Variations in the denitrifying microbial community and functional genes during mesophilic and thermophilic anaerobic digestion of cattle manure

Contribution of microorganisms from pit mud to volatile flavor compound synthesis in fermented grains for nongxiangxing baijiu brewing

BACKGROUND

Nongxiangxing baijiu (NB) is known for its distinct flavor profile, which is attributed to key aroma compounds. The exposed fermentation technique, utilizing daqu and solid-state fermentation in pit muds, plays a crucial role in flavor development. Though previous studies have investigated the impact of microorganisms from pit ?ud and fermented grains on flavor compound production, a comprehensive understanding of microbial functions in the entire pit fermentation system is lacking. Herein, we aimed to explore the role of pit-mud-derived microorganisms in shaping the microbial community and flavor compound synthesis in NB.

RESULTS

There are 76 volatile flavor compounds that have been identified in fermented grains during NB fermentation. The main flavor compounds in NB clustered within the same network module, and 27.27% of microorganisms in the core modules of the fermented grain co-occurrence network originated from pit mud. The relationship between pit mud microorganisms and flavor compounds revealed a significant positive correlation (92%). Notably, Prevotella and Sarocladium were identified as the main contributors to this effect on flavor.

CONCLUSION

Microorganisms originating from pit mud influenced the composition and activity of microorganisms in fermented grains and facilitated the production of flavor compounds in NB. © 2023 Society of Chemical Industry.

The performance, mechanism and greenhouse gas emission potential of nitrogen removal technology for low carbon source wastewater

Excessive nitrogen can lead to eutrophication of water bodies. However, the removal of nitrogen from low carbon source wastewater has always been challenging due to the limited availability of carbon sources as electron donors. Biological nitrogen removal technology can be classified into three categories: heterotrophic biological technology (HBT) that utilizes organic matter as electron donors, autotrophic biological technology (ABT) that relies on inorganic electrons as electron donors, and heterotrophic-autotrophic coupling technology (CBT) that combines multiple electron donors. This work reviews the research progress, microbial mechanism, greenhouse gas emission potential, and challenges of the three technologies. In summary, compared to HBT and ABT, CBT shows greater application potential, although pilot-scale implementation is yet to be achieved. The composition of nitrogen removal microorganisms is different, mainly driven by electron donors. ABT and CBT exhibit the lowest potential for greenhouse gas emissions compared to HBT. N2O, CH4, and CO2 emissions can be controlled by optimizing conditions and adding constructed wetlands. Furthermore, these technologies need further improvement to meet increasingly stringent emission standards and address emerging pollutants. Common measures include bioaugmentation in HBT, the development of novel materials to promote mass transfer efficiency of ABT, and the construction of BES-enhanced multi-electron donor systems to achieve pollutant prevention and removal. This work serves as a valuable reference for the development of clean and sustainable low carbon source wastewater treatment technology, as well as for addressing the challenges posed by global warming.

Carbon/nitrogen flows and associated microbial communities in full-scale foodwaste treatment plants

Microorganisms play key roles in the conversion of organic matter in foodwaste. However, both the microbially-mediated element (carbon/C and nitrogen/N) flows and associated microbial communities in foodwaste treatment plants (FWTPs) remain unclear. This study collected samples of different foodwaste treatment units from five full-scale FWTPs to analyze the C/N flows and microbial communities in foodwaste treatment processes. Results showed that 39.8-45.0% of organic carbon in foodwaste was converted into biogas. Hydrolytic acidogenic bacteria (e.g., Lactobacillus and Limosilactobacillus) and eukaryota (e.g., Cafeteriaceae, Saccharomycetales, and Agaricomycetes) were more abundant in feedstock and pretreatment units. Redundancy analyses showed that acidogens were major players in the transformation of foodwaste organic matter. Populations of W27 and Tepidanaerobacter were major contributors to the difference in conversion of C/N in these FWTPs. This study could support foodwaste treatment efficiencies improvement by providing insights into C/N flows and associated microbiota in FWTPs.

Buffered loofah supported Microalgae-Bacteria symbiotic (MBS) system for enhanced nitrogen removal from rare earth element tailings (REEs) wastewater: Performance and functional gene analysis

Rare earth element tailings (REEs) wastewater, which has the characteristics of high ammonia nitrogen (NH₄⁺-N) and low COD. It can cause eutrophication and biotoxicity in water which is produced in high volumes, requiring treatment before final disposal. Microalgae-Bacteria symbiotic (MBS) system can be applied in REEs wastewater, but its low extent of nitrogen removal and instability limit its application. By adding biodegradable carrier as both carbon source and carrier, the system can be stabilized and the efficiency can be improved. In this work, the extent of NH₄⁺-N removal reached 100% within 24 h in a MBS system after adding loofah under optimal conditions, and the removal rate reached 127.6 mg NH₄⁺-N·L^-1·d^-1. In addition, the carbon release from loofah in 3 d reached 408.7 mg/L, which could be used as a carbon source to support denitrification. During 90 d of operation of the MBS system loaded with loofah, the effluent NH₄⁺-N was less than 15 mg/L. At phylum level, Proteobacteria were dominant which accounted for 78.2%. Functional gene analysis showed that enhancement of microalgae assimilation was the main factor affecting NH₄⁺-N removal. This work expands our understanding of the enhanced role of carbon-based carriers in the denitrification of REEs wastewater.

Uncovering the diverse hosts of tigecycline resistance gene tet(X4) in anaerobic digestion systems treating swine manure by epicPCR

The tet(X4) gene is a clinically important tigecycline resistance gene and has shown high persistence in livestock-related environments. However, the bacterial hosts of tet(X4) remain unknown due to the lack of appropriate approaches. Herein, a culture-independent and high-throughput epicPCR (emulsion, paired isolation, and concatenation polymerase chain reaction) method was developed, optimized, and demonstrated for the identification of bacterial hosts carrying tet(X4) from environmental samples. Considering the high sequence similarity between tet(X4) and other tet(X)-variant genes, specific primers and amplification conditions were screened and optimized to identify tet(X4) accurately and link tet(X4) with the 16S rRNA gene, which were further validated using artificially constructed bacterial communities. The epicPCR targeting tet(X4) was applied for the identification of bacterial hosts carrying this resistance gene in anaerobic digestion systems treating swine manure. A total of 19 genera were identified as tet(X4) hosts, which were distributed in the phyla Proteobacteria, Bacteroidota, Firmicutes, and Caldatribacteriota. Sixteen genera and two phyla that were identified have not been previously reported as tet(X4) bacterial hosts. The results indicated that a far more diverse range of bacteria was involved in harboring tet(X4) than previously realized. Compared with the tet(X4) hosts determined by correlation-based network analysis and metagenomic binning, epicPCR revealed a high diversity of tet(X4) hosts even at the phylum level. The epicPCR method developed in this study could be effectively employed to reveal the presence of tet(X4) bacterial hosts from a holistic viewpoint.

Impact of soil amendments on nitrous oxide emissions and the associated denitrifying communities in a semi-arid environment

Denitrifying bacteria produce and utilize nitrous oxide (N2O), a potent greenhouse gas. However, there is little information on how organic fertilization treatments affect the denitrifying communities and N2O emissions in the semi-arid Loess Plateau. Here, we evaluated how the denitrifying communities are responsible for potential denitrification activity (PDA) and N2O emissions. A field experiment was conducted with five fertilization treatments, including no fertilization (CK), mineral fertilizer (MF), mineral fertilizer plus commercial organic fertilizer (MOF), commercial organic fertilizer (OFP), and maize straw (MSP). Our result showed that soil pH, soil organic carbon (SOC), and dissolved organic nitrogen (DON) were significantly increased under MSP treatment compared to MF treatment, while nitrate nitrogen (NO3−−N) followed the opposite trend. Organic fertilization treatments (MOF, OFP, and MSP treatments) significantly increased the abundance and diversity of nirS- and nosZ-harboring denitrifiers, and modified the community structure compared to CK treatment. The identified potential keystone taxa within the denitrifying bacterial networks belonged to the distinct genera. Denitrification potentials were significantly positively correlated with the abundance of nirS-harboring denitrifiers, rather than that of nirK- and nosZ-harboring denitrifiers. Random forest modeling and structural equation modeling consistently determined that the abundance, community composition, and network module I of nirS-harboring denitrifiers may contribute significantly to PDA and N2O emissions. Collectively, our findings highlight the ecological importance of the denitrifying communities in mediating denitrification potentials and the stimulatory impact of organic fertilization treatments on nitrogen dynamics in the semi-arid Loess Plateau.

Effect of wheat straw pretreated with liquid fraction of digestate from different substrates on anaerobic digestion performance and microbial community characteristics

The effects of liquid fraction of digestate (LFD) pretreatment on anaerobic digestion (AD) performance and microbial community characteristics were estimated. Prior to AD, LFD (LFD_SM, LFD_FW, and LFD_WS) collected separately from three continuously stirred tank reactors (CSTRs) using swine manure (SM), food waste (FW), and wheat straw (WS) as the mono-substrate was applied to pretreat WS. The results showed that AD with LFD pretreatment resulted in biomethane yields of 240.2-277.9 mL·gVS^-1, a 33.57%-54.54% improvement over the yield of the control, and also produced a time saving of 32.26%-46.77%. The pretreatment parameters were optimized for LFD pretreatment. The enhancement effect was in the order of LFD_FW > LFD_SM > LFD_WS. Simultaneously, the cellulose, hemicellulose and lignin contents in the WS and their characteristics (surface properties, crystallinity index, etc.) varied accordingly. The function of the microbial community was strengthened during the pretreatment stage, but the structure of the microbial community had a clear response to the LFD source substrates. Bacteroidetes was the most dominant phyla and was positively correlated with the hydrolysis rate. Consequently, the LFD from the different substrates used as pretreat agents could improve the AD performance of WS.

Environmental factors and interactions among microorganisms drive microbial community succession during fermentation of Nongxiangxing daqu

Nongxiangxing daqu (daqu), which is produced by the open fermentation of wheat, is the fermentation starter of baijiu. This work reports the occurrence and driving factors of microbial community succession (MCS), which determines daqu quality, during daqu fermentation. The co-occurrence networks of the 2 stages of the MCS showed that module 2 of stage P1 contained Mucoromycota and Actinobacteriota and was affected by temperature, humidity, CO₂, and moisture; module 8 of stage P2 contained Mucoromycota and Saccharomycetes and was influenced by acidity and moisture. Twenty-two genera were thebiomarkers of the MCS. The MCS was driven by temperature, humidity, CO₂, O₂, acidity,moisture and interactions between the biomarkes and Lactobacillales, Saccharomycetales, and Acetobacter. The main driving factors of the bacterial community succession were acidity, moisture, and temperature, and that of the fungal community succession was moisture. These results guide the control of MCS during daqu fermentation.

Succession of diazotroph community and functional gene response to inoculating swine manure compost with a lignocellulose-degrading consortium

Diazotroph community contributes to the nitrogen mass and improves the agronomic quality of composting product, but their responses to microbial inoculation during composting are unclear. In this study, the lignocellulose-degrading consortium was inoculated at different levels (0%: CK (control) and 10%: T) to investigate their effects on the variations in the diazotroph community and functional gene during composting. In the later composting phase, the nifH gene copy number was 17.50-25.28% higher in T than CK. The nitrogenase abundance in CK and T were 0.042% and 0.046% in composting product, respectively. Network analysis indicated that inoculation affected the co-occurrence patterns of the diazotroph community and changed the keystone species composition. Partial least-squares path modeling showed that available carbon sources and the succession of the diazotroph community mainly determined the increased abundance of nifH gene. Microbial inoculation stimulated the diazotrophs activities, and was conducive to the nitrogen production in composting product.

Simultaneous removal of nitrate and diethyl phthalate using a novel sponge-based biocarrier combined modified walnut shell biochar with Fe3O4 in the immobilized bioreactor

A novel biological carrier combining sponge and modified walnut shell biochar with Fe₃O₄ (MWSB@Fe₃O₄) was fabricated to achieve simultaneous removal of nitrate and diethyl phthalate (DEP). The optimal reaction conditions of the immobilized bioreactor were: carbon to nitrogen (C/N) ratio of 1.5, Fe²⁺ concentration of 20 mg L^-1, and hydraulic retention time (HRT) of 8 h. Under the optimal conditions and DEP concentration of 800 μg L^-1, the highest removal efficiency of DEP and nitrate in the immobilized bioreactor with the novel biological carrier were 67.87% and 83.97% (68.43 μg L^-1 h^-1 and 1.71 mg L^-1 h^-1), respectively. Scanning electron microscopy (SEM) showed that the novel biological carrier in this study carried more bio-sediments which is closely related to the denitrification efficiency. The gas chromatography (GC) data showed that the nitrogen production of the immobilized bioreactor (99.85%) was higher than that of another experimental group (97.84%). Fluorescence excitation-emission matrix (EEM) and Fourier transform infrared spectrometer (FTIR) indicated the immobilized bioreactor emerged more extracellular polymeric substances (EPS) which was related to favourable biological stability under the DEP environment. Moreover, according to high-throughput sequencing data, the Zoogloea sp. L2 responsible for iron-reduction and denitrification was the main strain in this immobilized bioreactor.

Simultaneous carbon dioxide reduction and enhancement of methane production in biogas via anaerobic digestion of cornstalk in continuous stirred-tank reactors: The influences of biochar, environmental parameters, and microorganisms

The composition of biogas produced by anaerobic digestion (AD) is typically not ideal due to high CO₂ content. In the study, cottonwood biochar was used as an enhanced mediator for the continuously stirred tank reactor AD of cornstalk. The effects of substrate loading and biochar dosage on biogas composition, volatile fatty acids (VFAs), NH₃-N, and microbial community characteristics were systematically explored. The results showed that the highest volumetric biogas production rate with biochar was 1.40 L/L/d, at the same time, the CO₂ content in the biogas decreased by 5.90%, while the CH₄ content increased by 7.40%, compared with the values in AD without biochar. Moreover, VFAs were degraded effectively, in particular, the propionic acid concentration decreased by 55.7%. Besides, microbial abundance had positive correlations with environmental parameters. This study could provide valuable information for both the elucidation of strengthening mechanisms of biochar and further large-scale engineering application.

Life Cycle Assessment of the Mesophilic, Thermophilic, and Temperature-Phased Anaerobic Digestion of Sewage Sludge

In this study the environmental impact of the anaerobic digestion (AD) of sewage sludge within an activated sludge wastewater treatment plant (WWTP) was investigated. Three alternative AD systems (mesophilic, thermophilic, and temperature-phased anaerobic digestion (TPAD)) were compared to determine which system may have the best environmental performance. Two life cycle assessments (LCA) were performed considering: (i) the whole WWTP (for a functional unit (FU) of 1 m3 of treated wastewater), and (ii) the sludge line (SL) alone (for FU of 1 m3 of produced methane). The data for the LCA were obtained from previous laboratory experimental work in combination with full-scale WWTP and literature. According to the results, the WWTP with TPAD outperforms those with mesophilic and thermophilic AD in most analyzed impact categories (i.e., Human toxicity, Ionizing radiation, Metal and Fossil depletion, Agricultural land occupation, Terrestrial acidification, Freshwater eutrophication, and Ozone depletion), except for Climate change where the WWTP with mesophilic AD performed better than with TPAD by 7%. In the case of the SL alone, the production of heat and electricity (here accounted for as avoided environmental impacts) led to credits in most of the analyzed impact categories except for Human toxicity where credits did not balance out the impacts caused by the wastewater treatment system. The best AD alternative was thermophilic concerning all environmental impact categories, besides Climate change and Human toxicity. Differences between both LCA results may be attributed to the FU.

Biological nitrogen removal via combined processes of denitrification, highly efficient partial nitritation and Anammox from mature landfill leachate

The combined processes of pre-denitrification, highly efficient partial nitritation and Anammox were developed to treat mature landfill leachate. In the partial nitritation stage, an outstanding nitrite production rate (NPR) of approximately 1.506 kg·(m³ day)^-1 of mature landfill leachate was achieved in a zeolite biological aerated filter (ZBAF) due to the inhibition of nitrite-oxidizing bacteria (NOB) by free ammonia (FA) and free nitrous acid (FNA). With respect to the nitrogen removal performance of the combined process, remarkable nitrogen removal efficiencies (NRE) and nitrogen removal rates (NRR), which exceeded 90.0% and 0.490 kg·(m³ day)^-1, respectively, were detected based on the stable and efficient partial nitritation performance and reasonable control of effluent nitrite to ammonium ratios (at approximately 1.2) in the ZBAF. High-throughput sequencing analysis further revealed that the dominant bacteria genera Paracoccus and Comamonas in the denitrification reactor, Nitrosomonas in the ZBAF and Candidatus Kuenenia and Candidatus Anammoxoglobus in the Anammox reactor were demonstrated to be responsible for denitrification, partial nitritation and Anammox process, respectively.

Reservoir water stratification and mixing affects microbial community structure and functional community composition in a stratified drinking reservoir

To investigate how the aquatic bacterial community of a stratified reservoir drives the evolution of water parameters, the microbial community structure and network characteristics of bacteria in a stratified reservoir were investigated using Illumina MiSeq sequencing technology. A total of 42 phyla and 689 distinct genera were identified, which showed significant seasonal variation. Additionally, stratified variations in the bacterial community strongly reflected the vertical gradient and seasonal changes in water temperature, dissolved oxygen, and nutrition concentration. Furthermore, principal coordinate analysis indicated that most microorganisms were likely influenced by changes in water stratification conditions, exhibiting significant differences during the stratification period and mixing period based on Adonis, MRPP, and Anosim. Compared to the stratification period, 123 enhanced operational taxonomic units (OTUs; 29%) and 226 depleted OTUs (52%) were identified during the mixing period. Linear discriminant analysis effect size results showed that 15 major genera were enriched in the mixing period and 10 major genera were enriched in the stratification period. Importantly, network analysis revealed that the keystone species belonged to hgcI_clade, CL500-29, Acidibacter, Paucimonas, Flavobacterium, Prochlorothrix, Xanthomonadales, Chloroflexia, Burkholderiales, OPB56, KI89A_clade, Synechococcus, Caulobacter or were unclassified. Redundancy analysis showed that temperature, dissolved oxygen, pH, chlorophyll-α, total phosphorus, nitrate, and ammonia were important factors influencing the water bacterial community and function composition, which were consistent with the results of the Mantel test analysis. Furthermore, random forest analysis showed that temperature, dissolved oxygen, ammonia, and total dissolved phosphorous were the most important variables predicting water bacterial community and function community α- and β-diversity (P < 0.05). Overall, these results provide insight into the interactions between the microbial community and water quality evolution mechanism in Zhoucun reservoir.

Effect of Cornstalk Biochar Immobilized Bacteria on Ammonia Reduction in Laying Hen Manure Composting

NH₃ emission has become one of the key factors for aerobic composting of animal manure. It has been reported that adding microbial agents during aerobic composting can reduce NH₃ emissions. However, environmental factors have a considerable influence on the activity and stability of the microbial agent. Therefore, this study used cornstalk biochar as carriers to find out the better biological immobilization method to examine the mitigation ability and mechanism of NH₃ production from laying hen manure composting. The results from different immobilized methods showed that NH₃ was reduced by 12.43%, 5.53%, 14.57%, and 22.61% in the cornstalk biochar group, free load bacteria group, mixed load bacteria group, and separate load bacteria group, respectively. Under the simulated composting condition, NH₃ production was 46.52, 38.14, 39.08, and 30.81 g in the treatment of the control, mixed bacteria, cornstalk biochar, and cornstalk biochar separate load immobilized mixed bacteria, respectively. The cornstalk biochar separate load immobilized mixed bacteria treatment significantly reduced NH₃ emission compared with the other treatments (p < 0.05). Compared with the control, adding cornstalk biochar immobilized mixed bacteria significantly decreased the electrical conductivity, water-soluble carbon, total nitrogen loss, and concentration of ammonium nitrogen (p < 0.05), and significantly increased the seed germination rate, total number of microorganisms, and relative abundance of lactic acid bacteria throughout the composting process (p < 0.05). Therefore, the reason for the low NH₃ emission might be due not only to the adsorption of the cornstalk biochar but also because of the role of complex bacteria, which increases the relative abundance of lactic acid bacteria and promotes the acid production of lactic acid bacteria to reduce NH₃ emissions. This result revealed the potential of using biological immobilization technology to reduce NH₃ emissions during laying hen manure composting.

Beneficial effects of bacterial agent/bentonite on nitrogen transformation and microbial community dynamics during aerobic composting of pig manure

This study investigated the effects of adding a bacterial agent (B) and bentonite (BT) on nitrogen transformation, nitrogen functional genes, and the microbial community dynamics during the aerobic composting of pig manure, as well as their contributions to NH₃ and N₂O emissions. Treatments B, BT, and BT + B reduced the NH₃ emissions by 31.34%, 18.82%, and 23.67%, respectively, and the N₂O emissions by 53.16%, 72.56%, and 63.41%. N₂O and NH₃ emissions were strongly related to the functional genes. Adding bacterial agent promoted the ammonia oxidation process to reduce NH₃ emissions, whereas the influence of bentonite on nitrogen conversion was mostly related to nirS and nirK in denitrification processes. Nitrification and denitrification were dominated by different functional microorganisms in various stages of composting, where Proteobacteria comprised the most important denitrifying microorganisms. Thus, the additives reduced NH₃ and N₂O emissions by regulating nitrification and denitrification processes, and adding both was highly advantageous.

Analysis for microbial denitrification and antibiotic resistance during anaerobic digestion of cattle manure containing antibiotic

This study investigated the effects of tylosin (0, 10, and 100 mg/kg dry weight) on the denitrification genes and microbial community during the anaerobic digestion of cattle manure. N₂ emissions were reduced and N₂O emissions were increased by 10 mg/kg tylosin. Adding 100 mg/kg tylosin increased the emission of both N₂O and N₂. The different responses of denitrifying bacteria and genes to tylosin may have been due to the presence of antibiotic resistance genes (ARGs). Network analysis indicated that denitrification genes and ARGs had the same potential host bacteria. intI1 was more important for the horizontal transfer of denitrification genes and ARGs during anaerobic digestion than intI2. The anaerobic digestion of manure containing tylosin may increase nitrogen losses and the associated ecological risk.

Treatment of polluted surface water with nylon silk carrier-aerated biofilm reactor (CABR)

Carrier aerated biofilm reactor (CABR) with nylon silk as the biofilm growth carrier was constructed to treatment of polluted surface water, which could improve the practical application in comparison with MABR process. The results show that CABR process can effectively improve the self-purification capacity of the polluted surface water, efficient removal of COD and NH₃-N, making water quality achieve the level V of Environmental Quality Standards for Surface Water (GB 3838-2002, China). Modified nylon silk can alter the community structures and increase bacteria during CABR process operation. Large pore size of nylon silk leads to the formation of special biofilm structure in CABR. Extracellular polymer (EPS) and membrane fouling resistance distribution indicated that the nylon silk fouling control ability of CABR reactor is much higher than that of membrane-aerated biofilm reactors (MABR). The results show that the CABR process can effectively purify surface water and improve the practical application.

Merely inoculating anammox sludge to achieve the start-up of anammox and autotrophic desulfurization-denitrification process

Anammox and autotrophic desulfurization-denitrification (AADD) process is feasible for the nitrogen and sulfide removal in the same reactor, and the influence of excess nitrate produced by anammox could also be alleviated simultaneously. This study firstly proposed a novel strategy with inoculating single anammox sludge to start up the AADD process. Results demonstrated that the 90% nitrogen removal efficiency (NRE), 2.55kgm^-3 d^-1 nitrogen removal rate (NRR), and 95% sulfide removal efficiency (SRE) were obtained at the influent total nitrogen of 280mgL^-1 and sulfide of 221.5mgL^-1, and the final effluent nitrate concentration was as low as 8mgL^-1 under the appropriate operation conditions. Tryptophan-like and protein-like substances were characterized as the main components in bound EPS. Thiobacillus (35.68%) and Pseudoxanthomonas (11.61%) were identified as the predominant genera. This study will pave a potential avenue to promote the treatment of high concentration nitrogen and sulfide in wastewater in the future.

Enhanced denitrification by nano ɑ-Fe2O3 induced self-assembled hybrid biofilm on particle electrodes of three-dimensional biofilm electrode reactors

Three-dimensional biofilm electrode reactors (3D-BERs) represent a novel technology for wastewater denitrification. Formation of mature electroactive biofilm on particle electrodes is crucial to realize successful denitrification in 3D-BERs. However, long start-up time and low electroactivity of the biofilm formed on particle electrodes limit the further application of 3D-BERs in wastewater treatment. In this work, self-assembled hybrid biofilms (SAHB) was cultivated on granular activate carbon particle electrodes of the 3D-BER by assembling nano ɑ-Fe₂O₃ into the biofilm. ɑ-Fe₂O₃ was selected due to its high affinity to bacterial outer-membrane cytochromes, an important mediator for microbial electron transfer. SAHB formed on particle electrodes were characterized and the denitrification performance of 3D-BERs was also investigated. Results indicate that nano ɑ-Fe₂O₃ plays positive roles in the start-up of 3D-BER, which captures more microbes into SAHB and constructs thick biofilm on particle electrodes. Special microorganisms with denitrification function related with genera of Hydrogenophaga and Opitutus are distinctively enriched in SAHB. Nano ɑ-Fe₂O₃ induced SAHB exhibit superior denitrification performance compared to natural biofilm. The average denitrification rate increases from 0.62 mg total nitrogen/L/h for natural biofilm to 1.73 mg total nitrogen/L/h for SAHB, mainly ascribed to accelerated nitrites reduction. Our work provides new technical solution to enhance nitrates removal in 3D-BERs and brings deep insights into application of bio-electrochemical system in wastewater treatment.

Temperature dependence of denitrification microbial communities and functional genes in an expanded granular sludge bed reactor treating nitrate-rich wastewater

The temperature dependence of denitrification was investigated for high nitrate nitrogen denitrification in an expanded granular sludge bed (EGSB) reactor. The optimal reaction temperatures were 15-35 °C in which nearly complete denitrification was achieved with the removal of COD maintained over 80%. Nitrite accumulation was observed at 10 °C indicating the incomplete denitrification at low temperature. However, almost complete denitrification was even accomplished as high as 52 °C. High-throughput sequencing detected a total of 84 bacterial genera and 7 phyla, and temperature variation resulted in the shift of microbial community structure and diversity. Proteobacteria thrived while Firmicutes and Bacteroidetes were inhibited by temperature stress. The predominance of Halomonas and the significant decrease of Azoarcus at low temperature indicated a more important role of these two genera in denitrification in an EGSB reactor. The results of qPCR indicated that temperature exerted effects on the abundance of denitrification function genes, nirK, nirS, narG, and nosZ, due to the shift of the bacterial community. This study provided a comprehensive understanding of temperature effects on the denitrification process in an EGSB reactor treating high concentration nitrate wastewater.