Bacterial community and nitrate removal by simultaneous heterotrophic and autotrophic denitrification in a bioelectrochemically-assisted constructed wetland

Enhancing manganese redox-driven nitrogen removal by integrating manganese ore with microelectrolysis into constructed wetlands

Microelectrolysis-integrated constructed wetlands (e-CWs) typically exhibit limited total phosphorus (TP) removal performance, while manganese ore-amended CWs (MOCWs) encounter challenges associated with the interplay between Mn redox cycling and nitrogen transformation. To overcome these limitations, this study introduced MO around the cathode or anode regions in e-CWs, designated as e-CMOCW and e-AMOCW, respectively. Results demonstrated that manganese oxides accelerated NH4+-N removal. Additionally, microelectrolysis enhanced the reductive dissolution of MO, increasing Mn2+ production. This process significantly enriched diverse denitrifying bacteria within e-CMOCW and e-AMOCW, promoting Mn redox cycling and nitrogen transformation, thereby achieving higher NO3--N and total nitrogen removal efficiencies. Moreover, elevated Mn2+ concentrations facilitated TP removal by forming Mn-P precipitates. Canna indica L. mitigated oxidative stress induced by MO and microelectrolysis through increased activity of superoxide dismutase (SOD) and catalase (CAT), ensuring its growth remained unaffected. This study proposes a novel optimization strategy to enhance pollutant removal efficiency in CWs.

Enhanced removal of perfluorooctanoic acid and perfluorooctane sulphonic acid by direct current in iron-based constructed wetlands

Iron minerals have been used for the treatment of PFOA and PFOS in constructed wetlands (CWs). Electron transfer that mediated by iron cycling is the primary mechanism for the removal of PFOA and PFOS. To further improve the electron transfer and enhance treatment efficiency of PFOA and PFOS, direct current with different voltages was applied in iron-based CWs. The results show that PFOA and PFOS removal efficiencies reached 63.2 ± 2.3 % and 57.5 ± 2.2 % at the voltage of 0.3 V, and further improved by 2.7 % and 3.5 % after the voltage increased to 0.8 V. The Cyt C that involved in electron transfer was increased to 174.9 ± 5.2 nmol/L in the cathode of voltage-added CWs. The contents of fulvic-like acids (18.2 %) and humic-like acids (9.5 %) materials that contribute to electron transfer were also 4.1 % and 2.6 % higher than that without direct current. The abundance of Geobacter that involved in electron transfer, PFOA and PFOS removal, was highly enriched in the application of direct current. Moreover, microbial pathways associated with PFOA and PFOS removal such as carbohydrate metabolism (sucrose metabolism), energy metabolism (oxidative phosphorylation), and membrane transfer (bacterial secretion system) were up-regulated. In general, the application of direct current showed excellent removal performance of PFAS through the enhanced electron transfer in iron minerals-based CWs.

Energy production and denitrogenation performance by sludge biochar based constructed wetlands-microbial fuel cells system: Overcoming carbon constraints in water

As freshwater demand grows globally, using reclaimed water in natural water bodies has become essential. Constructed wetlands (CWs) are widely used for advanced wastewater treatment due to their environmental benefits. However, low carbon/nitrogen (C/N) ratios in wastewater limit nitrogen removal, often leading to eutrophication. This study explores the use of sewage sludge biochar (SB) and activated carbon (AC) as electrodes in microbial fuel cell-constructed wetlands (MFC-CW) to enhance nitrogen removal and energy generation. Results indicated that the sludge biochar closed-circuit CW (MSBS-CW) achieved considerable total nitrogen removal (95.85 %) and maximum power density (9.05 mW/m²). Furthermore, high-throughput sequencing and functional gene analysis revealed substantial shifts in the microbial community within MSBS-CW, particularly in the electroactive bacteria (Geobacter), autotrophic denitrifying bacterium (Hydrogenophaga, Thiobacillus) and anaerobic ammonium oxidation bacteria (Candidatus_Brocadia). Electrochemical and material characterization showed that SB enhanced the cathode's electrochemical performance and the anode's biocompatibility, thereby improving denitrification and energy generation. This study demonstrates that sludge biochar is an effective low-cost electrode material for MFC-CW systems, offering a sustainable solution for nitrogen removal and energy production under carbon-constrained conditions.

Biogeochemical study of spent gunshot fate in the environment: Influence of aggressive bacterial strain

The hunting of waterfowl may contribute significantly to environmental contamination through the leaching of highly toxic elements (As, Pb, Sb) from spent gunshot deposited on hunting grounds. It is therefore clearly necessary to develop a biogeochemical protocol that might decipher the fate of spent gunshot in the environment. In that context, we present a study that follows the laboratory simulation approach and discusses the methodical pros and cons of the protocol. We performed an experimentally induced 60-day shot bio-weathering under three conditions: 1) neutral control conditions (WAT; ultrapure water), 2) control conditions (MED; sterile water with growth medium), and 3) biotic conditions (BAC; presence of Acidithiobacillus thiooxidans). We measured the elements leached from the shot (based on ICP-MS analysis), solution pH, and shot surface changes (using SEM-EDS observations). Lastly, we checked the efficiency with which plants bio-accumulate elements from MED and BAC leachates. We observed severe gunshot weathering and leaching of all the elements (reaching the maximum for Pb - 1120 μg/g in BAC treatment, which was 0.11% of the Pb burden of the shot). Among other elements studied (As, Cu, Sb, and Zn) the highest relative leaching (also in BAC) was observed for Zn and Cu, 83.6% and 61.0% respectively, a phenomenon that may be linked to the non-homogenous distribution of elements in the gunshot due to the potential existence of a Cu-Zn coating or a consequence of different solidification temperatures of the elements, which are important in the casting of gunshot. BAC treatment was also associated with the greatest surface alterations (seen by SEM-EDS), which were probably stimulated by the bacteria lowering of the pH. Plant response to metals released from shot was comparable for studied Brasica juncea and Festuca rubra, suggesting that similar bio-accumulation trends may take place in a field-weathering scenario.

Simultaneous partial nitrification, endogenous and autotrophic denitrification in a single-stage electrolysis-integrated sequencing batch biofilm reactor (E-SBBR) for stable and enhanced kitchen digested wastewater treatment

Simultaneous partial nitrification-denitrification (SPND) is a promising process for nitrogen (N) removal from kitchen digested wastewater characterized by a low C/N ratio. However, its widespread application is often restricted due to the unstable partial nitrification and unsatisfactory denitrification performance. This work developed a novel simultaneous partial nitrification, endogenous and autotrophic denitrification process using a single-stage electrolysis-integrated sequencing batch biofilm reactor (E-SBBR) with anoxic/electro-anaerobic/aerobic operating strategy. The novel process considerably enhanced the stability and N removal efficiency (NH4+-N>94.5% and TN>90.8%) of the SPND process. The pre-electro-anaerobic phase achieved alkalinity and H2 generation, and intracellular carbon storage. The increased alkalinity resulted in increased free ammonia (FA) which secured complete suppression of nitrite-oxidizing bacteria (NOB). SPND efficiency in the aerobic phase was dramatically improved using polyhydroxyalkanoates (PHAs) and H2 as electron donors for endogenous and autotrophic denitrification. Microbial community analysis indicated the successful washout of NOB and the enrichment of ammonia-oxidizing bacteria (AOB), denitrifying glycogen accumulating organisms (DGAOs), autotrophic and heterotrophic denitrifiers in the system. This research presents a distinctive SPND process for intensified kitchen digested wastewater treatment and gives insights into the underlying mechanism.

Effect of pathogen Globisporangium ultimum on plant growth and colonizing bacterial communities

Plants recruit plant-associated microbes from soil to enhance their growth and mitigate the adverse effects of pathogen invasion on plant health. How pathogens impact the interactions of the plant-associated microbes and plant growth is poorly understood. We established S-microsystems (sterile soil inoculated with 101 bacteria isolated from humus soil with Artemisia annua, Oryza sativa or Houttuynia cordata), and N-microsystems (natural soil with these plants) to evaluate the effects of the fungus Globisporangium ultimum on plant growth and their colonizing bacterial communities (CBCs). S-microsystems and N-microsystems were inoculated with and without G. ultimum, respectively. Their seedling growth and CBCs were investigated. Plant height and root numbers in A. annua, O. sativa and H. cordata S-microsystems with G. ultimum were 34.5 % and 52.8 %, 23.1 % and 31.3 %, 102.1 % and 45.0 % higher than those without G. ultimum, respectively. The CBCs were diverse among S-microsystems of A. annua, O. sativa and H. cordata, and the CBC abundances in the three S-microsystems without G. ultimum were higher than those with G. ultimum. The relative abundances of bacterial genera Rhizobium, Pseudomonas, Brevundimonas and Cupriavidus were significantly positively related to plant growth. We determined that the CBCs in A. annua, O. sativa and H. cordata were selective and related to the plant species, and can mitigate disadvantageous influences of G. ultimum on seedling growth. The plants and their CBCs' abundance and composition were differentially affected by G. ultimum. Our results provide evidence that CBCs promote plant growth due to dynamic changes in the composition and abundance of CBC members, which were affected by plant species and biotic factors.

Enhancing nitrogen removal from digested swine wastewater by anammox with aeration optimization coupling real-time control strategy

The nitrogen removal of anaerobically digested swine wastewater (ADSW) through partial nitritation and anammox is hindered by the challenge of balancing aeration between ammonia oxidizing bacteria (AOB) and anammox bacteria (AnAOB). This study focused on optimizing aeration through a real-time control strategy in an integrated fixed-film activated sludge reactor for treating ADSW. The system implemented a dual aeration mode that included both low dissolved oxygen (DO) (< 0.4 mg/L) and short-term high DO (0.6-1.2 mg/L), with pH, oxidation-reduction potential, and NH4+-N electrode values as real-time control parameters. NH4+-N removal rate increased from 3.37 to 12.82 mgN/(gVSS·h), and total nitrogen (TN) removal rate enhanced from 0.14 to 0.25 kgN/(m3·d). Increasing DO stimulated AOB activity by 31 % and provided sufficient NO2--N for AnAOB. The r-strategist AnAOB (Candidatus Kuenenia) proliferated well in the biofilm (0.25 % in flocs vs. 1.86 % in biofilm). The enrichment of denitrifiers improved organic matter and TN removal.

Autotrophic denitrification by sulfur-based immobilized electron donor for enhanced nitrogen removal: Denitrification performance, microbial interspecific interaction and functional traits

Sulfur-driven autotrophic denitrification (SdAD) is a promising nitrogen removing process, but its applications were generally constrained by conventional electron donors (i.e., thiosulfate (Na2S2O3)) with high valence and limited bioavailability. Herein, an immobilized electron donor by loading elemental sulfur on the surface of polyurethane foam (PFSF) was developed, and its feasibility for SdAD was investigated. The denitrification efficiency of PFSF was 97.3%, higher than that of Na2S2O3 (91.1%). Functional microorganisms (i.e., Thiobacillus and Sulfurimonas) and their metabolic activities (i.e., nir and nor) were substantially enhanced by PFSF. PFSF resulted in the enrichment of sulfate-reducing bacteria, which can reduce sulfate (SO42-). It attenuated the inhibitory effect of SO42-, whereas the generated product (hydrogen sulfide) also served as an electron donor for SdAD. According to the economic evaluation, PFSF exhibited strong market potential. This study proposes an efficient and low-cost immobilized electron donor for SdAD and provides theoretical support to its practical applications.

High-efficient nutrient removal in a single-stage electrolysis-integrated sequencing batch biofilm reactor (E-SBBR) for low C/N sanitary sewage treatment

To efficiently remove nutrients from low C/N sanitary sewage by conventional biological process is challenging due to the lack of sufficient electron donors. A novel electrolysis-integrated sequencing batch biofilm reactor (E-SBBR) was established to promote nitrogen and phosphorus removal for sanitary sewage with low C/N ratios (3.5-1.5). Highly efficient removal of nitrogen (>79%) and phosphorus (>97%) was achieved in the E-SBBR operating under alternating anoxic/electrolysis-anoxic/aerobic conditions. The coexistence of autotrophic nitrifiers, electron transfer-related bacteria, and heterotrophic and autohydrogenotrophic denitrifiers indicated synergistic nitrogen removal via multiple nitrogen-removing pathways. Electrolysis application induced microbial anoxic ammonia oxidation, autohydrogenotrophic denitrification and electrocoagulation processes. Deinococcus enriched on the electrodes were likely to mediate the electricity-driven ammonia oxidation which promoted ammonia removal. PICRUSt2 indicated that the relative abundances of key genes (hyaA and hyaB) associated with hydrogen oxidation significantly increased with the decreasing C/N ratios. The high autohydrogenotrophic denitrification rates during the electrolysis-anoxic period could compensate for the decreased heterotrophic rates resulting from insufficient carbon sources and nitrate removal was dramatically enhanced. Electrocoagulation with iron anode was responsible for phosphorus removal. This study provides insights into mechanisms by which electrochemically assisted biological systems enhance nutrient removal for low C/N sanitary sewage.

An overview of recent advances and future prospects of three-dimensional biofilm electrode reactors (3D-BERs)

Three-dimensional biofilm electrode reactors (3D-BERs) have attracted extensive attention in recent years due to their wide application range, high efficiency and energy saving. On the basis of traditional bio-electrochemical reactor, 3D-BERs are filled with particle electrodes, also known as the third electrodes, which can not only be used as a carrier for microbial growth, but also improve the electron transfer rate of the whole system. This paper reviews the constitution, advantages and basic principles of 3D-BERs as well as current research status and progress of 3D-BERs in recent years. The selection of electrode materials, including cathode, anode and particle electrode are listed and analyzed. Different constructions of reactors, like 3D-unipolar extended reactor and coupled 3D-BERs are introduced and discussed. Various contaminants degraded by 3D-BERs including nitrogen, azo dyes, antibiotics and the others are calculated and the corresponding degradation effects are described. The influencing factors and mechanisms are also introduced. At the same time, according to the research advances of 3D-BERs, the shortcomings and weakness of this technology in the current research process are analyzed, and the future research direction of this technology is prospected. This review aims to summarize recent studies of 3D-BERs in bio-electrochemical reaction and open a bright window to this booming research theme.

Nitrogen removal from mature landfill leachate through enhanced Partial Nitrification-Anammox process in an innovative multi-stage fixed biofilm reactor

In the current integrated PN/A method/process for mature landfill leachate treatment, microbial inhibition and low nitrogen removal capacity are the big barriers due to high ammonia concentration and low C/N. This study aimed to evaluate the performance of a high-rate nitrogen removal lab-scale reactor, which combines pre-denitrification and Partial Nitrification-Anammox (PN/A) in a multi-stage fixed biofilm reactor (MFBR), for mature landfill leachate treatment. A nitrogen removal efficiency (NRE) of 90.43 % and an average nitrogen removal rate (NRR) of 0.94 kg/m³·d were observed at an influent NH+ 4-N concentration of 2274.39 mg/L during the last operational phase. The nitrogen mass balance showed that the nitrogen concentration gradually decreases along the course, and nitrogen was mainly removed in the aerobic chambers, in which Anammox contributed to 86.4 % of the removed nitrogen, while the front anoxic chamber is mainly used to remove NO- 3-N from the recirculation. Redundancy analysis showed that the variation in NH+ 4-N concentration along the course was the main factor affecting microbial community succession, which shows that the reactor configuration enables efficient cooperation and distribution of different microorganisms. Moreover, economic analysis of MFBR process showed that the energy consumption and carbon addition were reduced by 58.9 % and 100 %, respectively. Therefore, the MFBR established in this study, with its new configuration, achieves efficient treatment of landfill leachate in a single reactor and is environmentally friendly, and could be considered as a reference for full-scale landfill leachate treatment.

Microelectrolysis-integrated constructed wetland with sponge iron filler to simultaneously enhance nitrogen and phosphorus removal

Integrating sponge iron (SI) and microelectrolysis individually into constructed wetlands (CWs) to enhance nitrogen and phosphorus removal are challenged by ammonia (NH₄⁺-N) accumulation and limited total phosphorus (TP) removal efficiency, respectively. In this study, a microelectrolysis-assisted CW using SI as filler surrounding the cathode (e-SICW) was successfully established. Results indicated that e-SICW reduced NH₄⁺-N accumulation and intensified nitrate (NO₃^--N), the total nitrogen (TN) and TP removal. The concentration of NH₄⁺-N in the effluent from e-SICW was lower than that from SICW in the whole process with 39.2-53.2 % decrease, and as the influent NO₃^--N concentration of 15 mg/L and COD/N ratio of 3, the removal efficiencies of NO₃^--N, TN and TP in e-SICW achieved 95.7 ± 1.9 %, 79.8 ± 2.5 % and 98.0 ± 1.3 %, respectively. Microbial community analysis revealed that hydrogen autotrophic denitrifying bacteria of Hydrogenophaga was highly enriched in e-SICW.

Efficient carbon removal and excellent anti-clogging performance have been achieved in multilayer quartz sand horizontal subsurface flow constructed wetland for domestic sewage treatment

The present study aimed to investigate the application of a multilayer quartz sand substrate horizontal subsurface flow constructed wetland (HSFCW) for campus sewage treatment. It aimed to assess the pollutant removal efficiency and anti-clogging performance under the suggested maximum organic loading rate (250 g/m²/d). The results of the multilayer HSFCW (CW6) were compared to the mololayer HSFCW (CW1) for the removal of the chemical oxygen demand (COD), solid accumulation, and microbial communities. During operation, the combination conditions of high hydraulic loading rate (HLR) with low COD concentration were better for COD removal under a high organic loading rate (OLR) of 200-300 g/m²/d. The maximum removal rate reached 80.4% in CW6 under high HLR, which was 13.8% higher than that in CW1, showing better adsorption and biodegradation ability of organic matter. Impressive clogging resistance capacity was found in CW6 due to the lower contents of the insoluble organic matter (IOM) that are prone to clogging, indicating full degradation of organic matters, particularly IOM, in CW6 under high HLR. Less abundance of unclassified Chitinophagaceae (under low HLR), Pedobacter and Saccharibacteria_genera_incertae_sedis (under high HLR) in CW6, which contributed to aerobic membrane fouling, helped to prevent clogging. Moreover, Brevundimonas, Cloacibacterium, Citrobacter, Luteimonas contributed to IOM degradation, thus further enhancing the anti-clogging performance. In view of the better clogging resistance performance, the application of CW6 operated under high HLR and low COD concentrations was recommended to achieve economical, efficient, and steady COD removal for domestic sewage treatment in long-term operation.

Understanding the impacts of intermittent electro field on the bioelectrochemical aniline degradation system: Performance, microbial community and functional enzyme

On account of the lack of a sustainable electron donor source and the inhibitory effect of aniline on denitrogenation make it tough to achieve simultaneous removal of aniline and nitrogen. Herein, the strategy of adjusting electric field mode was applied to the electro-enhanced sequential batch reactors (E-SBRs: R1 (continuous ON), R2 (2 h-ON/2 h-OFF), R3 (12 h-ON/12 h-OFF), R4 (in the aerobic phase ON), R5 (in the anoxic phase ON)) to treat aniline wastewater. Aniline removal rate reached approximately 99% in the five systems. Decreasing electrical stimulation interval from 12 to 2 h significantly improved the electron utilization efficiency for aniline degradation and nitrogen metabolism. The total nitrogen removal was achieved from 70.31% to 75.63%. Meanwhile, the hydrogenotrophic denitrifiers of Hydrogenophaga, Thauera, and Rhodospirillales, enriched in reactors of minor electrical stimulation interval. Accordingly, the expression of functional enzyme related to electron transport was incremental with the proper electrical stimulation frequency.

Performance and mechanism of sacrificed iron anode coupled with constructed wetlands (E-Fe) for simultaneous nitrogen and phosphorus removal

Three anodic biofilm electrode coupled CWs (BECWs) with graphite (E-C), aluminum (E-Al), and iron (E-Fe), respectively, and a control system (CK) were constructed to evaluate the removal performance of N and P in the secondary effluent of wastewater treatment plants (WWTPs) under different hydraulic retention time (HRT), electrified time (ET), and current density (CD). Microbial communities, and different P speciation, were analyzed to reveal the potential removal pathways and mechanism of N and P in BECWs. Results showed that the optimal average TN and TP removal rates of CK (34.10% and 55.66%), E-C (66.77% and 71.33%), E-Al (63.46% and 84.93%), and E-Fe (74.93% and 91.22%) were obtained under the optimum conditions (HRT 10 h, ET 4 h, CD 0.13 mA/cm²), which demonstrated that the biofilm electrode could significantly improve N and P removal. Microbial community analysis showed that E-Fe owned the highest abundance of chemotrophic Fe(II) (Dechloromonas) and hydrogen autotrophic denitrifying bacteria (Hydrogenophaga). N was mainly removed by hydrogen and iron autotrophic denitrification in E-Fe. Moreover, the highest TP removal rate of E-Fe was attributed to the iron ion formed on the anode, causing co-precipitation of Fe(II) or Fe(III) with PO₄^3--P. The Fe released from the anode acted as carriers for electron transport and accelerated the efficiency of biological and chemical reactions to enhance the simultaneous removal of N and P. Thus, BECWs provide a new perspective for the treatment of the secondary effluent from WWTPs.

Coupled mixotrophic denitrification and utilization of refractory organics driven by Mn redox circulation for significantly enhanced nitrogen removal

Coupled mixotrophic denitrification and degradation of organics driven by redox transition of Mn for nitrogen removal has attracted much attention. Herein, this study explored the removal performance and mechanisms for nitrogen and refractory organics from secondary effluent in up-flow MnOx biofilter. Results showed that the removal of organics and nitrate was significantly enhanced by the synergistic process of heterotrophic denitrification and Mn(II)-driven autotrophic denitrification (MnAD), which were originated from the facilitation of Mn circulation. But nitrate removal was closely related to the types of carbon source and Mn(II) concentration. Single small molecular carbon source (glucose) performed better than mixed carbon source (humic acid and glucose) in nitrate removal process (74.8% in stage 1-2 vs. 54.1% in stage 3-5). And raising external Mn(II) concentration increased the contribution of MnAD (60.2% in stage 5 vs. 46.5% in stage 3) to nitrate removal. Furthermore, the relationship between Mn/N transformation and microbial community structure shifts revealed that the redox transition between Mn(II) and Mn(IV) promoted the enrichment of denitrogenation bacteria and functional genes, thus contributing to pollutants removal. Our studies expand the knowledge of MnOx-mediated pollutants removal processes and support the potential application of MnOx for removal of residual refractory organics and nitrogen.

A comprehensive review on mixotrophic denitrification processes for biological nitrogen removal

Biological denitrification is the most widely used method for nitrogen removal in water treatment. Compared with heterotrophic and autotrophic denitrification, mixotrophic denitrification is later studied and used. Because mixotrophic denitrification can overcome some shortcomings of heterotrophic and autotrophic denitrification, such as a high carbon source demand for heterotrophic denitrification and a long start-up time for autotrophic denitrification. It has attracted extensive attention of researchers and is increasingly used in biological nitrogen removal processes. However, so far, a comprehensive review is lacking. This paper aims to review the current research status of mixotrophic denitrification and provide guidance for future research in this field. It is shown that mixotrophic denitrification processes can be divided into three main kinds based on different kinds of electron donors, mainly including sulfur-, hydrogen-, and iron-based reducing substances. Among them, sulfur-based mixotrophic denitrification is the most widely studied. The most concerned influencing factors of mixotrophic denitrification processes are hydraulic retention times (HRT) and ratio of chemical oxygen demand (COD) to total inorganic nitrogen (C/N). The dominant functional bacteria of sulfur-based mixotrophic denitrification system are Thiobacillus, Azoarcus, Pseudomonas, and Thauera. At present, mixotrophic denitrification processes are mainly applied for nitrogen removal in drinking water, groundwater, and wastewater treatment. Finally, challenges and future research directions are discussed.

Cold temperature mediated nitrate removal pathways in electrolysis-assisted constructed wetland systems under different influent C/N ratios and anode materials

Electrolysis had proven to be useful for the enhanced performance in constructed wetlands (CWs). While at cold temperature, the nitrate removal pathways, plant physiological characteristics and microbial community structure in electrolysis-assisted CWs were unclear. Therefore, the purification performance of three electrolysis-assisted horizontal subsurface-flow constructed wetlands (E-HSCWs) with different anodes and a control system in cold seasons were evaluated in this study. E-HSCWs showed a 2.02-83.21% increase of total nitrogen (TN) removal when compared to control, and the gaps were enlarged with increasing C/N (chemical oxygen demand/total nitrogen, COD/TN) ratios. Nitrite accumulation in E-HSCWs presented a first increase then went down trend with increasing C/N ratios, compared to a steady increase in control system. The optimum C/N ratio was 8 in E-HSCWs for both TN and COD removal. Moreover, Ti|IrO₂-Ta₂O₅ (Ti) anode showed the highest potential for TN and COD removal. Less root weight, shorter root length and reduced TN and total phosphorus (TP) contents in roots were observed in wetland plants (Iris sibirica) of E-HSCWs. In E-HSCWs with Fe and C anodes, the nitrate removal was mainly accomplished by autotrophic denitrifier Hydrogenophaga. While in E-HSCWs with Ti anode, the synergistic effect of autotrophic denitrifier Hydrogenophaga and heterotrophic denitrifiers Acidovorax, Simplicispira, Zoogloea accounted for the nitrate removal. These results showed that E-HSCWs at proper C/N ratio of 8 would be promising for nitrate removal at cold temperature.

Advanced bioelectrochemical system for nitrogen removal in wastewater

Nitrogen (N) pollution in water has become a serious issue that cannot be ignored due to the harm posed by excessive nitrogen to environmental safety and human health; as such, N concentrations in water are strictly limited. The bioelectrochemical system (BES) is a new method to remove excessive N from water, and has attracted considerable attention. Compared with other methods, it is highly efficient and has low energy consumption. However, the BES has not been applied for N removal in practice due to lack of in-depth research on the mechanism and construction of high-performance electrodes, separators, and reactor configurations; this highlights a need to review and examine the efforts in this field. This paper provides a comprehensive review on the current BES research for N removal focusing on the reaction principles, reactor configurations, electrodes and separators, and treatment of actual wastewater; the corresponding performances in these realms are also discussed. Finally, the prospects for N removal in water using the BES are presented.

The interaction between Pseudomonas C27 and Thiobacillus denitrificans in the integrated autotrophic and heterotrophic denitrification process

Compared to autotrophic and heterotrophic denitrification process, the integrated autotrophic and heterotrophic denitrification (IAHD) shows wider foreground of applications in the actual wastewaters with organic carbon, nitrogen and sulfur co-existing. The efficient co-removal of sulfur, nitrogen, and carbon in the IAHD system is guaranteed by the interaction between heterotrophic and autotrophic denitrificans. In order to further explore the interaction between functional bacteria, Pseudomonas C27 and Thiobacillus denitrifcans were selected as typical heterotrophic and autotrophic bacteria, and their characteristics metabolic responses to different sulfide concentrations were studied. Pseudomonas C27 had higher metabolic activity than T. denitrificans in the IAHD medium with sulfide concentration of 3.12-15.62 mmol/L. Moreover, the fastest sulfide removal rate (0.35 mmol/L·h) was achieved with a single inoculation of Pseudomonas C27. Meanwhile, in mixed inoculant conditions, the interaction between Pseudomonas C27 and T. denitrificans (P:T = 3:1, P:T = 1:1 and P:T = 1:3) yielded the highest sulfide removal efficiency (more than 85%) when sulfide concentration was 6.25-12.5 mmol/L. Additionally, the sulfide removal rate increased with the inoculation proportion of Pseudomonas C27. Thus, this apparent interaction provided a theoretical basis for further understanding and guidance on the efficient operation of IAHD system.

Nitrate Water Contamination from Industrial Activities and Complete Denitrification as a Remediation Option

Freshwater is a scarce resource that continues to be at high risk of pollution from anthropogenic activities, requiring remediation in such cases for its continuous use. The agricultural and mining industries extensively use water and nitrogen (N)-dependent products, mainly in fertilizers and explosives, respectively, with their excess accumulating in different water bodies. Although removal of NO3 from water and soil through the application of chemical, physical, and biological methods has been studied globally, these methods seldom yield N2 gas as a desired byproduct for nitrogen cycling. These methods predominantly cause secondary contamination with deposits of chemical waste such as slurry brine, nitrite (NO2), ammonia (NH3), and nitrous oxide (N2O), which are also harmful and fastidious to remove. This review focuses on complete denitrification facilitated by bacteria as a remedial option aimed at producing nitrogen gas as a terminal byproduct. Synergistic interaction of different nitrogen metabolisms from different bacteria is highlighted, with detailed attention to the optimization of their enzymatic activities. A biotechnological approach to mitigating industrial NO3 contamination using indigenous bacteria from wastewater is proposed, holding the prospect of optimizing to the point of complete denitrification. The approach was reviewed and found to be durable, sustainable, cost effective, and environmentally friendly, as opposed to current chemical and physical water remediation technologies.

Selecting suitable seed sludge for anammox enrichment: Role of influent characteristics and reactor operational conditions

The study investigated the suitability of three different sludge collected from diverse environments for anammox process establishment. Sludge was collected from SBR (S1) treating municipal wastewater, nitrification tank (S2), denitrification tank (S3) treating fertilizer industrial wastewater. The microbial community in the seed sludge was studied. The presence of anammox bacteria was detected only in seed sludge S2 treating high NH₄⁺-N wastewater. Seed sludge S3 showed high abundance of denitrifiers due to NO₃^--N and organic carbon rich environments in denitrification tank. The anammox start-up performances of sludge were assessed. S2 achieved start-up within 65 days whereas S1 and S3 showed longer start-up period of 79 and 93 days, respectively. S1, S2, S3 achieved nitrogen removal rate of 148.84 gN m^-3day^-1, 159.70 gNm^-3day^-1 and 120.90 gNm^-3day^-1, respectively. Influent NH₄⁺-N, NO₃^--N and organic carbon concentrations governed the abundance of anammox and denitrifying bacteria in seed sludge thereby impacting the anammox start-up.

Vertical-flow constructed wetland based on pyrite intensification: Mixotrophic denitrification performance and mechanism

Deep nitrogen removal from low-carbon wastewater is a pressing water treatment challenge as of yet. Eight sets of vertical-flow constructed wetland (VFCW) intensified by pyrite were designed and applied to treat with low C/N ratio wastewater in this research. The results showed that the addition of pyrite (100% added) significantly promoted TN removal with an efficiency higher than 27.05% under low C/N ratio conditions, indicating that mixotrophic denitrification was achieved in VFCW. Microbial analysis showed that the community structure and diversity of microorganisms were changed significantly, and the growth of autotrophic (Thiobacillus) and heterotrophic bacteria (Thauera) concomitantly enhanced. It is recommended that the addition amount of pyrite is 75% of the wetland volume, meantime, mixing evenly with 25% high porosity substrate (such as activated carbon, volcanic stone, etc.), which could enhance the effective adhesion of microorganisms and their contact area with pyrite, ultimately improve the denitrification capacity of the VFCW.

In situ COD monitoring with use of a hybrid of constructed wetland-microbial fuel cell

The hybrid system of constructed wetland and microbial fuel cell (CW-MFC) used as a biosensor is becoming a new research focus with the advantage of resisting the shock loading and enriching more electricigens. In this study, a structural parameter S integrating the size, the position and the spacing of the anode and the cathode was proposed. And the electrogenesis and biosensing performances of the vertical flow CW-MFC biosensors were evaluated at different S values. The results showed that all the three biosensors could achieve good monitoring for COD (R² > 0.97). And the coulombic yield was more suitable for the response signal than output voltage. But different biosensing properties including detection signal, detection range, detection time, correlation fitting degree and sensitivity were also displayed. Further, in order to optimize the biosensing performance, the coulombic yield in stable voltage stage (Qs) was proposed which can shorten the detection time by 70% at most. On the anodes, abundant nitrogen-transforming bacteria (NTB) were enriched as well as electrochemically active bacteria (EAB). The competition of NTB for substrates and electrons with EAB disturbed the output voltage signal but not affect the stability of coulombic yield signal. Moreover, the significant linear correlation between the S values and the ratios of EAB to NTB colonized both on anodes and on cathodes indicated the differences of the electricity generation and biosensing performance at the different structural parameters.

Enhanced nitrogen removal in an electrochemically coupled biochar-amended constructed wetland microcosms: The interactive effects of biochar and electrochemistry

The interactive effects of both biochar (BC) and electrochemistry (EC) can affect nitrogen (N) removal process. However, little is known about how this function in constructed wetland (CW) systems. In this study, an electrochemically (EC) coupled BC-amended saturated subsurface vertical flow constructed wetland (BECW) systems were established to enhance nitrogen (N) removal. Other three CW systems: without BC and EC (CW); with EC only (ECW); and with BC only (BCW) were performed as controls. Results indicated that the total nitrogen (59.88%-93.03%) and nitrate‑nitrogen (83.14%-100%) of the BECW system were significantly enhanced (p < 0.05) compared with the control systems. Treated WWTP tail-water could meet Class-IV of the Surface Water Quality Standard (GB3838-2002) in China by the BECW system. The enhanced N removal in the BECW system could be attributed to (1) the autotrophic denitrification process in which H₂ and Fe²⁺ provided by the cathode and anode acted as electron donors; and (2) BC addition acting as substrate could improve the activity, diversity and richness of microorganisms. Microbial community analysis further indicated that high N removal in the BECW system was significantly dependent on the synergy between the heterotrophic and autotrophic denitrifiers, facilitated by BC and EC interaction. Results illustrate that the BECW system is a feasible and eco-sustainable technology for treating low C/N tail-water from WWTPs. This work provides a novel and fundamental understanding of the electrochemically coupled biochar-amended CW system. These results could serve as a theoretical basis for the engineered applications in the deep purification of WWTPs' tail-water.

Enhanced fluoroglucocorticoid removal from groundwater in a bio-electrochemical system with polyaniline-loaded activated carbon three-dimensional electrodes: Performance and mechanisms

This study aimed to investigate the removal performance and mechanisms of dexamethasone (DEX), a representative fluoroglucocorticoid (FGC), from micro-polluted oligotrophic groundwater in a bio-electrochemical system amended with polyaniline-loaded activated carbon (PANI@AC) as three-dimensional particle electrodes (BES-3D). The BES-3D achieved a DEX removal efficiency of 95.7%, which was 39.0% and 14.1% higher than that of a single biological system (SBIO) and two-dimensional bio-electrochemical system (BES-2D), respectively. The preliminary metabolic mechanism of defluorination accounted for 53.5%, 41.1%, and 16.3% in BES-3D, BES-2D, and SBIO, respectively, which was accompanied by demethylation, side-chain fracture, and hydroxyl oxidation for ketone formation and final-ring opening. The main mechanism by which removal was improved in BES-3D was the enrichment of functional microbes and enhancement of the expression of dehalogenation genes. The relative abundance of functional microbes with electron transfer ability and reductive dehalogenating genera, i.e., Pseudomonas, Methylotenera, Desulfuromonas, Sphingomonas, and Microbacterium, in BES-3D was 3.7-6.1 times higher and the copy number of functional genes was 1.9 times higher than those of SBIO, which contributed to the high DEX removal.

Biological treatments of mercury and nitrogen oxides in flue gas: biochemical foundations, technological potentials, and recent advances

Nitrogen oxides (NO_x) and mercury (Hg) are commonly found coexistent pollutants in combustion flue gas. Ever-increasing emission of atmospheric Hg and NO_x has caused considerable environmental risks. Traditional flue gas demercuration and denitration techniques have many socioeconomic, technological and environmental drawbacks. Biotechnologies can be a promising and prospective alternative strategy. This article discusses theoretical foundation (biochemistry and genomic basis) and technical potentials (Hg⁰ bio-oxidation coupled to denitrification) of bioremoval of Hg and NO_x in flue gas and summarized recent experimental and technological advances. Finally, several specific technical perspectives have been put forward to better guide future researches.

Clogging in subsurface wastewater infiltration beds: genesis, influencing factors, identification methods and remediation strategies

Subsurface wastewater infiltration (SWI) is an environmentally friendly technology for the advanced treatment of domestic sewage. Clogging (including physical, chemical and biological clogging) of the porous medium not only directly reduces the hydraulic load (treatment efficiency), but also reduces the service life. Although clogging has become one of the key issues discussed in several reports, there are still several gaps in understanding, especially in its occurrence process and identification. SWI clogging causes, development process and solutions are different from those of constructed wetlands. This article quotes some reports on constructed wetlands to provide technical ideas and reference for revealing SWI clogging problems. Based on the analysis of the clogging genesis, this review gathers the main factors that affect the degree of clogging, and new methods for the identification of clogging conditions. Some preventive and unclogging measures/strategies are presented. Finally, it is suggested that to effectively alleviate the clogging phenomenon and extend the service life, priority should be given to the comprehensive analysis of wastewater quality and solid constituents accumulated in the pores. Then, the effectiveness of in-situ strategies, such as alternating operation will be the main focuses of future research.

Community Composition and Spatial Distribution of N-Removing Microorganisms Optimized by Fe-Modified Biochar in a Constructed Wetland

Microbial nitrogen (N) removal capability can be significantly enhanced in a horizontal subsurface flow constructed wetland (HSCW) amended by Fe-modified biochar (FeB). To further explore the microbiological mechanism of FeB enhancing N removal, nirS- and nirK-denitrifier community diversities, as well as spatial distributions of denitrifiers and anaerobic ammonium oxidation (anammox) bacteria, were investigated in HSCWs (C-HSCW: without biochar and FeB; B-HSCW: amended by biochar; FeB-HSCW: amended by FeB) treating tailwater from a wastewater treatment plant, with C-HSCW without biochar and FeB and B-HSCW amended by biochar as control. The community structures of nirS- and nirK-denitrifiers in FeB-HSCW were significantly optimized for improved N removal compared with the two other HSCWs, although no significant differences in their richness and diversity were detected among the HSCWs. The spatial distributions of the relative abundance of genes involved in denitrification and anammox were more heterogeneous and complex in FeB-HSCW than those in other HSCWs. More and larger high-value patches were observed in FeB-HSCW. These revealed that FeB provides more appropriate habitats for N-removing microorganisms, which can prompt the bacteria to use the habitats more differentially, without competitive exclusion. Overall, the Fe-modified biochar enhancement of the microbial N-removal capability of HSCWs was a result of optimized microbial community structures, higher functional gene abundance, and improved spatial distribution of N-removing microorganisms.

Sulfur transformation in sulfur autotrophic denitrification using thiosulfate as electron donor

Thiosulfate is frequently used as an energy source and electron donor in autotrophic denitrification (AD) for removing nitrate from wastewater. However, transforming pathways of S₂O₃^2- in this process is unclear. Herein, the aim of this study is to explore possible transforming pathways of sulfur compounds in thiosulfate-based AD process. After measuring the variation of NO₃^-, NO₂^-, and various sulfur compounds such as S⁰, SO₄^2-, S₂O₃^2-, acid volatile sulfide (AVS), and S^2- in the presence and absence of S₂O₃^2-, the variation process of S₂O₃^2- and the contribution of various sulfur compounds were analyzed. The results indicated that S⁰, AVS, and S^2- were the intermediate products when S₂O₃^2- was applied as an electron donor. All S₂O₃^2-, S⁰, AVS, and S^2- could act as electron donors in the nitrate removal process with the final products of SO₄^2-. The utilization priority of these four sulfur sources was presumed in the following order: S^2- > S₂O₃^2- > AVS ≈ S⁰. Furthermore, sulfur transformation and balance in nitrate removal process was also investigated. This suggests the transforming pathways of sulfur compounds in denitrification process. Nitrogen removal and sulfur conversion process are dependent on the presence of microorganisms in the sludge.

Nutrient Removal Process and Cathodic Microbial Community Composition in Integrated Vertical-Flow Constructed Wetland - Microbial Fuel Cells Filled With Different Substrates

An integrated vertical-flow constructed wetland-microbial fuel cell system (CW-MFC), consisting of an up-flow chamber and a down-flow chamber, was constructed to treat synthetic sewage wastewater. The performance of CW-MFCs filled with different substrates [i.e., ceramsite (CM-A), quartz (CM-B), and zeolite (CM-C) granules] under various hydraulic retention times (HRTs, 7.6, 4.0, and 2.8 d) was evaluated. Efficient and stable nitrogen (N) and phosphorus (P) removals were observed in CM-A under different HRTs, while the voltage outputs of the CW-MFCs was greatly reduced as the HRTs decreased. With an HRT of 2.8 d, the ammonium (NH4 +-N) and orthophosphate (PO4 3--P) removal efficiencies in CM-A were as high as 93.8 and 99.6%, respectively. Bacterial community analysis indicates that the N removal in the cathode area of CM-A could potentially benefit from the appearance of nitrifying bacteria (e.g., Nitrosomonas and Nitrospira) and relatively high abundance of denitrifiers involved in simultaneous nitrification and denitrification (e.g., Hydrogenophaga, Zoogloea, and Dechloromonas) and denitrifying sulfide removal (e.g., Thauera). Additionally, the difference in N removal efficiency among the CW-MFCs could be partly explained by higher iron (Fe) content in milled ceramsite granules and higher abundance of denitrifiers with nitrate reduction and ferrous ions oxidation capabilities in CM-A compared with that in CM-B and CM-C. Efficient PO4 3--P removal in CM-A was mainly ascribed to substrate adsorption and denitrifying phosphorus (P) removal. Concerning the substantial purification performance in CM-A, ceramsite granules could be used to improve the nutrient removal efficiency in integrated vertical-flow CW-MFC.

Origin and stability of uranium accumulation-layers in an Alpine histosol

Uranium (U) accumulation in organic soils is a common phenomenon that can lead to high U concentration in montane wetlands. The stability of the immobilized U in natural wetlands following redox fluctuations and re-oxidation events, however, is not currently known. In this study, we investigated a saturated histosol that had accumulated up to 6000 ppm of U at 30 cm below ground level (bgl). Uranium in the waters feeding the wetland originates from the weathering of surrounding gneiss rocks, a process releasing trace amounts (<3 ppb) of soluble U into nearby streams. Redox oscillations in the first 20 cm bgl led to the accumulation of U, Ca, S in low permeability layers at 30 and 45 cm bgl. XRF measurements along the core showed that U strongly correlates with sulfur (S) and calcium (Ca), but not iron (Fe). We tested the stability of uranium in the histosol over a nine-month laboratory amendment of a large core of the histosol (∅ 30 cm; length 55 cm) with up to 500 ppm nitrate. Nitrate addition was followed by complete nitrate reduction and re-generation of oxidizing E_h conditions in the top 25 cm of the soil without U release to the soil pore waters above background levels (1-2 ppb). Our results demonstrate that, fast reduction of nitrate, sulfate, and Fe(III) occur in the soil without U release. The remarkable stability of sorbed U in the histosol may result from buffering by sulfide and S_n° and/or strong U(IV)-OM or U(VI)-OM enhanced by organic S moieties or bridging complexation by Ca. That U in the soil was immobile under nitrate addition for up to 9 months can inform remediation strategies based on the use of artificial wetlands to limit U mobility in contaminated sites.

Simultaneous wastewater treatment and energy harvesting in microbial fuel cells: an update on the biocatalysts

The development of microbial fuel cell (MFC) makes it possible to generate clean electricity as well as remove pollutants from wastewater. Extensive studies on MFC have focused on structural design and performance optimization, and tremendous advances have been made in these fields. However, there is still a lack of systematic analysis on biocatalysts used in MFCs, especially when it comes to pollutant removal and simultaneous energy recovery. In this review, we aim to provide an update on MFC-based wastewater treatment and energy harvesting research, and analyze various biocatalysts used in MFCs and their underlying mechanisms in pollutant removal as well as energy recovery from wastewater. Lastly, we highlight key future research areas that will further our understanding in improving MFC performance for simultaneous wastewater treatment and sustainable energy harvesting.

Step-feeding ratios affect nitrogen removal and related microbial communities in multi-stage vertical flow constructed wetlands

Step-feeding (SF) strategies have been adopted in several types of constructed wetlands (CWs) to enhance nitrogen (N) removal. However, it is unclear how SF affects the N-transforming bacterial communities in CWs. Herein, four multi-stage vertical flow constructed wetlands (MS-VFCWs), each including three vertical flow stages (stage 1-3), were operated under different SF ratios (0%, 10%, 20% and 30%) in the stage 2. The physicochemical influent and effluent parameters, i.e., redox potential (ORP), pH value, chemical oxygen demand (COD), total nitrogen (TN), ammonia (NH₄⁺-N), nitrate (NO₃^--N), and nitrite (NO₂^--N), free-ammonia (FA) concentration, COD/TN ratio, as well as the abundance, structure, and activity of N-transforming bacteria were investigated. Results showed that N removal in a multi-stage vertical flow constructed wetland in the absence of SF was 45.0 ± 7.74%. Alternatively, a combined SF ratio of 20% increased N removal to 61.7% ± 4.50%, accounting for a 37.1% increase compared to the SF ratio of 0%. In the microbial community, FA was determined to be the primary physicochemical parameter governing nitrification processes in MS-VFCWs. Further, partial nitrification processes played an important role in ammonium removal during stage 1, while ammonia-oxidizing archaea were major contributors to ammonium removal in stage 3. Furthermore, abundance of nitrite reductase genes (nirS, nirK) and relative abundance of denitrifying bacteria increased with increasing SF ratio; while the nirS/nirK ratio and the alpha diversity of nirK denitrifiers were significantly affected by SF ratios, and the influent NO₃^--N concentration was related to a shift in denitrifier composition toward strains containing the nirS gene. Autotrophic (e.g., Thiobacillus, Sulfurimonas, Arenimonas, Gallionella and Methyloparacoccus) and facultative chemolithoautotrophic (e.g., Pseudomonas and Denitratisoma) denitrifying bacteria were enriched in stage 2. Hence, the synergy between heterotrophic and autotrophic denitrifying bacteria promoted excellent N removal efficiency with a low COD/TN ratio.

Microporous intermittent aeration vertical flow constructed wetlands for eutrophic water improvement

To enhance eutrophic water improvement effect, three parallel lab-scale oxidation pond-vertical subsurface flow constructed wetland-stable pond combined systems with different microporous intermittent aeration positions were constructed. The purification effect of each system was determined, and the contribution rate of each part of the system was also calculated. The characters of bacterial community under different aeration positions were also analyzed. Microporous intermittent aeration rate of 5 mg/L was chosen as the aeration rate for follow-up experiment. The result showed that the best COD_Cr, total nitrogen, and total phosphorus removal efficiencies were achieved by the combined system with bottom microporous intermittent aeration, and the efficiencies were 71.04%, 79.52%, and 95.10%, respectively. The best ammonium nitrogen removal efficiency was 92.62% and was achieved by the combined system with surface microporous intermittent aeration. After analyses, 14 strains of bacteria associated with the removal of N elements were found and 8 strains of bacteria associated with P element cycle were found.

Myriophyllum elatinoides growth and rhizosphere bacterial community structure under different nitrogen concentrations in swine wastewater

In this study, Myriophyllum elatinoides growth under different nitrogen (N) concentrations (2, 250, 300, 350 and 400 mg L^-1) and changes in rhizosphere bacterial community structure were investigated. High N (>300 mg L^-1) concentrations caused reduction in M. elatinoides biomass. Growth tended to stabilize at 49 days. N concentration in roots were higher than that in stems and leaves under high N conditions. TN and NH₄⁺ removal efficiencies reached 84.0% and 87.2%, respectively, in M. elatinoides surface flow constructed wetlands (SFCWs). Rhizosphere bacterial diversity increased over time. Proteobacteria, Firmicutes, Cyanobacteria, and Bacteroidetes dominated at the phylum level. Genera Turicibacter, Allochromatium, and Methylocystis increased at low N (<300 mg L^-1) concentrations, while Pseudomonas increased at high N concentrations over the experimental period. Redundancy analysis showed that pH was strongly correlated with changes in rhizosphere bacterial community structure. These findings helped to insight into N removal mechanism in M. elatinoides.

Thiosulfate-driven autotrophic and mixotrophic denitrification processes for secondary effluent treatment: Reducing sulfate production and nitrous oxide emission

Three ecological floating beds (EFBs) with different additional electron donors including sodium thiosulfate, mixed electron donors of sodium thiosulfate and sodium acetate and without additional electron donors were established to compare the differences of nitrogen removal efficiency, nitrous oxide emission, microbial community and functional gene between autotrophic and mixotrophic denitrification. Results showed denitrification efficiency was nearly 100% in both autotrophic and mixotrophic process when electron donors were sufficient while that ranged from 4 to 43% without additional electron donors. Sodium acetate addition could effectively decrease sulfate concentration in effluent and nitrogen oxide flux. In addition, high-throughput sequencing analysis revealed autotrophic denitrifying bacteria were dominant in autotrophic denitrification while autotrophic, facultative and heterotrophic denitrifying bacteria coexisted in mixotrophic denitrification, and there was no dominant genus. For EFB with mixed external autotrophic and heterotrophic electron donors, it can not only achieve better denitrification efficiency, but also reduce the emission of nitrous oxide.

Electron storage and resupply modes during sulfur cycle enhanced nitrogen removal stability in electrochemically assisted constructed wetlands under low temperature

In this work, an electrochemically assisted vertical flow constructed wetland (E-VFCW) achieved efficient PO₄^3--P (92.9-96.6%), NO₃^--N (50.8-91.8%) and TN (38.8-73.1%) removal from synthetic sewage effluent within 1-12 h at 12 °C. Abiotic reduction, Fe(II)-, S- and H₂-dependent denitrification, as well as coupling of fermentation, acetogenesis and heterotrophic denitrification might facilitate NO₃^--N removal in the E-VFCW. Particularly, electron resupply for NO₃^--N reduction by the in-situ deposited FeS, FeS₂ and S⁰ in the E-VFCW would occur during electron supply-demand disequilibrium situations (e.g., lower HRT or temperature). Stoichiometric results suggested that 21.7-278.7 mmol e^- d^-1 from the in-situ deposited S contributed to NO₃^--N reduction under HRT of 1-6 h at 12 °C, which improved the resilience capabilities of the E-VFCW to temperature and nitrogen loads fluctuations. Overall, this work provides new insights into the modes of S cycle mediating NO₃^--N conversions in the E-VFCW under low temperature.

Denitrification with non-organic electron donor for treating low C/N ratio wastewaters

Denitrification with non-organic electron donors for treating low C/N ratio wastewater has attracted growing interests. Hydrogen, reduced sulfur compounds and ferrous ions are mainly used in autotrophic denitrification, holding promise for achieving practical applications. Recently, the development of autotrophic denitrification-based processes, such as bioelectrochemically-supported hydrogenotrophic denitrification and sulfur-/iron-based denitrification assisted multi-contaminant removal, provide opportunities for applying these processes in wastewater treatment. Exploration of the autotrophic denitrification process in terms of contaminant removal mechanism, interaction among functional microorganisms, and potential full-scale applications is thus of great importance. Here, an overview of the commonly used non-organic electron donors, e.g., hydrogen, reduced sulfur compounds and ferrous ions, in denitrification for treating low C/N ratio wastewater is provided. Also, the feasibility of applying the combined processes based on autotrophic denitrification with the compounds is discussed. Furthermore, challenges and future possibilities as well as concerns about the practical applications are envisaged in this review.

Hydrilla verticillata-Sulfur-Based Heterotrophic and Autotrophic Denitrification Process for Nitrate-Rich Agricultural Runoff Treatment

Hydrilla verticillata-sulfur-based heterotrophic and autotrophic denitrification (HSHAD) process was developed in free water surface constructed wetland mesocosms for the treatment of nitrate-rich agricultural runoff with low chemical oxygen demand/total nitrogen (C/N) ratio, whose feasibility and mechanism were extensively studied and compared with those of H. verticillata heterotrophic denitrification (HHD) mesocosms through a 273-day operation. The results showed that the heterotrophic and autotrophic denitrification can be combined successfully in HSHAD mesocosms, and achieve satisfactory nitrate removal performance. The average NO₃^--N removal efficiency and denitrification rate of HSHAD were 94.4% and 1.3 g NO₃^--N m^-3·d^-1 in steady phase II (7-118 d). Most nitrate was reduced by heterotrophic denitrification with sufficient organic carbon in phase I (0-6 d) and II, i.e., the C/N ratio exceeded 4.0, and no significant difference of nitrate removal capacity was observed between HSHAD and HHD mesocosms. During phase III (119-273 d), sulfur autotrophic denitrification gradually dominated the HSHAD process with the C/N ratio less than 4.0, and HSHAD mesocosms obtained higher NO₃^--N removal efficiency and denitrification rate (79.1% and 1.1 g NO₃^--N m^-3·d^-1) than HHD mesocosms (65.3% and 1.0 g NO₃^--N m^-3·d^-1). As a whole, HSHAD mesocosms removed 58.8 mg NO₃^--N more than HHD mesocosms. pH fluctuated between 6.9-9.0 without any pH buffer. In general, HSHAD mesocosms were more stable and efficient than HHD mesocosms for NO₃^--N removal from agricultural runoff during long-term operation. The denitrificans containing narG (1.67 × 10⁸ ± 1.28 × 10⁷ copies g^-1 mixture-soil^-1), nirS (8.25 × 10⁷ ± 8.95 × 10⁶ copies g^-1 mixture-soil^-1), and nosZ (1.56 × 10⁶ ± 1.60 × 10⁵ copies g^-1 mixture-soil^-1) of litter bags and bottoms in HSHAD were higher than those in HHD, which indicated that the combined heterotrophic and autotrophic denitrification can increase the abundance of denitrificans containing narG, nirS, and nosZ, thus leading to better denitrification performance.

Improvement of Black-Odor Water by Pichia Strain GW1 under Optimized NH3-N Degradation Conditions

In this study, a yeast strain with an outstanding NH₃-N degradation ability was isolated from the sediment of a black-odor water channel in Guangdong Province, China. Based on phenotypic and phylogenetic analysis, this strain was identified as Pichia kudriavzevii GW1. The optimum conditions for NH₃-N degradation by the GW1 strain were as follows: 0.3% inoculum concentration, 1.5 L/min aeration, pH 7, and a temperature of 35°C. Under optimized conditions, the GW1 strain degraded 95.5% of the NH₃-N. The strain was then added to simulated black-odor water under optimal degradation conditions to investigate changes to the bacterial community over time. 16S rRNA sequencing of samples collected on days 0, 7, 14, and 21 showed that, in the presence of the GW1 strain, the relative abundances of the phyla Proteobacteria, Bacteroidetes, Chloroflexi, and Firmicutes increased in the black-odor water. In addition, the relative abundance of Propionivibrio, a known NH₃-N degrading genus, increased. This study will facilitate the use of microbiological methods to repair black-odor water.

Enhancing the pollutant removal performance and biological mechanisms by adding ferrous ions into aquaculture wastewater in constructed wetland

Aquaculture wastewater seriously threatens the human health. In this study, non-poisonous iron was added into constructed wetlands to purify aquaculture wastewater and the wastewater treatment performances of CWs were explored under the treatment conditions of different plant species and different dosages of ferrous ions. The optimal treatment conditions were experimentally determined as follows: 20 mg/L ferrous ions in CWs planted with Canna indica after 7-day operation, the removal efficiencies of TN, TP and COD were respectively 95 ± 1.9%, 77 ± 1.2% and 62 ± 2%. The improvements in the pollutant removal performance depended on biological mechanisms of plants and microorganisms. The optimal dosage of iron ions could adjust enzyme activities and functional amino acids. Specific functional bacteria (Paracoccus detected based on nirK genetic information and Hydrogenophaga detected based on pufM genetic information) were cultured and domesticated by iron ions. The functional bacteria promoted nitrogen and phosphorus removals.

Response of soil microbes after direct contact with pyraclostrobin in fluvo-aquic soil

Agricultural chemicals affect the daily life of food production. However, the abuse of pesticides led to the damage to the environment. Pyraclostrobin (PYR) is commonly used strobilurin fungicide which inhibits fungal respiration through mitochondrial cytochrome-b and c1 inhibition. There is increasing concerns that PYR may adversely impact the environment. Although impacts on ecological receptors have been detailed, little information is available regarding the toxicological impact of PYR on soil microbial community dynamics and functioning. Understanding the potential impact on soil microbial populations is important. The activity of enzymes (urease, dehydrogenase, and β-glucosidase) and diversity of microbial community structure using high-throughput 16S rRNA sequencing were evaluated at different soil-PYR concentrations (0.1, 1.0, and 2.5 mg/kg) over a 48 day exposure period. Urease activity remained stable in general. Pyraclostrobin inhibited dehydrogenase activity during the exposure period. The β-glucosidase activity was inhibited on day 28 and induced on day 48 at 1.0 and 2.5 mg/kg. The genera Gp6, Exiguobacterium, Gp4, and Gemmatimonas were both the dominant genera and significantly changed genera. Pyraclostrobin had different level of influence on soil microbes containg their enzyme activity and community structure. The purpose of the current study was to examine the impact of PYR addition on soil enzymes as an indicator of soil health and to have complementary data on the impact of microbial populations. Furthermore, the study may also be the guide for further rational pesticide selection.

Effects of current intensities on the performances and microbial communities in a combined bio-electrochemical and sulfur autotrophic denitrification (CBSAD) system

The lab-scale system combined bioelectrochemical and sulfur autotrophic denitrification (CBSAD) was established to evaluate the effects of currents (50-300 mA) on both the performances and microbial communities. Results showed that the nitrate removal rate increased significantly when the current increased from 50 to 200 mA, while it slightly decreased with higher currents. Mass balance results revealed that hydrogen autotrophic denitrification contributed almost three times (70.25-78.62%) to denitrification compared with that of the sulfur part (21.38-29.75%). Illumina MiSeq sequencing showed that the currents changed the bacterial richness and diversity in this system. Phylum Firmicutes and class Clostridia predominated >50% under each condition. And multiple key bacteria capable of denitrification such as Proteiniclasticum, Thauera and Family_XI_uncultured were identified and found in higher proportions when the current was 200 mA. Therefore, this study helps revealing the mechanisms of accelerating nitrate-reduction through applied currents in the CBSAD systems.

Aerobic denitrifiers with petroleum metabolizing ability isolated from caprolactam sewage treatment pool

To improve the biological nitrogen removal efficiency of petrochemical wastewater, three aerobic denitrifiers were isolated from caprolactam sewage treatment pool. They were identified as Acinetobacter sp. YY1, Sphingomonas sp. YY2 and Pseudomonas sp. YY3, respectively. The nitrification and denitrification enzyme genes could be detected using polymerase chain reaction (PCR). Moreover, the strain YY2 was a novel aerobic denitrifier belongs to genus of Sphingomonas, which showed great ability for metabolizing aromatic hydrocarbons. In the nitrification and denitrification process, the total nitrogen (TN) removal efficiency after 48 h was 94.22% and 90.10%, respectively. In the process of simultaneous nitrification and denitrification in mixed N-source, ammonia nitrogen was preferentially utilized. Furthermore, the strain YY2 exhibited excellent extracellular polymer secretion properties and excellent aerobic denitrification capacity using petroleum refractory organic compounds, which are beneficial for the formation of bacterial micelles and the engineering applications for the treatment of petrochemical wastewater.

Nitrification/denitrification shaped the mercury-oxidizing microbial community for simultaneous Hg0 and NO removal

A denitrifying/nitrifying membrane biofilm reactor for simultaneous removal of Hg⁰ and NO was investigated. Hg⁰ and NO removal efficiency attained 94.5% and 86%, respectively. The mercury-oxidizing microbial community was significantly shaped by nitrification/denitrification after the supply of gaseous Hg⁰and NO continuously. Dominant genera Rhodanobacter and Nitrosomonas participated in Hg⁰ oxidation, nitrification and denitrification simultaneously. Hg⁰ oxidizing bacteria (Gallionella, Rhodanobacter, Ottowia, Nitrosomonas and etc.), nitrifying bacteria (Nitrosomonas, Rhodanobacter, Diaphorobacte and etc.) and denitrifying bacteria (Nitrosomonas, Rhodanobacter, Castellaniella and etc.) co-existed in the MBfR, as shown by metagenomic sequencing. X-ray photoelectron spectroscopy (XPS) and high performance liquid chromatography-inductively coupled plasma mass spectrometry (HPLC-ICP-MS) confirmed the formation of a mercuric species (Hg²⁺) from mercury bio-oxidation. Mechanism of mercury oxidation can be described as the bacterial oxidation of Hg⁰ in which Hg⁰ serves as electron donor, NO serves as electron donor in nitrification and electron acceptor in denitrification, oxygen serves as electron acceptor.

Microbial community modulates electrochemical performance and denitrification rate in a biocathodic autotrophic and heterotrophic denitrifying microbial fuel cell

A comparison of autotrophic (AD) and heterotrophic (HD) cathodic denitrification in a Microbial Fuel Cell (MFC) was made in this study. Denitrifying microbial consortia were developed from cow manure and soil and acclimatized under AD and HD conditions. The AD MFC supported the power output of 4.45 W m^-3 while removing nitrate nitrogen (NO₃^--N) at the rate of 0.118 kg NO₃^--N m^-3 d^-1. Significant power output (3.02 W m^-3) and nitrate removal rate (2.06 kg NO₃^--N m^-3 d^-1) were achieved in HD MFC. Further, 16S rDNA based community analysis revealed higher diversity in HDMFC. The genus Thauera and Pseudomonas were predominant in ADMFC while genus Klebsiella and Alkaliphilus were abundant in HDMFC. The abundance of the denitrifying genes namely narG, nirS, and nosZ were assessed with the help of quantitative PCR and presence of all the genes in both the conditions ensured the necessary molecular requirements for complete denitrification.

Simultaneous autotrophic removal of sulphate and nitrate at different voltages in a bioelectrochemical reactor (BER): Evaluation of degradation efficiency and characterization of microbial communities

The autotrophic removal of sulphate and nitrate in bioelectrochemical reactors was investigated at different external voltages (0.2, 0.4, 0.6, 0.8 and 1.0 V) under anaerobic conditions. Sulphate and nitrate removal, nitrite accumulation, reduction trend of nitrate and sulphate and microbial community structure were explored. Results indicate the highest removal efficiencies of nitrate and sulphate at 43.3 ± 2.8 and 7.1 ± 0.2 mg·l^-1·d^-1 when the voltage is 0.6 V. Moreover, nitrite accumulation decreases with increased voltage from 0.2 V to 1.0 V. Illumina high-throughput sequencing results show similar richness and diversity of bacterial species with increased voltage from 0.2 V to 0.8 V. However, with further increased voltage to 1.0 V, bacterial diversity and richness decrease significantly. Overall, significant differences in community compositions are observed at different voltages.