures rise from 20 to 35 °C, with peak ammonia production at 30 °C. The presence of sulfur ions inhibits the DNRA process in the strain LZ-1. However, this inhibitory effect diminished as the S/N ratio increased from 1/4 to 1. These insights contribute to a deeper understanding of the impact of environmental factors on DNRA and serve as a valuable reference for the utilization of strain LZ-1 in nitrogen recovery from nitrate-rich wastewaters.

Understanding the potential role of microbial electrolysis cells in promoting electron transfer and microbial metabolism during the drying period in treating metformin-containing wastewater with an adsorption-biological coupling system

Effective removal of metformin from wastewater through biological treatment technology has been a challenging issue. Enhancing electron transfer was demonstrated to be an effective measure to improve the removal of refractory pollutants from wastewater. In this study, the effects of a microbial electrolysis cell (MEC) in strengthening an adsorption-biological coupling reactor during the drying period in treating metformin wastewater were investigated, along with its microbial community and metabolism. Compared to without a MEC, the removal rates of chemical oxygen demand (COD), total phosphorus (TP), ammonia-nitrogen, and metformin all increased with the increase of voltage; at 1.0 V, their removal rates were 77.26 %-91.45 %, 59.22 %-75.85 %, 79.52 %-91.56 %, and 57.45 %-70.15 % respectively. The main dominant bacteria in the two groups were Pseudomonadota (28.14 %-75.72 % and 13.51 %-84.79 %, respectively) and Actinobacteria (16.27 %-67.10 % and 6.11 %-84.27 %). The MEC increased the relative abundance of glycolytic glucokinase and pyruvate kinase genes. In nitrogen metabolism, dissimilar nitrate reduction was strengthened. In addition, the relative abundance of the functional genes involved in phosphate translocation, electron transport-linked phosphorylation, and phosphate metabolism were all increased after voltage addition, which promoted microbial activity and increased the TP removal rate.

The impact of endogenous organic detritus on differentiation of nitrate reduction pathway in sediments

In order to explore the impact of endogenous organic detritus on differentiation of nitrate reduction pathway and mechanism, our study hypothesizes that the source of organic matter leads to differences in its chemical structure, thus affecting the nitrate reduction pathway. An indoor incubation experiment was conducted by adding different organic detritus from nitrogen-fixing and non-nitrogen-fixing cyanobacteria, green algae, and submerged macrophyte (sediment mixed thoroughly with different algal detritus and topped with 60 cm of water). The chemical components of different organic detritus degradation were mainly composed of aliphatic and aromatic compounds from cyanobacterial detritus as well as from green algae and macrophyte detritus, respectively, but the proportion was entirely different. Although the abundance of functional genes involved in the nitrogen cycle is similar in all groups, the microbial community structures are vastly different. The dominant microbial community structure and nitrate reduction rate as well as their negative relationship all indicated the discrepancy between ecological function and dominant microbial community structure. This suggested that the minority microbial community plays a dominant role in the nitrate reduction process. However, there is a high consistency between nitrate reduction rates and nitrogen nutrient levels. In addition, the dissimilatory nitrate reduction to ammonium (DNRA) predominated (10 ~ 35 μmol/kg/h), followed by denitrification (0.2 ~ 1.4 μmol/kg/h) in the nitrate reduction process. Therefore, the degradation of endogenous organic detritus promoted the nitrogen retention process mainly carried out by a minority microbial community, contributing to maintain the original eutrophic state in water bodies.

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.

Achieving mainstream nitrogen removal by partial nitrification and anammox in the carriers-coupled membrane aerated biofilm reactor

The integration of partial nitrification-anammox (PN/A) into membrane-aerated biofilm reactor (MABR) is a promisingly energy-efficient and high-efficiency technology for nitrogen removal. The inhibition of nitrite oxidizing bacteria (NOB) remains as the most significant challenge for its development. In our investigation, we proposed a novel process to integrate carriers to MABR (CMABR), which combined the carriers enriched with anaerobic ammonium-oxidizing bacteria (AnAOB) and partial nitrifying MABR system. The effect of different hydraulic retention time (HRT) was explored in CMABR and it showed that the nitrogen removal rate of CMABR could reach more than 200 g-N/(m3·d) at an HRT of 3 h The increase of NOB activity was witnessed when the residual NH4+-N concentration was lower than 5 mg-N/L. Finally, the higher nitrogen removal rate and successful PN/A can be achieved by optimized condition through the operation of two-stage CMABRs with 30 % of carriers filling ratio and a total HRT of 6 h Superior NH4+-N removal efficiency (97 %) and total nitrogen removal efficiency (81 %) were reached compared with other MABR for PN/A processes. The CMABR exerted the special advantage that significant AnAOB attached on the carriers, rather than only on the membrane biofilm, thus it was beneficial to maintain the activity of ammonia oxidizing bacteria (AOB) and improve the nitrogen removal rate and effluent quality. This investigation provides creative and significant perspectives for the design and operation of PN/A processes in the future MABR applications.

Deciphering the key role of biofilm and mechanisms in high-strength nitrogen removal within the anammox coupled partial S0-driven autotrophic denitrification system

Anammox coupled partial S0-driven autotrophic denitrification (PS0AD) technology represents an innovative approach for removing nitrogen from wastewater. The research highlighted the crucial role of biofilm on sulfur particles in the nitrogen removal process. Further analysis revealed that sulfur-oxidizing bacteria (SOB) are primarily distributed in the inner layer of the biofilm, while anammox bacteria (AnAOB) are relatively evenly distributed in inner and outer layers, with Thiobacillus and Candidatus Brocadia being the dominant species, respectively. Except for anammox and PS0AD processes, 15N isotope labeling tests determined that sulfur reshaped nitrogen metabolism pathways, providing solid evidence for the occurrence of sulfammox process. SOB and AnAOB collaborate in nitrogen and sulfur conversion, with SOB-drived PS0AD processes reducing nitrate to nitrite for AnAOB to remove ammonia. Conversely, the nitrate produced from anammox process can be reused by SOB. Metagenomic analyses verified that SOB drove the PS0AD process through encoding soxBYZ gene, while AnAOB might play an important role in simultaneously driving the anammox and sulfammox processes. These findings underscore the importance of biofilm and clarify the nitrogen-sulfur cycle mechanisms within the coupled system.

Revealing the combined effect of hydroxylamine and hydrazine on nitrogen removal performance of completely autotrophic nitrogen removal over nitrite (CANON) process

This study explored capabilities of completely autotrophic nitrogen removal over nitrite (CANON) process under co-regulation of hydroxylamine (NH2OH) and hydrazine (N2H4). Results indicated that granular (R1) and flocculated (R2) sludge CANON systems were started-up respectively on day 14 and day 17 by co-introduction of 0.50 + 1.00 mg/L and 1.00 + 5.00 mg/L NH2OH and N2H4, with ammonia removal efficiency (ARE) of 53.75 % ± 8.30 % and 54.17 % ± 7.40 %, respectively. Furthermore, the relative abundance of Nitrosomonas, Nitrospira, Candidatus_Nitrotoga and Nitrobacter reduced after NH2OH and N2H4 was supplied into R1 and R2 for 30 days, thereby inhibiting ammonia-oxidizing bacteria (AOB) and nitrite-oxidizing bacteria (NOB) activity. Nevertheless, the relative abundance of Candidatus_Kuenenia and Candidatus_Jettenia recovered once NH2OH and N2H4 were absent, produced reversible inhibitory on anaerobic ammonia-oxidation bacteria (AnAOB) activity. Results will demystify the nitrogen elimination performance and mechanism of the CANON process from a novel viewpoint.

Insights into the impact of 6PPD-Q and 6PPD on nitrogen metabolism and microbial community in the anammox system

N-(1,3-Dimethylbutyl)-N'-phenyl-p-phenylenediamine (6PPD) is an antioxidant commonly used in tire manufacturing, and its release into the environment has significantly increased due to rapid urbanization. When subjected to ozonation, 6PPD converts into the harmful pollutant 6PPD quinone (6PPDQ). These substances enter wastewater treatment plants (WWTPs) via stormwater runoff and pipelines, posing significant risks to the functional microorganisms. Anammox, a strictly controlled and sensitive microbial nitrogen removal process, is especially susceptible to the effects of the pollutants. This study investigates the comprehensive impact of 6PPD-Q and 6PPD on anammox communities based on characterization analysis and metagenomics. At environmental concentrations, 6PPD-Q at 200 ng/L-1000 ng/L led to the disintegration of anammox granules. Extended exposure to both 6PPD-Q and 6PPD significantly reduces the population of anammox bacteria (AnAOB). By utilizing organic matter from dead cells and incoming carbonate as a carbon source, the system evolved into a nitrogen metabolism network primarily focused on denitrification and dissimilatory nitrate reduction to ammonium (DNRA). This transformation was accompanied by a reshuffling of the microbial community and associated genes, resulting in an accumulation of NH4+-N. These findings underscore the toxicity of 6PPD-Q and 6PPD to anammox and stress the importance of incorporating 6PPD into regulatory and preventive strategies.

Enhancement of the reactivation process of long-term starved anammox granular sludge with gravel balls: Microbial succession and metabolic impact

Anaerobic ammonium oxidation (Anammox) process is an economical and energy-efficient method of wastewater nitrogen removal. However, they are highly susceptible to starvation stress caused by sudden environmental changes. Rapid reactivation of starved anammox sludge is a crucial method to address seed sludge shortages and expand practical applications. This study investigated the impact of gravel balls on the reactivation of long-term starved anammox granular sludge (628 days). The results showed that gravel balls enhanced the recovery of nitrogen removal performance in starved anammox sludge, with nitrogen removal efficiency being 19.88% higher than the control group at the end of the recovery phase. The gravel balls also increased extracellular polymeric substance (EPS) secretion, contributing to the stability of the anammox system. Furthermore, the gravel balls promoted the proliferation of anammox bacteria, with the relative abundance of anammox bacteria reaching 38.25% on the 80th day. The analyses of microbial functions indicated that gravel balls facilitated cross-feeding and co-metabolism among microbes, while enhancing quorum sensing associated with anammox bacteria, forming a multifunctional community network centered on anammox bacteria. This indicates that gravel balls can effectively accelerate the reactivation process of long-term starved anammox sludge, aiding the reutilization of long-term starved anammox sludge.

Unique ecology of biofilms and flocs: Bacterial composition, assembly, interaction, and nitrogen metabolism within deteriorated bioreactor inoculated with mature partial nitrification-anammox sludge

This work unraveled discrepant ecological patterns between biofilms and flocs in a deteriorated bioreactor inoculated with mature partial nitrification-anammox (PN/A) sludge. Based on 16S rRNA analysis, a comprehensive evaluation of neutral and null models, along with niche width, delineated that the bacterial community assembly in biofilms and flocs was dominantly driven by the stochastic process, and dispersal limitation critically shaped the community assembly. Co-occurrence network analysis revealed that environmental stress caused decentralized and fragmented bacterial colonies, and anammox bacteria were mainly peripheral in biofilms network and less involved in interspecific interactions. Simultaneous PN/A and partial denitrification-anammox (PD/A) processes were identified, whereas PN and PD process primarily occurred in the biofilms and flocs, respectively, as evidenced by metagenomics. Collectively, these outcomes are expected to deepen the basic understanding of complex microbial community and nitrogen metabolism under environmental disturbance, thereby better characterizing and serving the artificial ecosystems.

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.

A comprehensive catalog encompassing 1376 species-level genomes reveals the core community and functional diversity of anammox microbiota

Research on the microbial community and function of the anammox process for environmentally friendly wastewater treatment has achieved certain success, which may mean more universal insights are needed. However, the comprehensive understanding of the anammox process is constrained by the limited taxonomic assignment and functional characterization of anammox microbiota, primarily due to the scarcity of high-quality genomes for most organisms. This study reported a global genome catalog of anammox microbiotas based on numerous metagenomes obtained from both lab- and full-scale systems. A total of 1376 candidate species from 7474 metagenome-assembled genomes were used to construct the genome catalog, providing extensive microbial coverage (averaged of 92.40 %) of anammox microbiota. Moreover, a total of 64 core genera and 44 core species were identified, accounting for approximately 64.25 % and 43.97 %, respectively, of anammox microbiota. The strict core genera encompassed not only functional bacteria (e.g., Brocadia, Desulfobacillus, Zeimonas, and Nitrosomonas) but also two candidate genera (UBA12294 and OLB14) affiliated with the order Anaerolineales. In particular, core denitrifying bacteria with observably taxonomic diversity exhibited diverse functional profiles; for instance, the potential of carbohydrate metabolism in Desulfobacillus and Zeimonas likely improves the mixotrophic lifestyle of anammox microbiota. Besides, a noteworthy association was detected between anammox microbiota and system type. Microbiota in coupling system exhibited complex diversity and interspecies interactions by limiting numerous core denitrifying bacteria. In summary, the constructed catalog substantially expands our understanding of the core community and their functions of anammox microbiota, providing a valuable resource for future studies on anammox systems.

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

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

Non-electroactive bacteria behave variously in AnMBR biofilm control using electric field

Electroactive bacteria are often regarded as key players responding to electric fields that are used to control biofilm development during AnMBR (anaerobic membrane bioreactor) operation. Consequently, little attention has been given to non-electroactive bacteria in the same systems because of their incapability to acquire and transfer electrons directly. However, in this study, we identified some functionally important non-electroactive bacteria from biofilm established under low-voltage (0, 0.3, 0.5 and 1 V) electric fields in AnMBRs, designated as E-AnMBRs in this study. During the whole experiment, non-electroactive bacteria, mainly belonging to Proteobacteria, Bacteroidetes, and Chloroflexi, were found in all biofilm samples taken from each E-AnMBR. Under 0.3 V and 1 V conditions, non-electroactive bacteria did not seem to contribute to the development of biofilm significantly. Whereas under 0.5 V conditions, the growth of non-electroactive bacteria contributed up to 0.61 kPa/day biofilm formation. Therefore, 0.5 V was identified as a critical voltage, leading to the most severe biofilm formation. The microbial community structure in the reactor with a 0.5 V electric field was distinctly unique, caused by the increase of non-electroactive bacterial activity and the upregulation of their metabolic pathways. Notably, functional genes involved in carbon metabolism and oxidative phosphorylation pathway were upregulated. Furthermore, the 0.5 V electric field enhanced the protein/polysaccharide ratio and increased zeta potential to 31.6 mV (p < 0.01) of the biofilm samples. This was because upregulating quorum sensing genes accelerated the coordinated gene regulations and functional activities among non-electroactive bacteria.

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.

Remediation of arsenic- and nitrate-contaminated groundwater through iron-dependent autotrophic denitrifying culture

Groundwater contamination with arsenic and nitrate poses a pressing concern for the safety of local communities. Bioremediation, utilizing Fe(II)-oxidizing nitrate reducing bacteria, shows promise as a solution to this problem. However, the relatively weak environmental adaptability of a single bacterium hampers practical application. Therefore, this study explored the feasibility and characteristics of a mixed iron-dependent autotrophic denitrifying (IDAD) culture for effectively removing arsenic and nitrate from synthetic groundwater. The IDAD biosystem exhibited stable performace and arsenic resistance, even at a high As(III) concentration of 800 μg/L. Although the nitrogen removal efficiency of the IDAD biosystem decreased from 71.4% to 64.7% in this case, the arsenic concentration in the effluent remained below the standard (10 μg/L) set by WHO. The crystallinity of the lepidocrocite produced by the IDAD culture decreased with increasing arsenic concentration, but the relative abundance of the key iron-oxidizing bacteria norank_f_Gallionellaceae in the culture showed an opposite trend. Metagenomic analysis revealed that the IDAD culture possess arsenic detoxification pathways, including redox, methylation, and efflux of arsenic, which enable it to mitigate the adverse impact of arsenic stress. This study provides theoretical understanding and technical support for the remediation of arsenic and nitrate-contaminated groundwater using the IDAD culture.

Cooperation and competition between denitrification and chromate reduction in a hydrogen-based membrane biofilm reactor

Competition and cooperation between denitrification and Cr(VI) reduction in a H2-based membrane biofilm reactor (H2-MBfR) were documented over 55 days of continuous operation. When nitrate (5 mg N/L) and chromate (0.5 mg Cr/L) were fed together, the H2-MBfR maintained approximately 100 % nitrate removal and 60 % chromate Cr(VI) removal, which means that nitrate outcompeted Cr(VI) for electrons from H2 oxidation. Removing nitrate from the influent led to an immediate increase in Cr(VI) removal (to 92 %), but Cr(VI) removal gradually deteriorated, with the removal ratio dropping to 14 % after five days. Cr(VI) removal resumed once nitrate was again added to the influent. 16S rDNA analyses showed that bacteria able to carry out H2-based denitrification and Cr(VI) reduction were in similar abundances throughout the experiment, but gene expression for Cr(VI)-reduction and export shifted. Functional genes encoding for energy-consuming chromate export (encoded by ChrA) as a means of bacterial resistance to toxicity were more abundant than genes encoding for the energy producing Cr(VI) respiration via the chromate reductase ChrR-NdFr. Thus, Cr(VI) transport and resistance to Cr(VI) toxicity depended on H2-based denitrification to supply energy. With Cr(VI) being exported from the cells, Cr(VI) reduction to Cr(III) was sustained. Thus, cooperation among H2-based denitrification, Cr(VI) export, and Cr(VI) reduction led to sustained Cr(VI) removal in the presence of nitrate, even though Cr(VI) reduction was at a competitive disadvantage for utilizing electrons from H2 oxidation.

Optimization of "sulfur-iron-nitrogen" cycle in constructed wetlands by adjusting siderite/sulfur (Fe/S) ratio

Sulfur-siderite autotrophic denitrification (SSAD) has been proved to solve the key problem of low nitrogen removal efficiency caused by the shortage of carbon source in constructed wetlands (CWs). In this study, five vertical flow constructed wetlands (VFCWs) were constructed with different Fe/S ratios (0/0, 0/1, 1/1, 2/1 and 1/2) to optimizing SSAD process, labeled S.0, S.1, S.2, S.3 and S.4. The results showed that the best NO3--N and TN removal rates were achieved with a Fe/S ratio of 2:1 (S.3), which were 96.26 ± 1.40% and 93.63 ± 3.12%, respectively. The abundance of denitrification genes (nirS, nirK and nosZ) in S.3 was significantly increased. Illumina high-throughput sequencing analysis indicated that the abundance and diversity of microorganisms involved in the "Sulfur-Iron-Nitrogen" cycle were enriched in S.3. The current study provided that the "Sulfur-Iron-Nitrogen" cycle in CWs was optimized by adjusting Fe/S ratio, and more types of denitrifying bacteria could be enriched, thereby enhancing nitrogen removal.

Synergistic Integration of Anammox and Endogenous Denitrification Processes for the Simultaneous Carbon, Nitrogen, and Phosphorus Removal

The feasibility of a synergistic endogenous partial denitrification-phosphorus removal coupled anammox (SEPD-PR/A) system was investigated in a modified anaerobic baffled reactor (mABR) for synchronous carbon, nitrogen, and phosphorus removal. The mABR comprising four identical compartments (i.e., C1-C4) was inoculated with precultured denitrifying glycogen-accumulating organisms (DGAOs), denitrifying polyphosphate-accumulating organisms, and anammox bacteria. After 136 days of operation, the chemical oxygen demand (COD), total nitrogen, and phosphorus removal efficiencies reached 88.6 ± 1.0, 97.2 ± 1.5, and 89.1 ± 4.2%, respectively. Network-based analysis revealed that the biofilmed community demonstrated stable nutrient removal performance under oligotrophic conditions in C4. The metagenome-assembled genomes (MAGs) such as MAG106, MAG127, MAG52, and MAG37 annotated as denitrifying phosphorus-accumulating organisms (DPAOs) and MAG146 as a DGAO were dominated in C1 and C2 and contributed to 89.2% of COD consumption. MAG54 and MAG16 annotated as Candidatus_Brocadia (total relative abundance of 16.5% in C3 and 4.3% in C4) were responsible for 74.4% of the total nitrogen removal through the anammox-mediated pathway. Functional gene analysis based on metagenomic sequencing confirmed that different compartments of the mABR were capable of performing distinct functions with specific advantageous microbial groups, facilitating targeted nutrient removal. Additionally, under oligotrophic conditions, the activity of the anammox bacteria-related genes of hzs was higher compared to that of hdh. Thus, an innovative method for the treatment of low-strength municipal and nitrate-containing wastewaters without aeration was presented, mediated by an anammox process with less land area and excellent quality effluent.

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.

Novel insights into intrinsic mechanisms of magnetic field on long-term performance of anaerobic ammonium oxidation process

The performance of an anaerobic ammonium oxidation (anammox) reactor with the magnetic field of 40 mT was systematically investigated. The total nitrogen removal rate was enhanced by 16% compared with that of the control group. The enhancing mechanism was elucidated from the improved mass transfer efficiency, the complicated symbiotic interspecific relationship and the improved levels of functional genes. The magnetic field promoted formation of the loose anammox granular sludge and the homogeneous and well-connected porous structure to enhance the mass transfer. Consequently, Candidatus Brocadia predominated in the sludge with an increase in abundance of 13%. Network analysis showed that the positive interactions between Candidatus Brocadia and heterotrophic bacteria were strengthened, which established a more complicated stable microbial community. Moreover, the magnetic field increased the levels of hdh by 26% and hzs by 35% to promote the nitrogen metabolic process. These results provided novel insights into the magnetic field-enhanced anammox process.

The interplay of hematite and photic biofilm triggers the acceleration of biotic nitrate removal

The soil-water interface is replete with photic biofilm and iron minerals; however, the potential of how iron minerals promote biotic nitrate removal is still unknown. This study investigates the physiological and ecological responses of photic biofilm to hematite (Fe2O3), in order to explore a practically feasible approach for in-situ nitrate removal. The nitrate removal by photic biofilm was significantly higher in the presence of Fe2O3 (92.5%) compared to the control (82.8%). Results show that the presence of Fe2O3 changed the microbial community composition of the photic biofilm, facilitates the thriving of Magnetospirillum and Pseudomonas, and promotes the growth of photic biofilm represented by the extracellular polymeric substance (EPS) and the content of chlorophyll. The presence of Fe2O3 also induces oxidative stress (•O2-) in the photic biofilm, which was demonstrated by electron spin resonance spectrometry. However, the photic biofilm could improve the EPS productivity to prevent the entrance of Fe2O3 to cells in the biofilm matrix and mitigate oxidative stress. The Fe2O3 then promoted the relative abundance of Magnetospirillum and Pseudomonas and the activity of nitrate reductase, which accelerates nitrate reduction by the photic biofilm. This study provides an insight into the interaction between iron minerals and photic biofilm and demonstrates the possibility of combining biotic and abiotic methods to improve the in-situ nitrate removal rate.

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.

Rapid start-up sulfur-driven autotrophic denitrification granular process: Extracellular electron transfer pathways and microbial community evolution

Sulfur-driven autotrophic denitrification (SAD) granular process has significant advantages in treating low-carbon/nitrogen wastewater; however, the slow growth rate of sulfur-oxidizing bacteria (SOB) results in a prolonged start-up duration. In this study, the thiosulfate-driven autotrophic denitrification (TAD) was successfully initiated by inoculating anaerobic granular sludge on Day 7. Additionally, the electron donor was successfully transferred to the cheaper elemental sulfur from Day 32 to Day 54 at the nitrogen loading rate of 176.2 g N m-3 d-1. During long term experiment, the granules maintained compact structures with the α-helix/(β-sheet + random coil) of 29.5-40.1 %. Extracellular electron transfer (EET) pathway shifted from indirect to direct when electron donors were switched thiosulfate to elemental sulfur. Microbial analysis suggested that thiosulfate improved EET involving enzymes activity. Thiobacillus and Sulfurimonas were dominant in TAD, whereas Longilinea was enriched in elemental sulfur-driven autotrophic denitrification. Overall, this strategy achieved in-situ enrichment of SOB in granules, thereby shortening start-up process.

Robust rehabilitation of anammox system by granular activated carbon under long-term starvation stress: Microbiota restoration and metabolic reinforcement

This article investigates the buffering capacity and recovery-enhancing ability of granular activated carbon (GAC) in a starved (influent total nitrogen: 20 mg/L) anaerobic ammonium oxidation (anammox) reactor. The findings revealed that anammox aggregated and sustained basal metabolism with shorter performance recovery lag (6 days) and better nitrogen removal efficiency (84.9 %) due to weak electron-repulsion and abundance redox-active groups on GAC's surface. GAC-supported enhanced extracellular polymeric substance secretion aided anammox in resisting starvation. GAC also facilitated anammox bacterial proliferation and expedited the restoration of anammox microbial community from a starved state to its initial-level. Metabolic function analyses unveiled that GAC improved the expression of genes involved in amino acid metabolism and sugar-nucleotide biosynthesis while promoted microbial cross-feeding, ultimately indicating the superior potential of GAC in stimulating more diverse metabolic networks in nutrient-depleted anammox consortia. This research sheds light on the microbial and metabolic mechanisms underlying GAC-mediated anammox system in low-substrate habitats.

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.

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

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

The impact of perfluorooctanoic acid shock on hydrogen-driven nitrate and arsenate removal

Perfluorooctanoic acid (PFOA) is a type of toxic per- and poly-fluoroalkyl substance (PFAS) commonly found in groundwater due to its use in firefighting and industrial applications. The main purpose of this study was to investigate the influence of PFOA shock on the biological performance of a hydrogen-driven bioreactor for nitrate and arsenate removal. Four hydrogen-driven removal reactors (HdBRs) used for the simultaneous removal of nitrate and arsenal were operated with concentrations of either 0, 1, 5, and 10 mg/L of PFOA to induce shock on the systems and examine the corresponding bacterial response. Our results showed that PFOA shock inhibited and decreased the maximum hydrogen-driven arsenate removal rate. Principal Component Analysis (PCA) confirmed that this performance decrease occurred due to a bacterial strike triggered by PFOA shock. PFOA toxicity also led to protein secretion and sludge density decreases. Bacterial analyses showed shifts in the community population due to PFOA shock. The dominant bacteria phylum Proteobacteria became more abundant, from 41.24% originally to 48.29% after exposure to 10 mg/L of PFOA. Other phyla, such as Euryarchaeota and Bacteroidetes, were more tolerant to PFOA shock. Although some of the predominant species within the sludge of each HdBR exhibited a decline, other species with similar functions persisted and assumed the functional responsibilities previously held by the dominant species.

Performance and microbial metabolic mechanism of imidacloprid removal in a microbial electrolysis cell-integrated adsorption biological coupling system

Coke used as a filler to treat imidacloprid (IMI) wastewater by both adsorption biological coupling and microbial electrolysis cells (MEC)-adsorption biological coupling technologies, the removal efficiencies on pollutions in wastewater containing IMI were investigated, and the key functional genes related to IMI degradation pathways were also revealed. Results showed that the removal rates of COD, ammonia nitrogen, TP, and IMI under the adsorption biological coupling treatment and MEC-adsorption biological coupling treatment were 94.61-95.54%, 93.37-95.79%, 73.69-83.80%, and 100%, respectively. MEC increased the relative abundance of Proteobacteria by 9.01% and transformed the dominant bacteria from Lysobacter and Reyranella to Brevundimonas and Aquincola. Moreover, MEC up-regulated the abundance of the coding genes PK (9.30%), narG (2.26%), pstS (3.63%), and phnD (1.32%), and converted the IMI degradation products to smaller molecular weight C6H8N2 and C6H6ClNO. This study provided an important reference information for efficient treatment of IMI wastewater using the MEC-adsorption biological coupling technology.

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

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

Performance changes in the anammox process under the stress of rare-earth element Ce(III) and the evolution of microbial community and functional genes

The high Ce(III) content in ionic rare-earth tailings wastewater has hindered the application of anammox process in this field. Here, the effect of Ce(III) on the performance of anammox processes was investigated, and the evolution of microbial communities and functional genes was explored using metagenomic sequencing. The results showed that the reactor nitrogen removal rate decreased when the Ce(III) concentration reached 25 mg/L, although ammonia nitrogen removal (92.31%) and nitrogen removal efficiency (81.33%) remained at a high level; however, both showed a significant decreasing trend. The relative abundance of anammox bacteria increased continuously from P1-P5, reaching 48.81%, whereas the relative abundance of Candidatus jettenia reached 33.71% at P5, which surpassed that of Candidatus brocadia as the most abundant anammox bacteria, and further analysis of functional genes and metabolic pathways revealed that Candidatus brocadia was richer in biochemical metabolic genes, whereas Candidatus jettenia had richer efflux genes.

Acorus calamus recycled as an additional carbon source in a microbial fuel cell-constructed wetland for enhanced nitrogen removal

Acorus calamus was recycled as an additional carbon source in microbial fuel cell-constructed wetlands (MFC-CWs), for efficient nitrogen removal of low carbon wastewater. The pretreatment methods, adding positions, and nitrogen transformations were investigated. Results indicated that alkali-pretreatment cleaved the benzene rings in dominant released organics, producing chemical oxygen demand of 164.5 mg from per gram of A. calamus. Pretreated biomass addition in the anode of MFC-CW attained the maximum total nitrogen removal of 97.6% and power generation of 12.5 mW/m², which were higher than those with biomass in the cathode (97.6% and 1.6 mW/m², respectively). However, the duration of a cycle with biomass in the cathode (20-25 days) was longer than that in the anode (10-15 days). Microbial metabolisms related to organics degradation, nitrification, denitrification, and anammox were intensified after biomass recycling. This study provides a promising method to improve nitrogen removal and energy recovery in MFC-CWs.