Performance of different macrophytes in the decontamination of and electricity generation from swine wastewater via an integrated constructed wetland-microbial fuel cell process

Promoting bioremediation of brewery wastewater, production of bioelectricity and microbial community shift by sludge microbial fuel cells using biochar as anode

Seeking low-cost and eco-friendly electrode catalyst of microbial fuel cell (MFC) reactor has received extensive attention in recent decades. In this study, a sludge MFC was coupled with biochar-modified-anode (BC-300, BC-400, and BC-500) for actual brewery wastewater treatment. The physicochemical properties of biochar largely depended on the pyrolysis temperature, further affecting the removal efficiency of wastewater indicators. BC-400 MFC proved to be efficient for TN and NH4+-N removal, while the maximum removal efficiencies of COD and TP were achieved by BC-500 MFC, reaching respectively 97.14 % and 89.67 %. Biochar could promote the degradation of dissolved organic matter (DOM) in wastewater by increasing the electrochemical performances of MFC. The maximum output voltage of BC-400 MFC reached 410.24 mV, and the maximum electricity generation of 108.05 mW/m2 was also obtained, surpassing the pristine MFC (BCC-MFC) by 4.67 times. High-throughput sequencing results illustrated that the enrichment of electrochemically active bacteria (EAB) and functional bacteria (Longilinea, Denitratisoma, and Pseudomonas) in BC-MFCs, contributed to pollutants degradation and electron transfer. Furthermore, biochar affected directly the electrical conductivity of wastewater, simultaneously changing microbial community composition of MFC anode. Considering both enhanced removal efficiency of pollutants and increased power generation, the results of this study would offer technical reference for the application of biochar as MFC catalyst for brewery wastewater treatment.

Removal and reduction mechanism of Cr (VI) in Leersia hexandra Swartz constructed wetland-microbial fuel cell coupling system

Cr (VI) is extremely harmful to both the environment and human health, and it can linger in the environment for a very long period. In this research, the Leersia hexandra Swartz constructed wetland-microbial fuel cell (CW-MFC) system was constructed to purify Cr (VI) wastewater. By comparing with the constructed wetland (CW) system, the system electricity generation, pollutants removal, Cr enrichment, and morphological transformation of the system were discussed. The results demonstrated that the L. hexandra CW-MFC system promoted removal of pollutants and production of electricity of the system. The maximum voltage of the system was 499 mV, the COD and Cr (VI) removal efficiency was 93.73% and 97.00%. At the same time, it enhanced the substrate and L. hexandra ability to absorb Cr and change it morphologically transformation. Additionally, the results of XPS and XANES showed that the majority of the Cr in the L. hexandra and substrate was present as Cr (III). In the L. hexandra CW-MFC system, Geobacter also functioned as the primary metal catabolic reducing and electrogenic bacteria. As a result, L. hexandra CW-MFC system possesses the added benefit of removing Cr (VI) while producing energy compared to the traditional CW system.

Sustainable in situ ammonia recovery from municipal solid waste leachate in a single-stream microbial desalination cell

Municipal solid waste (MSW) leachate is one of the most hazardous waste streams leading to great potential risk to environment, and a renewable resource with high concentrations of organic contaminant and ammonia. High energy consumption and chemical input are still the challenges for ammonia recovery from MSW leachate. Here, a single-stream microbial desalination cell (SMDC) was successfully developed for simultaneous energy extraction from organic contaminant and in-situ energy utilization for ammonia recovery. 70% of the organic contaminant from the actual MSW leachate was removed, and 24.9% of the total ammonia was recovered as high-purity (NH4)2SO4. The additional desalination chamber introduced into the SMDC can potentially enhance the NH4+ migration that was determined by the NH4+ concentration gradient and electric field. More than 30% of the total nitrogen was lost, as revealed by nitrogen mass balance analysis, probably resulting from the anodic denitrification process driven by denitrifying microorganisms, e.g., Thauera, which thrived in the anode chamber. Concomitantly, the chemical input for ammonia stripping can be reduced by up to 68% due to the relatively low buffer capacity of the catholyte and the OH- production from the cathode reaction. This SMDC can be an effective and environmentally sustainable solution for MSW leachate treatment and resource recovery.

Development of nature-based sustainable passive technologies for treating and disinfecting municipal wastewater: Experiences from constructed wetlands and slow sand filter

There is an urgent need to develop low-cost technology for effective wastewater treatment and its further disinfection to the level that makes it economically useful. This work has designed and evaluated the various types of constructed wetlands (CWs) followed by a slow sand filter (SSF) for wastewater treatment and disinfection. The studied CWs were, CWs with gravels (CW-G), free water surface-CW (FWS-CWs), and CWs integrated microbial fuel cell (MFC) with granular graphite (CW-MFC-GG) planted with Canna indica plant species. These CWs were operated as secondary wastewater treatment technologies followed by SSF for disinfection purposes. The highest total coliform removal was observed in the combination of CW-MFC-GG-SSF which achieved a final concentration of 172 CFU/100 mL, whereas faecal coliform removal was 100 % with the combinations of CW-G-SSF and CW-MFC-GG-SSF, achieving 0 CFU/100 mL in the effluent. In contrast, FWS-SSF achieved the lowest total and faecal coliform removal attaining a final concentration of 542 CFU/100 mL and 240 CFU/100 mL, respectively. Furthermore, E. coli were detected as negative/absent in CW-G-SSF and CW-MFC-GG-SSF, while it was positive for FWS-SSF. In addition, the highest turbidity removal was achieved in CW-MFC-GG and SSF combination of 92.75 % from the municipal wastewater influent turbidity of 82.8 NTU. Furthermore, in terms of overall treatment performance of CW-G-SSF and CW-MFC-GG-SSF, these systems were able to treat 72.7 ± 5.5 % and 67.0 ± 2.4 % of COD and 92.3 % and 87.6 % of phosphate, respectively. Additionally, CW-MFC-GG also exhibited a power density of 85.71 mA/m3 and a current density of 25.71 mW/m3 with 700 Ω of internal resistance. Thus, CW-G and CW-MFC-GG followed by SSF could be a promising solution for enhanced disinfection and wastewater treatment.

The race between classical microbial fuel cells, sediment-microbial fuel cells, plant-microbial fuel cells, and constructed wetlands-microbial fuel cells: Applications and technology readiness level

Microbial fuel cell (MFC) is an interesting technology capable of converting the chemical energy stored in organics to electricity. It has raised high hopes among researchers and end users as the world continues to face climate change, water, energy, and land crisis. This review aims to discuss the journey of continuously progressing MFC technology from the lab to the field so far. It evaluates the historical development of MFC, and the emergence of different variants of MFC or MFC-associated other technologies such as sediment-microbial fuel cell (S-MFC), plant-microbial fuel cell (P-MFC), and integrated constructed wetlands-microbial fuel cell (CW-MFC). This review has assessed primary applications and challenges to overcome existing limitations for commercialization of these technologies. In addition, it further illustrates the design and potential applications of S-MFC, P-MFC, and CW-MFC. Lastly, the maturity and readiness of MFC, S-MFC, P-MFC, and CW-MFC for real-world implementation were assessed by multicriteria-based assessment. Wastewater treatment efficiency, bioelectricity generation efficiency, energy demand, cost investment, and scale-up potential were mainly considered as key criteria. Other sustainability criteria, such as life cycle and environmental impact assessments were also evaluated.

The potential and sustainable strategy for swine wastewater treatment: Resource recovery

Swine wastewater is highly polluted with complex and harmful substances that require effective treatment to minimize environmental damage. There are three commonly used biological technologies for treating swine wastewater: conventional biological technology (CBT), microbial electrochemical technology (MET), and microalgae technology (MT). However, there is a lack of comparison among these technologies and a lack of understanding of their unique advantages and efficient operation strategies. This review aims to compare and contrast the characteristics, influencing factors, improvement methods, and microbial mechanisms of each technology. CBT is cost-effective but has low resource recovery efficiency, while MET and MT have the highest potential for resource recovery. However, all three technologies are affected by various factors and toxic substances such as heavy metals and antibiotics. Improved methods include exogenous/endogenous enhancement, series reactor operation, algal-bacterial symbiosis system construction, etc. Though MET is limited by construction costs, CBT and MT have practical applications. While swine wastewater treatment processes have developed automatic control systems, the application need further promotion. Furthermore, key functional microorganisms involved in CBT's pollutant removal or transformation have been detected, as have related genes. The unique electroactive microbial cooperation mode and symbiotic mode of MET and MT were also revealed, respectively. Importantly, the future research should focus on broadening the scope and scale of engineering applications, preventing and controlling emerging pollutants, improving automated management level, focusing on microbial synergistic metabolism, enhancing resource recovery performance, and building a circular economy based on low-cost and resource utilization.

Comparative study on the treatment of swine wastewater by VFCW-MFC and VFCW: Pollutants removal, electricity generation, microorganism community

Swine wastewater, characterized by high organic and nutrient content, poses significant environmental challenges. This study aims to compare the effectiveness of two treatment technologies, namely Vertical Flow Constructed Wetland-Microbial Fuel Cell (VFCW-MFC) and Vertical Flow Constructed Wetland (VFCW), in terms of pollutant removal, electricity generation, and microorganism community dynamics. The results showed that the average removal efficiencies of chemical oxygen demand (COD), ammonia nitrogen, total nitrogen (TN), total phosphorus (TP) and sulfadiazine antibiotics (SDZ) by VFCW-MFC were as high as 94.15%, 95.01%, 42.24%, 97.16% and 82.88%, respectively, which were all higher than that by VFCW. Both VFCW-MFC and VFCW have good tolerance to SDZ. In addition, VFCW-MFC has excellent electrical performance, with output voltage, power density, coulombic efficiency and net energy recovery up to 443.59 mV, 51.2 mW/m³, 52.91% and 2.04 W/(g·s), respectively, during stable operation. Moreover, the microbial community diversity of VFCW-MFC was more abundant, and the species abundance distribution in cathode region was more rich and even than in anode region. At phylum level, the dominant microorganisms in VFCW-MFC included Proteobacteria, Bacteroidota, Firmicutes and Actinobacteriota, which showed good degradation effect on SDZ. Proteobacteria and Firmicutes are also involved in electricity production. Chloroflexi, Proteobacteria and Bacteroidota play a major role in nitrogen reduction.

Both species richness and growth forms affect nutrient removal in constructed wetlands: A mesocosm experiment

Introduction

Plant richness is thought to improve the function of constructed wetlands (CWs), but most CWs are planted with monocultures, with only a few employed polycultures, which have drawn contradictory conclusions. We suppose functional diversity is the key to better performance of plant communities and hypothesize that CWs planted with diverse growth forms are superior in plant growth and nutrient removal.

Methods

In this study, six emergent plant species categorized into slender type (Schoenoplectus tabernaemontani, Typha orientalis), fan type (Iris sibirica, Acorus calamus) and large type (Canna indica and Thalia dealbata) were planted in monocultures, combinations (two species of the same growth form) and mixed polycultures (six species of three growth forms). We then compared how plant growth and nutrient uptake differed among treatments.

Results

It showed that the polyculture considerably increased the removal of total nitrogen (TN) and total phosphorus (TP), but the combination did not outperform monoculture. High consistency in the patterns between underground biomass and total biomass indicated that plant roots were essential for nutrient consumption. Compared with slender and fan plants, the large plants had a greater biomass increase in polycultures, which greatly accelerated the absorption and assimilation of TN and TP.

Conclusion

Our study indicated that plant community with various growth forms reduced the intensity of interspecific competition, increased the functional diversity, and greatly enhanced the ability of pollutant removal. Our results also provide some suggestions for plant selection and combination designs in CWs.

Investigation of electrochemical properties, leachate purification, organic matter characteristics, and microbial diversity in a sludge treatment wetland- microbial fuel cell

Sludge treatment wetland-microbial fuel cell (STW-MFC) is a unique sludge treatment process that produces bioelectricity, but its technology is still in its infancy. This study investigated the electrochemical properties, organic matter characteristics, leachate purification, and microbial community structure of STW-MFCs as affected by electrode location. When electrodes were placed in the filler layer, the STW-MFC system presented a higher power generation capacity (maximum output power density: 0.498 W/m³; peak cell voltage: 0.879 V) and organic matter degradation efficiency. The hydrophilic fraction was the main dissolved organic carbon fraction in sludge extracellular biological organic matter (EBOM) and leachate dissolved organic matter (DOM). Aromatics were mainly concentrated in the hydrophobic acid fraction. The UV-254 content of sludge EBOM decreased mainly in the hydrophilic and transphilic acid fractions. The excitation-emission matrix analysis showed that tryptophan-like protein was more easily eliminated than tyrosine-like protein. In addition, there was a strong correlation between voltage and NH₄⁺ removal efficiency; a negative correlation between total chemical oxygen demand (TCOD), total nitrogen (TN), and total phosphorus (TP) removal efficiency, and a negative correlation between pH and TN, TP, and NH₄⁺ removal efficiencies. High-throughput sequencing showed that the system was most abundant in Thermomonas, Geothrix and Geobacter when the electrodes were placed in the filled layer, while the levels of genes for membrane transport, carbohydrate metabolism and energy metabolism functions were higher than in other systems. This work will support STW- MFC widespread implementation by illuminating the underlying mechanics of different anode positions.

Performance assessment of normal and electrode-assisted floating wetlands: influence of input pollutant loads, surface area, and positioning of anode electrodes

This study reports the design and development of microbial fuel cell (MFC) assisted floating wetlands and compares treatment removal performance with a normal (without electrodes) floating wetland. Both types of floating wetlands were planted with Phragmites plant and evaluated for real municipal wastewater treatment. The effective volume of each floating wetland was 0.5 m³. The floating wetlands were operated under variable hydraulic load rates, i.e., 20 and 60 mm/day. Mean 5-day biochemical oxygen demand (BOD₅), chemical oxygen demand (COD), ammoniacal nitrogen (NH₄-N), total nitrogen (TN), total phosphorus (TP), total suspended solids (TSS), and coliform removal percentages ranged between 71 and 96%, 72 and 94%, 62 and 86%, 58 and 75%, 82 and 97%, 64 and 92%, and 72 and 93%, respectively within the normal and electrode-assisted MFC integrated floating wetlands. The electrode-integrated floating wetlands showed better pollutant removal performance than the normal system under unstable input pollutant loading conditions. Nitrogen and organic matter removals were achieved through both electrochemically active and inactive microbial removal routes. Physical separation processes, such as filtration and sedimentation, contributed to phosphorus, solids, and coliform removal. Plant uptake contributed to micro-scale nitrogen (≤ 1%) and phosphorus (≤ 0.1%) removal. Increment of hydraulic/pollutant load improved organic removal but decreased nutrient removal performance of the normal, electrode-integrated floating wetlands. The electrode-integrated floating wetlands produced power densities ranging between 0.7 and 1.4 mW/m³, and 0.2 and 2.3 mW/m³ during lower, upper input loading ranges, respectively. Bioenergy production of the electrode-integrated floating wetlands varied within the two operational periods due to a wider range of electrochemically inactive microbial populations in real wastewater that interfered with electrochemical organic matter oxidation. The positioning difference of the anode electrodes was a significant factor that improved pollutant removal within the electrode-integrated floating wetlands compared to the other variable, i.e., anode electrodes surface area.

Constructed Wetland Coupled Microbial Fuel Cell: A Clean Technology for Sustainable Treatment of Wastewater and Bioelectricity Generation

The availability of clean water and the depletion of non-renewable resources provide challenges to modern society. The widespread use of conventional wastewater treatment necessitates significant financial and energy expenditure. Constructed Wetland Microbial Fuel Cells (CW-MFCs), a more recent alternative technology that incorporates a Microbial Fuel Cell (MFC) inside a Constructed Wetland (CW), can alleviate these problems. By utilizing a CW’s inherent redox gradient, MFC can produce electricity while also improving a CW’s capacity for wastewater treatment. Electroactive bacteria in the anaerobic zone oxidize the organic contaminants in the wastewater, releasing electrons and protons in the process. Through an external circuit, these electrons travel to the cathode and produce electricity. Researchers have demonstrated the potential of CW-MFC technology in harnessing bio-electricity from wastewater while achieving pollutant removal at the lab and pilot scales, using both domestic and industrial wastewater. However, several limitations, such as inadequate removal of nitrogen, phosphates, and toxic organic/inorganic pollutants, limits its applicability on a large scale. In addition, the whole system must be well optimized to achieve effective wastewater treatment along with energy, as the ecosystem of the CW-MFC is large, and has diverse biotic and abiotic components which interact with each other in a dynamic manner. Therefore, by modifying important components and optimizing various influencing factors, the performance of this hybrid system in terms of wastewater treatment and power generation can be improved, making CW-MFCs a cost-effective, cleaner, and more sustainable approach for wastewater treatment that can be used in real-world applications in the future.

Recent progress in microbial fuel cells using substrates from diverse sources

Increasing untreated environmental outputs from industry and the rising human population have increased the burden of wastewater and other waste streams on the environment. The most prevalent wastewater treatment methods include the activated sludge process, which requires aeration and is, therefore, energy and cost-intensive. The current trend towards a circular economy facilitates the recovery of waste materials as a resource. Along with the amount, the complexity of wastewater is increasing day by day. Therefore, wastewater treatment processes must be transformed into cost-effective and sustainable methods. Microbial fuel cells (MFCs) use electroactive microbes to extract chemical energy from waste organic molecules to generate electricity via waste treatment. This review focuses use of MFCs as an energy converter using wastewater from various sources. The different substrate sources that are evaluated include industrial, agricultural, domestic, and pharmaceutical types. The article also highlights the effect of operational parameters such as organic load, pH, current, and concentration on the MFC output. The article also covers MFC functioning with respect to the substrate, and the associated performance parameters, such as power generation and wastewater treatment matrices, are given. The review also illustrates the success stories of various MFC configurations. We emphasize the significant measures required to fill in the gaps related to the effect of substrate type on different MFC configurations, identification of microbes for use as biocatalysts, and development of biocathodes for the further improvement of the system. Finally, we shortlisted the best performing substrates based on the maximum current and power, Coulombic efficiency, and chemical oxygen demand removal upon the treatment of substrates in MFCs. This information will guide industries that wish to use MFC technology to treat generated effluent from various processes.

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

For the deep reduction of nitrogen, the microbial fuel cell-constructed wetland (MFC-CW) was conducted for treating carbon constraint wastewater. Results indicated that nitrogen removal decreased from 94.96% to 24.96% with influent COD/TN (chemical oxygen demand/total nitrogen) from 4 to 0. MFC-CW was seriously affected by low organic wastewater. Wetland plants contributed to denitrification, with TN removal increasing from 46.13% to 64.87%. The bioenergy output showed a linear relationship with influent COD, and the maximum power density of 1.17 mW/m² was obtained. Correlation analysis indicated that functional genera of Paenibacillus, Trichococcus, norank_KD4-96, norank_OLB14 played a crucial role in nitrogen removal. Influent COD and wetland plants affected carbon and nitrogen metabolisms, and key genes related to denitrification were more abundant in the cathode. Findings illustrated the nitrogen metabolism in MFC-CW with carbon constraint wastewater and will extend the application of MFC-CW in secondary effluent treatment from wastewater treatment plants.

Microbial electro deionization for waste water treatment - A critical review on methods, applications and mechanism

Electro deionization using microbial communities has been proven as a competent method for desalination and abatement of water pollution by removing ionic chemicals from the target waters. Microbial Desalination Cell (MDC) facilitates microbial deionization which can either support or be a substitute for the conventional desalination methods. Generation of electricity is accomplished by the bio electrochemical oxidation of organic compounds present as contaminants in wastewater which in turn attribute to the migration of ions in MDC system. The present review aims to elucidate the theory, principles and the application of microbial desalination cell and microbial fuel cell (MFC) in treatment of saline and wastewaters. Air cathode MDC and stacked MDC for purification of saline water are found to give promising results. Air pump assisted microbial desalination cell reported 150.39 ppm h^-1 of salt removal with an operational time period of 80 h and showed consistent results. Hence the air cathode assisted MDC showed dominant capacity of salt removal compared to stacked MDC. Also, three major types of microbial fuel cell, namely photosynthetic biofilm MFC, constructive wetland MFC and ceramic membrane supported MFC are reviewed for their potentials in wastewater treatment by deionization method and electricity generation. Complete (100%) removal of chemical oxygen demand was reported by photosynthetic microbial fuel cell operated for 16 days having 435.8 Ω of external resistance. When constructive wetland microbial fuel cell was operated for 10 days with 1000 ohms of external resistance, it exhibited complete (100%) removal of chemical oxygen demand from the wastewater. About 92% of chemical oxygen demand removal was demonstrated by ceramic membrane supported microbial fuel. Compared to ceramic membrane microbial fuel cell, photosynthetic and constructive wetland microbial fuel cell displayed better performance in terms of pollutant removal capacity and economical factor. Ability of the electrogenic species, namely Geobacter, Shewanella, Clostridium and Bacillus and the photosynthetic species, namely Chorella Vulgaris Rhodopsuedomonas, and Scenedesmus abundans in microbial deionization methods and their performance levels reported by several researchers are presented.

A comparative landfill leachate treatment performance in normal and electrodes integrated hybrid constructed wetlands under unstable pollutant loadings

This study provides a comparative pollutant removal performance assessment between organic or construction materials-based four hybrid wetland systems that received landfill leachate. The hybrid systems included vertical flow (VF) followed by horizontal flow (HF)-based unplanted and planted systems, and planted electrodes incorporated microbial fuel cell (MFC) integrated hybrid wetlands systems. All the systems were run in free-draining mode. Overall mean chemical oxygen demand (COD), total nitrogen (TN), and total phosphorus (TP) removal percentage of the hybrid systems ranged between 81 and 99%, 82 and 96%, 74 and 99%, respectively, under unstable input pollutant loading conditions. Additionally, up to 27% organic and up to 14% nitrogen removal improvement was observed in electrodes integrated free-draining VF wetlands. Free-draining and additional oxygen availability from atmospheric diffusion, rootzone improved the removal performance of MFC-based VF wetlands. Input load increment decreased organic, nutrient removals in second stage HF units due to saturated media. The chemical composition of the employed media supported biotic, abiotic organic, nutrient removal pathways. Nutrient accumulation percentage in plants tissue was very low, i.e., ≤3%. Bioenergy production across the MFC-based VF-HF wetlands decreased with input pollutant load increment. The single anode electrode-based VF wetland achieved maximum power density production, i.e., 294 mW/m².. The electrodes integrated hybrid systems achieved comparatively stable removal performance despite input pollutant/hydraulic load variation.

Enhanced performance and microbial interactions of shallow wetland bed coupling with functional biocathode microbial electrochemical system (MES)

It is essential to remediate the polluted urban river, which endangers the aquatic creatures and affected human body's senses. The treatment wetland combined with microbial electrochemical system (MES) used for the remediation is becoming a new research focus due to its ideal pollutants removal efficiency and small footprint. Here this paper provided a kind of novel shallow wetland bed coupling with close-circuit microbial electrochemical system (WB-CMES) to remove pollutants in surface water. In contrast to the shallow wetland bed coupling with open-circuit MES (WB-OMES) and the shallow wetland bed without MES (WB), the enhancing effects and pollutants removal pathway were evaluated. After 62-day operation, average TN removal efficiency in WB-CMES was 87.7%, which was 19.7% and 13.8% higher than that of WB-OMES and WB respectively. The rate coefficient k of NO₃^--N degradation in WB-CMES was 1.6 and 1.8 times higher than that in WB-OMES and WB. The results of chlorophyll, protein and superoxide dismutase (SOD) in WB-CMES were 27.3%, 44.3% and 12.9% higher than those in WB. The microbial community structure analysis indicated that electroactive bacteria on anode like Desulfobulbus could oxidize organics and generate electrons to compensate cathode, meanwhile, cathode could enrich more species of functional bacteria like Rhodobacter, Pirellula, Hyphomicrobium, Thauera, which had a synergistic effect on oxygen reduction, nitrogen removal and plant growth. The results indicated that oxygen produced by submerged plants could be utilized by the oxygen-reducing functional biocathode of MES and the proper aerobic and anoxic environment might enhance nitrate removal mainly through simultaneous nitrification and denitrification (SND), aerobic denitrification and anammox. This research provided a novel technology with advantages of simple operation, flexible configuration, easy scale-up and low cost for application in remediation of highly polluted surface water.

Interrelation between macrophytes roots and cathode in constructed wetland-microbial fuel cells: Further evidence

As an essential component in constructed wetland-microbial fuel cells (CW-MFC) system, the macrophytes play multiple roles in bioelectricity generation and decontaminants performance. However, the interrelation between macrophytes roots and cathode has not been fully investigated despite the fact that plant cultivation strategy is a critical issue in practice. For the first time, this study was designed to explore the interaction between macrophytes and cathode in CW-MFC by planting Cyperus altrnlifolius at relatively different positions from the cathode. The results showed that plants exhibited higher bioelectricity generation and dramatically improved pollution removal, as well as the improved richness and diversity of cathode microbes. More significantly, the relative locations between the plant roots and the cathode could lead to different cathode working patterns, while the optimal cathode pattern "plant root-assisted bio- & air-cathode" was formed when the plant roots are directly placed on the air-cathode layer in CW-MFC. The insight into the plant root and cathode relationship lies in whether the "multi-function cathode" can be established. This study contributes to increase the knowledge regarding the presence and behavior of plant roots and cathode throughout a CW-MFC system.

Effects of ammonia on electrochemical active biofilm in microbial electrolysis cells for synthetic swine wastewater treatment

When facing wastewater with high organic and ammonia, e. g. swine wastewater, microbial electrolysis cell (MEC) is emerging for energy extraction as hydrogen and methane. However, the effects of highly concentrated ammonia on MEC haven't been fully evaluated. In this study, single-chamber MECs were operated with acetate and sucrose as substrates under various ammonia concentrations. The current generally increased with ammonia loading from 80 to 3000 mg L^-1. Yet, the substrate consumption in MECs was inhibited with ammonia concentrations above 1000 mg L^-1. As a combined result, the energy recovery efficiency of MECs was stable. The electrochemical activity of anode biofilm reached the peak under 1000 mg L^-1 ammonia and was restricted under higher ammonia loadings. Under neutral pH, the NH₄⁺ increases the cell membrane permeability, which benefited the electrochemical activity of exoelectrogens to a proper extent. Nevertheless, the toxic ammonia also accelerated the anode biomass loss and stimulated the extracellular polymeric substance (EPS) secretion. Due to the current increase, the abundance of exoelectrogens generally raised with ammonia loading from 80 to 3000 mg L^-1. However, except for anode biomass loss, the carbon and methane metabolism pathways were inhibited in acetate-fed MEC, while the glycolysis acted as the rate-limiting step for substrate degradation in sucrose-fed conditions. This study systematically examined the influences of high ammonia loading on MEC performances, bio-community and anode electrochemical activities, and evaluated practical feasibility and application inch of MECs for the energy recovery and pollutant removal of high concentration organic and ammonia wastewater.

The reduction and fate of antibiotic resistance genes (ARGs) and mobile genetic elements (MGEs) in microbial fuel cell (MFC) during treatment of livestock wastewater

The fate and removal efficiency of antibiotic resistance genes (ARGs) and mobile genetic elements (MGEs) in livestock wastewater by microbial fuel cell (MFC) was evaluated by High-throughput quantitative PCR. The results showed that 137 ARGs and 9 MGEs were detected in untreated livestock wastewater. The ARG number of macrolide-lincosamide-streptogramin group B (MLSB), tetracycline and sulfonamide were relatively higher. Throughout the treatment process, the number and abundance of ARGs and MGEs significantly decreased. The relative abundance of tetracycline, sulfonamide and chloramphenicol resistance genes showed the most obvious decreasing trend, and the relative abundance of MGEs decreased by 75% (from 0.012 copies/16S rRNA copies to 0.003 copies/16S rRNA copies). However, the absolute abundance of beta-lactamase resistance genes slightly increased. The operation process of MFC produces selective pressure on microorganisms, and Actinobacteria were predominant and had the ability to decompose antibiotics. The COD removal rate and TN removal rate of livestock wastewater were 67.81% and 62.09%, and the maximum power density and coulomb efficiency (CE) reached 11.49% and 38.40% respectively. This study demonstrated that although the removal of COD and TN by MFC was limited, MFC was quite effective in reducing the risk of antibiotic toxicity and horizontal gene transfer.

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.

Hybrid constructed wetlands integrated with microbial fuel cells and reactive bed filter for wastewater treatment and bioelectricity generation

The present study aimed to develop a pilot-scale integrated system composed of anaerobic biofilter (AF), a floating treatment wetland (FTW) unit, and a vertical flow constructed wetland coupled with a microbial fuel cell (CW-MFC) and a reactive bed filter (RBF) for simultaneously decentralized urban wastewater treatment and bioelectricity generation. The first treatment stage (AF) had 1450 L and two compartments: a settler and a second one filled with plastic conduits. The two CWs (1000 L each) were vegetated with mixed plant species, the first supported in a buoyant expanded polyethylene foam and the second (CW-MFC) filled with pebbles and gravel, whereas the RBF unit was filled with P adsorbent material (light expanded clay aggregate, or LECA) and sand. In the CW-MFC units, 4 pairs of electrode chambers were placed in different spacing. First, both cathode and anode electrodes were composed of graphite sticks and monitored as open circuit. Later, the cathode electrodes were replaced by granular activated carbon (GAC) and monitored as open and closed circuits. The combined system efficiently reduced COD (> 64.65%), BOD₅ (81.95%), N-NH₃ (93.17%), TP (86.93%), turbidity (94.3%), and total coliforms (removal of three log units). Concerning bioenergy, highest voltage values were obtained with GAC electrodes, reaching up to 557 mV (open circuit) and considerably lower voltage outputs with closed circuit (23.1 mV). Maximum power densities were obtained with 20 cm (0.325 mW/m²) and 30 cm (0.251 mW/m²). Besides the electrode superficial areas, the HRT and the water level may have influenced the voltage values, impacting DO and COD concentrations in the wastewater.

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.

A review on recent advancements in bioenergy production using microbial fuel cells

The generation of energy and its efficient use in industries and agriculture are critical to any country's growth. A country like India, which is still developing, faces a major challenge in terms of generating adequate electricity. With the current crisis and environmental concerns, the government must look past carbon-based energy sources and into long-term energy sources. Microbial fuel cells (MFCs) are a form of technology that can be used to both treat wastewater and generate electricity on a large scale. Researchers play a critical role in making this technology practical and effective enough to be implemented. However, since the charge of building microbial fuel cells is superior than the cost of fossil fuels, it is unlikely that power production will continually be aggressive with existing energy generation approaches. However, improvements in power densities and lower material expenses could render microbial fuel cells a viable option for energy making in the future. Following a thorough literature review, the analysis resumes the role of micro-organisms and substrates in the anode chamber. Microbial fuel cells are discussed in terms of their forms, materials, mechanism, and activity. This analysis discusses the various factors that influence microbial fuel cells, as well as contemporary challenges and applications in the development of sustainable electrical power.

Improving the outcomes from electroactive constructed wetlands by mixing wastewaters from different beverage-processing industries

Denitrification in electroactive constructed wetland (EW) systems is constrained by the carbon source and the carbon/nitrogen (C/N) ratio (the COD/TN ratio). In this study, wastewater with a high C/N from a brewery was added to wastewater with a low C/N (dairy wastewater) in an EW system, and the pollutant removal, bioelectricity generation, transformations of dissolved organic matter, and microbial community structures were evaluated. The results showed that the average removal rates of ammonium nitrogen, total nitrogen, and chemical oxygen demand from the wastewater mixture were 6.40%, 46.44%, and 23.85% higher than those from the wastewater with a low C/N, respectively. Dissimilatory nitrate reduction to ammonium was effectively inhibited, and the NH₄⁺-N removal was 25.52% higher, when the wastewater mixture was used instead of the high C/N wastewater. Similarly, the output voltage was significantly increased, and the internal resistance of the device was reduced, for the wastewater mixture. The structure of the microbial community improved, the relative abundance of electrochemically active bacteria was higher, and the protein-like and humic-like components were lower, in the mixture treatment than in the individual treatment. The results show that the nitrogen removal and biopower generation improved in an EW system when high C/N wastewater was used as the carbon source.

The Role of Wetland Plants on Wastewater Treatment and Electricity Generation in Constructed Wetland Coupled with Microbial Fuel Cell

CWMFC is a novel technology that has been used for almost a decade for concurrent wastewater treatment and electricity generation in varying scopes of domestic, municipal, and industrial applications since its implementation in 2012. Its advantage of low-cost enhanced wastewater treatment and sustainable bioelectricity generation has gained considerable attention. Nevertheless, the overall efficiency of this novel technology is inclined by several operating factors and configuration strands, such as pH, sewage composition, organic loading, electrode material, filter media, electrogens, hydraulic retention time, and macrophytes. Here, we investigate the effect of the wetland plant component on the overall performance of CWMFCs. The macrophyte’s involvement in the oxygen input, nutrient uptake, and direct degradation of pollutants for the required treatment effect and bioelectricity production are discussed in more detail. The review identifies and compares planted and unplanted CWMFC with their efficiency on COD removal and electricity generation based on previous and recent studies.

A taxonomy of design factors in constructed wetland-microbial fuel cell performance: A review

The past decade has seen the rapid development of constructed wetland-microbial fuel cell (CW-MFC) technology in many aspects. The first publication on the combination of constructed wetland (CW) and microbial fuel cell (MFC) appeared in 2012, subsequently, research on the subject has grown exponentially to improve the performance of CW-MFCs in their dual roles of wastewater treatment and power generation. Although significant research has been conducted on this technology worldwide, a comprehensive and critical review of effective controlling parameters is lacking. More broadly, research is needed to draw up-to-date conclusions on recent developments and to identify knowledge gaps for further studies. This review paper systematically enumerates and reviews research studies published in this area to determine the key design factors and their role in CW-MFC performance. Moreover, a taxonomy of all CW-MFC design parameters has been synthesised from the literature. Importantly, this original work provides a comprehensive conceptual framework for future researchers, designers, builders, and users to understand CW-MFC technology. Within the taxonomy, parameters are placed in three main categories (physical/environmental, chemical, and biological/electrochemical) and comprehensive details are given for each parameter. Finally, a comprehensive summary of the parameters has been tabulated showing their impact on CW-MFC operation, design recommendations from literature, and the significant research gaps that this review has identified within the existing literature. It is hoped that this paper will provide a clear and rich picture of this technology at its current stage of development and furthermore, will facilitate a deeper understanding of CW-MFC performance for long-term and large-scale development.

A review on China's constructed wetlands in recent three decades: Application and practice

Constructed wetlands (CWs) have been introduced to and developed in China for environmental engineering over the most prosperous three decades (1990-2020). To study the origin, development process, and future trend of CWs, this review summarized a wide range of literatures between 1990 and 2020 by Chinese authors. Firstly, the publication number over years, research highlights, and the author contributions with the most published papers in this field were conducted through bibliometric analysis. Secondly, the most principal components of CWs, substrates and macrophytes were summarized and analyzed. Thirdly, the typical application cases from traditional CWs, pond systems to combined pond-wetland systems were presented. In China, CWs were predominately distributed in the east of the so-called 'Hu Huanyong Line'. Therefore CWs were limited by the socio-economic level and climatic conditions. It is unquestionable that the overall level of China's CWs has improved significantly, and one of the most prominent features has started towards the plural pattern development. There has been a trend of large-scale or low-cost CW application in the recent years. However, lifecycle research and management are required for better strategies in the future.

Intensified constructed wetlands for the treatment of municipal wastewater: experimental investigation and kinetic modelling

This study reports organics and nutrient removal performances of the intensified constructed wetlands, i.e., tidal flow-based microbial fuel cell (MFC) and tidal flow wetlands that received municipal wastewater. The wetland systems were filled with organic (coco peat, biochar) or waste (Jhama brick, steel slag) materials, planted with Phragmites australis or Chrysopogon zizanioides (Vetiver) species, and operated under three flood periods: 8, 16, 24 h. Input ammonia nitrogen (NH3-N), total nitrogen (TN), phosphorus (P), chemical oxygen demand (COD), and biochemical oxygen demand (BOD) load across the wetland systems ranged between 3-27, 12-78, 0.1-23, 36-1130, and 11-281 g/m2day, respectively; mean removal percentages were 60-83, 74-84, 95-100, 94-98, and 93-97%, respectively, throughout the experimental run. The wetland systems achieved similar organics and P removals; operational and media variation did not influence removal kinetics. All wetland systems achieved the highest TN removal (76-87%) when subjected to 24-h flood period. TN removal performances of waste material-based wetlands were comparable to organic media-based systems. Tidal flow-based MFC wetlands achieved better TN removal than tidal flow wetlands because of supplementary electron production through fuel cell-based organics degradation kinetics. Maximum power production rates across the tidal flow-based MFC wetlands ranged between 53 and 57 mW/m2. Monod kinetics-based continuous stirred tank reactor (CSTR) models predicted NH3-N, TN, and COD removals (in wetland systems) more accurately. Kinetic models confirmed the influence of substrate (i.e., pollutant) and environmental parameters on pollutant removal routes.

Mapping the field of constructed wetland-microbial fuel cell: A review and bibliometric analysis

The embedding microbial fuel cell (MFC) into constructed wetlands (CW) to form CW-MFC bears the potential to obtain bioelectricity and a clean environment. In this study, a bibliometric analysis using VOSviewer based on Web of Science data was conducted to provide an overview by tracing the development footprint of this technology. The countries, institutions, authors, key terms, and keywords were tracked and corresponding mapping was generated. From 2012 to September 2020, 442 authors from 129 organizations in 26 countries published 135 publications in 42 journals with total citation of 3139 times were found. The key terms analysis showed four clusters: bioelectricity generation performance, mechanism study, refractory pollutants removal, and enhanced conventional contaminants removal. Further research themes include exploring the biochemical properties of electrochemically active bacteria, emerging contaminants removal, effective bioelectricity harvest and the use, and biosensor development as well as scaling-up for real field application. The bibliometric results provide valuable references and information on potential research directions for future studies.

Dual role of macrophytes in constructed wetland-microbial fuel cells using pyrrhotite as cathode material: A comparative assessment

In the recent years many studies have shown that wetland plants play beneficial roles in bioelectricity enhancement in constructed wetland-microbial fuel cell (CW-MFC) because of the exudation of root oxygen and root exudates. In this study, the long-term roles of plants on the bioelectricity generation and contaminant removal were investigated in multi-anode (Anode1 and Anode2) and single cathode CW-MFCs. The electrode distances were 20 cm between Anode1-cathode and 10 cm between Anode2-cathode, respectively. Additionally, the employment of natural conductive pyrrhotite mineral as cathode material was firstly investigated in CW-MFC system. A cathode potential of -98 ± 52 mV to -175 ± 60 mV was achieved in the unplanted (CW-MFC 1), and planted CW-MFCs with Iris pseudacorus (CW-MFC 2), Lythrum salicaria (CW-MFC 3), and Phragmites australis (CW-MFC 4). The maximum power densities of Anode1-cathode and Anode2-cathode were 8.23 and 15.29 mW/m² in CW-MFC 1, 8.51 and 1.67 mW/m² in CW-MFC 2, 5.67 and 3.15 mW/m² in CW-MFC 3, and 7.59 and 14.71 mW/m² in CW-MFC 4, respectively. Interestingly, smaller power density was observed at Anode2-cathode, which has shorter electrode distance than Anode1-cathode in both CW-MFC 2 and CW-MFC 3, which indicates the negative role of oxygen released from the flourished plant roots at Anode2 micro-environment in power production. Therefore, recovering power from commercial CW-MFCs with flourished plants will be a challenge. The contradiction between keeping short electrode distance and avoiding the interference from plant roots to maintain anaerobic anode may be solved by the proposed modular CW-MFCs.

Influence of plant radial oxygen loss in constructed wetland combined with microbial fuel cell on nitrobenzene removal from aqueous solution

This study investigated the effects of radial oxygen loss (ROL) of three different plants on nitrobenzene (NB) wastewater treatment and bioelectricity generation performance in constructed wetland-microbial fuel cell (CW-MFC). ROL and root biomass from wetland plants showed positive effects on NB wastewater compared to unplanted CW-MFC. Scirpus validus exhibited higher tolerance to NB than Typha orientalis and Iris pseudacorus at 20-200 mg/L NB. As NB concentration reached 200 mg/L, the CW-MFC with Scirpus validus had relatively high DO (2.57 ± 0.17 mg/L) and root biomass (16.42 ± 0.18 g/m²), which resulted in the highest power density and voltage (19.5 mW/m², 590 mV) as well as NB removal efficiency (93.9 %) among four reactors. High-throughput sequencing results suggested that electrochemically active bacteria (EAB) (e.g., Geobacter, Ferruginibacter) and dominant NB-degrading bacteria (e.g., Comamonas, Pseudomonas) could be enhanced by wetland plants, especially in CW-MFC with Scirpus validus. Therefore, Scirpus validus was a good option for simultaneously treating NB wastewater and producing bioelectricity.

Response Characteristics of Nitrifying Bacteria and Archaea Community Involved in Nitrogen Removal and Bioelectricity Generation in Integrated Tidal Flow Constructed Wetland-Microbial Fuel Cell

This study explores nitrogen removal performance, bioelectricity generation, and the response of microbial community in two novel tidal flow constructed wetland-microbial fuel cells (TFCW-MFCs) when treating synthetic wastewater under two different chemical oxygen demand/total nitrogen (COD/TN, or simplified as C/N) ratios (10:1 and 5:1). The results showed that they achieved high and stable COD, NH₄ ⁺-N, and TN removal efficiencies. Besides, TN removal rate of TFCW-MFC was increased by 5-10% compared with that of traditional CW-MFC. Molecular biological analysis revealed that during the stabilization period, a low C/N ratio remarkably promoted diversities of ammonia-oxidizing archaea (AOA) and ammonia-oxidizing bacteria (AOB) in the cathode layer, whereas a high one enhanced the richness of nitrite-oxidizing bacteria (NOB) in each medium; the dominant genera in AOA, AOB, and NOB were Candidatus Nitrosotenuis, Nitrosomonas, and Nitrobacter. Moreover, a high C/N ratio facilitated the growth of Nitrosomonas, while it inhibited the growth of Candidatus Nitrosotenuis. The distribution of microbial community structures in NOB was separated by space rather than time or C/N ratio, except for Nitrobacter. This is caused by the differences of pH, dissolved oxygen (DO), and nitrogen concentration. The response of microbial community characteristics to nitrogen transformations and bioelectricity generation demonstrated that TN concentration is significantly negatively correlated with AOA-shannon, AOA-chao, 16S rRNA V4-V5-shannon, and 16S rRNA V4-V5-chao, particularly due to the crucial functions of Nitrosopumilus, Planctomyces, and Aquicella. Additionally, voltage output was primarily influenced by microorganisms in the genera of Nitrosopumilus, Nitrosospira, Altererythrobacter, Gemmata, and Aquicella. This study not only presents an applicable tool to treat high nitrogen-containing wastewater, but also provides a theoretical basis for the use of TFCW-MFC and the regulation of microbial community in nitrogen removal and electricity production.

A Novel Phenolic Foam-Derived Magnetic Carbon Foam Treated as Adsorbent for Rhodamine B: Characterization and Adsorption Kinetics

In recent decades, dye wastewaters produced by dye-manufacturing and dye-consuming industries have become a growing water pollution problem. Herein, a novel phenolic foam-derived magnetic carbon foam is synthesized by in-situ pyrolysis of iron acetylacetonate (Fe(acac)3) containing phenolic resin and its corresponding application as an adsorbent for the removal of Rhodamine B from effluent is investigated. The characterization of the as-prepared adsorbent is carried out by SEM, EDS, XRD, XPS, VSM, FT-IR, Raman, and BET. The magnetic carbon foam is observed to consist of a CFe15.1/C matrix modified with α-Fe/Fe3C/Fe2O3 composites, possessing a 3D porous architecture formed by inter-connected cells with diameters of 50–200 μm and narrow ligaments with thicknesses of ~20 μm. Experimental tests demonstrate that the equilibrium of adsorption behavior of Rhodamine B onto the obtained adsorbent can be reached within 40 min and the corresponding maximum adsorption capacity is 258.03 mg/g. The effects of contact time and adsorbent dosage on the adsorption performance are investigated. Besides, four models are introduced to fit the experimental data to evaluate the adsorption kinetics. The overall rate constant is determined by the chemisorption process, according to the pseudo-second order adsorption kinetics mechanism. Besides, the pH effect on RhB adsorption onto magnetic carbon foam is investigated, with the pH values varying from 3 to 10. The above results prove the application prospect of magnetic carbon foam prepared in this work in dye wastewater treatment.