Effective ammonium removal by anaerobic oxidation in microbial fuel cells

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.

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.

Scale-up of the bioelectrochemical system: Strategic perspectives and normalization of performance indices

Electrochemists and ecological engineers find environmental bioelectrochemistry appealing; however, there is a big gap between expectations and actual progress in bioelectrochemical system (BES). Implementing such technology opens new opportunities for novel electrochemical reactions for resource recovery and effective wastewater treatment. Loopholes of BES exist in its scaling-up applications, and numerous attempts toward practical applications (200, 1000, and 1500 L) are key successive indicators toward its commercialization. This review emphasized the critical rethinking of standardization of performance indices i.e. current generation (A/m²), net energy recovery (kWh/kg·COD), product/resource yield (mM), and economic feasibility ($/kWh) to make fair comparison with the existing treatment system. Therefore, directional perspectives, including modularity, energy-cost balance, energy and resource recovery, have been proposed for the sustainable market of BES. The current state of the art and up-gradation in resource recovery and contaminant removal warrants a systematic rethinking of functional worth and niches of BES for practical applications.

Integrating Human Waste with Microbial Fuel Cells to Elevate the Production of Bioelectricity

Due to the continuous depletion of natural resources currently used for electricity generation, it is imperative to develop alternative energy sources. Human waste is nowadays being explored as an efficient source to produce bio-energy. Human waste is renewable and can be used as a source for an uninterrupted energy supply in bioelectricity or biofuel. Annually, human waste such as urine is produced in trillions of liters globally. Hence, utilizing the waste to produce bioenergy is bio-economically suitable and ecologically balanced. Microbial fuel cells (MFCs) play a crucial role in providing an effective mode of bioelectricity production by implementing the role of transducers. MFCs convert organic matter into energy using bio-electro-oxidation of material to produce electricity. Over the years, MFCs have been explored prominently in various fields to find a backup for providing bioenergy and biofuel. MFCs involve the role of exoelectrogens which work as transducers to convert the material into electricity by catalyzing redox reactions. This review paper demonstrates how human waste is useful for producing electricity and how this innovation would be beneficial in the long term, considering the current scenario of increasing demand for the supply of products and shortages of natural resources used to produce biofuel and bioelectricity.

Effectiveness of constructed wetland integrated with microbial fuel cell for domestic wastewater treatment and to facilitate power generation

Constructed wetlands (CWs) have gained a lot of attention for wastewater treatment due to robustness and natural pollutant mitigation characteristics. This widely acknowledged technology possesses enough merits to derive direct electricity in collaboration with microbial fuel cell (MFC), thus taking advantage of microbial metabolic activities in the anoxic zone of CWs. In the present study, two identical lab-scale CWs were selected, each having 56 L capacity. One of the CW integrated with MFC (CW-MFC) contains two pairs of electrodes, i.e., carbon felt and graphite plate. The first pair of CW-MFC consists of a carbon felt cathode with a graphite plate anode, and the second pair contains a graphite plate cathode with a carbon felt anode. The other CW was not integrated with MFC and operated as a traditional CW for evaluating the performance. CW-MFC and CW were operated in continuous up-flow mode with a hydraulic retention time of 3 days and at different organic loading rates (OLRs) per unit surface area, such as 1.45 g m^-2 day^-1 (OLR-1), 2.43 g m^-2 day^-1 (OLR-2), and 7.25 g m^-2 day^-1 (OLR-3). The CW-MFC was able to reduce the organic matter, phosphate, and total nitrogen by 92%, 93%, and 70%, respectively, at OLR of 1.45 g m^-2 day^-1, which was found to be higher than that obtained in conventional CW. With increase in electrochemical redox activities, the second pair of electrodes made way for 3 times higher power density of 16.33 mW m^-2 as compared to the first pair of electrodes in CW-MFC (5.35 mW m^-2), asserting carbon felt as a good anode material to be used in CW-MFC. The CW-MFC with carbon felt as an anode material is proposed to improve the electro-kinetic activities for scalable applications to achieve efficient domestic wastewater treatment and electricity production.

Enhancement in Cathodic Redox Reactions of Single-Chambered Microbial Fuel Cells with Castor Oil-Emitted Powder as Cathode Material

Microbial fuel cell (MFC) would be a standalone solution for clean, sustainable energy and rural electrification. It can be used in addition to wastewater treatment for bioelectricity generation. Materials chosen for the membrane and electrodes are of low cost with suitable conducting ions and electrical properties. The prime objective of the present work is to enhance redox reactions by using novel and low-cost cathode catalysts synthesized from waste castor oil. Synthesized graphene has been used as an anode, castor oil-emitted carbon powder serves as a cathode, and clay material acts as a membrane. Three single-chambered MFC modules developed were used in the current study, and continuous readings were recorded. The maximum voltage achieved was 0.36 V for a 100 mL mixture of domestic wastewater and cow dung for an anodic chamber of 200 mL. The maximum power density obtained was 7280 mW/m². In addition, a performance test was evaluated for another MFC with inoculums slurry, and a maximum voltage of 0.78 V and power density of 34.4093 mW/m² with an anodic chamber of 50 mL was reported. The present study’s findings show that such cathode catalysts can be a suitable option for practical applications of microbial fuel cells.

Simultaneous enhancement of treatment performance and energy recovery using pyrite as anodic filling material in constructed wetland coupled with microbial fuel cells

Constructed wetland coupled with microbial fuel cells (CW-MFCs) are a promising technology for sustainable wastewater treatment. However, the performance of CW-MFCs has long been constrained by the limited size of its anode. In this study, we developed an alternative CW-MFC configuration that uses inexpensive natural conductive pyrite as an anodic filling material (PyAno) to extend the electroactive scope of the anode. As a result, the PyAno configuration significantly facilitated the removal of chemical oxygen demand, ammonium nitrogen, total nitrogen, and total phosphorus. Meanwhile, the PyAno increased the maximum power density by 52.7% as compared to that of the quartz sand control. Further, a typical exoelectrogen Geobacter was found enriched in the anodic zone of PyAno, indicating that the electroactive scope was extended by conductive pyrite. In addition, a substantial electron donating potential was observed for the anodic filling material of PyAno, which explained the higher electricity output. Meanwhile, a higher dissimilatory iron reducing potential was observed for the anodic sediment of PyAno, demonstrating the integrity of an iron redox cycling in the system and its promotive effect for the wastewater treatment. Together, these results implied that the PyAno CW-MFCs can be a competitive technology to enhance wastewater treatment and energy recovery simultaneously.

Ammonium as a Carbon-Free Electron and Proton Source in Microbial Electrosynthesis Processes

Biogas upgrading to biomethane with microbial electrosynthesis (MES) is receiving much attention due to increasing biomethane demands and surplus renewable energy. Research has demonstrated the feasibility of MES to increase methane yield by reducing CO2 in anaerobic digestion (AD). Such CO2 reduction occurs at the cathode and requires the supply of both protons and electrons. The most studied sources of protons and electrons are oxidation of organic substances and water, generated at the anode. These anodic reactions, however, also imply the production of CO2 and O2, respectively, both with negative implications for the AD process. A source of protons and electrons without CO2 and O2 as by-products would be beneficial for MES-enhanced biomethane production. This opinion article discusses the possibility of ammonium to serve as a sustainable proton and electron source.

Use of Cyclic Voltammetry to Describe the Electrochemical Behavior of a Dual-Chamber Microbial Fuel Cell

Cyclic voltammetry (CV) was used in this work to describe the electrochemical behavior of a dual-chamber microbial fuel cell (MFC). The system performance was evaluated under vacuum and non-pressurized conditions, different reaction times, two sweep potentials, 25 and 50 mVs−1 and under different analyte solutions, such as distilled water and domestic wastewater. CV experiments were conducted by using a potentiostat with three different configurations to collect the measurements. A dual-chamber MFC system was equipped with a DupontTM Nafion® 117 proton exchange membrane (PEM), graphite electrodes (8.0 cm × 2.5 cm × 0.2 cm) and an external electric circuit with a 100-Ω resistor. An electrolyte (0.1 M HCl, pH ≈ 1.8) was used in the cathode chamber. It was found that the proton exchange membrane plays a major role on the electrochemical behavior of the MFC when CV measurements allow observing the conductivity performance in the MFC in the absence of a reference electrode; under this potentiostat setting, less current density values are obtained on the scanned window potentials. Therefore, potentiostat setting is essential to obtain information in complex electrochemical processes present in biological systems, such as it is the case in the MFCs. Results of the study showed that wastewater constituents and the biomass suspended or attached (biofilm) over the electrode limited the electron charge transfer through the interface electrode-biofilm-liquor. This limitation can be overcome by: (i) Enhancing the conductivity of the liquor, which is a reduction of the ohmic drop, (ii) reducing the activation losses by a better catalysis, and (iii) by limiting the diffusional gradients in the bulk liquor, for instance, by forced convection. The use of the electrolyte (0.1 M HCl, pH ≈ 1.8) and its diffusion from the cathode to the anode chamber reduces the resistance to the flow of ions through the PEM and the flow of electrons through the anodic and cathodic electrolytes. Also reduces the activation losses during the electron transfer from the substrate to the electrode surface due to the electrode catalysis improvement. On the other hand, vacuum also demonstrated that it enhances the electrochemical performance of the dual-chamber MFC due to the fact that higher current densities in the system are favored.

Suppressing methanogens and enriching electrogens in bioelectrochemical systems

Suppression of methanogens is considered as one of the main challenges in achieving the practical application of several types of bioelectrochemical system (BES). Feasibility of mixed culture as an inoculum in BES is mainly restricted by methanogenic population. Methanogens compete with electrogens (in bioanodes) or acetogens (in biocathodes) for substrate which results in diminishing Coulombic efficiency. Selection of particular inoculum pretreatment method affects the microbial diversity in anodic/cathodic microenvironments and hence the performance of BES. This review discusses various physical, chemical and biological pretreatment methods for suppressing the growth of methanogens. Selective microbial enrichment in anodic/cathodic biofilm can be promoted with bioaugmentation and/or applied external potential approach to harvest maximum Coulombs from the substrate. For field application of BES, physical pretreatment methods can be proposed with intermittent addition of chemical inhibitors and conversion of methane to electricity in order to make the process inexpensive along with recovering the maximum energy.

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.

Relaxing the formation of hypoxic bottom water with sediment microbial fuel cells

The method of improving bottom water environment using industrial wastes to suppress diffusion substances from bottom sediment has recently captured the attention of many researchers. In this study, wastewater discharge-derived sediment was used to examine an alternative approach involving the use of sediment microbial fuel cells (SMFCs) in relaxing the formation of hypoxic bottom water, and removing reduced substances from sediment. Concentrations of dissolved oxygen (DO) and other ions were measured in overlying water and sediment pore water with and without the application of SMFCs. The results suggest that SMFCs can markedly reduce hydrogen sulfide and manganese ion concentrations in overlying water, and decrease the depletions of redox potential and DO concentration. In addition, SMFCs can dissolve ferric compounds in the sediment and thereby release the ferric ion available to fix phosphate in the sediment. Our results indicate that SMFCs can be used as an alternative method to relax the formation of hypoxic bottom water and to remove reduced substances from the sediment, thus improving the quality of both water and sediment environments.

Scaling-up of a novel, simplified MFC stack based on a self-stratifying urine column

BACKGROUND

The microbial fuel cell (MFC) is a technology in which microorganisms employ an electrode (anode) as a solid electron acceptor for anaerobic respiration. This results in direct transformation of chemical energy into electrical energy, which in essence, renders organic wastewater into fuel. Amongst the various types of organic waste, urine is particularly interesting since it is the source of 75 % of the nitrogen present in domestic wastewater despite only accounting for 1 % of the total volume. However, there is a persistent problem for efficient MFC scale-up, since the higher the surface area of electrode to volume ratio, the higher the volumetric power density. Hence, to reach usable power levels for practical applications, a plurality of MFC units could be connected together to produce higher voltage and current outputs; this can be done by combinations of series/parallel connections implemented both horizontally and vertically as a stack. This plurality implies that the units have a simple design for the whole system to be cost-effective. The goal of this work was to address the built configuration of these multiple MFCs into stacks used for treating human urine.

RESULTS

We report a novel, membraneless stack design using ceramic plates, with fully submerged anodes and partially submerged cathodes in the same urine solution. The cathodes covered the top of each ceramic plate whilst the anodes, were on the lower half of each plate, and this would constitute a module. The MFC elements within each module (anode, ceramic, and cathode) were connected in parallel, and the different modules connected in series. This allowed for the self-stratification of the collective environment (urine column) under the natural activity of the microbial consortia thriving in the system. Two different module sizes were investigated, where one module (or box) had a footprint of 900 mL and a larger module (or box) had a footprint of 5000 mL. This scaling-up increased power but did not negatively affect power density (≈12 W/m(3)), a factor that has proven to be an obstacle in previous studies.

CONCLUSION

The scaling-up approach, with limited power-density losses, was achieved by maintaining a plurality of microenvironments within the module, and resulted in a simple and robust system fuelled by urine. This scaling-up approach, within the tested range, was successful in converting chemical energy in urine into electricity.

The tubular MFC with carbon tube air-cathode for power generation and N,N-dimethylacetamide treatment

A continuous flow microbial fuel cell (MFC) was assembled with carbon tube air-cathode and carbon felt anode. The organic solvent N,N-dimethylacetamide (DMAC) was used as the only carbon source for power generation. After the adaptive phase, the cell potential was gradually increased from 0.15 to 0.45 V with 200 Ω of external resistor during 150 h of operation. The calculated power density of this MFC was 100 mW L(-1) when the cell potential was 0.45 V. The reversible redox peaks of carbon tube were obtained in cyclic voltammogram between -0.5 and -0.25 V under aerobic circumstance. The removal rate of DMAC was 15-50% after treatment with hydraulic retention time of 12 min. The results indicated that it is possible to realize the power extraction from DMAC wastewater in the form of electricity by the bioconversion process of MFC.