Anodic ammonia oxidation to nitrogen gas catalyzed by mixed biofilms in bioelectrochemical systems

Oxygen-to-ammonium-nitrogen ratio as an indicator for oxygen supply management in microoxic bioanodic ammonium oxidation

Microbial electrolysis cells (MECs) have been proven effective for oxidizing ammonium (NH4+), where the anode acts as an electron acceptor, reducing the energy input by substituting oxygen (O2). However, O2 has been proved to be essential for achieving high removal rates MECs. Thus, precise control of oxygen supply is crucial for optimizing treatment performance and minimizing energy consumption. Unlike previous studies focusing on dissolved oxygen (DO) levels, this study introduces the O2/NH4+-N ratio as a novel control parameter for balancing oxidation rates and the selectivity of NH4+ oxidation towards dinitrogen gas (N2) under limited oxygen condition. Our results demonstrated that the O2/NH4+-N ratio is a more relevant oxygen supply indicator compared to DO level. Oxygen served as a more favorable electron acceptor than the electrode, increasing NH4+ oxidation rates but also resulting in more oxidized products such as nitrate (NO3-). Additionally, nitrous oxide (N2O) and N2 production were higher with the electrode as the electron acceptor compared to oxygen alone. An O2/NH4+-N ratio of 0.5 was found to be optimal, achieving a balance between product selectivity for N2 (51.4 % ± 4.5 %) and oxidation rates (344.6 ± 14.7 mg-N/L*d), with the columbic efficiency of 30.7 % ± 2.0 %. Microbial community analysis revealed that nitrifiers and denitrifiers were the primary bacteria involved, with oxygen promoting the growth of nitrite-oxidizing bacteria, thus facilitating complete NH4+ oxidation to NO3-. Our study provides new insights and guidelines on the appropriate oxygen dosage, offering strategies into optimizing operational conditions for NH4+ removal using MECs.

Distinct microbial communities enriched in water-saturated and unsaturated reactors influence performance of integrated hydroponics-microbial electrochemical technology

This study aimed to understand the wastewater treatment and electricity generation performance besides the microbial communities of the integrated Hydroponics-Microbial Electrochemical Technology (iHydroMET) systems operated with water-saturated and water-unsaturated reactors. The organics removal was slightly higher in the water-unsaturated system (93 ± 4 %) than in the water-saturated system (87 ± 2 %). The total nitrogen removal and electric voltage were considerably higher in the water-saturated system (42 ± 5 %; 111 ± 8 V per reactor) than in the water-unsaturated system (18 ± 3 %; 95 ± 9 V per reactor). The enhanced organics and nitrogen removal and high voltage output in respective conditions were due to the dominance of polysaccharide-degrading aerobes (e.g., Pirellula), anammox bacteria (e.g., Anammoximicrobium), denitrifiers (e.g., Thauera and Rheinheimera), and electroactive microorganisms (e.g., Geobacter). The differential performance governed by distinct microbial communities under the tested conditions indicates that an appropriate balancing of water saturation and unsaturation in reactors is crucial to achieving optimum iHydroMET performance.

Influence of increasing anode surface area on nitrite-absent ammonium oxidation in a continuous single-chamber bio-electrochemical system

Reducing energy consumption in conventional nitrogen removal processes is a crucial and urgent requirement. This study proposes an efficient electrode-dependent bio-electrochemical anaerobic ammonium (NH4+-N) oxidation (BE-ANAMMOX) process, employing a carbon brush as the electron acceptor and voltage of 0.8 V. The applied voltage facilitated the removal of NH4+-N with a maximum removal efficiency of 41% and a Coulombic efficiency of 40.92%, without the addition of nitrite (NO2--N). Furthermore, the NH4+-N removal efficiency demonstrated an increase corresponding to the increase in the anodic surface area. The bio-electrochemical NH4+-N removal achieved remarkable reductions, eliminating the need for O2 and NO2--N by 100%, lowering energy consumption by 67%, and reducing CO2 emissions by 66% when treating 1 kg of NH4+-N. An analysis of the microbial community revealed an increase in nitrifiers and denitrifiers, including Exiguobacterium aestuarii, Alishewanella aestuarii, Comamonas granuli, and Acinetobacter baumannii. This intricate process involved the direct conversion of NH4+-N to N2 by ANAMMOX bacteria through extracellular electron transfer, all without NO2--N. Thus, bio-electrochemical NH4+-N removal exhibits promising potential for effective nitrogen removal in wastewater treatment facilities.

Bioelectrochemical system for nitrogen removal: Fundamentals, current status, trends, and challenges

Biological nitrogen removal (BNR) is essential for the treatment of nitrogen-containing wastewater. However, the requirement for aeration and the addition of external carbon sources, resulting in greenhouse gas emissions and additional costs, are disadvantages of the traditional BNR process. Alternative technologies have been devised to overcome these drawbacks. Bioelectrochemical nitrogen removal (BENR) has been proposed for efficient nitrogen removal, demonstrating flexibility and versatility. BENR can be performed by combining nitrification, denitrification, anaerobic ammonium oxidation (ANAMMOX), or organic carbon oxidation. Bioelectrochemical-ANAMMOX (BE-ANAMMOX) is the most promising method for nitrogen removal, as it can directly convert NH4+ to N2 and H2 in one step when the electrode is arranged as an electron acceptor. High-value-added hydrogen can potentially be recovered with efficient nitrogen removal using this concept, maximizing the benefits of BENR. Using alternative electron acceptors, such as electrodes and metal ions, for complete total nitrogen removal is a promising technology to substitute NO2- production from NH4+ oxidation by aeration. However, the requirement of electron donors for NO3- reduction, low NH4+ removal efficiency, and low competitiveness of exoelectrogenic bacteria still remain the main obstacles. The future direction for successful BENR should aim to achieve complete anaerobic NH4+ oxidation without any electron acceptor and to maximize selectivity in H2 production. Therefore, the bioelectrochemical pathways and balances between efficient nitrogen removal and high-value-added chemical production should be further studied for carbon and energy neutralities.

Enhancement of Ammonium Oxidation at Microoxic Bioanodes

Bioelectrochemical systems (BESs) are considered to be energy-efficient to convert ammonium, which is present in wastewater. The application of BESs as a technology to treat wastewater on an industrial scale is hindered by the slow removal rate and lack of understanding of the underlying ammonium conversion pathways. This study shows ammonium oxidation rates up to 228 ± 0.4 g-N m-3 d-1 under microoxic conditions (dissolved oxygen at 0.02-0.2 mg-O2/L), which is a significant improvement compared to anoxic conditions (120 ± 21 g-N m-3 d-1). We found that this enhancement was related to the formation of hydroxylamine (NH2OH), which is rate limiting in ammonium oxidation by ammonia-oxidizing microorganisms. NH2OH was intermediate in both the absence and presence of oxygen. The dominant end-product of ammonium oxidation was dinitrogen gas, with about 75% conversion efficiency in the presence of a microoxic level of dissolved oxygen and 100% conversion efficiency in the absence of oxygen. This work elucidates the dominant pathways under microoxic and anoxic conditions which is a step toward the application of BESs for ammonium removal in wastewater treatment.

Direct ammonium oxidation to nitrogen gas (Dirammox) in Alcaligenes strain HO-1: The electrode role

It has been recently suggested that Alcaligenes use a previously unknown pathway to convert ammonium into dinitrogen gas (Dirammox) via hydroxylamine (NH₂OH). This fact alone already implies a significant decrease in the aeration requirements for the process, but the process would still be dependent on external aeration. This work studied the potential use of a polarised electrode as an electron acceptor for ammonium oxidation using the recently described Alcaligenes strain HO-1 as a model heterotrophic nitrifier. Results indicated that Alcaligenes strain HO-1 requires aeration for metabolism, a requirement that cannot be replaced for a polarised electrode alone. However, concomitant elimination of succinate and ammonium was observed when operating a previously grown Alcaligenes strain HO-1 culture in the presence of a polarised electrode and without aeration. The usage of a polarised electrode together with aeration did not increase the succinate nor the nitrogen removal rates observed with aeration alone. However, current density generation was observed along a feeding batch test representing an electron share of 3% of the ammonium removed in the presence of aeration and 16% without aeration. Additional tests suggested that hydroxylamine oxidation to dinitrogen gas could have a relevant role in the electron discharge onto the anode. Therefore, the presence of a polarised electrode supported the metabolic functions of Alcaligenes strain HO-1 on the simultaneous oxidation of succinate and ammonium.

Comparative assessment of energy generation from ammonia oxidation by different functional bacterial communities

Bioelectrochemical ammonia oxidation (BEAO) in a microbial fuel cell (MFC) is a recently discovered process that has the potential to reduce energy consumption in wastewater treatment. However, level of energy and limiting factors of this process in different microbial groups are not fully understood. This study comparatively investigated the BEAO in wastewater treatment by MFCs enriched with different functional groups of bacteria (confirmed by 16S rRNA gene sequencing): electroactive bacteria (EAB), ammonia oxidizing bacteria (AOB), and anammox bacteria (AnAOB). Ammonia oxidation rates of 0.066, 0.083 and 0.082 g NH₄⁺-N L^-1 d^-1 were achieved by biofilms enriched with EAB, AOB, and AnAOB, respectively. With influent 444 ± 65 mg NH₄⁺-N d^-1, nitrite accumulation between 84 and 105 mg N d^-1 was observed independently of the biofilm type. The AnAOB-enriched biofilm released electrons at higher potential energy levels (anode potential of 0.253 V vs. SHE) but had high internal resistance (R_int) of 299 Ω, which limits its power density (0.2 W m^-3). For AnAOB enriched biofilm, accumulation of nitrite was a limiting factor for power output by allowing conventional anammox activity without current generation. AOB enriched biofilm had R_int of 18 ± 1 Ω and yielded power density of up to 1.4 W m^-3. The activity of the AOB-enriched biofilm was not dependent on the accumulation of dissolved oxygen and achieved 1.5 fold higher coulombic efficiency when sulfate was not available. The EAB-enriched biofilm adapted to oxidize ammonia without organic carbon, with R_int of 19 ± 1 Ω and achieved the highest power density of 11 W m^-3. Based on lab-scale experiments (scaling-up factors not considered) energy savings of up to 7 % (AnAOB), 44 % (AOB) and 475 % (EAB) (positive energy balance), compared to conventional nitrification, are projected from the applications of BEAO in wastewater treatment plants.

Realizable wastewater treatment process for carbon neutrality and energy sustainability: A review

Despite a quick shift of global goals toward carbon-neutral infrastructure, activated sludge based conventional systems inhibit the Green New Deal. Here, a municipal wastewater treatment plant (MWWTP) for carbon neutrality and energy sustainability is suggested and discussed based on realizable technical aspects. Organics have been recovered using variously enhanced primary treatment techniques, thereby reducing oxygen demand for the oxidation of organics and maximizing biogas production in biological processes. Meanwhile, ammonium in organic-separated wastewater is bio-electrochemically oxidized to N₂ and reduced to H₂ under completely anaerobic conditions, resulting in the minimization of energy requirements and waste sludge production, which are the main problems in activated sludge based conventional processes. The anaerobic digestion process converts concentrated primary sludge to biomethane, and H₂ gas recovered from nitrogen upgrades the biomethane quality by reducing carbon dioxide in biogas. Based on these results, MWWTPs can be simplified and improved with high process and energy efficiencies.

SRB-based bioelectrochemical system: A potential multipollutant combatant for enhanced landfill waste stabilization

Due to the lower proportion of organic matter and higher toxicity of the aged landfill, most of the advanced treatment technologies are not effective from economic, environmental, and social perspectives. This study evaluates the potential of sulfate-reducing bacteria (SRB) based bioelectrochemical-system (BES) in the decontamination of landfill wastes by reducing GHGs emissions and levels of soluble pollutants. The landfill waste (solid/leachate) collected from the Pirana Landfill site was assessed for economical long-term treatment and scaling up the feasibility of the designed system. The present system demonstrated significant improvement in volumetric hydrogen production of 3.1:1 (H2:CH4) by suppressing methanogenesis with a significant reduction in heavy metals concentration and other organic components. Despite being amended with 0.1 N ammonia, the treated leachate level of NO3 (2.350 ± 1.077 mg/L) was reduced by 5.3 times, hence reducing further groundwater pollution from landfill leaching. The BES-treated solid waste was more stabilized as shown by a fivefold increase in surface area and can be potentially applied for leachate immobilization and bio-fortification of agricultural fields. The vector arrangement and magnitude with differences in magnitudes for both leachate and solid waste supported the on-site applicability of BES treatment. Concerning the affinity in various treatment systems, the dendrogram clearly showed Ca and Fe placed far from each other (3506.08), in comparison to Fe and Mg (1186.6), followed by Fe and Cu (1544.6). Voltammograms proved the efficacy of the enriched electrochemically active bacteria (EAB), to support the treatment of landfill solid waste and leachate sustainably.

A novel electrochemical sensor based on nitrite-oxidizing bacteria for highly specific and sensitive detection of nitrites

Real-time nitrite control in water is necessary for environmental safety and human health, and has triggered the research and development of novel detection methods. Previous studies have made great progress on enzyme-free and enzyme electrochemical sensors. However, enzyme-free sensors have low selectivity and a complex preparation process, and enzyme sensors have short lifetimes, and these issues need to be addressed. In this work, we proposed for the first time a highly specific and sensitive biofilm sensor based on nitrite-oxidizing bacteria (NOB) for the bio-electrochemical detection of nitrite in water. The mechanism of nitrite detection was attributed to the competition of oxygen between aerobic respiration of the NOB and the cathode oxygen reduction on the carbon felt electrode, resulting in a decrease in current. This decrease in current (ΔI) had a linear relationship with the nitrite concentration in the range of 0.1 to 1 mg L^-1 and 1 to 10 mg L^-1, which was corresponding to the sensitivities of 48.62 and 2.24 μA mM^-1 cm^-2, respectively. And the limit of detection (LOD) was calculated to be 0.033 mg L^-1 (2.39 μM) with a signal-to-noise ratio of 3. Moreover, several common interfering ions had no effect on the nitrite detection owing to the functional microbial species (NOB) and weakly electrochemical behavior of electrode at the low potential of -0.1 V, showing high specificity for nitrite detection of biofilm sensor. Therefore, the actual nitrified wastewater was well detected by the biofilm sensor. In addition, allylthiourea (ATU) took good effect on the resistance of the influence of ammonia oxidizing bacteria (AOB) in the biofilm sensor, maintaining the high selectivity of biofilm sensor in case the biofilm sensor was fouled with AOB. The biofilm sensor in our work showed good selectivity, sensitivity and stability in long-term detection.

Anaerobic ammonium oxidation coupled to Fe(III) reduction: Discovery, mechanism and application prospects in wastewater treatment

Fe(III) reduction coupled with anaerobic ammonium oxidation is known as Feammox. Feammox, which was first discovered in wetland ecosystems, has the potential to be used in wastewater treatment systems due to its ability to remove ammonium. Feammox can produce N₂, NO₂^- or NO₃^- through the reduction of Fe(III) and oxidation of ammonium, which is a potential process to nitrogen loss from aquatic ecosystems and terrestrial ecosystems. The Acidimicrobiaceae sp. A6 was the first Feammox functional bacteria that was successfully isolated from wetlands. The nitrogen removal effect of Feammox can be influenced by many environmental factors, such as pH, organic matter, and different sources of Fe(III). Feammox has broad application prospects, but more exploration is needed to apply this principle to wastewater treatment. This review introduces the development, mechanism, functional microbes and factors affecting the Feammox process, and discusses its potential applications.

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.

Microbial Electrochemical Technologies for Sustainable Nitrogen Removal in Marine and Coastal Environments

For many years, the world's coastal marine ecosystems have received industrial waste with high nitrogen concentrations, generating the eutrophication of these ecosystems. Different physicochemical-biological technologies have been developed to remove the nitrogen present in wastewater. However, conventional technologies have high operating costs and excessive production of brines or sludge which compromise the sustainability of the treatment. Microbial electrochemical technologies (METs) have begun to gain attention due to their cost-efficiency in removing nitrogen and organic matter using the metabolic capacity of microorganisms. This article combines a critical review of the environmental problems associated with the discharge of the excess nitrogen and the biological processes involved in its biogeochemical cycle; with a comparative analysis of conventional treatment technologies and METs especially designed for nitrogen removal. Finally, current METs limitations and perspectives as a sustainable nitrogen treatment alternative and efficient microbial enrichment techniques are included.

Bioelectrochemical system for dehalogenation: A review

Halogenated organic compounds are persistent pollutants, whose persistent contamination and rapid spread seriously threaten human health and the safety of ecosystems. It is difficult to remove them completely by traditional physicochemical techniques. In-situ remediation utilizing bioelectrochemical technology represents a promising strategy for degradation of halogenated organic compounds, which can be achieved through potential modulation. In this review, we summarize the reactor configuration of microbial electrochemical dehalogenation systems and relevant organohalide-respiring bacteria. We also highlight the mechanisms of electrode potential regulation of microbial dehalogenation and the role of extracellular electron transfer in dehalogenation process, and further discuss the application of bioelectrochemical technology in bioremediation of halogenated organic compounds. Therefore, this review summarizes the status of research on microbial electrochemical dehalogenation systems from macroscopic to microscopic levels, providing theoretical support for the development of rapid and efficient in situ bioremediation technologies for halogenated organic compounds contaminated sites, as well as insights for the removal of refractory fluorides.

Conversion of nitrogen and carbon in enriched paddy soil by denitrification coupled with anammox in a bioelectrochemical system

The aim of this study is to investigate conversion of nitrogen and COD in enriched paddy soil by nitrification coupled with anammox process in a dual chamber bioelectrochemical system. The paddy soil was enriched for denitrification coupled with anammox by microbial consortia and was acclimatized in the cathodic chamber of microbial fuel cells (MFCs). The bioelectrochemical systems were treated with different ammonium concentrations in the cathodic chamber: the MFC with low concentration ammonium (LA-MFC, 50 mg/L ammonium), the MFC with medium concentration ammonium (MA-MFC, 500 mg/L ammonium), and MFC with high concentration ammonium (HA-MFC, 1000 mg/L ammonium), and the initial COD in the anodic chamber was 1200 mg/L. The CK treatments were conducted with 1000 mg/L ammonium under the same conditions, except without inoculum in the cathode chamber. The consumption rate of ammonium in the cathodic chambers of CK, LA-MFC, MA-MFC, and HA-MFC were 9%, 64%, 84%, and 84%, respectively. The degradation rate for COD achieved in the anode chambers of CK, LA-MFC, MA-MFC, and HA-MFC were 70%, 86%, 93%, and 93%, respectively. The analysis of the microbial community of three treated MFCs in the cathode chamber indicated that the nitrification-denitrification process occurs in the cathode chamber. The dominant species for nitrification was Nitrospira, and the dominant species for denitrification were Denitratisoma, Dechloromonas, and Candidatus_Competibacter. Moreover, anammox process also observed in the cathode chamber. The functional genes nirS/K, hzsB, and 16S rDNA were assessed by qPCR analysis, and the results confirmed the presence of denitrification-coupled anammox in the cathodic chamber.

Integrated simultaneous nitrification/denitrification and comammox consortia as efficient biocatalysts enhance treatment of domestic wastewater in different up-flow bioelectrochemical reactors

Simultaneous nitrification/denitrification (SND) can efficiently deplete NH₄⁺ by using air-exposed biocathode (AEB) in bioelectrochemical reactors. However, the fluctuation of wastewater adversely affects the functional biofilms and therefore the performance. In this work, four up-flow bioelectrochemical reactors (UBERs) with some novel inocula were investigated to improve domestic wastewater treatment. The UBERs exhibited favorable removal of chemical oxygen demand (COD, 95%), NH₄⁺-N (99%), and total nitrogen (TN, 99%). The maximum of current (2.7 A/m³), power density (136 mW/m³) and coulombic efficiency (20.5%) were obtained. Cyclic voltammetry analysis showed all the electrodes were of diversified catalytic reactions. Illumina pyrosequencing showed the predominant Ignavibacterium, Thauera, Nitrosomonas, Geminicoccus and Nitrospira were in all electrodes, contributing functional biofilms performing SND, comammox, and bioelectrochemical reactions. FAPROTAX analysis confirmed twenty-one functional groups with obvious changes related to chemoheterotrophy, respiration/oxidation/denitrification of nitrite and nitrate. Comfortingly, such novel diversified consortia in UBERs enhance the microbial metabolisms to treat domestic wastewater.

Bioelectrochemical anoxic ammonium nitrogen removal by an MFC driven single chamber microbial electrolysis cell

Nitrogen removal from wastewater is an indispensable but highly energy-demanding process, and thus more energy-saving treatment processes are required. Here, we investigated the performance of bioelectrochemical ammonium nitrogen (NH₄⁺-N) removal from real domestic wastewater without energy-intensive aeration by a single chamber microbial electrolysis cell (MEC) that was electrically powered by a double chamber microbial fuel cell (MFC). Anoxic NH₄⁺-N oxidation and total nitrogen (TN) removal rates were determined at various applied voltages (0-1.2 V), provided by the MFC. The MEC achieved a NH₄⁺-N oxidation rate of 151 ± 42 g NH₄⁺-N m^-3 d^-1 and TN removal rate of 95 ± 42 g-TN m^-3 d^-1 without aeration at the applied voltage of 0.8 V (the anode potential E_anode = +0.633 ± 0.218 V vs. SHE). These removal rates were much higher than the previously reported values and conventional biological nitrogen removal processes. Open and closed-circuit MEC batch experiments confirmed that anoxic NH₄⁺-N oxidation was an electrochemically mediated biological process (that is, an anode acted as an electron acceptor) and denitrification occurred simultaneously without NO₂^- and NO₃^- accumulation. Moreover, ex-situ¹⁵N tracer experiment and microbial community analysis revealed that anammox and heterotrophic denitrification mainly contributed to the TN removal. Thus, the bioelectrochemical anodic NH₄⁺-N oxidation was coupled with anammox and denitrification in this MFC-assisted MEC system. Taken together, our MFC-driven single chamber MEC could be a high rate energy-saving nitrogen removal process without external carbon and energy input and high energy-demanding aeration.

One-pot degradation of urine wastewater by combining simultaneous halophilic nitrification and aerobic denitrification in air-exposed biocathode microbial fuel cells (AEB-MFCs)

Urine wastewater is used as fuel in microbial fuel cells to generate power for several applications. However, the knowledge on the removal efficiencies of pollutants and bacterial composition of electrode biofilm is still lacking. In this study, two air-exposed biocathode microbial fuel cells (AEB-MFCs) were constructed and some nitrogen-removing consortium were inoculated to fabricate multifunctional AEBs for urine treatment and energy recovery. Results demonstrated that urine wastewater can be degraded through one-pot degradation without positive aeration. The removal efficiencies of NH₄⁺-N, total nitrogen and chemical oxygen demand reached 86.8% ± 1.5%, 62.7% ± 2.3%, and 52.7% ± 1.6% respectively. Cyclic voltammetry illustrated several catalytic activities related to C/N metabolism occurred in both biofilms and varied with the operation continuing in a single stable cycle. In addition, the community structure analysis revealed that many active microorganisms, including nitrogen-removing bacteria, heterotrophs, and electrochemically active bacteria were enriched in both electrodes, especially many halophilic nitrifiers/denitrifiers occupied in AEBs and directed the system toward the integrated pathways of halophilic nitrogen removal and energy recovery. This study presented a novel method for the energy conversion and effective degradation of urine, which can serve as a promising technology for urine wastewater treatment.

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.

Comparison of Carbon Dioxide with Anaerobic Digester Biogas as a Methanogenic Biocathode Feedstock

BES biogas upgrading studies have typically used bicarbonate or commercial gas mixtures as a biocathode substrate instead of anaerobic digester biogas. Therefore, the objective of this study was to (i) compare the performance of a methanogenic BES between CO₂-fed and biogas-fed cycles; (ii) investigate possible factors that may account for observed performance differences; and (iii) assess the performance of a biogas-fed biocathode at various applied cathode potentials. The maximum 1-d CH₄ production rate in a biogas-fed biocathode (3003 mmol/m²-d) was 350% higher than in a CO₂-fed biocathode (666 mmol/m²-d), and the biogas-fed biocathode was capable of maintaining high performance despite a variable biogas feed composition. Anode oxidation of reduced gases (e.g., CH₄ and H₂S) from biogas may theoretically contribute 4% to 35% of the total charge transfer from anode to cathode at applied cathode potentials of -0.80 to -0.55 V (vs SHE). The introduction of biogas did not significantly change the biocathode archaeal community (dominated by a Methanobrevibacter sp. phylotype), but the bacterial community shifted away from Bacteroidetes and toward Proteobacteria, which may have contributed to the improved performance of the biogas-fed system. This study shows that anaerobic digester biogas is a promising biocathode feedstock for BES biogas upgrading.

Electrochemical Bacterial Enrichment from Natural Seawater and Its Implications in Biocorrosion of Stainless-Steel Electrodes

Microbial electrochemical technologies have revealed the opportunity of electrochemical enrichment for specific bacterial groups that are able to catalyze reactions of interest. However, there are unsolved challenges towards their application under aggressive environmental conditions, such as in the sea. This study demonstrates the impact of surface electrochemical potential on community composition and its corrosivity. Electrochemical bacterial enrichment was successfully carried out in natural seawater without nutrient amendments. Experiments were carried out for ten days of exposure in a closed-flow system over 316L stainless steel electrodes under three different poised potentials (−150 mV, +100 mV, and +310 mV vs. Ag/AgCl). Weight loss and atomic force microscopy showed a significant difference in corrosion when +310 mV (vs. Ag/AgCl) was applied in comparison to that produced under the other tested potentials (and an unpoised control). Bacterial community analysis conducted using 16S rRNA gene profiles showed that poised potentials are more positive as +310 mV (vs. Ag/AgCl) resulted in strong enrichment for Rhodobacteraceae and Sulfitobacter. Hence, even though significant enrichment of the known electrochemically active bacteria from the Rhodobacteraceae family was accomplished, the resultant bacterial community could accelerate pitting corrosion in 316 L stainless steel, thereby compromising the durability of the electrodes and the microbial electrochemical technologies.

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

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

Influence of conductive material on the bioelectrochemical removal of organic matter and nitrogen from low strength wastewater

The treatment of low strength wastewater that has the level of discharge standard for wastewater treatment plant was studied using an upflow bioelectrochemical reactor with an applied voltage of 0.6 V. The direct interspecies electron transfer (DIET) between electroactive bacteria was activated in the upflow bioelectrochemical reactor, which improved the substrate affinity of bacteria. The effluent qualities in COD and ammonia nitrogen was stable at less than 3.5 mg/L and 7.46 mg/L at 1 h of hydraulic retention time, respectively. The conductive materials, including conductive sheets and conductive particles, further increased the biomass retention and the DIET by altering the abundance of dominant bacterial groups. The effluent qualities in COD and ammonia nitrogen was improved up to 1.98 mg/L and 2.65 mg/L, respectively, by the conductive sheets. The upflow bioelectrochemical reactor with conductive materials is a good tertiary treatment process for improving the quality of the final effluent discharged from wastewater treatment plant.

The hunt for the most-wanted chemolithoautotrophic spookmicrobes

Microorganisms are the drivers of biogeochemical methane and nitrogen cycles. Essential roles of chemolithoautotrophic microorganisms in these cycles were predicted long before their identification. Dedicated enrichment procedures, metagenomics surveys and single-cell technologies have enabled the identification of several new groups of most-wanted spookmicrobes, including novel methoxydotrophic methanogens that produce methane from methylated coal compounds and acetoclastic 'Candidatus Methanothrix paradoxum', which is active in oxic soils. The resultant energy-rich methane can be oxidized via a suite of electron acceptors. Recently, 'Candidatus Methanoperedens nitroreducens' ANME-2d archaea and 'Candidatus Methylomirabilis oxyfera' bacteria were enriched on nitrate and nitrite under anoxic conditions with methane as an electron donor. Although 'Candidatus Methanoperedens nitroreducens' and other ANME archaea can use iron citrate as an electron acceptor in batch experiments, the quest for anaerobic methane oxidizers that grow via iron reduction continues. In recent years, the nitrogen cycle has been expanded by the discovery of various ammonium-oxidizing prokaryotes, including ammonium-oxidizing archaea, versatile anaerobic ammonium-oxidizing (anammox) bacteria and complete ammonium-oxidizing (comammox) Nitrospira bacteria. Several biogeochemical studies have indicated that ammonium conversion occurs under iron-reducing conditions, but thus far no microorganism has been identified. Ultimately, iron-reducing and sulfate-dependent ammonium-oxidizing microorganisms await discovery.

Microbial electricity driven anoxic ammonium removal

Removal of nitrogen, mainly in form of ammonium (NH₄⁺), in wastewater treatment plants (WWTPs) is a highly energy demanding process, mainly due to aeration. It causes costs of about half a million Euros per year in an average European WWTP. Alternative, more economical technologies for the removal of nitrogen compounds from wastewater are required. This study proves the complete anoxic conversion of ammonium (NH₄⁺) to dinitrogen gas (N₂) in continuously operated bioelectrochemical systems at the litre-scale. The removal rate is comparable to conventional WWTPs with 35 ± 10 g N m^-3 d^-1 with low accumulation of NO₂^-, NO₃^-, N₂O. In contrast to classical aerobic nitrification, the energy consumption is considerable lower (1.16 ± 0.21 kWh kg^-1 N, being more than 35 times less than for the conventional wastewater treatment). Biotic and abiotic control experiments confirmed that the anoxic nitrification was an electrochemical biological process mainly performed by Nitrosomonas with hydroxylamine as the main substrate (mid-point potential, E_ox = +0.67 ± 0.08 V vs. SHE). This article proves the technical feasibility and reduction of costs for ammonium removal from wastewater, investigates the underlying mechanisms and discusses future engineering needs.

Acceleration of electroactive anammox (electroammox) start-up by switching acetate pre-acclimated biofilms to electroammox biofilms

In this study, an operational method of switching acetate media to ammonium media after the formation of stable acetate-oxidizing biofilms (ACAM mode), was developed. The results showed that the start-up time was shortened to 48days in the ACAM mode compared to the AM (always ammonium media) mode (>120days), and an ammonia removal rate of 82±3% was achieved successfully and sustainably in the ACAM mode during the following long-term operation of more than 2months. Moreover, the ACAM mode was more efficient in enriching both electroammox bacteria and electricigens with Ignavibacteriaceae, Geobacteraceae and Nitrosomonadaceae as dominant families, which could favour the formation of high-performance electroammox biofilms. Thus, the ACAM mode might promote the widespread implementation of the electroammox process.

Opportunities for groundwater microbial electro-remediation

Groundwater pollution is a serious worldwide concern. Aromatic compounds, chlorinated hydrocarbons, metals and nutrients among others can be widely found in different aquifers all over the world. However, there is a lack of sustainable technologies able to treat these kinds of compounds. Microbial electro‐remediation, by the means of microbial electrochemical technologies (MET), can become a promising alternative in the near future. MET can be applied for groundwater treatment in situ or ex situ, as well as for monitoring the chemical state or the microbiological activity. This document reviews the current knowledge achieved on microbial electro‐remediation of groundwater and its applications.