Microbial nitrate removal in biologically enhanced treated coal gasification wastewater of low COD to nitrate ratio by coupling biological denitrification with iron and carbon micro-electrolysis

Optimization of operating parameters and microbiological mechanism of a low C/N wastewater treatment system dominated by iron-dependent autotrophic denitrification

Ferrous iron (Fe2+) reduces the amount of external carbon source used for the denitrification of low-C/N wastewater. The effects of key operating parameters on the efficiency of ferrous-dependent autotrophic denitrification (FDAD) and the functioning mechanism of the microbiome can provide a regulatory strategy for improving the denitrification efficiency of low C/N wastewater. In this study, the response surface method (RSM) was used to explore the influence of four important parameters-the molar ratio of Fe2+ to NO3--N (Fe/N), total organic carbon (TOC), the molar ratio of inorganic carbon to NO3--N (IC/N) and sludge volume (SV, %)-on the FDAD efficiency. Functional prediction and molecular ecological networks based on high-throughputs sequencing techniques were used to explore changes in the structure, function, and biomarkers of the sludge microbial community. The results showed that Fe/N and TOC were the main parameters affecting FDAD efficiency. Higher concentrations of TOC and high Fe/N ratios provided more electron donors and improved denitrification efficiency, but weakened the importance of biomarkers (Rhodanobacter, Thermomonas, Comamonas, Thauera, Geothrix and unclassified genus of family Gallionellaceae) in the sludge ecological network. When Fe/N > 4, the denitrification efficiency fluctuated significantly. Functional prediction results indicated that genes that dominated N2O and NO reduction and the genes that dominated Fe2+ transport showed a slight decrease in abundance at high Fe/N levels. In light of these findings, we recommend the following optimization ranges of parameters: Fe/N (3.5-4); TOC/N (0.36-0.42); IC/N (3.5-4); and SV (approximately 35%).

Subtle magnesium liberation of self-fabricated functional filler actuates highly efficient phosphorus removal from source-separated urine by SBBR

Domestic wastewater source-separated treatment has attracted wide attention due to the efficiency improvement of sewage treatment systems, energy saving, resource reuse, and the construction and operation cost saving of pipeline networks. Nonetheless, the excess source-separated urine still demands further harmless treatment. Sequencing batch biofilm reactor (SBBR), a new type of composite biofilm reactor developed by filling different fillers into the sequential batch reactor (SBR) reactor, has higher pollutant removal performance and simpler operation and maintenance. However, the phosphorus removal ability of the SBBR filling with conventional fillers is still limited and needs further improvement. In this study, we developed two new fillers, the self-fabricated filler A and B (SFA/SFB), and compared their source-separated urine treatment performance. Long-term treatment experimental results demonstrated that the SBBR systems with different fillers had good removal performance on the COD and TN in the influent, and the removal rate increased with the increasing HRT. However, only the SBBR system with the SFA showed excellent PO43--P and TP removal performance, with the removal rates being 83.7 ± 11.9% and 77.3 ± 13.7% when the HRT was 1 d. Microbial community analysis results indicated that no special bacteria with strong phosphorus removal ability were present on the surface of the SFA. Adsorption experimental results suggested that the SFA had better adsorption performance for phosphorus than the SFB, but it could not always have stronger phosphorus adsorption and removal performance during long-term operation due to the adsorption saturation. Through a series of characterizations such as SEM, XRD, and BET, it was found that the SFA had a looser structure due to the use of different binder and production processes, and the magnesium in the SFA gradually released and reacted with PO43- and NH4+ in the source-separated urine to form dittmarite and struvite, thus achieving efficient phosphorus removal. This study provides a feasible manner for the efficient treatment of source-separated urine using the SBBR system with self-fabricated fillers.

Response of nutrients removal efficiency, enzyme activities and microbial community to current and voltage in a bio-electrical anammox system

The effects of different current intensities and voltage levels on nutrient removal performance and microbial community evolution in a Bio-Electrical Anammox (BEA) membrane bioreactor (MBR) were evaluated. The nitrogen removal efficiency increased with the current intensity within the range of 64-83 mA, but this improvement was limited at the current further increased. The phosphorus removal in the BEA MBR was attributed to the release of Fe2+, which was closely associated with the applied current to the electrodes. Heme c concentration, enzyme activities, and specific anammox activity exhibited a decreasing trend, while the functional denitrification genes showed a positive correlation with rising voltage. The nitrogen removal efficiency of the BEA system initially increased and then decreased with the voltage rose from 1.5V to 3.5V, peaking at 2.0V of 94.02% ± 1.19%. Transmission electron microscopy and flow cytometry results indicated that accelerated cell apoptosis/lysis led to an irreversible collapse of the biological nitrogen removal system at 3.5V. Candidatus Brocadia was the predominant anammox bacteria in the BEA system. In contrast, closely related Candidatus Kuenenia and Chloroflexi bacteria were gradually eliminated in electrolytic environment. The abundances of Proteobacteria-affiliated denitrifiers were increased with the voltage rising since the organic matter released by the cell apoptosis/lysis was accelerated at a high voltage level.

Promoting aerobic denitrification in reservoir water with iron-activated carbon: Enhanced nitrogen and organics removal efficiency, and biological mechanisms

Carbon scarcity limits denitrification in micropolluted water, especially in drinking water reservoirs. Therefore, a Fe-activated carbon (AC) carrier was used in this study to enhance the nitrogen removal capacity of aboriginal denitrification in drinking water reservoirs under aerobic conditions. Following carrier addition, total nitrogen (TN) and permanganate index (CODMn) removal efficiencies reached 81.89% and 72.66%, respectively, and were enhanced by 40.45% and 39.65%. Nitrogen balance analysis indicated that 77.86% of the initial TN was converted into gaseous nitrogen. Biolog analysis suggested that the metabolic activity of denitrifying bacteria was substantially enhanced. 16S rRNA gene sequencing indicated that organic degradation bacteria, hydrogen-consuming, Fe-oxidizing, and Fe-reducing denitrifying bacteria (e.g., Arenimonas, Hydrogenophaga, Zoogloea, Methylibium, and Piscinibacter) evolved into the dominant species. Additionally, napA, nirS, nirK, and nosZ genes were enriched by 3.17, 6.68, 0.40, and 6.70 folds, respectively, which is conducive to complete denitrification. These results provide a novel pathway for the use of Fe-AC to promote aerobic denitrification in micropolluted drinking water reservoirs.

Nitrate removal in iron sulfide-driven autotrophic denitrification biofilter: Biochemical and chemical transformation pathways and its underlying microbial mechanism

Iron sulfides-based autotrophic denitrification (IAD) is effective for treating nitrate-contaminated wastewater. However, the complex nitrate transformation pathways coupled with sulfur and iron cycles in IADs are still unclear. In this study, two columns (abiotic vs biotic) with iron sulfides (FeS) as the packing materials were constructed and operated continuously. In the abiotic column, FeS chemically reduced nitrate to ammonium under the ambient condition; this chemical reduction reaction pathway was spontaneous and has been overlooked in IAD reactors. In the biotic column (IAD biofilter), the complex nitrogen-transformation network was composed of chemical reduction, autotrophic denitrification, dissimilatory nitrate reduction to ammonium (DNRA) and sulfate reducing ammonium oxidation (Sulfammox). Metagenomic analysis and XPS characterization of the IAD biofilter further validated the roles of functional microbial communities (e.g., Acidovorax, Diaphorobacter, Desulfuromonas) in nitrate reduction process coupled with iron and sulfur cycles. This study gives an in-depth insight into the nitrogen transformations in IAD system and provides fundamental evidence about the underlying microbial mechanism for its further application in biological nitrogen removal.

Microbial mixotrophic denitrification using iron(II) as an assisted electron donor

Mixotrophic denitrification processes have a great potential in nitrogen removal in biological wastewater treatment processes. However, so far, few studies have focused on the mixotrophic denitrification system using Fe(II) as an exclusively assisted electron donors and the underlying mechanisms in such a process remain unclear. Furthermore, the mechanisms by which microorganisms cover carbon, nitrogen, phosphorus and iron in an iron-assisted mixotrophic system remain unrevealed. In this work, we explore the feasibility of using Fe(II) as an assisted electron donor for enhancing simultaneous nitrogen and phosphorus removal via long-term reactor operation and batch tests. The results show that Fe(II) could provide electrons for efficient nitrate reduction and that biological reactions played a predominant role in these systems. In these systems Thermomonas, a strain of nitrate-reduction Fe(II)-oxidation bacterium, was enriched and accounted for a maximum abundance of 60.2%. These findings indicate a great potential of the Fe(II)-assisted mixotrophic denitrification system for practical use as an efficient simultaneous nitrogen and phosphorus removal process.

Mechanism insights into enhanced treatment of wasted activated sludge by hydrogen-mediated anaerobic digestion

In the current study, different forms of added gas including H₂, CO₂, and mixed gas (V_H2:V_CO2 = 4:1), as well as different hydrogen partial pressures (0.10, 0.30, and 0.50 atm) were investigated for the influence on anaerobic performance in waste activated sludge (WAS) treatment. The mixed gas significantly improved methane production by over 20%, which positively correlated with the hydrogen partial pressure. However, pure H₂ (0.5 atm) heavily inhibited methane production by 76.5%. Combined with the microbial metabolic activity study, H₂ accelerated the hydrolysis process. Afterward, mixing with CO₂ accelerated H₂ and organic consumption, thus promoting WAS degradation and methane production. Based on the most extra release of organics, the mixed group exerted the superior performance with hydrogen partial pressure at 0.3 atm. The microbial community analysis evidenced that mixed gas enriched proteolytic and homoacetogenic bacteria and hybrid-trophic methanogens. By metagenomics study, hydrolysis, acetogenic, and methanogenesis pathways were all enhanced via the exogenous addition of H₂ and CO₂, sustainably transforming WAS towards CH₄. This study discovered the mechanism of the enhanced conversion from WAS to CH₄ by exogenous H₂ and provided a promising approach for WAS reduction and energy recovery.

A new concept of waste iron recycling for the enhancement of the anammox process

As a by-product of industry, waste iron scraps (WIS) are low-cost and widely available, which was potential for the development of iron-assisted anammox. In this study, the feasibility of adding WIS to enhance the nitrogen removal of the anammox process (also called WIS-assisted anammox) was demonstrated. Results indicated that the WIS-assisted anammox reactors performed a 15-35% higher nitrogen removal efficiency than that of the control. Compared to the sludge from the control, the sludge from the WIS-assisted anammox reactors had a higher iron content (78-113 g kg^-1 SS) and a better specific anammox activity (10.8-15.5 mg N g^-1 VSS h^-1). The enhanced growth of the anammox bacteria (related to Ca. Kuenenia stuttgartiensis with 99% similarity) in the WIS-assisted anammox reactors was also confirmed by high-throughput sequencing and qPCR. Furthermore, the functional genes predicted by PICRUSt2 revealed a higher level of hydroxylamine oxidoreductase (hao)-like proteins expression of the biomass from the WIS-assisted anammox reactors, implying that the hydroxylamine-related anammox pathway was promoted. Additionally, the observation of cytoplasmic nitrate reductase (narG), copper-containing nitrite reductase (nirK), and nitric oxide reductase (norB) suggested that the introduction of WIS might promote the denitrification ability. This was correlated to the lower ΔNO₃^-/ΔNH₄⁺ ratio observed in these WIS-assisted anammox reactors. Overall, the WIS-assisted anammox offers a sustainable nitrogen removal process for wastewater treatment with waste iron recycling.

Synchronous N and P Removal in Carbon-Coated Nanoscale Zerovalent Iron Autotrophic Denitrification─The Synergy of the Carbon Shell and P Removal

Fe⁰ is a promising electron donor for autotrophic denitrification in the simultaneous removal of nitrate and phosphorus in low C/N wastewater. However, P removal may inevitably inhibit bio-denitrification. It has not been well recognized and led to an overdose of iron materials. This study employed carbon-coated zerovalent iron (Fe⁰@C) to support autotrophic denitrification to mitigate the inhibition effects of P removal and enhance both N and P removal. The critical role of the carbon shell in Fe⁰@C was to block the direct contact between Fe⁰ and P and NO₃^--N, to maintain the Fe⁰ activity. Besides, P inhibited the chemical reduction of NO₃^--N by competing for Fe⁰ active sites. This indirectly boosted H₂ generation and promoted bio-denitrification. P removal displayed negligible effects on microbial species but indirectly enhanced the nitrogen metabolic activities because of promoted H₂ in Fe⁰@C-based autotrophic denitrification. Bio-denitrification, in turn, strengthened Fe-P co-precipitation by promoting the formation of ferric hydroxide as a secondary adsorbent for P removal. This study demonstrated an efficient method for simultaneous N and P removal in autotrophic denitrification and revealed the synergistic interactions among N and P removal processes.

Preparation of three dimensional bimetallic Cu-Ni/NiF electrodes for efficient electrochemical removal of nitrate nitrogen

It still remains a hotspot and great challenge to efficiently remove the nitrate nitrogen from high salt wastewater. Herein, a novel three dimensional porous bimetallic copper-nickel alloy electrode was fabricated with Ni foam (NiF) as substrate. The physicochemical and electrochemical characterization results showed Cu-Ni/NiF electrode possessed the smaller particle size (0.3-1.0 μm) and electrode film resistance comparing with Ni/NiF and Cu/NiF electrodes. Besides, higher double layer capacitance (C_dl) for Cu-Ni/NiF electrode indicated more electrochemical active sites could be used in the electrochemical nitrate nitrogen (NO₃^--N) removal. The electrochemical experiments showed the Cu-Ni/NiF electrode had the optimal NO₃^--N reduction ability and almost 100% NO₃^--N removal could be achieved with 30 min. All NO₃^--N removal processes were in accord with the pseudo-first-order reaction kinetics completely. The gaseous nitrogen selectivity for Cu-Ni/NiF electrode could reach 80.9% within 300 min. Stability assessment experiments indicated the Cu-Ni/NiF electrode all kept an excellent stability with Na₂SO₄ or NaCl electrolyte and the Cl^- addition could significantly improve the gaseous nitrogen selectivity. Finally, a possible removal mechanism of NO₃^--N was proposed. This work offered a direction for designing non-noble bimetallic electrodes for nitrate removal.

Simultaneously advanced removal of nitrogen and phosphorus in a biofilter packed with ZVI/PHBV/sawdust composite: Deciphering the succession of dominant bacteria and keystone species

In this study, a biofilter was developed with a ZVI/PHBV/sawdust (ZPS) composite for treating simulative secondary effluent from wastewater treatment plants. Results showed that effluent concentrations of NO₃^--N and TP in the ZPS biofilter were stable below 2.0 mg/L and 0.1 mg/L, corresponding to 95% NO₃^--N removal and 99% TP removal, respectively. Microbial community analysis revealed that the transformation of dominant taxa from Dechloromonas to Clostridium sensu stricto_7 from 30 d to 120 d suggested that the ZVI-induced succession of dominant fermentation bacteria ensured the stable carbon supply for denitrification. Co-occurrence network analysis showed that the ZVI directly enhanced the interaction of microbial community. Fe-related bacteria occupied a key position in the rare species, which might maintain the function of iron-mediated organic matter decomposition and denitrification. These findings provide an alternative for advanced removal of nitrogen and phosphorus in biofilters packed with ZPS composites.

Tuning Nitrate Electroreduction Activity via an Equilibrium Adsorption Strategy: A Computational Study

Converting nitrates into industrial-value chemicals is of great significance for environmental sustainability. Herein, the electrochemical nitrate reduction reaction (NO₃RR) performances of transition metal (TM) anchored g-C₃N₄ (TM/g-C₃N₄) were systematically evaluated using density functional theory and ab initio molecular dynamics. A novel equilibrium adsorption model was constructed to screen the excellent catalysts, accelerating high-throughput calculations. As found, Hf/g-C₃N₄ exhibits remarkable activity with a limiting potential of 0.11 V. Besides, the superior electrode selectivity overwhelmingly depresses the formation of byproducts and greatly speeds up the NO₃RR. The electronic structure analysis discloses the origin of the electrocatalytic activity for TM/g-C₃N₄ and indicates why the IVB group elements have an outstanding role. Finally, the formation energy of the clusters and ab initio molecular dynamics simulations prove their structure has fine stability. This work not only offers high-performance electrode candidates for the NO₃RR but also opens up a precedent of electrocatalysis in the field of wastewater treatment of explosives.

Boosting electrocatalytic activity of carbon fiber@fusiform-like copper-nickel LDHs: Sensing of nitrate as biomarker for NOB detection

Morphological evolution of layered double hydroxides (LDHs) with preferential crystal facets has appealed gigantic attention of research community. Herein, we prepare hierarchical hybrid material by structurally integrating fusiform-like CuNiAl LDHs petals on conductive backbone of CF (CF@CuNiAl LDHs) and investigate electrocatalytic behavior in nitrate reduction over a potential window of -0.7 V to +0.7 V. The CF@CuNiAl LDHs electrode exhibits remarkable electrocatalytic aptitude in nitrate sensing including broad linear ranges of 5 nM to 40 µM and 75 µM to 2.4 mM with lowest detection limit of 0.02 nM (S/N = 3). The sensor shows sensitivity of 830.5 ± 1.84 µA mM^1- cm^2- and response time within 3 s. Owing to synergistic collaboration of improved electron transfer kinetics, specific fusiform-like morphology, presence of more catalytically active {111} facets and superb catalytic activity of LDHs, CF@CuNiAl LDHs electrode has outperformed as electrochemical sensor. Encouraged from incredible performance, CF@CuNiAl LDHs flexible electrode has been applied in real-time in-vitro detection of nitrite oxidizing bacteria (NOB) through the sensing of nitrate because NOB convert nitrite into nitrate by characteristic metabolic process to obtain their energy. Further, CF@CuNiAl LDHs based sensing podium has also been employed in in-vitro detection of nitrates from mineral water, tap water and Pepsi drink.

Insight into the effect of N-acyl-homoserine lactones-mediated quorum sensing on the microbial social behaviors in a UASB with the regulation of alkalinity

N-acyl-homoserine lactones (AHLs)-mediated quorum sensing (QS) has been reported as the inducers of microbial social behaviors in anaerobic digestion (AD) processes. However, it is not well understood that how to intentionally change the secretion of AHLs by conventional engineering control such as the regulation of alkalinity. The present research investigated the effect of endogenous AHLs-mediated QS on the microbial social behaviors in an upflow anaerobic sludge bed (UASB) reactor with the influent alkalinity decreased from 2800 mg/L to 700 mg/L by stages. The results showed that the alkalinity of 1800-2200 mg/L was more favorable for the AD in the UASB, with an excellent specific methanogenic activity (SMA) and better microbial aggregation statuses. The alkalinity out of the favorable alkalinity range would decrease the SMA with the accumulation of VFAs in the reactor. It was found that signal molecule C₄-HSL was always the dominant AHL in the UASB along with the decrease of influent alkalinity, while 3-oxo-C₆-HSL, 3-oxo-C₁₂-HSL and C₁₄-HSL were remarkably improved only within the favorable range of alkalinity. Pearson correlation concluded that the dominant signal molecule C₄-HSL was the specific AHL in enhancing the synthesis of extracellular polysaccharide and the metabolism of acidogens. The co-occurrence network revealed that Mesotoga, Sulfurospirillum and Methanoregula were the key hubs in the microbial interaction network, and the AHLs-mediated QS indirectly facilitated the methanogenic metabolism. The present work provided a revealing insight into the effect of AHLs-mediated QS on the microbial social behaviors in AD process with the regulation of alkalinity.

Influence of modified biochar supported Fe-Cu/polyvinylpyrrolidone on nitrate removal and high selectivity towards nitrogen in constructed wetlands

In this study, the biochar (BC) supported Fe-Cu bimetallic stabilized by PVP (Fe-Cu/PVP/BC) were prepared and utilized to enhance the nitrate (NO₃^-) removal and the selectivity toward nitrogen (N₂). Results showed the optimum Fe:Cu:BC ratio and the dosage of the BC (pyrolysis at 700 °C) supported Fe-Cu bimetallic stabilized by polyvinylpyrrolidone (PVP) (Fe-Cu/PVP/BC₇₀₀) were respectively 1:2:3 and 1 mg L^-1 with the selectivity toward N₂ of 31 %. This was mainly due to the synergy among Fe⁰, Cu⁰ and BC in the Fe-Cu/PVP/BC. The addition of Fe⁰ could reduce the NO₃^- through providing electron. The Cu⁰ and BC improved the selectivity of NO₃^- to N₂ through forming [Cu-NO₂^-_ads] and adjusting redox potential. The addition of Fe-Cu/PVP/BC could supply electrons for denitrification and enhance the relative abundances of Azospira and Thauera related to denitrification to improve NO₃^- removal. This result was further confirmed by the variations of denitrifying functional genes (narG, nirK, nirS and nosZ). This research provided an effective method to improve NO₃^- removal during surface water treatment in constructed wetlands (CWs) by adding Fe-Cu/PVP/BC.

Enhanced anaerobic degradation of quinoline and indole with the coupling of sodium citrate and polyurethane

A coupling system of sodium citrate and biofilm based on polyurethane was prepared to analyse the coupling enhancement degradation on quinoline and indole. Four reactors (R1: sludge, R2: sludge + sodium citrate, R3: biofilm on polyurethane, and R4: biofilm + sodium citrate) were operated 120 days to compare the degradation efficiency. During whole running phases, R4 showed high degradation efficiency on quinoline (≥98.55%) and indole (≥95.44%). Analysis of bacterial colony showed anaerobic sludge reactors benefited the enrichment of Aminicenantes, Levilinea, and Longilinea, while anaerobic biofilm reactors benefited the enrichment of Giesbergeria and Comamonas. Furthermore, analysis of archaea colony showed acetic acid metabolism to produce methane was the main mode in anaerobic sludge reactors, while acetic acid and hydrogen metabolism to produce methane were both the main modes in biofilm reactors. This study can provide some references for the treatment of nitrogen heterocyclic wastewater.

Self-Activated Ni Cathode for Electrocatalytic Nitrate Reduction to Ammonia: From Fundamentals to Scale-Up for Treatment of Industrial Wastewater

Electrocatalytic reduction has recently received increasing attention as a method of converting waste nitrate into value-added ammonia, but most studies have focused on complex strategies of catalyst preparation and little has been done in the way of large-scale demonstrations. Herein, we report that in situ activation of a pristine Ni electrode, either on a lab scale or a pilot scale, is effective in facilitating nitrate reduction to ammonia, exhibiting extraordinarily high activity, selectivity, and stability. The self-activated Ni cathode has a robust capacity to reduce nitrate over a wide range of concentrations and achieves great conversion yield, NH₄⁺-N selectivity, and Faradaic efficiency, respectively, 95.3, 95.5, and 64.4% at 200 mg L^-1 NO₃^--N and 97.8, 97.1, and 90.4% at 2000 mg L^-1 NO₃^--N, for example. Fundamental research indicates that Ni(OH)₂ nanoparticles are formed on the Ni electrode surface upon self-activation, which play crucial roles in governing nitrate reduction reaction (NO₃RR) through the atomic H*-mediated pathway and accordingly suppressing hydrogen evolution reaction. More importantly, the self-activated Ni(OH)₂@Ni cathode can be easily scaled up to allow large volumes of real industrial wastewater to be processed, successfully transferring nitrate into ammonia with high yields and Faradaic efficiency. This study demonstrates a new, mild, and promising method of cleaning nitrate-laden wastewater that produces ammonia as a valuable byproduct.

Effects of polyurethane foam carrier addition on anoxic/aerobic membrane bioreactor (A/O-MBR) for coal gasification wastewater (CGW) treatment: Performance and microbial community structure

Coal gasification wastewater (CGW) is a typical toxic and refractory industrial wastewater with abundant phenols contained. Two identical anoxic/aerobic membrane bioreactors (with (R2) and without (R1) polyurethane (PU) foam) were carried out in parallel to investigate the role of PU foam addition in enhancing pollutants removal in CGW. Results showed that both systems exhibited effective removal of chemical oxygen demand (>93%) and total phenols (>97%) but poor ammonia nitrogen removal (<35%) constrained by ammonia oxidation process. GC-MS analysis revealed that aromatic and other refractory intermediates were dramatically reduced in R2. Moreover, the PU addition had negligible influence on the total soluble microbial products and extracellular polymeric substances contents but significantly alleviated membrane fouling with the operating time 33% prolonged. Microbial community revealed that Flavobacterium, Holophaga, and Geobacter were enriched on PU. Influent type might be a main driver for microbial community succession.

Metagenomic analysis of aromatic ring-cleavage mechanism in nano-Fe3O4@activated coke enhanced bio-system for coal pyrolysis wastewater treatment

In current study, nano-Fe₃O₄@activated coke enhanced bio-system (FEBS) under limited-oxygen condition was applied for efficient treatment of aromatic organics in coal pyrolysis wastewater. Metagenomic analyses revealed functional microbiome linkages and mechanism involved in aromatic ring-cleavage. Based on biodegradation efficiency in different reactors, FEBS supplementation conferred the best organic removal (avg. 92.29%). It also showed a remarkable advantage in biodegradability maintenance (>40%) over control reactors. Metagenomics profiling revealed the degradation processes were driven by Fe₃O₄ redox reactions and microbial biofilm, while the suspended sludge was the principal force for aromatic mineralization. Based on the analysis of functional species and genes, most bacteria cleaved the benzene ring preferably through the aerobic pathways, mediated by catechol 1, 2-dioxygenase, catechol 2, 3-dioxygenase and protocatechuate 3, 4-dioxygenase (66-84%). Ecological network showed that Comamonas testosterone-centered microbiome and Azotobacter linked to the nitrogen (N)-heterocyclic ring-cleavage. Network linkage further demonstrated that Alicycliphilus and Acidovorax were the key tone taxa involved in benzene ring-cleavage. Finally, combined with analysis of degradation products, bacteria degraded N-heterocyclic ring containing organic aromatic compounds (quinoline) mainly through anaerobic processes, whereas cleavage of benzene ring preferred aerobic pathways. The enriched functional species were the primary reason for the enhanced biodegradation in FEBS.

Performance and microbial community of a novel PVA/iron-carbon (Fe–C) immobilized bioreactor for nitrate removal from groundwater

An efficient immobilized denitrification bioreactor functioning under anaerobic conditions was developed by combining bacterial immobilization technology with iron-carbon (Fe–C) particles. The effects of key factors on nitrate (NO3 −–N) removal efficiency were invested, such as the carbon-nitrogen ratio (C/N), pH and hydraulic retention time (HRT). Experimental results show that 100.00% NO3 −–N removal efficiency and a low level of nitrite (NO2 −–N) accumulation less than 0.05 mg L−1 were obtained under the condition of a C/N ratio of 3, pH 7.0 and HRT of 6 h. Meteorological chromatographic analysis showed that the final product of denitrification was mainly nitrogen (N2). The main component of precipitation formed in the bioreactor was characterized as Fe3O4 by X-ray diffraction. High-throughput sequencing analysis indicated that the dominant bacterial class in the Fe–C bioreactor was Gammaproteobacteria, while the dominant genera were Zoogloea and Azospira, the relative abundances of which were as high as 23.25 and 15.43%, respectively.

Effect of calcinated pyrite on simultaneous ammonia, nitrate and phosphorus removal in the BAF system and the Fe2+ regulatory mechanisms: Electron transfer and biofilm properties

To efficiently remove nitrogen and phosphorus from secondary effluent with low values of COD/TN, a novel biological aerated filter (BAF) utilizing calcined pyrite with a large specific surface area (SSA) and pore diameter (PD) was designed to address this challenge. From the perspective of nutrients removal performance, and the corresponding effluent total nitrogen (TN) and PO₄^3--P in the calcined pyrite autotrophic denitrification (CPAD) process decreased from 40.21 to 1.07 mg/L to 1.22 and 0.14 mg/L, respectively. Furthermore, the nutrients removal kinetics analysis showed that the CPAD and pyrite autotrophic denitrification (PAD) processes could be fitted with Half-order and Zero-order reactions via kinetics analysis, respectively, indicating that the TN removal performance of CPAD processes was better than that of the PAD process. Moreover, CPAD combined with sulfur autotrophic denitrification (SAD) processes was fitted by First-order reaction, and the TN removal performance was further enhanced over the CPAD process. From the perspective of microregulation, Fe²⁺ production in the PAD and CPAD processes could accelerate the electron transfer rate by increasing electron transport system activity (ETSA) and reducing electrochemical impedance spectroscopy (EIS). Moreover, Fe²⁺ stimulated microbes to produce more proteins (PN) and C₁₀-HSL, which improved biofilm stability and interspecific communication processes. Notably, nitrifiers and autotrophic denitrifiers were simultaneously enriched via detection of high-throughput sequencing of 16 S rRNA genes, which verified the feasibility of simultaneous nitrification and autotrophic denitrification. Therefore, BAF with calcined pyrite and sulfur as composite fillers have a considerable advantage in nutrients removal.

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.

Role of porous polymer carriers and iron-carbon bioreactor combined micro-electrolysis and biological denitrification in efficient removal of nitrate from wastewater under low carbon to nitrogen ratio

In the current research, a novel bioreactor composed of porous polymer carriers and iron-carbon (PPC@FeC) was established through bacterial immobilized technology. The influence of key factors was studied on the nitrate removal performance of the PPC@FeC bioreactor. The experimental results showed that the highest removal rate of nitrate (7.33 mg L^-1 h^-1) can be obtained with short hydraulic retention times (HRT = 2.0 h) and low carbon-to-nitrogen ratio (C/N = 2.0). The results of high-throughput sequencing revealed that Zoogloea sp. L2 was the dominant strain in bioreactor responsible for nitrate removal. Moreover, the SEM and XRD analyses elucidated that Fe₂O₃ was the final product produced by the interaction of FeC and strain L2. These findings showed that the PPC@FeC bioreactor successfully combined micro-electrolysis and biological denitrification, which exhibited great potential in removing nitrate effectively from wastewater under low C/N ratio and short HRT conditions.

ALLODUST augmented activated sludge single batch anaerobic reactor (AS-SBAnR) for high concentration nitrate removal from agricultural wastewater

Nitrate is among the most widespread contaminants that threaten water bodies and waterways. Under favourable environmental conditions, high nitrate concentrations in water can contribute to eutrophication, thus presenting a high potential for risk to ecosystems and human health. Low-cost allophanic soil material and carbon-based bio-wastes have great potential to reduce nutrient concentrations from contaminated waters. This study investigated the mechanisms that underpin the reduction of nitrate concentrations and nitrous oxide (N₂O) emission in the presence of novel developed media in an activated sludge process. A new operating approach, employing a newly developed media (ALLODUST), was evaluated for enhanced NO^-₃-N removal from agricultural wastewater. Two anaerobic-aerobic batch reactors were developed, where the coupled bottom aeration method was used for efficient agitation and aeration in the aerobic reactor. The reactor was run at high NO^-₃-N concentrations (110 mg L^-1), under anoxic conditions at low- to long-term contact times (2, 12, and 22 h), while the aerobic period (clarification) was constant for all the experimental designs (2 h). ALLODUST retained its integrity and stability over the long-term operation. Low ALLODUST concentrations (5.95 g L^-1) removed 87% of the NO^-₃-N from the wastewater within 12 h. Further exploration revealed that the same amount of the media was optimal for decreasing N₂O emissions from the anaerobic activated sludge reactor by 80%.

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.

Microbial community responses to agricultural biomass addition in aerated constructed wetlands treating low carbon wastewater

Using agricultural biomasses as solid carbon substrates in constructed wetlands (CWs) could be an effective way to achieve sustainable nitrogen removal for carbon-limited wastewater treatments. This study investigated the response of bacteria community in CWs to the addition of agricultural biomasses (wheat straw, walnut shell and apricot pit). Results indicated that the addition of different agricultural biomasses had distinct influence on bacterial communities in CWs. Both wheat straw and walnut shell increased the diversity of microbial communities and optimized the structure of microorganisms. The effect of apricot pit on the richness and evenness of microbial communities was not significant, but the composition of microorganisms was significantly affected at the phylum level, especially the relative abundance of phylum Saccharibacteria. Moreover, the addition of agricultural biomasses in CWs acclimatized more functional bacteria including nitrifier and denitrifier, which were proved to be positively correlated with the high-rate denitrification performance. The obtained results would be beneficial to understand the underlying microbial mechanism of nitrogen removal in CWs with agricultural biomass and provide some guidance on the practical application of CWs.

Biodegradable chitosan-ethylene glycol hydrogel effectively adsorbs nitrate in water

Nitrate, existing as inorganic anions in water, possesses high water-solubility and has caused lots of contaminations around the world. It is thus extremely urgent to develop an effective method to effectively remove nitrate from water in a sustainable way. In this study, chitosan-ethylene glycol hydrogel (CEGH) was synthesized using the repeated freezing-thawing procedure. A range of batch sorption experiments were conducted to evaluate CEGH as a nitrate sorbent. The adsorption isotherms of nitrate onto CEGH followed the Langmuir model with coefficient of determination of 0.98 and a maximum Langmuir adsorption capacity of 49.04 mg/g, which is higher than that of other adsorbents. The adsorption of nitrate onto CEGH was affected by pH value and temperature. The results indicate that the main removal mechanism was polarity of CEGH molecules given by functional group O-H and N-H and hydrogen bond interaction between CEGH and nitrate molecules under acidic conditions. Therefore, CEGH, a biodegradable carbon-rich adsorbent, can be widely applied to remove nitrate in wastewater treatment and water body remediation.

Insights into electroactive biofilms for enhanced phenolic degradation of coal pyrolysis wastewater (CPW) by magnetic activated coke (MAC): Metagenomic analysis in attached biofilm and suspended sludge

To investigate the role of electroactive biofilms for enhanced phenolic degradation, lignite activated coke (LAC) and MAC were used as carriers in moving-bed biofilm reactor (MBBR) for CPW treatment. In contrast to activated sludge (AS) reactor, the carriers improved degradation performance of MBBR. Although two MBBRs exerted similar degradation capacity with over 92% of COD and 93% phenols removal under the highest phenolics concentration (500 mg/L), the effluent of MAC-based MBBR remained higher biodegradability (BOD₅/COD = 0.34 vs 0.18) than that of LAC-based MBBR. Metagenomic analysis revealed that electroactive biofilms determined phenolic degradation of MAC-based MBBR. Primarily, Geobacter (17.33%) started Fe redox cycle on biofilms and developed syntrophy with Syntrophorhabdus (6.47%), which fermented phenols into easily biodegradable substrates. Subsequently, Ignavibacterium (3.38% to 2.52%) and Acidovorax (0.46% to 8.83%) conducted biological electricity from electroactive biofilms to suspended sludge. They synergized with dominated genus in suspended sludge, Alicycliphilus (19.56%) that accounted for phenolic oxidation and nitrate reduction. Consequently, the significantly advantage of Geobater and Syntrophorhabdus was the keystone reason for superior biodegradability maintenance of MAC-based MBBR.

Effect of low-intensity electric current field and iron anode on biological nitrate removal in wastewater with low COD to nitrogen ratio from coal pyrolysis

Mixotrophic nitrate removal in wastewater from coal pyrolysis was achieved in microbial electrolysis cell with iron anode (iron-MEC). The effect of voltage, iron anode and conductivity were investigated. The effluent TN concentration was 8.35 ± 1.94 mg/L in iron-MEC when the conductivity of the wastewater was adjusted to 3.97 ± 0.08 mS/cm, which was lower than that in no-treated reactor. The increase of current density, which was resulted from the elevation of conductivity, promoted the iron corrosion and Fe2+ ion generation. Therefore, more Fe2+ ion was utilized by nitrate reducing ferrous oxidation bacteria (NRFOB) used to reduce nitrate. The microbial community analysis demonstrated that NRFOB, including Acidovorax and Bradyrhizobium, possessed a higher abundance in iron-MEC. The enrichment of Geobacter in iron-MEC might imply that the part of Fe(III) produced by ferrous oxidation was reduced by Geobacter, which established an iron cycle. Moreover, the production of N2O was decreased by the formation of Fe2+ ion.

Metagenomic insights into microbial characterizations in explaining the distinction of biofilter performance during start-up

As an effective alternative for dissolved nitrogen removal, biofilter closely associates its treatment performance to structural and/or operational conditions. In this study, a set of four different biofilters including MAVF (mature aerated vertical flow), NAVF (new aerated vertical flow), NVF (new non-aerated vertical flow), and BHF (baffled non-aerated horizontal flow) were employed to purify low C/N ratio (3.8) domestic wastewater. All the filters were packed with round ceramsite operated under varying hydraulic loading rates (HLRs) of 0.024-0.18 m/day. During the start-up, both the physicochemical and microbial characterizations were investigated. It was found that, carbon and nitrogen could achieve ideal removal in MAVF once added with further sedimentation, while phosphorus displayed an unsatisfactory sequestration in any of the four filters probably due to the high inflow load and/or lack of alternate anaerobic/aerobic conditions. Filter clustering based on percent removal and removal rate constant displayed a consistent pattern, which was similar to that based on taxa of phylum from 16S rRNA sequencing, or phylum/genus/species from shotgun metagenomic sequencing although there were obvious distinctions in taxa compositions among direct comparison. Meanwhile, gene function annotation revealed that filter clustering based on metabolic pathways was consistent with that based on purification performance. These consistencies might imply that the treatment performance was mainly determined by microbial degradation. The enrichment of specific functional microbes responsible for the degradation of certain pollutants, such as carbohydrates, matched well with the defined purification performance.

Electrochemical nitrate removal with simultaneous magnesium recovery from a mimicked RO brine assisted by in situ chloride ions

Electrochemical reduction is effective to remove nitrate but byproducts such as ammonia and nitrite would need chloride addition for indirect oxidation to nitrogen gas. Herein, electrochemical nitrate reduction was investigated to remove nitrate from a mimicked reverse osmosis (RO) brine containing chloride that eliminates the need for external chloride addition. Both Cu/Zn and Ti nano cathodes exhibited the best performance of nitrate removal with >97 % removal in either Na₂SO₄ or NaCl electrolyte, although with different products. Complete nitrate reduction to nitrogen gas was realized in the RO brine whose complex composition decreased the electrode efficiency, for example from 71.4 ± 0.2%-49.4 ± 0.3 % with the Cu/Zn cathode after 5 cycles of operation. Magnesium was recovered at the same time of nitrate removal and the purity of Mg(II) could reach 96.8 ± 2.0 % after proper pH pre-treatment. In a preliminary adsorption study, a key byproduct - chlorate was reduced by 49.8 ± 2.7 % after 3-h adsorption by 100 g L^-1 activated carbon. These results have demonstrated the simultaneous electrochemical nitrate removal and resource recovery from a complex water like a RO brine and provided new information such as byproduct management and electrode deterioration.

A novel enhanced anaerobic biodegradation method using biochar and Fe(OH)3@biochar for the removal of nitrogen heterocyclic compounds from coal gasification wastewater

Three identical lab-scale sequencing batch reactors (SBR) were operated for 120 days for raw (R₁), biochar (R₂), and Fe(OH)₃@biochar (R₃) enhanced anaerobic degradation of selected nitrogen heterocyclic compounds (NHCs). The occurrence of Fe-OH ensured the successful attachment of Fe(OH)₃ to biochar as evidenced by the Fourier transform infrared (FTIR) spectra of biochar and Fe(OH)₃@biochar. Acute biotoxicity experiments revealed that enhancing biochar and Fe(OH)₃@biochar effectively decreased the toxicity of microorganisms. Additionally, the introduction of biochar and Fe(OH)₃@biochar improved the settling performance of anaerobic sludge. Further, it was concluded that enriched Longilinea and Comamonas might be the major genera that function to degrade selected NHCs in anaerobic conditions.

Modified solid carbon sources with nitrate adsorption capability combined with nZVI improve the denitrification performance of constructed wetlands

In this study, various modified agricultural wastes (modified canna leaves (MCL), modified rice straw (MRS) and modified peanut shells (MPS)) as solid carbon sources (SCSs) were used to remove nitrate in constructed wetlands (CWs). Then, modified SCSs combined with nZVI (SCSN) as co-electrons further enhanced both heterotrophic denitrification (HD) and autotrophic denitrification (AD) performance of CWs. The results showed that NO₃^--N removal efficiencies in CWs with SCSNs (75.3-91.1%) and in CWs with SCSs (63.3-65.5%) were significantly higher than that in CK-CW (47.0%). The presence of SCSs reduced the accumulation of NO₂^--N in CWs. Compared to the addition of SCSs, the addition of SCSNs decreased the effluent COD concentration in CWs, avoiding secondary pollution. In addition, the solid-phase denitrifiers Silanimonas and Thauera were enriched in MPS-CW. Thermomonas, an autotrophic denitrifying bacteria (ADB), and Azospira, a nitrate-reducing Fe (II) oxidation bacteria (NRFOB), exhibited high relative abundance in MPN-CW.

Identification and nitrogen removal characteristics of Thauera sp. FDN-01 and application in sequencing batch biofilm reactor

A strain FDN-01 was isolated from the sequencing batch biofilm reactor (SBBR) which was seeded with wasted activated sludge from a municipal wastewater treatment plant in Shanghai. Bacterium FDN-01 was identified as Thauera sp., and Genbank Sequence_ID was KY393097. By comparing inorganic total nitrogen (TN) removal efficiency by strain FDN-01 under different conditions, the optimal initial pH, carbon source and the ratio of carbon to nitrogen were 7.5, sodium succinate and 4.0, respectively. Inorganic TN removal efficiency was 93% within 3 d while the concentration of nitrate was 100 mg/L, and the type of substrates affected extracellular polymeric substances (EPS) production and the ratio of protein to polysaccharide in the EPS. Further investigation for the application of strain FDN-01 in the SBBRs showed that anoxic ammonia oxidation occurred at room temperature, and the removal efficiencies of inorganic TN were noticeably enhanced by the augmentation of bacterium FDN-01 back into the SBBR. This study provided a promising method of TN removal requiring less carbon source in the wastewater.

Comparison of Clay Ceramsite and Biodegradable Polymers as Carriers in Pack-bed Biofilm Reactor for Nitrate Removal

In recent years, there is a trend of low C/N ratio in municipal domestic wastewater, which results in serious problems for nitrogen removal from wastewater. The addition of an external soluble carbon source has been the usual procedure to achieve denitrification. However, the disadvantage of this treatment process is the need of a closed, rather sophisticated and costly process control as well as the risk of overdosing. Solid-phase denitrification using biodegradable polymers as biofilm carrier and carbon source was considered as an attractive alternative for biological denitrification. The start-up time of the novel process using PCL (polycaprolactone) as biofilm carrier and carbon source was comparable with that of conventional process using ceramsite as biofilm carrier and acetate as carbon source. Further, the solid-phase denitrification process showed higher nitrogen removal efficiency under shorter hydraulic retention time (HRT) and low carbon to nitrogen (C/N) ratio since the biofilm was firmly attached to the clear pores on the surface of PCL carriers and in this process bacteria that could degrade PCL carriers to obtain electron donor for denitrification was found. In addition, solid-phase denitrification process had a stronger resistance of shock loading than that in conventional process. This study revealed, for the first time, that the physical properties of the biodegradable polymer played a vital role in denitrification, and the different microbial compositions of the two processes was the main reason for the different denitrification performances under low C/N ratio.

Enhanced biodegradation of coal gasification wastewater with anaerobic biofilm on polyurethane (PU), powdered activated carbon (PAC), and biochar

The primary objective was to explore the feasibility of anaerobic biofilm on polyurethane (PU), powdered activated carbon (PAC), and biochar in strengthening anaerobic degradation of phenolic compounds and selected nitrogen heterocyclic compounds (NHCs) in coal gasification wastewater (CGW). When total phenols (TPh) was less than 300 mg/L, PAC-based biofilm was more efficient. Whereas, when the TPh concentration was more than 450 mg/L, PU-based biofilm performed the optimal degradation efficiency. Furthermore, microbial community structure analysis showed that PAC and biochar had little effect on the microbial community structure after 120 days of operation, while the addition of PU could lead to the enrichment of Giesbergeria, Caldisericum, Thauera, Methanolinea, and Methanoregula.

Effect of low-intensity direct current electric field on microbial nitrate removal in coal pyrolysis wastewater with low COD to nitrogen ratio

The coupling of bioelectrochemical system with the biological denitrification process (R1) was constructed for nitrate removal in coal pyrolysis wastewater (CPW) and the effect of low-intensity direct current electric field was investigated. Compared with control reactor (R2), the total nitrogen (TN) removal efficiency in R1 at the optimized voltage of 0.9 V was 94.20 ± 2.14%, which was 14.42% higher than that in R2. The biofilm on the cathode of R1 enhanced the nitrate reducing, however, nitrite was only reduced by bacteria in suspended activated sludge, which was confirmed by cyclic voltammetry measurement (CV). Microbial community network analysis revealed that exoelectrogenic bacteria of Pseudomonas was enriched on the anode of R1, and the "small world", including Zoogloea, Pseudomonas and Arenimonas, was established under the stimulation of voltage. Therefore, Pseudomonas transferred electron to anode, and Arenimonas could utilize electron from anode to reduce nitrate, which enhanced TN removal in R1.

Comparative investigation on carbon-based moving bed biofilm reactor (MBBR) for synchronous removal of phenols and ammonia in treating coal pyrolysis wastewater at pilot-scale

By regulating the extraction solvent and alkali in pretreatment, two carbon-based MBBRs were compared in pilot-scale to synchronously remove phenols and ammonia of coal pyrolysis wastewater (CPW) under fluctuant phenols-ammonia loadings. It revealed that lignite activated coke (LAC)-based MBBR performed more stable with phenols increasing (250-550 mg/L), and reached higher tolerance limit to ammonia (>320 mg/L) than activated carbon (AC)-based MBBR under fluctuant ammonia loadings. During the phenols-ammonia synchronous removal process, the LAC provided the firm basis for shock resistance due to superior resilient adsorption capacity, enhanced sludge property and microbial cooperation. Furthermore, microbial analysis revealed that the strengthened collaboration between archaea and facultative bacteria played the primary role in phenols-ammonia synchronous degradation. Specifically, the heterotrophic bacteria consumed phenols-ammonia by partial nitrification process and ammonia assimilation, following by denitrifying process to further eliminate phenols. The multifunctional Comamonas was the critical genus participating in all procedures.

Enhanced nitrogen removal of coal pyrolysis wastewater with low COD to nitrogen ratio by partial nitrification-denitrification bioprocess assisted with polycaprolactone

The purpose of this study is to investigate the enhancement of polycaprolactone (PCL) on total nitrogen (TN) removal of coal pyrolysis wastewater (CPW) with low COD to nitrogen ratio by partial nitrification-denitrification bioprocess (PNDB) in one single reactor. With the innovative combination of PCL and PNDB, the TN removal efficiency in the experimental reactor (signed as R1) was 10.21% higher than control reactor (R2). Nitrite accumulation percentage (NAP) in R1 was 82.02%, which was 17.49% higher than R2 at the dissolved oxygen (DO) concentration of 0.9-1.5 mg/L, for the reason that the extra DO was consumed by PCL biodegradation at the aerobic period. Gel permeation chromatography (GPC) results demonstrated that organics with the molecular weight of 185 Da, which could serve as additional carbon sources for denitrifiers, were generated during the PCL hydrolysis process at the anoxic period. PCL was hydrolyzed by extracellular enzymes with the break of the ester bond which was confirmed by FT-IR spectrometer. Microbial community analysis revealed that Ferruginibacter was the dominant hydrolysis bacteria in R1. Nitrosomonas were the main ammonium-oxidizing bacteria (AOB) and Hyphomicrobium were the denitrifiers in this study.

The enhancement of iron fuel cell on bio-cathode denitrification and its mechanism as well as the microbial community analysis of bio-cathode

To address the issue of insufficient electrons during denitrification, an iron fuel cell (IFC) bioreactor using iron as abiotic anode was designed. The nitrogen removal efficiency (NRE) of IFC (2.54 ± 0.016%) was significantly lower than microbial fuel cell (MFC) (32.58 ± 0.033%) with same bio-cathode under autotrophic conditions, which was due to the permeation of acetate on proton exchange membrane (PEM) affected the process of enriching autotrophic denitrifying bacteria by MFC. When used in heterotrophic conditions, the NRE of the closed-circuits of IFC was 29.04%, 10.53%, 8.33% higher than open-circuits, respectively, when the COD/nitrogen (C/N) ratios was 1, 2 and 3. The enhancement of IFC was the iron anode could convert a portion of nitrate to nitrite according to the abiotic cathode control experiments. The mainly functional bacteria of bio-cathode was Paracoccus (53.04%). In conclusion, the IFC could be a theoretical model for using inorganic electron donor during denitrification.

Montmorillonite supported nanoscale zero-valent iron immobilized in sodium alginate (SA/Mt-NZVI) enhanced the nitrogen removal in vertical flow constructed wetlands (VFCWs)

Lacking of electron donor generally causes the low denitrification performance of constructed wetlands (CWs). Montmorillonite supported nanoscale zero-valent iron immobilized in sodium alginate (SA/Mt-NZVI) as novel electron donor-acceptor compounds were added in the denitrification zone of vertical flow constructed wetlands (VFCWs) to enhance the nitrogen removal. The key factors of the SA/Mt-NZVI dosage, the hydraulic retention time (HRT) of VFCWs, and the C/N ratios of influent were explored. SA/Mt-NZVI significantly improved the nitrogen (NO₃^--N) removal efficiency in VFCWs. When the optimal dosage of SA/Mt-NZVI was set as 2 g and the C/N was set as 6, the highest NO₃^--N removal efficiency was improved by 32.5 ± 1.0%. The microbial community analysis of by 16S rRNA had revealed that Proteobacteria and Bacteroidetes at phylum level and Betaproteobacteria, Gammaproteobacteria, and Alphaproteobacteria at class level played an important role in nitrogen removal.