Denitrifying microbial community with the ability to bromate reduction in a rotating biofilm-electrode reactor

Robust Activity and Stability of P-Doped Fe-Carbon Composites Derived from MOF for Bromate Reduction

Iron-based materials are effective for the reductive removal of the disinfection byproduct bromate in water, while the construction of highly stable and active Fe-based materials with wide pH adaptability remains greatly challenging. In this study, highly dispersed iron phosphide-decorated porous carbon (Fe2P(x)@P(z)NC-y) was prepared via the thermal hydrolysis of Fe@ZIF-8, followed by phosphorus doping (P-doping) and pyrolysis. The reduction performances of Fe2P(x)@P(z)NC-y for bromate reduction were evaluated. Characterization results showed that the Fe, P, and N elements were homogeneously distributed in the carbonaceous matrix. P-doping regulated the coordination environment of Fe atoms and enhanced the conductivity, porosity, and wettability of the carbonaceous matrix. As a result, Fe2P(x)@P(1.0)NC-950 exhibited enhanced reactivity and stability with an intrinsic reduction kinetic constant (kint) 1.53-1.85 times higher than Fe(x)@NC-950 without P-doping. Furthermore, Fe2P(0.125)@P(1.0)NC-950 displayed superior reduction efficiency and prominent stability with very low Fe leaching (4.53-22.98 μg L-1) in a wide pH range of 4.0-10.0. The used Fe2P(0.125)@P(1.0)NC-950 could be regenerated by phosphating, and the regenerated Fe2P(0.125)@P(1.0)NC-950 maintained 85% of its primary reduction activity after five reuse cycles. The study clearly demonstrates that Fe2P-decorated porous carbon can be applied as a robust and stable Fe-based material in aqueous bromate reduction.

Biological bromate reduction coupled with in situ gas fermentation in H2/CO2-based membrane biofilm reactor

Bromate, a carcinogenic contaminant generated in water disinfection, presents a pressing environmental concern. While biological bromate reduction is an effective remediation approach, its implementation often necessitates the addition of organics, incurring high operational costs. This study demonstrated the efficient biological bromate reduction using H2/CO2 mixture as the feedstock. A membrane biofilm reactor (MBfR) was used for the efficient delivery of gases. Long-term reactor operation showed a high-level bromate removal efficiency of above 95 %, yielding harmless bromide as the final product. Corresponding to the short hydraulic retention time of 0.25 d, a high bromate removal rate of 4 mg Br/L/d was achieved. During the long-term operation, in situ production of volatile fatty acids (VFAs) by gas fermentation was observed, which can be regulated by controlling the gas flow. Three sets of in situ batch tests and two groups of ex situ batch tests jointly unravelled the mechanisms underpinning the efficient bromate removal, showing that the microbial bromate reduction was primarily driven by the VFAs produced from in situ gas fermentation. Microbial community analysis showed an increased abundance of Bacteroidota group from 4.0 % to 18.5 %, which is capable of performing syngas fermentation, and the presence of heterotrophic denitrifiers (e.g., Thauera and Brachymonas), which are known to perform bromate reduction. Together these results for the first time demonstrated the feasibility of using H2/CO2 mixture for bromate removal coupled with in situ VFAs production. The findings can facilitate the development of cost-effective strategies for groundwater and drinking water remediation.

Enhanced removal of phenanthrene and nutrients in wetland sediment with metallic biochar: Performance and mechanisms

Polycyclic aromatic hydrocarbon (PAHs) are persistent organic pollutants and pose high risk in aquatic environment. The utilization of biochar is a strategy for PAHs-contaminated remediation but is challenging due to the adsorption saturation and reoccurrence of PAHs desorbed back into water. In this study, iron (Fe) and manganese (Mn) were provided as electron acceptors for biochar modification to enhance anaerobic biodegradation of phenanthrene (Phe). Results revealed that, the Mn(Ⅳ) and Fe(Ⅲ) modification improved the removal of Phe by 24.2% and 31.4% than that of biochar, respectively. Additionally, nitrate removal was improved by 19.5% with Fe(Ⅲ) amendment. The Mn-and Fe-biochar decreased Phe contents by 8.7% and 17.4% in sediment, 10.3% and 13.8% in biochar than that of biochar. Much higher DOC contents were observed with Mn- and Fe-biochar, which provided bioavailable carbon source for microbes and contributed to microbial degradation of Phe. The greater degree of humification, higher proportions of humic and fulvic acid like components in metallic biochar participated in electron transport and further enhancing the degradation of PAHs. Microbial analysis proved the high abundance of Phe-degrading bacteria (e.g. PAH-RHD_α, Flavobacterium and Vibrio), nitrogen removal microbes (e.g. amoA, nxrA, and nir), Fe and Mn bioreduction or oxidation (e. g. Bacillus, Thermomonas, Deferribacter) with metallic biochar. Based on the results, the Fe and Mn modification, especially Fe-modified biochar provided well performance for PAHs removal in aquatic sediment.

Seasonal variations of microbial community structure, assembly processes, and influencing factors in karst river

The physicochemical properties and microbial communities have significant annual and seasonal changes in karst aquifers. To explore the changes of microbial community and their relationships with environmental factors, water samples were collected from a typical karst river. Microbial communities in winter (Jan-2017 and Jan-2019) were stable with high similarity in spite of the 2 years sampling interval, but the microbial communities in Aug-2017 was different from that in Aug-2018. In four sampling times, there were 275 shared genera, whose average relative abundance ranging from 89.04 to 96.27%. The winter and summer specific genera were mainly from the recharge of tributary site K6 and discharge of waste water treatment plant (K2 and K3), respectively. The deterministic processes had a more significant effect on the microbial community assembly in winter than that in summer, which was affected by environmental pressure from pollution. Furthermore, antibiotics and inorganic nitrogen pollution affected element cycles of nitrogen and sulfur indirectly through microbial ecological modules in karst river, and the denitrification and desulfurization processes were potentially inhibited. These findings contributed to understand the changes and its assembly mechanism of microbial community, as well as the feedback to environment in polluted karst river.

Spatial and temporal conversion of nitrogen using Arthrobacter sp. 24S4-2, a strain obtained from Antarctica

According to average nucleotide identity (ANI) analysis of the complete genomes, strain 24S4-2 isolated from Antarctica is considered as a potential novel Arthrobacter species. Arthrobacter sp. 24S4-2 could grow and produce ammonium in nitrate or nitrite or even nitrogen free medium. Strain 24S4-2 was discovered to accumulate nitrate/nitrite and subsequently convert nitrate to nitrite intracellularly when incubated in a nitrate/nitrite medium. In nitrogen-free medium, strain 24S4-2 not only reduced the accumulated nitrite for growth, but also secreted ammonia to the extracellular under aerobic condition, which was thought to be linked to nitrite reductase genes nirB, nirD, and nasA by the transcriptome and RT-qPCR analysis. A membrane-like vesicle structure was detected in the cell of strain 24S4-2 by transmission electron microscopy, which was thought to be the site of intracellular nitrogen supply accumulation and conversion. This spatial and temporal conversion process of nitrogen source helps the strain maintain development in the absence of nitrogen supply or a harsh environment, which is part of its adaption strategy to the Antarctic environment. This process may also play an important ecological role, that other bacteria in the environment would benefit from its extracellular nitrogen source secretion and nitrite consumption characteristics.

Bromate reduction by Shewanella oneidensis MR-1 is mediated by dimethylsulfoxide reductase

Microbial bromate reduction plays an important role in remediating bromate-contaminated waters as well as biogeochemical cycling of bromine. However, little is known about the molecular mechanism of microbial bromate reduction so far. Since the model strain Shewanella oneidensis MR-1 is capable of reducing a variety of oxyanions such as iodate, which has a high similarity to bromate, we hypothesize that S. oneidensis MR-1 can reduce bromate. Here, we conducted an experiment to investigate whether S. oneidensis MR-1 can reduce bromate, and report bromate reduction mediated by a dimethylsulfoxide reductase encoded with dmsA. S. oneidensis MR-1 is not a bromate-respiring bacterium but can reduce bromate to bromide under microaerobic conditions. When exposed to 0.15, 0.2, 0.25, 0.5, and 1 mM bromate, S. oneidensis MR-1 reduced bromate by around 100, 75, 64, 48, and 23%, respectively, within 12 h. In vivo evidence from gene deletion mutants and complemented strains of S. oneidensis MR-1 indicates that MtrB, MtrC, CymA, GspD, and DmsA are involved in bromate reduction, but not NapA, FccA, or SYE4. Based on our results as well as previous findings, a proposed molecular mechanism for bromate reduction is presented in this study. Moreover, a genomic survey indicates that 9 of the other 56 reported Shewanella species encode proteins highly homologous to CymA, GspD, and DmsA of S. oneidensis MR-1 by sequence alignment. The results of this study contribute to understanding a pathway for microbial bromate reduction.

Model Evaluation of the Microbial Metabolic Processes in a Hydrogen-Based Membrane Biofilm Reactor for Simultaneous Bromate and Nitrate Reduction

The H₂-based membrane biofilm reactor (H₂-MBfR) has been acknowledged as a cost-effective microbial reduction technology for oxyanion removal from drinking water sources, but it remains unknown how the evolution of biofilm characteristics responds to the changing critical operating parameters of the H₂-MBfR for simultaneous bromate (BrO₃^-) and nitrate (NO₃^-) elimination. Therefore, an expanded multispecies model, applicable to mechanistically interpret the bromate-reducing bacteria (BRB)- and denitrifying bacteria (DNB)-dominated metabolic processes in the biofilm of the H₂-MBfR, was developed in this study. The model outputs indicate that (1) increased BrO₃^- loading facilitated the metabolism of BRB by increasing BRB fraction and BrO₃^- gradients in the biofilm, but had a marginal influence on NO₃^- reduction; (2) H₂ pressure of 0.04 MPa enabled the minimal loss of H₂ and the extension of the active region of BRB and DNB in the biofilm; (3) once the influent NO₃^- concentration was beyond 10 mg N/L, the fraction and activity of BRB significantly declined; (4) BRB was more tolerant than DNB for the acidic aquatic environment incurred by the CO₂ pressure over 0.02 MPa. The results corroborate that the degree of microbial competition for substrates and space in the biofilm was dependent on system operating parameters.

Enhanced production of short-chain fatty acids from sludge by thermal hydrolysis and acidogenic fermentation for organic resource recovery

Acidogenic fermentation (FM treatment) converts organics in waste sludge to valuable short-chain fatty acids (SCFAs). To maintain a favorable condition for the production of SCFAs, an alkali is often added continuously to maintain an alkaline pH in the fermenter. However, this chemical adjustment is costly and biotic hydrolysis is slow. In this research, thermal hydrolysis (TH) was introduced as a pretreatment to enhance fermentation and SCFA production. The results were compared with those obtained from the untreated sludge that underwent fermentation with a daily pH 10 adjustment (NT-FM_pH10). The TH pretreatment resulted in rapid abiotic hydrolysis within a short period (1 h), releasing more than 30.5% of organics into the liquid phase of the sludge. These dissolved organics in sludge promoted rapid acidogenesis and SCFA production. TH together with a one-time alkali pretreatment further increased the production of SCFAs during sludge fermentation (TH&Alk-FM): it produced 22.8% more SCFAs than the non-treated NT-FM_pH10 sludge with alkaline pH control during fermentation. Semicontinuous fermentation further showed the advantage of the TH&Alk-FM process, as a rapid and high production of SCFAs was achieved when the fermentation time was shortened from 5 d to 2 d. The microbial community analysis revealed that TH&Alk-FM and NT-FM_pH10 sludge samples had simple but varied microbial communities. The dominant genera in the TH&Alk-FM sludge were unclassified Ruminococcaceae (18.9%) and unclassified Porphyromonadaceae (22.3%), belonging to the classes Clostridia and Bacteroidia, respectively. NT-FM_pH10 was dominated by Tissierella (23.7%) and Proteiniborus (13.5%), which belong to Clostridia. Compared with NT-FM_pH10, the microbial consortia in TH&Alk-FM were supplied with sufficient soluble organics and performed better in fermentation and SCFA production, without the need for the daily alkali addition to control pH.

Transcriptome analysis provides new insights into the tolerance and aerobic reduction of Shewanella decolorationis Ni1-3 to bromate

As a possible human carcinogen, bromate is easily formed in drinking water and wastewater treatments using advanced oxidation technology. Microbial reduction is a promising method to remove bromate, but little is known about aerobic bromate reduction as well as the molecular mechanism of tolerance and reduction to bromate in bacteria. Herein, bromate reduction by isolate under aerobic conditions was reported for the first time. Shewanella decolorationis Ni1-3, isolated from an activated sludge recently, was identified to reduce bromate to bromide under both aerobic and anaerobic conditions. RNA-Seq together with differential gene expression analysis and Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analysis was performed to identify that bromate triggered the expression of genes for oxidative stress protection (e.g., ohr, msrQ, dsbC, gpo, gorA, and gst), DNA damage repair (e.g., dprA, parA, and recJ), and sulfur metabolism (e.g., cysH, cysK, and cysP). However, the genes for lactate utilization (e.g., lldF and dld), nitrate reduction (e.g., napA and narG), and dissimilatory metal reduction (e.g., mtrC and omcA) were down-regulated in the presence of bromate. The results contribute to revealing the molecular mechanism of resistance and reduction in S. decolorationis Ni1-3 to bromate under aerobic conditions and clarifying the biogeochemical cycle of bromine. KEY POINTS: • Aerobic bromate reduction by pure culture was observed for the first time • Strain Ni1-3 effectively reduced bromate under both aerobic and anaerobic conditions • ROS and SOS response genes were strongly induced in the presence of bromate.

Isolation of Anaerobic Bromate-Reducing Bacteria Using Different Carbon Sources and Transcriptomic Insights From Klebsiella variicola Glu3

Bromate, a possible human carcinogen, can be reduced to innocuous bromide by microorganisms. To characterize bromate reducers, microbes were enriched anaerobically from activated sludge by using bromate as the sole electron acceptor and different carbon sources as the electron donor. Bacteria that showed significant bromate-reducing activity but not coupled to cell growth were isolated. Two whole genomes of the isolates, namely, Raoultella electrica Lac1 and Klebsiella variicola Glu3, were reconstructed by Illumina and Nanopore sequencing. Transcriptomic analysis suggested that neither the respiratory nitrate reductase, the selenate reductase, nor the dimethylsulfoxide reductase was involved in the bromate reduction process, and strain K. variicola Glu3 reduced bromate via a yet undiscovered enzymatic mechanism. The results provide novel phylogenetic insights into bromate-reducing microorganisms and clues in putative genes encoding enzymes related to bromate reduction.

Re-evaluation of the environmental hazards of nZnO to denitrification: Performance and mechanism

Numerous studies have shown that zinc oxide nanoparticles (nZnO) have an inhibitory effect on wastewater biotreatment, where doses exceeding ambient concentrations are used. However, the effect of ambient concentrations of ZnO (<1 mg/L) on anaerobic digestion processes is not clear. Herein, this study comprehensively explored the impact of nZnO on the denitrification performance and core microbial community of activated sludge under ambient concentrations. Results showed that only 0.075 mg/L nZnO had shown a beneficial effect on nitrogen removal by activated sludge. When nZnO concentration reached 0.75 mg/L, significant enhancement of nitrate reduction and mitigation of nitrite accumulation were observed, indicating a remarkable stimulatory effect on nitrogen removal. Simultaneously, nZnO could weaken the sludge surface charge and improve the secretion of extracellular polymeric substances, thus enhancing sludge flocculation for denitrification. Microbial community analysis revealed that nZnO exposure increased the relative abundance of denitrifying bacteria, which could contribute to the reinforcement of traditional denitrification. Furthermore, exogenous addition of NH₄⁺ significantly inhibited the accumulation of nitrite, implying that nZnO had a potential to improve the denitrification process via a partial denitrification-anammox pathway. Considering current ambient concentration, the stimulatory effect shown in our work may better represent the actual behavior of ZnO in wastewater biotreatment.

Facial fabricated biocompatible homogeneous biocarriers involving biochar to enhance denitrification performance in an anoxic moving bed biofilm reactor

Biochar prepared from pineapple peel was facially combined with polyurethane sponges for the first time to form homogeneous biocompatible biocarriers, which can enhance denitrification performance in an anoxic MBBR. The experiments showed that a higher NO₃^--N removal efficiency (96.24 ± 1.3%) and kinetic constant (0.26 h^-1) were obtained in the MBBR employing these new biocarriers (B-MBBR), compared with a control MBBR with polyurethane sponges (C-MBBR). The attached and suspended biomass of the B-MBBR was increased by 47% and 26%, respectively. Biochar significantly enhanced the abundance of functional bacteria in terms of promoting biofilm (i.e., Leptonema), denitrifying bacteria (i.e., Thauera, Enterobacter and Pseudomonas) and electroactive bacteria (i.e., Geobacter) in the B-MBBR. Meanwhile, based on the content of coenzyme I (NADH) and denitrifying enzymes, biochar would also enhance electron transport activity for denitrification. Consequently, these facial prepared biocarriers are effective to enhance denitrification performance in MBBR with application significance.

Fate and reduction of bromate formed in advanced water treatment ozonation systems: A critical review

Disinfection in water treatment and reclamation systems eliminates the potential health risks associated with waterborne pathogens, however it may produce disinfection by-products (DBPs) harmful to human health. Potentially carcinogenic bromate is a DBP formed during the ozonation of bromide-containing waters. To mitigate the problem of bromate formation, different physical/chemical or biological reduction methods of bromate have been investigated. Until now, adsorption-based physical method has proven to be more effective than chemical methods in potable water treatment. Though several studies on biological reduction methods have been carried out in a variety of bioreactor systems, such as in biologically active carbon filters and denitrifying bioreactors, the microbiological mechanisms or biochemical pathways of bromate minimization have not been clearly determined to date. Genetic analysis could provide a broader picture of microorganisms involved in bromate reduction which might show cometabolic or respiratory pathways, and affirm the synergy functions between different contributing groups. The hypothesis established from the diffusion coefficients of different electron donor and acceptors, illustrates that some microorganisms preferring bromate over oxygen contain specific enzymes which lower the activation energy required for bromate reduction. In addition, considering microbial bromate reduction as an effective treatment strategy; field scale investigations are required to observe quantitative correlations of various influencing parameters such as pH, ozone dose, additives or constituents such as ammonia, hydrogen peroxide, and/or chloramine, dissolved organic carbon levels, dissolved oxygen gradient within biofilm, and empty bed contact time on bromate removal or reduction.

Current intensities altered the performance and microbial community structure of a bio-electrochemical system

A novel integrated bio-electrochemical system with sulfur autotrophic denitrification (SAD) and electrocoagulation (BESAD-EC) system was established to remove nitrate (NO₃^--N) and phosphorus from contaminated groundwater. The impacts of a current intensity gradient on the system's performance and microbial community were investigated. The results showed that NO₃^--N and total phosphorus (TP) could be effectively removed with maximum NO₃^--N reduction and TP removal efficiencies of 94.2% and 75.8% at current intensities of 200 and 400 mA, respectively. Lower current intensities could improve the removal efficiencies of NO₃^--N (≤200 mA) and phosphorus (≤400 mA), while higher current intensity (600 mA) caused the inhibition of nutrients removal in the system. MiSeq sequencing analysis revealed that low electrical stimulation improved the diversity and richness of microbial community, while high electrical stimulation reduced their diversity and richness. The relative abundance of some genus involved in denitrification and phosphorus removal processes such as Rhizobium, Hydrogenophaga, Denitratisoma and Gemmobacter, significantly (P < 0.05) reduced under high current conditions. This could be one of the main reasons for the deterioration of denitrification and phosphorus removal performance. The results of this study could be helpful to enhance the nutrient removal performance of bio-electrochemical systems in groundwater treatment processes.

Electro-assisted autohydrogenotrophic reduction of perchlorate and microbial community in a dual-chamber biofilm-electrode reactor

The electro-assisted autohydrogenotrophic reduction of perchlorate (ClO₄^-) was investigated in a dual-chamber biofilm-electrode reactor (BER), in which the microbial community was inoculated from natural sediments. To avoid the effect of extreme pH and direct electron transfer on perchlorate reduction, a novel cathode configuration was designed. The pH of the cathode compartment was successfully controlled in the range of 7.2-8.4 during whole experiment. The effective biological autohydrogenotrophic reduction of perchlorate was achieved using hydrogen generated in-situ on the electrode surface, and the removal rate of 10 mg L^-1 perchlorate reached 98.16% at HRT of 48 h. The highest perchlorate removal flux reached to 1498.420 mg m^-2·d^-1 with a 0.410 kW·h g-perchlorate^-1 energy consumption. The microbial community evolution in the BER was determined by high-throughput sequencing and the results indicated that the Firmicutes and Bacteroidetes were dominant at phylum level when perchlorate concentration was 10 mg L^-1 or lower. And the Proteobacteria became ascendant at the perchlorate concentration of 20 mg L^-1. The functional populations for perchlorate reduction were successfully enriched including Nitrosomonas (30%), Thermomonas (9%), Comamonas (8%) and Hydrogenophaga (3%). Meanwhile, the proportion of functional population in biofilm linked to perchlorate concentration. With the increase of influent perchlorate concentration, the perchlorate-reducing bacteria (PRB) were enriched successfully and became ascendant.

Bromate removal from water by acid activated and surfactant enriched Red Mud - the case of cooling water

Red Mud is a hazardous by-product of the Bayer process, used to produce alumina from bauxite, with ability to adsorb anions from water. Acid activation and enrichment with CetylTrimethylAmmonium Chloride (CTAC), a cationic surfactant, are employed to enable it to remove bromate initially from spiked double-distilled water. CTAC enrichment is found to substantially improve Red Mud's bromate removal ability in comparison to acid activation alone. Fourier Transformation Infrared Spectroscopy is used to evaluate the effectiveness of the enrichment process. Maximum CTAC loading is 0.037 g per g acid activated Red Mud (AARM). Adsorption is faster after CTAC enrichment. pH increase is found to adversely affect both AARM and acid activated CTAC enriched Red Mud's (CTAC-AARM) bromate removal capability, yet CTAC-AARM's ability proves more resistant to pH changes. Adsorption data fit best the Langmuir isotherm model for both adsorbers. The R² values for AARM and CTAC-AARM are 0.955 and 0.964 respectively. Maximum adsorbable bromate quantity is almost 2.5 times higher for CTAC-AARM in comparison to AARM. Finally, both Red Mud adsorbers are compared with respect to their ability to remove bromate from cooling water; an industrial matrix rich in competing ions. As cycles of concentration and pH appreciate, bromate adsorption is hindered regardless of the adsorber used. However, CTAC-AARM still performs better in removing bromate. It is proven that after suitable processing, Red Mud can re-enter the industrial cycle by playing a role in bromate removal from industrial waters.

Effect of heterotrophic anodic denitrification on anolyte pH control and bioelectricity generation enhancement of bufferless microbial fuel cells

Heterotrophic anodic denitrification (HAD) in the single-chamber microbial fuel cell (MFC) is a promising nitrogen removal technology. In this paper, the benefit (anolyte pH increase) and challenge (substrate consumption) brought by the heterotrophic anodic denitrification process for the electricity generation of bufferless MFCs were studied for the first time. Substrate anaerobic hydrolysis dramatically decreased the anolyte pH to 5.1, which seriously restricted the electric power output of the Control. The anolyte pH of the heterotrophic anodic denitrification MFCs (HADMFCs) with 60 mg/L (HADMFC-60), 90 mg/L (HADMFC-90), and 120 mg/L (HADMFC-120) nitrate nitrogen (NO₃^--N), retained above 6.0, 6.5, and 6.8 in every running cycles, due to the protons (H⁺) consumption by nitrate reduction. In the HADMFC-60 and HADMFC-90, 17.6% and 26.1% of the total organic carbons (TOC) were used for the nitrate reduction, but their electric power output significantly increased. The maximum power densities of the HADMFC-60 and HADMFC-90 were 3.3 and 5.4 times higher than that of the Control. However, when the proportion of TOC consumption for nitrate reduction increased to 35.8%, substrate insufficiency became a serious limitation for the electricity generation. The P_max of the HADMFC-120 dramatically decreased to 17.3 mW/m². Dysgonomonas was the dominant electro-active genus, and Petrimonas, Acidovorax and Devosia appeared as the denitrifying bacteria genera.

Microbial network succession along a current gradient in a bio-electrochemical system

A lab-scale three dimensional biofilm-electrode reactor (3DBER) coupled with sulfur/iron (3DBER-Fe/S) system was established to examine the impacts of current gradient on the performances and microbial network dynamics. Results showed that generally low current could promote nitrogen and phosphorus removal, while high current caused the inhibition of nutrients removal. Molecular ecological network (MEN) analysis showed that the current altered the overall architecture of the networks, and low currents could improve the scale and complexity of networks (<100 mA), while high current (≥100 mA) likely decrease the networks scale and complexity. Stronger competition was observed among Proteobacteria and Chloroflexi at high current conditions, which may be relevant to the deterioration of nutrients removal. In addition, the current dramatically altered the network interactions among denitrifiers, and the keystone species were intensively dynamic among various networks under the current gradient.

Synchronous degradation of aqueous benzotriazole and bromate reduction in catalytic ozonation: Effect of matrix factor, degradation mechanism and application strategy in water treatment

Ozone-based technologies are used for micro-pollutants removal in wastewater treatment. However, the generation of the toxic by-product bromate (BrO₃^-) is of a great concern. LaCoO₃ (LCO) catalytic ozonation has been used to overcome this significant drawback in the sole ozonation, achieving better BrO₃^- elimination efficiency. However, a key challenge is how to enhance micro-pollutant (benzotriazole, BZA) degradation efficiency and to eliminate formed BrO₃^- synchronously under various water qualities in drinking water or wastewater treatment. Therefore, the objective of this study is to propose a practical strategy of BZA removal and BrO₃^- reduction synchronously in water or wastewater treatment. In this study, important factors influencing BZA removal and BrO₃^- reduction were investigated, including [catalyst], [BZA], initial pH solution, [NH₄⁺-N] and [(bi)carbonate alkalinity]. Based on the performance and mechanism of these effects, a practical strategy for BZA degradation and BrO₃^- elimination with and without Br^- in the influent was developed. Additionally, the density functional theory (DFT) calculation successfully predicted the attack site on BZA by molecular ozone and formed hydroxyl radical (HO·) during LCO catalytic ozonation. Fukui indexes of f⁺ and f⁰ were calculated to forecast direct ozone molecule and HO· attack, respectively. Combination of DFT calculation with intermediates that identified through liquid chromatography-quadrupole time-of-flight mass spectrometry (LC-Q-TOF-MS), BZA degradation pathway was established more accurately. Additionally, four new intermediates were identified in this study. Overall, this study proposes a useful strategy for synchronous micro-pollutants degradation and BrO₃^- elimination, while also suggesting the feasibility of LCO catalytic ozonation for water or wastewater purification.

Energy recovery and carbon/nitrogen removal from sewage and contaminated groundwater in a coupled hydrolytic-acidogenic sequencing batch reactor and denitrifying biocathode microbial fuel cell

Developing cost-effective technology for treatment of sewage and nitrogen-containing groundwater is one of the crucial challenges of global water industries. Microbial fuel cells (MFCs) oxidize organics from sewage by exoelectrogens on anode to produce electricity while denitrifiers on cathode utilize the generated electricity to reduce nitrogen from contaminated groundwater. As the exoelectrogens are incapable of oxidizing insoluble, polymeric, and complex organics, a novel integration of an anaerobic sequencing batch reactor (ASBR) prior to the MFC simultaneously achieve hydrolytic-acidogenic conversion of complex organics, boost power recovery, and remove Carbon/Nitrogen (C/N) from the sewage and groundwater. The results obtained revealed increases in the fractions of soluble organics and volatile fatty acids in pretreated sewage by 52 ± 19% and 120 ± 40%, respectively. The optimum power and current generation with the pretreated sewage were 7.1 W m^-3 and 45.88 A m^-3, respectively, corresponding to 8% and 10% improvements compared to untreated sewage. Moreover, the integration of the ASBR with the biocathode MFC led to 217% higher carbon and 136% higher nitrogen removal efficiencies compared to the similar system without ASBR. The outcomes of the present study represent the promising prospects of using ASBR pretreatment and successive utilization of solubilized organics in denitrifying biocathode MFCs for simultaneous energy recovery and C/N removal from both sewage and nitrate nitrogen-contaminated groundwater.

Synergistic adsorption and electrocatalytic reduction of bromate by Pd/N-doped loofah sponge-derived biochar electrode

In this work, a novel Pd/N-doped loofah sponge-derived biochar (Pd/NLSBC) material with three-dimensional (3D) network structure was prepared through the carbonization-impregnation method and applied as cathode for electrocatalytic bromate removal. The N-doped biochar not only increased the adsorption capacity of electrode, but also facilitated electron transfer, subsequently resulting in the high electrocatalytic activity for bromate removal. The results indicated higher bromate adsorption capacity of Pd/NLSBC electrode was favorable to the electrocatalytic bromate removal. The influences of significant operating factors including calcination temperature, initial solution pH, applied current intensity, and initial bromate concentration on electrocatalytic bromate removal were also optimized. Under the current intensity of 10 mA, Pd/NLSBC-800 exhibited the highest bromate removal efficiency (96.7 %) and the bromide conversion rate reached almost 100 % at the initial bromate concentration of 0.781 μmol L^-1. This process could be effectively performed over a wide range of pH (2.0-9.0) and be well fitted to the pseudo-first-order kinetic model under different conditions. The reaction mechanism study indicated that both direct electron transfer and indirect reduction by the active hydrogen atom (H*) contributed to the elctrocatalytic bromate removal. Meanwhile, Pd/NLSBC-800 electrode could maintain its high electrocatalytic activity for bromate removal after five cycles.

Performance and microbial characterization of aerobic granular sludge in a sequencing batch reactor performing simultaneous nitrification, denitrification and phosphorus removal with varying C/N ratios

The mechanism and effect of C/N ratios on the aerobic granules simultaneous nitrification, denitrification and phosphorus removal (SNDPR) system are still unclear. The reactor performance and microbial community dynamics of the system were investigated under variable C/N ratios (20, 10 and 5). The COD, TP and NH₄⁺-N removal remained unaffected with variable C/N ratios. The decreased C/N ratio of five strongly influenced the nitrogen removal. Further investigations revealed that Candidatus_Accumulibacter, Acinetobacter, Candidatus_Competibacter were the predominant genera. Classification of key groups involved in nitrogen and phosphorus removal indicated the lowest C/N ratio resulting in a large microbial community shift. This study might contribute to the application of SNDPR system for the treatment of wastewater. Different C/N ratios led to shift on the microbial community and the dominant was phosphorus-accumulating bacteria. The nitrogen removal efficiency decreased while the removal of COD, TP and NH₄⁺-N remained remarkable with the decreased C/N ratios.

Phylogenetic characterization of bromate-reducing microbial community enriched anaerobically from activated sludge

Bromate is a weakly carcinogenic disinfection by-product generated from naturally existing bromide during ozonation. In order to uncover the phylogenetic diversity of bromate-reducing aquatic bacteria, the microbial mixed cultures were enriched anaerobically from various activated sludge samples with a high concentration of 3.6 mmol/L bromate which served as the electron acceptor in batch-fed reactors. Then the phylogenetic diversity was characterized by employing 16S rRNA gene-based clone libraries and high-throughput sequencing. Results showed that as highest as 48.65 mg/L [BrO₃^--Br] (0.61 mmol/L) could be reduced to Br^- within an operational cycle of 48 h, indicating the presence of bromate-reducing bacteria. The microbial analysis based on 16S rRNA gene clone libraries indicated that the dominant bacteria in the enriched consortium were affiliated to the genera of Aeromonas, which had been reported and implied its functional application to reduce bromate. In addition, some other subdivisions of bromate-reducing bacteria were enriched at different operational cycles. The abundance of the genus Rivibacter belonging to Comamonadaceae were increased after 10 cycles enrichment, which might represent another type of novel bromate bio-reducers. The study provided new insights for the phylogenetic diversity of the bromate-reducing microorganisms while further researches are required to verify the bromate bio-reduction mechanism.

Microbial reduction of bromate: current status and prospects

Bromate is a disinfection byproduct (DBP) that forms during the ozonation of bromide-containing natural water, which may cause health risks to humans. In this review, we provide an overview of the mechanism of bromate formation, microbial communities and bioreactors that are responsible for bromate reduction. Bromate can be formed through two pathways of bromide oxidation by ozone or by ^·OH, and it can be removed by biological approaches. Members belonging to phyla of Spirochaetes, Proteobacteria, Firmicutes, Actinobacteria, Clostridium, Deinococcus-Thermus and Bacteroidetes have been identified as capable of reducing bromate to bromide. Multiple configurations of biofilm bioreactors have been employed to cultivate microbial communities to perform bromate removal. The rapid development of multiomics has and will continue to accelerate the elucidation of the mechanisms involved in bromate and other DBP conversions, as well as the interaction patterns among different bacterial subdivisions in the bioremoval of DBPs.

pH control of an upflow pyrite-oxidizing denitrifying bioreactor via electrohydrogenesis

Maintenance of stable pH during pyrite-oxidizing denitrification process is important. Here, we demonstrated effective pH control (7.80 ± 0.20-8.40 ± 0.30) in an electrochemical-H₂ and pyrite-oxidizing denitrifying bioreactor (HPR) through in situ electrohydrogenesis. HPR achieved a higher nitrate removal activity (maximum:19.66 ± 0.63 mg NO₃^--N/(L·h)) with excellent resistance to high nitrate loading (up to 400 mg/L NO₃^--N) compared to that of the control groups. Nitrate removal rate of HPR fitted the Michaelis-Menten kinetic model (R² = 0.98, p < 0.01) well, and the denitrification followed the zero-order rate law. The results of the biofilm community analyses suggested that Thauera was the dominant bacteria in the cathode biofilm of HPR and may prefer hydrogen as an electron donor for autotrophic denitrification, while the relative abundance of Pseudomonas were similar in the cathode biofilm and pyrite biofilm. This study provides a new alternative for effective pH control in denitrifying bioreactors with pyrite as a packing material.

Perchlorate bioreduction linked to methane oxidation in a membrane biofilm reactor: Performance and microbial community structure

Perchlorate bioreduction coupled to methane oxidation was successfully achieved without the addition of nitrate or nitrite in a membrane biofilm reactor (MBfR) inoculated with a mixture of freshwater sediments and anaerobic digester sludge as well as return activated sludge. The reactor was operated at different methane pressures (60, 40 and 20 Kpa) and influent perchlorate concentrations (1, 5 and 10 mg/L) to evaluate the biochemical process of perchlorate bioreduction coupled to methane oxidation. Perchlorate was completely reduced with a higher removal flux of 92.75 mg/m²·d using methane as the sole carbon source and electron donor, other than hydrogen or other limiting organics. Quantitative real-time PCR showed that bacteria prevailed over archaea and the abundances of mcrA, pMMO, pcrA, and nirS genes were correlated with the influent perchlorate flux. High-throughput sequencing of 16S rRNA genes demonstrated that the functional community consisted of methanotrophs, methylotrophs, perchlorate-reducing bacteria, as well as various denitrifiers.

Toxic effects of vanadium (V) on a combined autotrophic denitrification system using sulfur and hydrogen as electron donors

Vanadium (V) is a common heavy metal and often co-occurs with nitrate in effluents from mining and metal finishing industry. In the present study, the toxic effects of V(V) were examined in a sulfur and hydrogen based autotrophic denitrification system. This combined system achieved simultaneously microbial denitrification and V(V) reduction. High concentration of V(V) (60 and 100 mg/L) inhibited the denitrification activities, while 30 mg/L V(V) had a very slight effect. V(V) induced increases of lactate dehydrogenase release and reactive oxygen species production, which may inhibit nitrate and nitrite reductases activities and abundances of denitrifying functional genes. Moreover, the extracellular polymeric substance production was also suppressed under V(V) stress, thereby decreasing the amount of biofilm biomass. Microbial community analyses suggesting the genus Bacillus may have higher tolerance to V(V). These findings can provide scientific basis for the optimized design of treatment system to remove nitrate and V(V) simultaneously.