Enhancement of the denitrification activity by exoelectrogens in single-chamber air cathode microbial fuel cells

Refractory degradable dissolved organic matter (R-DOM) driving nitrogen removal by the electric field coupled iron‑carbon biofilter (E-ICBF): Performance and microbial mechanisms

Researches on the advanced nitrogen (N) removal of municipal tailwater always overlooked the value of refractory degradable dissolved organic matter (R-DOM). In this study, a novel electric field coupled iron‑carbon biofilter (E-ICBF) was utilized to explore the performance and microbial changes with polyethylene glycol (PEG) as the representative R-DOM. Results demonstrated that the removal efficiencies of E-ICBF for nitrate nitrogen (NO3--N), ammonia nitrogen (NH4+-N), and total nitrogen (TN) improved by 28.76 %, 12.96 %, and 28.45 %, compared to quartz sand biofilter (SBF). Moreover, removal efficiencies of NO3--N and TN in E-ICBF with R-DOM went up by 12.11 % and 14.02 % compared to methanol. Additionally, both PEG and the electric field reduced the microbial richness and diversity. However, PEG promoted the increase of denitrifying bacteria abundance including unclassified_f_Comamonadaceae, Thauera, and unclassified_f_Gallionellaceae. The electric field improved the abundances of genes related to N removal (hao, nasC, nasA, nifH, nifD, nifK) and PEG further enhanced the effect. The abundances of key enzymes [EC:1.7.5.1], [EC:1.7.2.1], [EC:1.7.2.4], and [EC:1.7.2.5] decreased due to the addition of PEG and the electric field mitigated the negative influence. Additionally, the electric field changed relationships between microorganisms and pollutant removal, and improved interspecific relationships between denitrifying bacterial genera and other genera in E-ICBF.

Enhancing electrochemical nitrite sensing with a novel nanocomposite of activated carbon/carbon cloth derived from microbial biofilm

A novel approach was employed to fabricate a biofilm-derived activated carbon (BioAc) electrode on a carbon cloth (Cc) substrate for electrochemical nitrite sensing in water samples. The biofilm/Cc electrode was developed using a bioelectrochemical reactor, featuring a three-electrode system with nutrient media and microbial sources. The resultant biofilm electrode was activated at 450 °C for 2 h to eliminate impurities and enhance porosity. Morphological analysis of the BioAc/Cc electrode revealed a surface characterized by a compact film composed of numerous carbon nanoparticles. X-ray diffraction (XRD) analysis exhibited broad, highly crystalline peaks, enhancing both the electrode surface area and conductivity. Amperometry tests on the modified BioAc/Cc electrodes demonstrated a detection limit of 0.015 μM, a sensitivity of 1946.54 μA mM-1 cm-2, and a linear range spanning 0.35-478.21 μM at neutral pH conditions. Moreover, the electrodes demonstrated good stability with a RSD of 2.25% after 60 days and high reproducibility with an RSD of 1.64%. Real-time results showed 99.2 and 100.1% recovery for tap water and drinking water, respectively, highlighting the potential for commercialization in the future. These findings suggest that the BioAc/Cc electrode holds substantial potential for precise nitrite detection in environmental and wastewater applications.

Study of a three-dimensional biofilm-electrode reactor (3D-BER) that combined heterotrophic and autotrophic denitrification (HAD) to remove nitrate from water

A three-dimensional biofilm-electrode reactor (3D-BER) that combined heterotrophic and autotrophic denitrification (HAD) was developed to remove nitrate. The denitrification performance of the 3D-BER was evaluated under different experimental conditions, including current intensities (0-80 mA), COD/N ratios (0.5-5), and hydraulic retention times (2-12 h). The results showed that excessive current limited the nitrate removal efficiency. However, a longer hydraulic retention time was not required to achieve a better denitrification effect in the 3D-BER. Moreover, the nitrate could be effectively reduced over a broad range of COD/Ns (1-2.5), and its removal rate peaked at 89% at I = 40 mA, HRT = 8 h, and COD/N = 2. Although the current reduced the diversity of microorganisms in the system, it promoted the growth of dominant species. Nitrification microorganisms were enriched in the reactor, especially Thauera and Hydrogenophaga, which were crucial to the denitrification process. Thus, the combination of autotrophic denitrification and heterotrophic denitrification was promoted by the 3D-BER system to increase the efficiency of nitrogen removal.

Coupling mixotrophic denitrification and electroactive anodic nitrification by nitrate addition for promoting current generation and nitrogen removal

Nitrate promotes anodic denitrification and fasts organic matter removal in microbial fuel cells (MFCs). However, it suffers from poor total nitrogen (TN) removal and current recovery. In this study, some novel electroactive nitrifying/denitrifying bacteria (ENDB) were introduced in a single chambered air-cathode MFC to investigate the performance of this device and the microbial community shift by adding nitrate. Results showed a similar disturbance in current output by adding nitrate during a short-term operation. However, a stable and reproducible current increase was achieved in the continuous experiment. A maximum current of 0.76 A m^-3 and a maximum TN removal of >99 % were accomplished. The corresponding corrected coulombic efficiency was approximately 18 %. Under repeatable batches, a sharp decrease in chemical oxygen demand (COD) with feeding nitrate confirmed the temporary competition on electron donors through heterotrophic denitrification. The later current increase and nitrite detection occurring without metabolized COD could be considered evidence of electroactive anodic nitrification. The ENDB biofilm successfully coupled mixotrophic denitrification and electroactive anodic nitrification. It eventually promoted TN removal. In the process, genera Pseudoxanthomonas, Thauera, and Pseudomonas were enriched in the anodic ENDB biofilms. Cyclic voltammetry data confirmed the promotion of the electron transfer process by biofilms. The bacterial function predication revealed that the genes related to nitrogen removal and electron transfer were upregulated. Therefore, mixotrophic denitrification and electroactive anodic nitrification processes facilitated power recovery with the high efficiency of pollutant removal, finally ensuring water body security.

Nano-α-Fe2 O3 for enhanced denitrification in a heterotrophic/biofilm-electrode autotrophic denitrification reactor

In this study, a heterotrophic/biofilm-electrode autotrophic denitrification reactor (HAD-BER) was constructed and nano-ɑ-Fe₂ O₃ was coated on granular activated carbon (GAC) as a third electrode to enhance the nitrate removal performance. The introduction of nano-ɑ-Fe₂ O₃ could stimulate microorganisms to secrete more extracellular polymeric substances (EPS), accelerating the electron transfer. Moreover, more denitrification bacteria were enriched on the particle electrodes, especially Pseudomonas and Thermomonas, which played a significant role in denitrification. The denitrification performance at different COD/N ratios (0.65-3.23) and current intensities (0-150 mA) was investigated in depth. When the nitrate concentration of the influent was 60 mg/L, nitrate was almost completely removed at the optimal current intensity (60 mA) and COD/N ratio (1.29). At the same time, there was almost no nitrite (<0.10 mg/L) and ammonia nitrogen (0 mg/L) accumulation in the effluent. This study provided a new direction for the advancement of HAD-BER and accelerated its implementation. PRACTITIONER POINTS: By introducing nano-a-Fe2O3 into HAD-BER, more denitrification bacteria were enriched on the particle electrodes. The increased contents of polysaccharide and protein content could accelerate the electron transfer. Almost completely denitrification could be achieved at current = 60 mA and COD/N = 1.29. The study provided a new direction for the further development of HAD-BERs.

Insight into the role of microbial community interactions on nitrogen removal facilitated by a bioelectrochemical system in an osmotic membrane bioreactor

Insufficient nitrogen removal is a key challenge for the application of an osmotic membrane bioreactor (OMBR). The integration of a bioelectrochemical system (BES) and an OMBR was constructed to enhance nitrogen removal.To optimize the operation, five aeration intensities and three draw solutes (DSs) were applied in the proposed system. The results showed that the proposed system obtained the highest nitrogen removal efficiency of 77.36 ± 3.55 % with an aeration intensity of 0.6 L/min, and it was further increased to 94.99 ± 2.83 % and 99.92 ± 0.14 %with the NaOAc DS and the glucose DS, respectively.The analysis ofmetabolic pathways implied that species interactions existed,andthe following different mechanisms of enhanced nitrogen removal for the two organic DSs were proposed. The growth of denitrifying bacteria was enhanced by using reverse-fluxed organic NaOAc DS as a carbon source;glucoseDS stimulated electron transfer system activity to accelerate denitrification.

Fast and simultaneous detection of dissolved BOD and nitrite in wastewater by using bioelectrode with bidirectional extracellular electron transport

Timely and simultaneously detecting BOD and nitrite concentrations is of great significance for curbing of water pollution and adjusting wastewater treatment strategies. However, existing BOD and nitrite biosensors cannot perform synchronous detection due to their single electroactivity and differences in detection time. This study reported a novel dual-function electrochemical biosensor (DFEB) that could perform fast, simultaneous detection of nitrite and dissolved BOD. DFEB conducted a potential-step chronoamperometry on the mixed-bacteria bioelectrode with bidirectional electron transfer ability to obtain response signals. DFEB accurately measured dissolved BOD in the range of 5 ∼ 100 mg BOD L-1 and nitrite in the range of 0.05 ∼ 16 mg NO2--N L-1 within 20 min and maintain stable performance over 200 tests. DFEB performed well in artificial wastewater, aquatic wastewater, anaerobic tank effluent and anammox effluent, with relative errors < 15.7% and 16.8% in detecting nitrite and dissolved BOD, respectively. Our study provided a feasible way to develop multifunctional biosensors for detecting pollutants with different redox properties in wastewater.

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.

Reinforced nitrite supplement by cathode nitrate reduction with a bio-electrochemical system coupled anammox reactor

Anammox has been widely used for the treatment of nitrogen wastewater. However, the problem of stable NO₂^- supplement becomes one of the limiting factors. It is an effective method to obtain NO₂^- by denitrifying the NO₃^-, including the by-product of Anammox. In this study, NO₂^- was reinforced by bio-electrochemical system (BES) through the reaction of partial denitrification in situ in an Anammox reactor. Our results showed that both NO₃^- and NO₂^- can be reduced on the cathode with different Coulombic efficiencies. The reduction of NO₃^- amount increased with an increase in Inf-NO₃^-, which was greater than that of NO₂^-. The conversion amount of NO₃^- was 2.50% ± 17.25% to the theoretical Eff-NO₃^-, and the maximum reduction amount was 23.24% with the highest Coulombic efficiency of 3.56%. High throughput results showed that denitrifying bacteria, such as Limnobacter, Thauera, Denitratisoma, Nitrosomonas and Nitrospira, were attached to the cathode surface and in Anammox granular sludge. This study showed that NO₂^- can be supplied by reducing the by-product NO₃^- with denitrification cathode at Anammox environment in-situ.

Effect of poised cathodic potential on anodic ammonium nitrogen removal from domestic wastewater by air-cathode microbial fuel cells

Performances of anodic ammonia oxidation have been investigated for various bioelectrochemical systems at a wide range of poised anodic potentials in the literature. The effect of poised cathodic potential on ammonium nitrogen (NH₄⁺-N) and total nitrogen (TN, sum of NH₄⁺-N, NO₂^--N, and NO₃^--N) removal from domestic wastewater by single chamber air-cathode microbial fuel cells (MFCs) was investigated. Poising the air-cathode potential at +0.7 V vs. SHE significantly increased current generation (from 11 ± 1 mA to 22.8 ± 5 mA) and oxygen permeation into the MFC through the air-cathode (from 75.4 ± 1.2 g-O₂/m³/d to 151 ± 3.7 g-O₂/m³/d), which consequently resulted in a high NH₄⁺-N removal rate of 150 ± 13 g-NH₄⁺-N/m³/d and TN removal rate of 63 ± 16 g-TN/m³/d. These high NH₄⁺-N and TN removal rates could be attributed to the enhancement of dual respiratory pathways: the electrode-assisted anodic and aerobic NH₄⁺ oxidation.

Modifying Ti3C2 MXene with NH4+ as an excellent anode material for improving the performance of microbial fuel cells

Poor anode performance is one of the main bottlenecks in the development of microbial fuel cells (MFCs) for practical applications. Multilayered Ti₃C₂ MXene (m-MXene) is an alternative anode modification material because of its high specific surface area and electrical conductivity. However, the multilayered structure, negatively charged surface, and electropositivity of m-MXene could limit its modification effects. In this work, we used a solution-phase flocculation method (ammonium ion method) to restack and aggregate MXene nanosheets as an anode modification material (n-MXene). The n-MXene-modified anode had a higher specific surface area, surface hydrophilicity and surface electropositivity than the m-MXene-modified anode. The n-MXene-modified anode obtained a maximum current density of 2.1 A m^-2, which was 31.2% and 61.5% higher than that of the m-MXene-modified anode (1.6 A m^-2) and bare carbon fiber cloth anode (1.3 A m^-2). This improved anode performance was attributed to both the decrease in the charge transfer resistance and diffusion resistance, which were related to the increased quantity of biomass and microbial nanowire (or pili)-shaped filaments on the electrode surface. This work provides a simple and cost-effective approach to prepare MXene nanosheets for the modification of MFC anodes.

Efficiency of microbial fuel cells in the treatment and energy recovery from food wastes: Trends and applications - A review

The rising global population and their food habits result in food wastage and cause an obstacle in its treatment and disposal. Due to the rapid shift in the lifestyle of the human population and urbanization, almost one-third of the food produced is wasted from various sectors like domestic sources, agricultural sectors, and industrial sectors. These food resources squandered are rich in organic biomolecules which can cause complications upon direct disposal in the environment. Conventional disposal methods like composting, landfills and incineration demand high costs besides causing severe environmental and health issues. To overcome these demerits of the conventional methods and to avoid the loss of rich organic food resources, there is an immediate need for a sustainable and eco-friendly solution for the valorization of the food wastes. Microbial fuel cells (MFCs) are gaining attention, due to their ideal approach in the production of electricity and parallel treatment of organic food wastes. The MFCs are significant as an innovative approach using microorganisms and oxidizing the organic food wastes into bio-electricity. In this review, the recent advancements and practices of the MFCs in the field of food waste treatment and management along with electricity production are discussed. The major outcome of this work highlights the setting up of MFC for the treatment of higher volumes of food waste residues and enhancing the bioelectricity production in an optimal condition. For further improvements in the food waste treatments using MFCs, greater understanding and more research needs are to be focused on the commercialization, different operational modes, operational types, and low-cost fabrication coupled with careful examination of scale-up factors.

Effect of hydraulic retention time on the denitrification performance and metabolic mechanism of a multi-chambered bio-electrochemical system

The effects of hydraulic retention time (HRT) on the denitrification performance of the multi-chambered bio-electrochemistry system and the metabolic mechanism of the microbial community were investigated. Results indicated that the NO₃^--N and NO₂^--N removal efficiency was up to 99.5% and 99.9%, respectively. The electricity generation performance of the system was optimum at 24 h HRT, with the maximum power density and output voltage of the fourth chamber to be 471.2 mW/m³ and 602.4 mV, respectively. With the decrease of HRT from 24 h to 8 h, the protein-like substance in extracellular polymeric substance (EPS) of granular sludge was reduced and the fluorescence intensities were weakened. Besides, the abundance of metabolism pathway was the highest at 50.0% and 49.9%, respectively, and the methane metabolism (1.8% and 2.0%, respectively) and the nitrogen metabolism (0.8% and 0.9%, respectively) in Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway played important roles in providing guaranteed stability and efficient removal of organic matter and nitrogen from the system.

Rapid start-up strategy of partial denitrification and microbially driven mechanism of nitrite accumulation mediated by dissolved organic matter

The rapid start-up of Partial denitrification (PD; nitrate to nitrite) was investigated based on the analysis of microbially driven mechanism of nitrite accumulation mediated by Dissolved organic matter (DOM) in this study. The nitrate to Nitrite transformation ratio (NTR) > 90% and effluent nitrate < 5 mg/L were achieved in 17 days by feeding with lower nitrate of ~ 35 mg/L and removing the idling period. And the enhanced nitrite accumulation when applying the above strategy is related to the decreased utilization of the aliphatic DOM during nitrite reduction process. Additionally, the rapid enriched Thauera and OLB13 (37.21%) and inhibited norank_f__Blastocatellaceae (2.86%), and the increased disparity (2.0-fold) between the genes involved in nitrite generation (e.g., narH) and for nitrite reduction (e.g., nirK) jointly contributed to PD start-up. While the genes (e.g., DLD) related to producing electrons from aliphatic DOM also up-regulated by 0.1-fold, which led to the increased nitrate removal and NTR.

Impact of anodophilic biofilm bioelectroactivity on the denitrification behavior of air-cathode microbial fuel cells

Generally, high bioelectroactivity of anodophilic biofilm favors high power generation of microbial fuel cell (MFC); however, it is not clear whether it can promote denitrification of MFC synchronously. In this study, we studied the impact of anodophilic biofilm bioelectroactivity on the denitrification behavior of air-cathode MFC (AC-MFC) in steady state and found that high bioelectroactivity of anodophilic biofilm not only favored high power generation of the AC-MFC, but also promoted the growth of denitrifers at the anodes and strengthened denitrification. Anodophilic biofilms of AC-MFC with various bioelectroactivity were acclimated at conditions of open circuit (OC), R_ext of 1000 Ω and 20 Ω (denoted as AC-MFC-OC, AC-MFC-1000Ω, and AC-MFC-20Ω, respectively) and performed for over 100 days. Electrochemical tests and microbial analysis results showed that the anode of the AC-MFC-20Ω delivered higher current response of both oxidation and denitrification and had higher abundance of electroactive bacteria than the AC-MFC-OC, AC-MFC-1000Ω, demonstrating a higher bioelectroactivity of the anodophilic biofilms. Moreover, these electroactive bacteria favored the accumulation of denitrifers, like Thauera and Alicycliphilus, probably by consuming trace oxygen through catalyzing oxygen reduction. The AC-MFC-20Ω not only delivered a 61.7% higher power than the AC-MFC-1000Ω, but also achieved a stable and high denitrification rate constant (k_DN ) of 1.9 h^-1 , which was 50% and 40% higher than that of the AC-MFC-OC and AC-MFC-1000Ω, respectively. It could be concluded that the high bioelectroactivity of the anodophilic biofilms not only favored high power generation of the AC-MFC, but also promoted the enrichment of denitrifers at the anodes and strengthened denitrification. This study provided an effective method for enhancing power generation and denitrification performance of the AC-MFC synchronously.

Denitrifying anaerobic methane oxidation process responses to the addition of growth factor betaine in the MFC-granular sludge coupling system: Enhancing mechanism and metagenomic analysis

To solve the problem of the slow growth of denitrifying anaerobic methane oxidation (DAMO) bacteria during the enrichment process, betaine was added as a growth factor and its influence on the mechanism of DAMO process along with the metagenomic analysis of the process in a MFC-granular sludge coupling system was explored. When the addition of betaine was increased to 0.5 g/L and 1.0 g/L, the NO₃^--N removal increased to 210 mg/L. Also, the increasing betaine dosage in 1st to 4th chambers resulted in a significant increase in dissolved methane concentration which reached a maximum value of 16.6 ± 1.19 mg/L. When the dosage of betaine was increased from 0 g/L to 1.0 g/L, the dominant bacterial phyla in the 1st to 4th chambers changed to Proteobacteria (20.8-50.7%) from Euryarchaeota (42.0-54.1%) and Methanothrix which was significantly decreased by 17.9-37.4%. There was a slight decline in the DAMO microorganism abundance, possibly due to the increased methyl donors limiting the DAMO microorganism growth. Denitrification metabolism pathway module (increased from 0.10% to 0.15%) of Nitrogen metabolism and Formaldehyde assimilation, and serine pathway of Methane metabolism presented an ascendant trend with the increased betaine dosage as determined by the metagenomics analysis of KEGG metabolism pathway.

Concurrent treatment of raw and aerated swine wastewater using an electrotrophic denitrification system

Enhanced nitrate removal in the cathode chamber of bioelectrochemical systems (BES) using aerated swine wastewater under high nitrate levels and low organic carbon was investigated in this study, focusing on the relationship between nitrogen and bacterial communities involved in denitrification pathways. BESs with the anion exchange membrane (AEM) under cathodic applied potentials of -0.6 V vs. AgCl/AgCl reference electrode showed a removal rate of 99 ± 2 mg L^-1 d^-1. Moreover, organic compounds from the untreated full-strength wastewater were simultaneously eliminated in the anode chamber with a removal rate of 0.46 g COD L^-1 d^-1 with achieved efficiency of 61.4 ± 0.5% from an initial concentration of around 5 g of COD L^-1, measured over the course of 7 days. The highest microbial diversity was detected in BESs under potentials of -0.6 V, which include autotrophic denitrifiers such as Syderoxidans, Gallionela and Thiobacillus.

Comprehensive performance, bacterial community structure of single-chamber microbial fuel cell affected by COD/N ratio and physiological stratifications in cathode biofilm

This study compares the effects and bacterial community structure of single-chamber microbial fuel cells (MFCs) in the treatment of NH₄⁺-containing wastewater with different chemical oxygen demand (COD)/N ratios, whilst simultaneously conducting stratification research on the cathode biofilm. To this end, five nitrifier pre-enriched single-chamber MFC reactors are established to treat five different COD/N wastewaters, respectively. The results show that MFCs with low COD/N have better NH₄⁺-N removal, electrochemical performance, but the removal stability and COD removal effect are lower than MFCs with high COD/N. High-throughput sequencing reveals that the anode community structure is weakly affected by the COD/N and is dominated by Geobacter; however, the cathode community is complex and susceptible to the COD/N. Furthermore, the pH profile in the cathode biofilm is characterized by a pH microelectrode and fluorescence in situ hybridization (FISH) is used to confirm that the distribution trend of nitrifiers and denitrifiers in cathode biofilm.

A novel electrochemical oxidation-methanogenesis system for simultaneously degrading antibiotics and reducing CO2 to CH4 with low energy costs

A novel electrochemical oxidation-methanogenesis (EO-M) system was proposed for the first time to simultaneously achieve antibiotic degradation and a bioelectrochemical conversion of CO₂ to CH₄ with low energy costs. A dual-chamber system was installed with an antimony-doped tin oxide anode (Ti/SnO₂-Sb) for the electrocatalytic generation of hydroxyl radicals to degrade ciprofloxacin (CIP), and a CO₂-reducing methanogenic biocathode was enriched based on a three-dimensional (3D) graphitized granular activated carbon (GGAC) for microbial electromethanogenesis. The anode achieved removal efficiencies as high as 99.99% and 90.53% for CIP (14 mL, 50 mg L^-1) and the chemical oxygen demand (COD, 89 mg L^-1), respectively. The biocathode was rapidly enriched within 15 days and exhibited a methane production rate that stabilized at 15.12 ± 1.82 m³ m^-3 d^-1; additionally, the cathodic coulombic efficiency reached 71.76 ± 17.24%. The energy consumption of CIP degradation was reduced by 3.03 Wh L^-1 compared to that of a single electrochemical oxidation system due to the lower cathodic overpotential of CO₂ bioelectrochemical reduction in the EO-M system. A detailed analysis of the biofilm evolution in the 3D biocathode during the start-up process demonstrated that the enhanced absorption of extracellular polymeric substances by the GGAC cathode accelerated the enrichment of methanogens and induced the formation of methanogens with a large number of flagella. An analysis of the microbial community showed that a high relative abundance of Methanobacterium movens could promote a flagella-mediated direct electron transfer of the biocathode, eventually reducing the cathodic overpotential and energy costs of the EO-M system.

Nitric oxide reduction by microbial fuel cell with carbon based gas diffusion cathode for power generation and gas purification

Nitric oxide (NO) from anthropogenic emission is one of the main air contaminants and induces many environmental problems. Microbial fuel cells (MFCs) with gas diffusion cathode provide an alternative technology for NO reduction. In this work, pure NO as the sole electron acceptor of MFCs with gas diffusion cathode (NO-MFCs) was verified. The NO-MFCs obtained a maximum power density of 489 ± 50 mW/m². Compared with MFCs using O₂ in air as electron acceptor (Air-MFCs), the columbic efficiency increased from 23.2% ± 4.3% (Air-MFCs) to 55.7% ± 4.6% (NO-MFCs). The NO removal rate was 12.33 ± 0.14 mg/L/h and N₂ was the main reduction product. Cathode reduction was the dominant pathway of NO conversion in NO-MFCs, including abiotic electrochemical reduction and microbial denitrification process. The predominant genera in anodic microbial community changed from exoelectrogenic bacteria in Air-MFCs to denitrifying bacteria in NO-MFCs and effected the power generation.

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

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

Potential applications of endogenous sulfide for enhanced denitrification of low C/N domestic wastewater in anodic mixotrophic denitrification microbial fuel cell: The mechanism of electrons transfer and microbial community

Anodic mixotrophic denitrification microbial fuel cell (MFC) was developed for pollutants removal and electricity generation in treatment of low C/N domestic wastewater. The experimental results show that the MFC achieved up to 100% of acetate, 100% of sulfide, and more than 91% of nitrate removal efficiency in all the MFCs. Particularly, thiosulfate was generated as the main intermediate of sulfide oxidation, and the sulfate generation ratio ranged from 66.93% to 73.76%. Those electrons produced during the acetate and sulfide oxidation were mainly used for denitrification and electricity generation. The microbial community analysis revealed that heterotrophic denitrifying bacteria (HDB) and sulfide-based autotrophic denitrifying bacteria (SADB) were the dominant bacteria for pollutants removal, and those facultative autotrophic bacterium (FAB) were key functional genera for high sulfate generation under both low and high sulfide concentrations. Meanwhile, the microbial functional prediction revealed that sulfide oxidation gene of Sqr and Sox were highly expressed. Moreover, a preliminary sulfide-based autotrophic denitrification (SAD) potential estimation indicated that the sulfide generated in the WWTPs had great potential for denitrification.

Performance and functional microbial communities of denitrification process of a novel MFC-granular sludge coupling system

The performance, microbial communities and functional gene metabolism of the novel microbial fuel cell (MFC)-granular sludge coupling system was investigated. The results showed that COD and nitrogen removal can be up to 1.3-2.0 kg COD/L, 20-30 mg NO2--N/L, and 60-70 mg NO3--N/L, respectively. Proteobacteria, Chloroflexi, and Firmicutes were the dominant bacterial phyla, and the denitrification process was mainly consisted of the dominant denitrifying bacteria: Thauera (26.21%) and Pseudomonas (14.79%) in the first compartment, combining with denitrifying anaerobic methane oxidation bacteria: NC10 phylum of 0.072% (the first compartment) and 0.089% (the fourth compartment), Candidatus Methylomirabilis oxyfera of 0.044% (the first compartment) and 0.048% (the fourth compartment). According to functional gene classification for Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis, metabolism was the main cluster for the whole sequence in the KEGG (7.17-11.41%), indicating that the dominant metabolic pathway played an important role in the degradation of pollutants.