Denitrification performance, bioelectricity generation and microbial response in microbial fuel cell - constructed wetland treating carbon constraint wastewater

Iron-carbon enhanced constructed wetland microbial fuel cells for tetracycline wastewater treatment: Efficacy, power generation, and the role of iron-carbon

Tetracycline (TC) antibiotics wastewater is a serious threat to human health and environment. In this study, four groups of laboratory-scale constructed wetlands (CWs) with different configurations were constructed to evaluate the removal efficiency of iron-carbon (Ic) coupled constructed wetland microbial fuel cells (CW-MFC) system for different pollutants removal and bioelectricity production. The results showed that the addition of Ic significantly promoted the removal of contaminants. The maximum removal rates of COD, TN, NH4+-N, and TP were 86.13 %, 81.60 %, 79.07 %, and 97.35 %, respectively. In particular, the removal rates of TC reached 100 %. 3D-EEM analysis further confirmed the role of Ic in promoting organic degradation. The Ic-CW-MFC system also showed superiority in power generation performance with peak power density of 7.90 mW/m2 (internal resistance is 10 Ω), 88.07 % higher than the traditional CW-MFC, while the internal resistance was 68.21 % lower. Therefore, when Ic is used as the substrate of CW-MFC system, its decontamination and electricity generation performance is the best. Analysis of RDA was used to elucidate the relationship of four CWs, dominant strains and environmental factors (pH, ORP and DO). The performance of traditional CWs decreased significantly after TC addition (5-20 mg/L), but Ic-CW-MFC could effectively alleviate the inhibition effect caused by high-concentration TC wastewater. The working mechanism of Ic-CW-MFC in TC wastewater was further analyzed through typical cycle experiment and characterization. The results showed that Ic-CW-MFC is an efficient and economical wastewater treatment technology, which has great potential application value in the treatment of wastewater containing TC.

The substrate configuration influences pollutant removal in constructed wetlands: From the aspects of submerged status of substrate and carbon-felt distribution

Redox regulation dominates the pollutant removal in constructed wetlands (CWs). To enhance efficient and cost-effective nitrogen removal, this study intended to build an unsaturated zone and add carbon-felt material for electron donor/acceptor adjustment. The unsaturated zone heights (0, 10, 20 cm) and carbon-felt distribution patterns (evenly scattered (CWSE), continuously linked (CWL), and head-tail linked like microbial fuel cells (CWMFC)) were simultaneously adjusted. Moreover, their effects and underlying microbial mechanisms on water purification were investigated. Results indicated that CWs with a 20 cm unsaturated zone achieved over 99 % ammonia nitrogen removal. CWSE facilitated optimal pollutant-microbe contact, enabling efficient in-situ electron utilization for 64.27 % total nitrogen removal through simultaneous nitrification-denitrification and anammox. In CWL, continuous carbon-felt distribution allowed efficient electron transport at a relatively macro-area and enhanced electron consumption by oxygen at the surface, leading to superior ammonia oxidation (82.97 %) in the middle area of CWL. Conversely, CWMFC facilitated direct electron transfer through the whole CW, enriched Geobacter at the top and Vibrio at the bottom, achieving 84.23 % total nitrogen removal through nitrification-denitrification under high oxygenation. This study elucidated microbial community niche differentiation in CWs mediated by carbon-felt electron transport and proposed optimal application scenarios for different carbon-felt configurations.

Optimized feeding schemes of heterotrophic anodic denitrification coupled with cathodic phosphate recovery from wastewater using a microbial fuel cell

Enhanced water quality standards and increasing resource scarcity have prompted extensive research into low-cost nitrogen removal and phosphate recovery from wastewater. Microbial fuel cells (MFCs) offer a viable solution by simultaneously removing nitrogen, recovering phosphorus, and generating electrical energy. This study employed MFCs to achieve simultaneous nitrogen removal and phosphorus recovery, investigating the impact of different feeding schemes. The experimental results indicated that replacing the entire anode chamber solution and recycling the anode effluent to the cathode chamber effectively prevented the accumulation of nitrifying bacteria while achieving the highest pollutant removal performance. Under closed circuit conditions, the system consistently maintained low nitrite concentrations, achieving an average nitrate removal efficiency of 68.09 ± 1.86 % and phosphate recovery efficiency of 83.46 ± 5.30 %. Furthermore, this feeding scheme facilitated microbial growth and reproduction while also improving operational convenience. The study utilized metagenomics and other technologies to comprehensively analyze the system's operation mechanism and reasons for its excellent performance.

Reversing inhibition to promotion in phenol-ammonium metabolism via algal-microbial fuel cell: Mechanisms of phenol-ammonium interaction and synergistic removal

Addressing the challenge of metabolic inhibition between phenol and ammonium in coal gasification wastewater (CGW), this study introduced a novel algal-microbial fuel cell (AMFC). It combined the advantages of electroactive bacteria and Synechocystis to achieve synergistic metabolism, establishing a cooperative mechanism for pollutant separation and enhanced transformation to achieve the mutual promotion of phenol and ammonium removal. Remarkably, raising phenol to 1500 mg COD/L boosted ammonium removal by 31.51 % in AMFC, due to a consistently higher potential difference than the control, which enhanced extracellular electron transfer (EET) via conductive nanowire and drove ammonium migration. Similarly, elevating ammonium concentration to 150 mg/L resulted in an 11.79 % increase in phenol removal efficiency, driven by superior solution conductivity and EET, as well as more electron acceptors (oxygen) from the algal cathode. This system challenged the conventional understanding of the antagonistic relationship between phenol and ammonium. Under high phenol conditions, the electroactive bacteria Clostridium sensu stricto 1 and Acinetobacter, Perlucidibaca formed a synergistic metabolic network, whereas Zoogloea, Ideonella, and other phenol-degrading bacteria were significantly enriched in high ammonium environments. The AMFC represented a breakthrough in reversing the metabolic inhibition between phenol and ammonium, providing a novel and energy-efficient strategy for treating complex industrial wastewater.

Efficiently intensified nitrogen transformation in electrolysis-integrated constructed wetlands: Comparative performance and mechanism

This study developed constructed wetlands (CWs) with direct current (DC) electrolysis and iron-carbon (IC) microelectrolysis. Ammonium removal was more significantly enhanced with IC by 12.7-27.3 %, while the promotion of DC only existed with lower voltage. DC electrolysis continuously promoted nitrate conversion by 5.7-13.3 % compared to the control. Total nitrogen removal was 75.0-92.7 % and 85.4-93.4 % with DC and IC electrolysis, respectively limited by ammonium and nitrite accumulation. Bioelectrolysis enriched nitrifying bacteria such as Ellin6067, Nitrospira, and Nitrosomonas. Moreover, IC stimulated hydrogenotrophic denitrifying bacteria (Paracoccus, Pseudomonas, and Dechloromonas) and up-regulated gene narG, narH, and narI, aligned with higher nitrate reductase activity. In contrast, DC caused higher abundance involved in electroautotrophic denitrifying bacteria (Thiobacillus) and genes of assimilatory nitrate and nitrite reduction (narB and nirA). Additionally, both electrolysis stimulated electron production and energy metabolism for nitrogen cycling. This work provided comparative methods to intensify nitrogen transformation in CW systems.

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.

Simplification and simulation of evaluation process for low efficiency constructed wetlands based on principal component analysis and machine learning

The existing performance evaluation process of constructed wetlands (CWs) is complex, with shortcomings in both simplification of method and construction of simulation model, especially for low-efficiency CWs (LECWs, with an average close-degree calculated by the entropy weight method being <0.6). This study presents a case study of LECWs in the Ningxia region (comprising 13 subsurface flow constructed wetlands (SSF CWs) and 7 surface flow constructed wetlands (SF CWs)), employs the entropy weight method (EWM) to construct an evaluation of CW operational efficiency, simplifies evaluation indicators through principal component analysis (PCA), develops two random forest (RF) models to validate the rationality of the simplified indicators, and establishes simulation models by logistic regression (LR). The results demonstrate that the evaluation indicators of CWs can be simplified to chemical oxygen demand (COD) and total nitrogen (TN), with no significant difference observed between the evaluation results and the original model (P < 0.05), thereby indicating reliability. Moreover, the simulation model performs well with R2 values for fitting SSF CWs and SF CWs exceeding 0.8. According to the simulated results of the model, the operational efficiency of LECWs is more significantly affected by the COD removal rates compared to the TN removal rates. In comparison to influent with 0 < COD/TN < 3 and 5 < COD/TN < 8, the operational efficiency of SSF CWs and SF CWs is optimal when COD/TN is between 3 and 5. These research findings may provide valuable support for streamlining evaluation processes and daily management for LECWs.

Baffled flow constructed wetland-microbial fuel cell coupling systems for combined secondary and tertiary wastewater treatment with simultaneous bioelectricity generation

Baffled flow constructed wetland-microbial fuel cell (BFCW-MFC) coupling systems were constructed with baffles embedded in cathode chamber. The performance of BFCW-MFCs operated at different hydraulic retention times (HRTs) was evaluated. At the representative HRT of 48 h, embedding 1 or 2 baffles (i.e., BFCW-MFC1 and BFCW-MFC2) produced 32.9 % (29.5 mW/m3) and 53.2 % (34.0 mW/m3) more power density than control system (22.2 mW/m3), respectively. Comparable organics biodegradation efficiencies were observed in BFCW-MFCs at the same HRTs. BFCW-MFC1 and BFCW-MFC2 had higher ammonium and total nitrogen removal efficiency. All systems had decreased nitrogen removal performance as shortening HRT from 72 to 12 h. Multiple nitrogen removal processes were involved, including ammonium oxidation, anammox, and heterotrophic and autotrophic denitrification. The predominant bacteria on electrodes were identified for analyzing bioelectricity generation and wastewater treatment processes. Generally, simultaneous wastewater treatment and bioelectricity generation were obtained in BFCW-MFCs, and embedding 1 or 2 baffles was preferable.

Review of cathodic electroactive bacteria: Species, properties, applications and electron transfer mechanisms

Cathodic electroactive bacteria (C-EAB) which are capable of accepting electrons from solid electrodes provide fresh avenues for pollutant removal, biosensor design, and electrosynthesis. This review systematically summarized the burgeoning applications of the C-EAB over the past decade, including 1) removal of nitrate, aromatic derivatives, and metal ions; 2) biosensing based on biocathode; 3) electrosynthesis of CH4, H2, organic carbon, NH3, and protein. In addition, the mechanisms of electron transfer by the C-EAB are also classified and summarized. Extracellular electron transfer and interspecies electron transfer have been introduced, and the electron transport mechanism of typical C-EAB, such as Shewanella oneidensis MR-1, has been combed in detail. By bringing to light this cutting-edge area of the C-EAB, this review aims to stimulate more interest and research on not only exploring great potential applications of these electron-accepting bacteria, but also developing steady and scalable processes harnessing biocathodes.

Simulation and optimisation of magnetic and experimental study of magnetic field coupling constructed wetland

This study developed a novel constructed wetland (CW) coupled with a magnetic field for treating domestic wastewater, and the magnetic field distribution was solved and optimised by the finite element method. Herein, we investigated the effects of optimising magnetic field optimisation and studied its impact on CW treatment performance and the responses of a microbial community. The optimisation results showed that the average magnetic field strength of the CW unit increases from 3 to 8 mT, and the proportion of areas with magnetic field strength greater than 5 mT also increases from 30% to 74%. The water quality analysis results showed that the removal of chemical oxygen demand (COD) and NH4+-N (p < 0.01) was significantly increased by the magnetic field (average 3 mT), increasing by 12.2% and 8.49%, respectively. Moreover, the removal of COD and NH4+-N (p < 0.01) was more significantly increased by M-VFCW(O) (average 8 mT), increasing by 15.58% and 49.1%, respectively. The magnetic field application shifted significantly the abundance of dominant bacteria in CWs. Relative abundance of dominant bacteria such as Proteobacteria (63.3%), Firmicutes (4.72%) and Actinobacteria (2.11%) that played an important role in organics removal and nitrification and denitrification-related bacteria such as Nitrospirae (1.48%) and Planctomycetes (9.58%) significantly promoted in M-VFCW(O). These results suggest that introducing a magnetic field into CWs may improve organics and nitrogen removal via the biological process, and the optimisation of the magnetic field was significant in enhancing the performance of VFCWs.

Insight into the microbial community of denitrification process using different solid carbon sources: Not only bacteria

There is a lack of understanding about the bacterial, fungal and archaeal communities' composition of solid-phase denitrification (SPD) systems. We investigated four SPD systems with different carbon sources by analyzing microbial gene sequences based on operational taxonomic unit (OTU) and amplicon sequence variant (ASV). The results showed that the corncob-polyvinyl alcohol sodium alginate-polycaprolactone (CPSP, 0.86±0.04 mg NO3--N/(g·day)) and corncob (0.85±0.06 mg NO3--N/(g·day)) had better denitrification efficiency than polycaprolactone (PCL, 0.29±0.11 mg NO3--N/(g·day)) and polyvinyl alcohol-sodium alginate (PVA-SA, 0.24±0.07 mg NO3--N/(g·day)). The bacterial, fungal and archaeal microbial composition was significantly different among carbon source types such as Proteobacteria in PCL (OTU: 83.72%, ASV: 82.49%) and Rozellomycota in PVA-SA (OTU: 71.99%, ASV: 81.30%). ASV methods can read more microbial units than that of OTU and exhibit higher alpha diversity and classify some species that had not been identified by OTU such as Nanoarchaeota phylum, unclassified_ f_ Xanthobacteraceae genus, etc., indicating ASV may be more conducive to understand SPD microbial communities. The co-occurring network showed some correlation between the bacteria fungi and archaea species, indicating different species may collaborate in SPD systems. Similar KEGG function prediction results were obtained in two bioinformatic methods generally and some fungi and archaea functions should not be ignored in SPD systems. These results may be beneficial for understanding microbial communities in SPD systems.

Performance of micro-pressure double-cycle coupled membrane integrated bioreactor for the treatment of urban sewage

Based on the theory of nitrogen and phosphorus removal and technical requirements, a micro-pressure double-cycle bioreactor coupled with membrane components was used to treat municipal wastewater. The method realized the simultaneous removal of organic matter, nitrogen, and phosphorus in the same reactor and had the characteristics of membrane bioreactor process. Results showed that the average removal efficiency of COD, NH+4-N, TN, and TP were 93.74%, 95.1%, 71.85%, and 81.03%, respectively. During operation, Proteobacteria and Bacteroidetes were the main dominant bacteria, and they had complete nitrogen and phosphorus metabolic pathways. Owing to the low protein content in the mixture, the design of film placement in the micro-precipitation zone was conducive to alleviating the membrane pollution caused by the accumulation of protein, thereby improving the effluent quality and extending the service life of the membrane components.

The influence of various chemical oxygen demands on microbial fuel cells performance using leachate as a substrate

Microbial fuel cells (MFCs), hailed as a promising technology, hold the potential to combat various wastewater pollutants while simultaneously converting their chemical energy into electricity through biocatalysts. This study explores the applicability of a dual compartment MFC (DC-MFC) under varying conditions, targeting the removal of chemical oxygen demand (COD) from landfill leachate and electricity generation. In this setup, anaerobic sludge from a wastewater treatment plant serves as the inoculum in the anode compartment of the MFC, with a Nafion117 membrane acting as the separator between MFC units. The cathode compartments are filled with distilled water and continually aerated for 24 h to enhance air supply. The study assesses the MFC's performance across different COD concentrations, focusing on COD removal, power generation, and Coulombic efficiency. The findings reveal that COD removal efficiency is notably enhanced at higher concentrations of organic matter. Specifically, at a COD concentration of 3325.0 mg L-1, the MFC exhibited the highest COD removal efficiency (89%) and maximum power density (339.41 mWm-2), accompanied by a Coulombic efficiency of 25.5%. However, as the initial substrate concentration increased to 3825 mg L-1, the efficiency decreased to 72%, with a Coulombic efficiency of 13.56% and a power density of 262.34 mWm-2. Optical density levels increased due to bacterial growth at ambient temperature and neutral pH, reflecting the dynamic microbial response within the system.

Effect of ibuprofen (IBU) on the sulfur-based and calcined pyrite-based autotrophic denitrification (SCPAD) systems with two filling modes: Performance and toxic response mechanism

To investigate the effect of ibuprofen (IBU) on the sulfur-based and calcined pyrite-based autotrophic denitrification (SCPAD) systems, two individual reactors with the layered filling (L-SCPAD) and mixed filling (M-SCPAD) systems were established via sulfur and calcined pyrite. Effluent NO3--N concentration of the L-SCPAD and M-SCPAD systems was first increased to 6.44, 0.93 mg/L under 0.5 mg/L IBU exposure and gradually decreased to 1.66 mg/L, 0 mg/L under 4.0 mg/L IBU exposure, indicating that NO3--N removal performance of the M-SCPAD system was better than that of the L-SCPAD system. The variation of extracellular polymeric substances (EPS) characteristics demonstrated that more EPS was secreted in the M-SCPAD system compared to the L-SCPAD system, which contributed to forming a more stable biofilm structure and protecting microorganisms against the toxicity of IBU in the M-SCPAD system. Moreover, the increased electron transfer impedance and decreased cytochrome c implied that IBU inhibited the electron transfer efficiency of the L-SCPAD and M-SCPAD systems. The decreased adenosine triphosphate (ATP) and electron transfer system activity (ETSA) content showed that IBU inhibited metabolic activity, but the M-SCPAD system exhibited higher metabolic activity compared to the L-SCPAD system. In addition, the analysis of the bacterial community indicated a more stable abundance of nitrogen removal function bacteria (Bacillus) in the M-SCPAD system compared to the L-SCPAD system, which was conducive to maintaining a stable denitrification performance. The toxic response mechanism based on the biogeobattery effect was proposed in the SCPAD systems under IBU exposure. This study provided an important reference for the long-term toxic effect of IBU on the SCPAD systems.

Salinity effect on an anaerobic methane- and ammonium-oxidising consortium: Shifts in activity, morphology, osmoregulation and syntrophic relationship

Nitrate-dependent anaerobic methane oxidation (AOM) is a microbial process of both ecological significance for global methane mitigation and application potential for wastewater treatment. It is mediated by organisms belonging to the archaeal family 'Candidatus Methanoperedenaceae', which have so far mainly been found in freshwater environments. Their potential distribution in saline environments and their physiological responses to salinity variation were still poorly understood. In this study, the responses of the freshwater 'Candidatus Methanoperedens nitroreducens'-dominated consortium to different salinities were investigated using short- and long-term setups. Short-term exposure to salt stress significantly affected nitrate reduction and methane oxidation activities over the tested concentration range of 15-200‰ NaCl, and 'Ca. M. nitroreducens' showed the higher tolerance to high salinity stress than its partner of anammox bacteria. At high salinity concentration, near marine conditions of 37‰, the target organism 'Ca. M. nitroreducens' showed stabilized nitrate reduction activity of 208.5 µmol day-1 gCDW-1 in long-term bioreactors over 300 days, in comparison to 362.9 and 334.3 µmol day-1 gCDW-1 under low-salinity conditions (1.7‰ NaCl) and control conditions (∼15‰ NaCl). Different partners of 'Ca.  M. nitroreducens' evolved in the consortia with three different salinity conditions, suggesting the different syntrophic mechanisms shaped by changes in salinity. A new syntrophic relationship between 'Ca. M. nitroreducens' and Fimicutes and/or Chloroflexi denitrifying populations was identified under the marine salinity condition. Metaproteomic analysis shows that the salinity changes lead to higher expression of response regulators and selective ion (Na+/H+) channeling proteins that can regulate the osmotic pressure between the cell and its environment. The reverse methanogenesis pathway was, however, not impacted. The finding of this study has important implications for the ecological distribution of the nitrate-dependent AOM process in marine environments and the potential of this biotechnological process for the treatment of high-salinity industrial wastewater.

Long-term operation of cathode-enhanced ecological floating bed coupled with microbial electrochemical system for urban surface water remediation: From lab-scale research to engineering application

Ecological floating bed coupled with microbial electrochemical system (ECOFB-MES) has great application potential in micro-polluted water remediation yet limited by low electron transfer efficiency on the microbial/electrode interface. Here, an innovative cathode-enhanced EOCFB-MES was constructed with nano-Fe₃O₄ modification and applied for in-situ remediation both at lab scale (6 L, 62-day operation) and demonstration scale (2300 m², 1-year operation). The cathode-enhanced ECOFB-MES exhibited superior removal in TOC (81.43 ± 2.05%), TN (85.12% ± 1.46%) and TP (59.80 ± 2.27%), much better than those of original ECOFB-MES and anode-enhanced ECOFB-MES in the laboratory test. Meanwhile, cathode-enhanced ECOFB-MES boosted current output by 33% than that of original ECOFB-MES, which made a great contribution to the improvement of ectopic electronic compensation for pollutant decontamination. Notably, cathode-enhanced ECOFB-MES presented high efficiency, stability and durability in the demonstration test, and fulfilled the average concentration of COD (9.5 ± 2.81 mg/L), TN (1.00 ± 0.21 mg/L) and TP (0.10 ± 0.04 mg/L) of effluent water to meet the Grade III (GB 3838-2002) with stable operation stage. Based on the KOSIM calculation, the removal loads of cathode-enhanced ECOFB-MES in carbon, nitrogen and phosphorus could reach 37.14 g COD/(d·m²), 2.62 g TN/(d·m²) and 0.55 g TP/(d·m²), respectively. According to the analysis of microbial communities and functional genes, the cathode modified by Fe₃O₄ made a sensible enrichment in electroactive bacteria (EAB) and nitrogen-converting bacteria (NCB) as well as facilitated the functional genes expression in electron transfer and nitrogen metabolism, resulting in the synergistic removal of carbon in sediment and nitrite in water. This study provided a brandnew technique reference for in-situ remediation of surface water in practical application.

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

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

Effects of plants and soil microorganisms on organic carbon and the relationship between carbon and nitrogen in constructed wetlands

Constructed wetland is an ideal place for studying the effects of plants and microorganisms on the nutrient cycling and carbon-nitrogen coupling in wetland for their clear background. This study examined both bare plots and others with plants (Phragmites australis or Typha angustifolia) in constructed wetlands and vegetation and soil samples were collected to investigate the effects of plants and soil microorganisms on carbon and nitrogen content. Results showed that the soil organic carbon content was high in plots with high plant biomass, and the increase of soil organic carbon driven by plant biomass was mainly from light fraction organic carbon (LFOC). Correlation analysis and redundancy analysis (RDA) suggested that plants play an important role in the cycle of carbon and nitrogen elements in constructed wetland soils, and that plant nitrogen components were key factors influencing wetland soil carbon and nitrogen. In addition, this study found that most of the main microbial taxa were significantly correlated with dissolved organic carbon (DOC), ammonium nitrogen (NH4+), and nitrate and nitrite nitrogen (NOx-) indicating that microorganisms might play an important role in regulating soil element cycles in constructed wetlands by affecting the metabolism of activated carbon and reactive nitrogen. This study has implications for increasing the carbon sink of constructed wetlands to mitigate the effects of global warming.

Under-loaded operation of an anaerobic-anoxic-aerobic system in dry and wet weather dynamics to prevent overflow pollution: Impacts on process performance and microbial community

Effects of low hydraulic loading rate (HLR) in dry weather and high HLR in wet weather on pollutant removal, microbial community, and sludge properties of a full-scale wastewater treatment plant (WWTP) were extensively studied to explore the risk of under-loaded operation for overflow pollution control. Long-term low HLR operation had an insignificant effect on the pollutant removal performance of the full-scale WWTP, and the system could withstand high-load shocks in wet weather. Low HLR resulted in higher oxygen and nitrate uptake rate due to the storage mechanism under the alternating feast/famine condition, and lower nitrifying rate. Low HLR operation enlarged particle size, deteriorated floc aggregation and sludge settleability, and reduced sludge viscosity due to the overgrowth of filamentous bacteria and inhibition of floc-forming bacteria. The remarkable increase in Thuricola and the contract morphology of Vorticella in microfauna observation confirmed the risk of flocs disintegration in low HLR operation.

Roles of microalgae-based biofertilizer in sustainability of green agriculture and food-water-energy security nexus

For years, agrochemical fertilizers have been used in agriculture for crop production. However, intensive utilization of chemical fertilizers is not an ecological and environmental choice since they are destroying soil health and causing an emerging threat to agricultural production on a global scale. Under the circumstances of the increasing utilization of chemical fertilizers, cultivating microalgae to produce biofertilizers would be a wise solution since desired environmental targets will be obtained including (1) replacing chemical fertilizer while improving crop yields and soil health; (2) reducing the harvest of non-renewable elements from limited natural resources for chemical fertilizers production, and (3) mitigating negative influences of climate change through CO₂ capture through microalgae cultivation. Recent improvements in microalgae-derived-biofertilizer-applied agriculture will be summarized in this review article. At last, the recent challenges of applying biofertilizers will be discussed as well as the perspective regarding the concept of circular bio-economy and sustainable development goals (SDGs).

Degradation and transformation of linear alkyl-benzene sulfonates (LAS) in integrated constructed wetland-microbial fuel cell systems

Linear alkylbenzene sulfonates (LAS) are the most commonly-used anionic surfactants in cleaning agents and detergents. Taking sodium dodecyl benzene sulfonate (SDBS) as the target LAS, this study investigated the degradation and transformation of LAS in integrated constructed wetland-microbial fuel cell (CW-MFC) systems. Results showed that, SDBS was able to improve the power output and reduce the internal resistance of CW-MFCs by reducing transmembrane transfer resistance of organics and electrons because of the amphiphilicity and solubilization, however, SDBS with relatively high concentration had a great potential to inhibit electricity generation and organics biodegradation of CW-MFCs because of the toxic effects on microorganisms. C atoms on alkyl group and O atoms on sulfonic acid group of SDBS had greater electronegativity and were prone to oxidation reaction. The biodegradation of SDBS in CW-MFCs was a process of alkyl chain degradation, desulfonation and benzene ring cleavage in sequence via ω, β and/or α-oxidations and radical attacks under the action of coenzymes and oxygen, in which 19 intermediates were produced, including four anaerobic degradation products (toluene, phenol, cyclohexanone and acetic acid). Especially, for the first time cyclohexanone was detected during the biodegradation of LAS. The bioaccumulation potential of SDBS was greatly reduced through the degradation by CW-MFCs, and thus the environmental risk of SDBS was effectively reduced.

Denitrification efficiency, microbial communities and metabolic mechanisms of corn cob hydrolysate as denitrifying carbon source

In this study, the denitrification efficacy of corn cob hydrolysate (CCH) was compared and analyzed with that of glucose and acetate to determine its feasibility as an additional carbon source, and its metabolic mechanism as a denitrification carbon source was investigated in depth. By constructing a denitrification reactor, it was found that the TN removal rate exceeded 97% and the effluent COD remained below 70 mg/L during the stable operation with CCH as the carbon source, and the denitrification effect was comparable to that of the glucose stage (GS) and the acetate stage (AS). The analysis of the microbial community showed that the dominant phylum was Proteobacteria and Bacteroidota, where the abundance of Bacteroidota in the hydrolysate stage (HS) (24.37%) was significantly higher than that of GS (4.89%) and AS (11.93%). And the analysis at the genus level showed the presence of a large number of genera of organic matter hydrolysis and acid production in HS that were almost absent in other stages, such as Paludibacter (12.83%), Gracilibacteria (4.27%), f__Prolixibacteraceae_Unclassified (2.94%). In addition, the higher fatty acid metabolism and lower sugar metabolism of HS during carbon metabolism were similar to the ratio of AS, suggesting that CCH was mainly fermented to acids and then involved in the tricarboxylic acid (TCA) cycle. During nitrogen metabolism, the high relative abundance of narG, nirS, and nosZ ensured the denitrification process. The results of this study were expected to provide a theoretical basis and data support for promoting denitrification from novel carbon sources.

Critical review of biochemical pathways to transformation of waste and biomass into bioenergy

In recent years, biofuel or biogas have become the primary source of bio-energy, providing an alternative to conventionally used energy that can meet the growing energy demand for people all over the world while reducing greenhouse gas emissions. Enzyme hydrolysis in bioethanol production is a critical step in obtaining sugars fermented during the final fermentation process. More efficient enzymes are being researched to provide a more cost-effective technique during enzymatic hydrolysis. The exploitation of microbial catabolic biochemical reactions to produce electric energy can be used for complex renewable biomasses and organic wastes in microbial fuel cells. In hydrolysis methods, a variety of diverse enzyme strategies are used to promote efficient bioethanol production from various lignocellulosic biomasses like agricultural wastes, wood feedstocks, and sea algae. This paper investigates the most recent enzyme hydrolysis pathways, microbial fermentation, microbial fuel cells, and anaerobic digestion in the manufacture of bioethanol/bioenergy from lignocellulose biomass.

Refractory azo dye wastewater treatment by combined process of microbial electrolytic reactor and plant-microbial fuel cell

An innovative design of microbial electrolytic reactor (MER) coupled with Ipomoea aquaticaForsk. plant microbial fuel cell (IAF-PMFC) was developed for azo dye wastewater treatment and electricity generation. This study aims to assess the sequential degradation of azo dye and the feasibility of energy self-sufficiency in the MER/IAF-PMFC system. The decomposition of azo dye into aromatic amines and dye decolorization occurred in the MER at high hydraulic loading of 0.28 m³/(m²·d), while dye intermediates were mainly mineralized in the IAF-PMFC at low hydraulic loading of 0.06 m³/(m²·d). The final decolorization efficiency and COD removal of the combined system reached 99.64% and 92.06% respectively, even at influent dye concentration of 1000 mg/L. The effects of open/closed circuit conditions, presence/absence of aquatic plant and different cathode areas on the performance of the IAF-PMFC for treating the effluent of the MER were systematically tested, and the results showed that closed-circuit condition, plant involvement and larger cathode area were more beneficial to decolorization, detoxification and mineralization of dye wastewater, bioelectricity output, plant growth and photosynthetic rate. The power consumption by the MER was 0.0163 kWh/m³ of dye wastewater, while the highest power generation of the IAF-PMFC reached 0.0183 kWh/m³. The current efficiency of the MER for dye decolorization was as high as 942.83%, while the maximum coulombic efficiency of the IAF-PMFC for intermediates metabolism was only 6.30%, which still had much space of bioelectricity generation promotion. The MER/IAF-PMFC technology can simultaneously realize refractory wastewater treatment and balance of electricity production and consumption.

Study on the performance and mechanism of bio-electrochemical system to mitigate membrane fouling in bioreactors

To alleviate membrane fouling, a membrane of the membrane bioreactor was directly used as the anode of the bio-electrochemical system. On the 14th day, the control group had blocked, while the experimental group with a current of 0.44 mA, the increase in ΔTMP was only 2.2 kPa. The polysaccharide and protein concentrations in the open-circuit group were 4.2 and 2.9 times higher than those in the closed-circuit group, respectively. Three-dimensional fluorescence spectroscopy and gas chromatography mass spectrometry showed that most of the deposition in the control group contained high-molecular-weight compounds, especially long-chain ester derivatives, phenols, and complex hydrocarbons, whereas the experimental group was the opposite. Therefore, current (electrons) can change the composition of the cake layer. High-throughput sequencing indicated that a significantly higher abundance of electroactive microorganisms on the experimental than control group. Two-dimensional correlation spectroscopy showed that electrons promote the degradation of polysaccharides, thereby alleviating membrane fouling.