Bioenergy generation and rhizodegradation as affected by microbial community distribution in a coupled constructed wetland-microbial fuel cell system associated with three macrophytes

Advancements in constructed wetland technology: a state-of-the-art review on bio-electrochemical processes, tidal flow dynamics, and resilience to shock loads

This research article presents a comprehensive examination of recent advancements in constructed wetland technology, with a primary focus on bio-electrochemical processes, including electrolysis and microbial fuel cells, as well as the impact of tidal flow and shock loads on the constructed wetland performance. To date, extensive studies and in-depth analyses in these aspects are limited, highlighting a significant research gap. Electrolysis is explored for its efficacy in dephosphorization and denitrification, particularly under conditions of low carbon availability. Additionally, microbial fuel cell technology is investigated for its dual benefits of bioenergy generation and climate change mitigation. The tidal flow component is highlighted for its ability to create anaerobic, anoxic, and aerobic environments within and between cells, crucial for effective nitrogen removal. The study emphasizes the importance of constructed wetland resilience to shock loads, whether from increased discharge due to rainfall or heightened contaminant levels. The research employs bibliographic analysis and microbial community profiling and investigates factors such as nutrient removal, polarization curves, and the effects of flood/rest and flood/drain in tidal flow. Furthermore, the article delves into the impacts of hydraulic and organic shock loads on constructed wetland systems, providing a comprehensive overview of the current state of the field.

Evolutionary trends and development of constructed wetland coupled microbial fuel cell: A decade of development

Constructed wetland coupled microbial fuel cells (CW-MFC) have gained considerable attention as an innovative sustainable technology for simultaneous wastewater treatment and bio-electricity generation since its inception in 2012. This study, which employed bibliometrics of research articles and patent analysis, is a comprehensive investigation into the evolutionary trend and development status of CW-MFC. We analyzed 85 patents and 247 research articles using structured and unstructured data. By using the logistic model and econometric analysis, we predicted the technological life cycle and investigated the factors driving the development of CW-MFC. In addition, this study discusses the innovative hotspots, research and innovative trends, and key contributors at the forefront of this green infrastructure. The study results show that CW-MFC technology has gained greater interest under the influence of national and international policies, market demand, economic growth, and national R&D expenditure, highlighting its global impact. The study also showed that CW-MFC was predominantly distributed in Asia, with China at the forefront. The technology evolution path from both patents and papers showed the transition of research and development from the performance evaluation of CW-MFC with conventional-based technology to the current study focused on system optimization by exploring designs and component factors to enhance efficiency. Furthermore, the technological maturity (TMR) of CW-MFC was determined using the logistic model to be 50.5%, with significant potential for further development (R2 = 0.98). Overall, this study provides a comprehensive understanding of the current trends of CW-MFC development and provides forecasting and direction for future developments.

Constructed wetland microbial fuel cell as enhancing pollutants treatment technology to produce green energy

The persistent challenge of water pollution, exacerbated by slow progress in ecofriendly technologies and accumulating pollutants, underscores the need for innovative solutions. Constructed Wetland Microbial Fuel Cell (CW-MFC) emerges as an intriguing environmental technology capable of adressing this issue by eliminating contaminants from wastewater while simultaneously producing green energy as an additional bonus. In recent years, CW-MFC technology has gained attention due to its sustainability and promising prospects for a circular waste-free industry. However, due to various technological and biological challenges, it has not yet achieved wide-scale application. This review examines the current state of CW-MFC technology and identifies both biotic and abiotic strategies for optimization through operational and structural improvements affecting biocomponents. Our review highlights several key findings: (1) Plants play an important role in reducing the system's inner resistance through mechanisms such as radial oxygen loss, evapotranspiration, and high photosynthetic flow, which facilitate electroactive bacteria and affect redox potential. (2) Plant characteristics such as root porosity, phloem and aerenchyma development, chlorophyll content, and plant biomass are key indicators of CW-MFC performance and significantly impact both pollutant removal and energy harvesting. (3) We expand the criteria for selecting suitable plants to include mesophytes and C3 pollutant-tolerant species, in addition to traditional aquatic and C4 plants. Additionally, the review presents several technical approaches that enhance CW-MFC efficiency: (1) design optimization, (2) use of novel materials, and (3) application of external electrical fields, aeration, light, and temperature adjustments. CW-MFCs are capable of nearly complete elimination of a wide range of contaminants, including organic matter (84 % ± 10), total nitrogen (80 % ± 7) and phosphorus (79 % ± 18) compounds, metals (86 % ± 10), pharmaceuticals (87 % ± 7), dyes (90 % ± 8), and other complex pollutants, while generating green energy. We hope our findings will be useful in optimizing CW-MFC design and providing insights for researchers aiming to advance the technology and facilitate its future scaling.

Plant Microbial Fuel Cells in a botanical perspective: Nomenclatural constraints and new insights on plant traits potentially affecting bioelectrical perfomance

Plant microbial fuel cells represent an innovative type of microbial fuel cell technology, utilizing plant rhizodeposition to fuel electrochemically active bacteria on the anode surface, thereby generating bioelectricity. This study delves into some botanical aspects of plant species employed in PMFCs and Constructed Wetland PMFCs, aiming to investigate whether their bioelectrical performance is influenced by Raunkiær life forms and root architecture. Our study involved 40 plant species described in 38 documents. In some cases, nomenclature issues prevented the interpretation of actual species used in the experiments. The bioelectrical performance of PMFCs appeared to be significantly affected by both life forms and root architecture. Therophytes and Hemicriptophytes exhibited higher median values than the other life forms, while the Geophyte group showed very high power density values despite a lower median value. In contrast, CW-PMFCs do not appeared to be significantly affected by the botanical traits considered, likely due to the limited data collected on this experimental configuration. The plant species that performed the best in PMFCs include Carex hirta, Alisma plantago-aquatica, Glyceria maxima and Canna indica, all of which have an adventitious root system. C. hirta, G. maxima and C. indica are geophytes, while A. plantago-aquatica is a hydrophyte. Consequently, epiphytes, chamaephytes and nanophanerophytes, as well as plants with fibrous root systems, appeared to be not recommended for PMFCs. Nevertheless, the results of our study may have certain limitation due to nomenclature issues that prevented the accurate identification of species used in the PMFCs, the absence of a standardized benchmark for electrical measurement, and the lack of clear match between each species and its bioelectrical performance, reducing the data pool.

Constructed wetlands combined with microbial fuel cells (CW-MFCs) as a sustainable technology for leachate treatment and power generation

The physical and chemical treatment processes of leachate are not only costly but can also possibly produce harmful by products. Constructed wetlands (CW) has been considered a promising alternative technology for leachate treatment due to less demand for energy, economic, ecological benefits, and simplicity of operations. Various trends and approaches for the application of CW for leachate treatment have been discussed in this review along with offering an informatics peek of the recent innovative developments in CW technology and its perspectives. In addition, coupling CW with microbial fuel cells (MFCs) has proven to produce renewable energy (electricity) while treating contaminants in leachate wastewaters (CW-MFC). The combination of CW-MFC is a promising bio electrochemical that plays symbiotic among plant microorganisms in the rhizosphere of an aquatic plant that convert sun electricity is transformed into bioelectricity with the aid of using the formation of radical secretions, as endogenous substrates, and microbial activity. Several researchers study and try to find out the application of CW-MFC for leachate treatment, along with this system and performance. Several key elements for the advancement of CW-MFC technology such as bioelectricity, reactor configurations, plant species, and electrode materials, has been comprehensively discussed and future research directions were suggested for further improving the performance. Overall, CW-MFC may offer an eco-friendly approach to protecting the aquatic environment and come with built-in advantages for visual appeal and animal habitats using natural materials such as gravel, soil, electroactive bacteria, and plants under controlled condition.

Advancement in constructed wetland microbial fuel cell process for wastewater treatment and electricity generation: a review

The constructed wetland coupled with a microbial fuel cell (CW-MFC) is a wastewater treatment process that combines contaminant removal with electricity production, making it an environmentally friendly option. This hybrid system primarily relies on anaerobic bioprocesses for wastewater treatment, although other processes such as aerobic bioprocesses, plant uptake, and chemical oxidation also contribute to the removal of organic matter and nutrients. CW-MFCs have been successfully used to treat various types of wastewater, including urban, pharmaceutical, paper and pulp industry, metal-contaminated, and swine wastewater. In CW-MFC, macrophytes such as rice plants, Spartina angalica, Canna indica, and Phragmites australis are used. The treatment process can achieve a chemical oxygen demand removal rate of between 80 and 100%. Initially, research focused on enhancing power generation from CW-MFC, but recent studies have shifted towards resource recovery from wastewater. This review paper provides an overview of the development of constructed wetland microbial fuel cell technology, from its early stages to its current applications. The paper also highlights research gaps and potential directions for future research.

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.

Key issues to consider toward an efficient constructed wetland-microbial fuel cell: the idea and the reality

The research on constructed wetland (CW) and microbial fuel cell (MFC) has been separate studies worldwide with crucial achievements being made in both fields. Due to environmentally friendly feature (of CW) and rich microbial population and excellent electrode catalytic activity (of MFC), CW and MFC have their own anticipated application prospect in wastewater purification and biological electricity generation. More significantly, the idea of embedding MFC into CW to form CW-MFC expands the scope for both of them and this has received much interest in recent years due to its striking features of sewage treatment efficiency, electricity generation, sustainability, and environmental friendliness. The increasing interest and the lack of soul of CW-MFC emerging to the new researchers reflect the need to recall the idea and summarize its development with regard to achieving its reality via some key issues This forms the basis of the paper. The paper also includes how to enhance the efficiency of electricity generation and supplement energy consumption, the degradation of emerging pollutants, and the degradation mechanism as well as the potential joint application of CW-MFC with other treatment technique. A mass of CW-MFC design parameters has been synthesized from the literature. Challenges and potential directions of CW-MFC in the future are prospected. It is expected that the paper can serve as a linkage for bridging knowledge gaps for further studies of CW-MFC.

Identification of sugars as root exudates of the macrophyte species Juncus effusus and Philodendron cordatum in constructed wetland-microbial fuel cells during bioelectricity production

Constructed wetland-microbial fuel cells (CW-MFCs) systems are a sustainable technology capable of producing bioelectricity and treating wastewater simultaneously. It is also possible to obtain bioelectricity from the photosynthetic substrates obtained by the rhizodeposition of macrophytes, where the electroactive microorganisms present in the rhizosphere use these compounds as biofuel. In the present study, the bioelectricity production capacity of Juncus effusus and Philodendron cordatum species was evaluated in a CW-MFC without an external carbon source. The Juncus effusus species showed a higher bioelectrochemical performance, as they recorded a maximum voltage of 399 mV, a power density of 63.7 mW/m2, a volumetric power density of 15.9 W/m3, an internal resistance of 200 Ω, an anodic potential of -368 mV, and a cathodic potential of 229 mV. In addition, different types of carbohydrates in the form of sugars (sucrose, fructose, galactose, and glucose) were quantified by liquid chromatography, with concentrations of 100-450 μg/L. Chromatographic analysis were performed from the root exudates released in the effluent of both species of macrophyte. Sucrose and glucose were the types of sugars that produced the largest amount with portions of up to 35% and 24%, respectively. Sugars are compounds that worked as electron donors for the production of bioelectricity by using endogenous substrates that fed the anodic biofilm. Consumption was 45-55% for sucrose and 40-65% for glucose. Of the different macrophytes evaluated in the CW-MFCs, it was observed that the production of bioelectricity differs mainly due to the quantity of the root exudates released in the rhizosphere.

Enhanced degradation of ibuprofen in an integrated constructed wetland-microbial fuel cell: treatment efficiency, electrochemical characterization, and microbial community dynamics

Over the past few decades, there has been a growing concern regarding the fate and transport of pharmaceuticals, particularly antibiotics, as emerging contaminants in the environment. It has been proposed that the presence of antibiotics at concentrations typically found in wastewater can impact the dynamics of bacterial populations and facilitate the spread of antibiotic resistance. The efficiency of currently-used wastewater treatment technologies in eliminating pharmaceuticals is often insufficient, resulting in the release of low concentrations of these compounds into the environment. In this study, we addressed these challenges by evaluating how different influent ibuprofen (IBU) concentrations influenced the efficiency of a laboratory-scale, integrated constructed wetland-microbial fuel cell (CW-MFC) system seeded with Eichhornia crassipes, in terms of organic matter removal, electricity generation, and change of bacterial community structure compared to unplanted, sediment MFC (S-MFC) and abiotic S-MFC (AS-MFC). We observed that the addition of IBU (5 mg L-1) resulted in a notable decrease in chemical oxygen demand (COD) and electricity generation, suggesting that high influent IBU concentrations caused partial inhibition for the electroactive microbial community due to its complexity and aromaticity. However, CW-MFC could recover from IBU inhibition after an acclimation period compared to unplanted S-MFC, even though the influent IBU level was increased up to 20 mg L-1, suggesting that plants in CW-MFCs have a beneficial role in relieving the inhibition of anode respiration due to the presence of high levels of IBU; thus, promoting the metabolic activity of the electroactive microbial community. Similarly, IBU removal efficiency for CW-MFC (i.e., 49-62%) was much higher compared to SMFC (i.e., 29-42%), and AS-MFC (i.e., 20-22%) during all experimental phases. In addition, our high throughput sequencing revealed that the high performance of CW-MFCs compared to S-MFC was associated with increasing the relative abundances of several microbial groups that are closely affiliated with anode respiration and organic matter fermentation. In summary, our results show that the CW-MFC system demonstrates suitability for high removal efficiency of IBU and effective electricity generation.

Removal of chloramphenicol and resistance gene changes in electric-integrated vertical flow constructed wetlands

The performance of an electric-integrated vertical flow constructed wetland (E-VFCW) for chloramphenicol (CAP) removal, changes in microbial community structure, and the fate of antibiotic resistance genes (ARGs) were evaluated. CAP removal in the E-VFCW system was 92.73% ± 0.78% (planted) and 90.80% ± 0.61% (unplanted), both were higher than the control system which was 68.17% ± 1.27%. The contribution of anaerobic cathodic chambers in CAP removal was higher than the aerobic anodic chambers. Plant physiochemical indicators in the reactor revealed electrical stimulation increased oxidase activity. Electrical stimulation enhanced the enrichment of ARGs in the electrode layer of the E-VFCW system (except floR). Plant ARGs and intI1 levels were higher in the E-VFCW than in the control system, suggesting electrical stimulation induces plants to absorb ARGs, reducing ARGs in the wetland. The distribution of intI1 and sul1 genes in plants suggests that horizontal transfer may be the main mechanism dispersing ARGs in plants. High throughput sequencing analysis revealed electrical stimulation selectively enriched CAP degrading functional bacteria (Geobacter and Trichlorobacter). Quantitative correlation analysis between bacterial communities and ARGs confirmed the abundance of ARGs relates to the distribution of potential hosts and mobile genetic elements (intI1). E-VFCW is effective in treating antibiotic wastewater, however ARGs potentially accumulate.

Performance of vertical up-flow-constructed wetlands integrating with microbial fuel cell (VFCW-MFC) treating ammonium in domestic wastewater

Vertical up-flow-constructed wetlands integrating with microbial fuel cell (VFCW-MFC) were evaluated for NH4+-N removal and bioelectricity recovery. The experiments were carried out in lab-scale VFCW-MFC microcosms treating synthetic domestic wastewater under different operational conditions of pH, hydraulic retention time, and mass loading rate. Effects of wild ornamental grass (Cenchrus setaceus) on treatment performance and voltage output were investigated simultaneously. Experiments demonstrated that the neutral pH of influents favoured NH4+-N removal and power generation. Extended retention time improved treatment capacity and power output but likely depended on the substrate availability. COD removal and power output increased, while NH4+-N removal decreased with increasing mass loading rates. At the loading rate of 88.31 mg COD/L.day, planted VFCW-MFCs exhibited better NH4+-N treatment performance (36.9%) and higher voltage output (132%-143%) than unplanted systems. Plants did not affect the COD removal efficiency of VFCW-MFCs (>95%). Power density was in the range of 1.26-1.59 mW/m² in planted microcosms with a maximum CE of 13.6%. The anode layer accounted for a major proportion of NH4+-N removal in VFCW-MFCs. This study implies that NH4+-N in domestic wastewaters with relatively high COD:N ratios can be treated effectively in up-flow CW-MFCs via anaerobic processes, including anammox and heterotrophic denitrifying processes. The mass loading rate could be a critical parameter to balance different microbial processes, thus, coincidently determining the potential of power recovery from wastewaters.

Effects of multiple key factors on the performance of petroleum coke-based constructed wetland-microbial fuel cell

In this study, two constructed wetland-microbial fuel cells (CW-MFC), including a closed-circuit system (CCW-MFC) and an open-circuit system (OCW-MFC) with petroleum coke as electrode and substrate, were constructed to explore the effect of multiple key factors on their operation performances. Compared to a traditional CW, the CCW-MFC system showed better performance, achieving an average removal efficiency of COD, NH₄⁺-N, and TN of 94.49 ± 1.81%, 94.99 ± 4.81%, and 84.67 ± 5.6%, respectively, when the aeration rate, COD concentration, and hydraulic retention time were 0.4 L/min, 300 mg/L, and 3 days. The maximum output voltage (425.2 mV) of the CCW-MFC system was achieved when the aeration rate was 0.2 L/min. In addition, the CCW-MFC system showed a greater denitrification ability due to the higher abundance of Thiothrix that might attract other denitrifying bacteria, such as Methylotenera and Hyphomicrobium, to participate in the denitrifying process, indicating the quorum sensing could be stimulated within the denitrifying microbial community.

Effects of the bioelectrochemical technique on methane emission and energy recovery in constructed wetlands (CWs) and related biological mechanisms

In this study, effects of bioelectrochemical technique on methane emission and energy recovery, and related mechanism underlying microbial competition were investigated. The results showed that running MFC was beneficial in reducing CH₄ emissions and promoting COD removal rates, regardless of whether the plant roots were located at the anode or the cathode. CH₄ emission was significantly higher in open-circuit reactors (6.2 mg m^-2 h^-1) than in closed-circuit reactors (3.1 mg m^-2 h^-1). Plant roots at the cathode had the highest electricity generation and the lowest CH4 emissions. The highest power generation (0.49 V, 0.33 w m^-3) and the lowest CH₄ emissions (2.3 mg m^-2 h^-1) were observed in the reactors where Typha orientalis was planted with plant roots at the cathode. The role of plants in strengthening electron acceptor was greater than that of plant rhizodeposits in strengthening electron donors. Real-time quantitative PCR (q-PCR) and correlation analysis indicated that the mcrA genes and CH₄ emissions were positively correlated (r = 0.98, p < 0.01), while no significant relationship between CH₄ emissions and pmoA genes was observed. Illumina sequencing revealed that more abundant exoelectrogens and denitrifying bacteria were observed when plant roots were located in cathodes. Strictly acetotrophic archae (Methanosaetaceae) were likely the main electron donor competitors with exoelectrogens. The results showed that the location of both plant species and plant roots at the electrode played an important role in CH₄ control and electricity generation. Therefore, it is necessary to strengthen plant configuration to reduce CH₄ emissions, to promote sustainable development of wastewater treatment.

Cladophora can mitigate the shock of glyphosate-containing wastewater on constructed wetlands coupled with microbial fuel cells

This study investigated the performance of constructed wetlands coupled with microbial fuel cells (CW-MFCs) treating agricultural wastewater containing glyphosate (N-phosphonomethyl glycine, PMG), and the use of Cladophora as a cathode plant in this system. Ten devices were divided into Cladophora groups (CGs) and no Cladophora groups (NGs), with five PMG concentrations (0, 10, 25, 50, and 100 mg/L). PMG removal efficiency significantly decreased with increasing PMG (P < 0.01) and was higher in CG devices than in NG devices at low PMG concentrations (<50 mg/L). The removal efficiency of chemical oxygen demand (COD) and NH₄⁺ in CGs was significantly higher than in NGs (P < 0.01). The highest power densities of 6.37 (CGs) and 6.26 mW/m² (NGs) were obtained at 50 mg/L PMG, and the average voltage was significantly higher in CGs than in NGs (p < 0.01). Moreover, PMG had a negative effect on the enrichment of electrochemically active bacteria, but Cladophora could mitigate this effect. The abundance of the resistance gene epsps was stabilized; The phnJ gene increased with increasing PMG in NGs and was downregulated at high PMG concentration in CGs, indicating better microbial adaptation to PMG in CGs throughout the experiment.

Current Status of Ponds in India: A Framework for Restoration, Policies and Circular Economy

Healthy pond ecosystems are critical for achieving several sustainable development goals (SDG) through numerous ecosystem services (e.g., flood control, nutrient retention, and carbon sequestration). However, the socio-economic and ecological value of ponds is often underestimated compared to the larger water bodies. Ponds are highly vulnerable to mounting land-use pressures (e.g., urban expansion, and agriculture intensification) and environmental changes, leading to degradation and loss of the pond ecosystem. The narrow utilitarian use-based conservation fails to recognize the multiple anthropogenic pressures and provides narrow solutions which are inefficient to regenerate the degraded pond ecosystem. In this paper, we holistically examined the legal challenges (policies) and key anthropogenic and environmental pressures responsible for pond degradation in India. The country is strongly dedicated to attaining SDG and circular economy (CE) through aquatic ecosystem conservation and restoration. Considerable efforts are required at the administration level to recognize the contribution of pond ecosystem services in attaining global environmental goals and targets. Worldwide restoration strategies were reviewed, and a framework for pond restoration and conservation was proposed, which includes policies and incentives, technologies such as environmental-DNA (e-DNA), life cycle assessment (LCA), and other ecohydrological measures. Nature-based solutions (NBS) offer a sustainable and cost-effective approach to restoring the pond's natural processes. Furthermore, linkage between the pond ecosystem and the CE was assessed to encourage a regenerative system for biodiversity conservation. This study informs the need for extensive actions and legislative reforms to restore and conserve the pond ecosystems.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s13157-022-01624-9.

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.

Stereoselective degradation pathway of amide chiral herbicides and its impacts on plant and bacterial communities in integrated vertical flow constructed wetlands

This study demonstrates the stereoselective degradation patterns and biodegradation mechanisms of metolachlor (MET) and napropamide (NAP) in integrated vertical flow constructed wetlands (IVCW). The higher interphase transferability of NAP resulted in higher degradation rates of 90.60 ± 4.09%. The enantiomeric fraction (EF) values of 0.38 ± 0.02 and 0.54 ± 0.03, respectively, recorded for the enantiomers S-MET and R-NAP, with higher herbicidal activities, demonstrated their highly selective biodegradation patterns. The antioxidant enzyme activities and fluorescence parameters of plants showed positive correlations with the degradation efficiency and enantioselectivity of MET and NAP. Adaptive regulations by plants promoted the proliferation of microbial genera like Enterobacter and unclassified_Burkholderiales, which could facilitate plant growth. Moreover, enrichment of the herbicide-degrading functional bacteria Terrimonas (5.10%), Comamonas (4.05%) Pseudoxanthomonas (4.49%) and Mycobacterium (1.42%) demonstrably promoted the preferential degradation of S-MET and R-NAP. Furthermore, the abundance of Ferruginibacter favored the use of R-NAP as carbon source to achieve co-removal of R-NAP and NO₃^--N.

Constructed wetlands for textile wastewater remediation: A review on concept, pollutant removal mechanisms, and integrated technologies for efficiency enhancement

Textile industries are among the ecologically unsustainable industries that release voluminous wastewater threatening ecosystem health. The constructed wetlands (CWs) are low-cost eco-technological interventions for the management of industrial wastewaters. The CWs are self-sustaining remediation systems that do not require an external source of energy and encompass simple operational mechanisms including biological (bioremediation and phytoremediation), chemical, and physical processes for pollutant removal. This review idiosyncratically scrutinizes the recent advances and developments in CWs, and their types employed for textile wastewater treatment. The major focus is on mechanisms involved during the removal of contaminants from textile wastewater in CWs and factors affecting the performance of the system. The article also discusses the State-of-the-Art integrated technologies e.g., CW-MFCs/algal ponds/sponge iron coupled systems, for the performance and sustainability enhancement of CWs. All the important aspects together with the technology amalgamation are critically synthesized for establishing suitable strategies for CW-based textile wastewater treatment systems.

Innovative constructed wetland coupled with microbial fuel cell for enhancing diazo dye degradation with simultaneous electricity generation

A novel earthen separator-based dual-chambered unplanted core of constructed wetland coupled with microbial fuel cell was developed for studying the microbe-material interaction and their effect on treatment performance and electricity generation. The constructed wetland integrated microbial fuel cell was evaluated for the degradation of high molecular weight diazo Congo red dye as a model pollutant. The system exhibited 89.99 ± 0.04% of dye decolorization and 95.80 ± 0.71% of chemical oxygen demand removal efficiency from an initial concentration of 50 ± 10 mg/L and 750 ± 50 mg/L, respectively. Ultraviolet-Visible spectrophotometric and gas chromatography-mass spectrometric analysis revealed naphthalene and phenol as mineralized products. The developed system achieved high power density and current density generation of 235.94 mW/m³ and 1176.4 mA/m³, respectively. Results manifested that dual-chambered constructed wetland coupled with microbial fuel cell has a high capability of dye decolorization and toxicity abatement with appreciable simultaneous bioelectricity generation owing to the significantly low internal resistance of 100 Ω.

Integration of various technology-based approaches for enhancing the performance of microbial fuel cell technology: A review

The conflict between climate change and growing global energy demand is an immense sustainability challenge that requires noteworthy scientific and technological developments. Recently the importance of microbial fuel cell (MFC) on this issue has seen profound investigation due to its inherent ability of simultaneous wastewater treatment, and power production. However, the challenges of economy-related manufacturing and operation costs should be lowered to achieve positive field-scale demonstration. Also, a variety of different field deployments will lead to improvisation. Hence, this review article discusses the possibility of integration of MFC technology with various technologies of recent times leading to advanced sustainable MFC technology. Technological innovation in the field of nanotechnology, genetic engineering, additive manufacturing, artificial intelligence, adaptive control, and few other hybrid systems integrated with MFCs is discussed. This comprehensive and state-of-the-art study elaborates hybrid MFCs integrated with various technology and its working principles, modified electrode material, complex and easy to manufacture reactor designs, and the effects of various operating parameters on system performances. Although integrated systems are promising, much future research work is needed to overcome the challenges and commercialize hybrid MFC technology.

Performance and mechanism of sulfamethoxazole removal in different bioelectrochemical technology-integrated constructed wetlands

Sulfamethoxazole (SMX) has a high concentration and detection frequency in aquatic environments due to the poor removal efficiency of traditional biological treatment processes. Bioelectrochemical technology-integrated constructed wetlands (CWs) have great potential for SMX removal; however, the process of SMX removal in different bioelectrochemical technology-integrated CWs (microbial fuel cell (MFC) and direct current (EC)) remains unclear. To address this, we examined the mechanism of SMX removal in MFCCW and ECCW. The results revealed that the SMX removal efficiency can reach 96.0 ± 2.4% in the ECCW and 97.2 ± 2.2% in the MFCCW. The enhancement of MFC for SMX removal in CW was slightly better than that in direct current (p > 0.05). It was found that the adsorption process of SMX in the substrate promoted by EC was more enhanced than that by MFC. Furthermore, bioelectrochemical technology improved plant activity, including root and enzymatic (superoxide dismutase, peroxidase, and catalase) activities, and fluorescence parameters (photochemical quenching coefficient, non-photochemical quenching coefficient, and quantum efficiency of PS II). Significant differences were found between CW and ECCW (p < 0.05), while no significant differences were found between CW and MFCCW (p > 0.05). The microbial activity and abundance in CW were improved by bioelectrochemical technology, and the microbial community structure was optimised to be simpler and more stable. However, EC tended to promote microbial and plant activity in CW, whereas MFC tended to optimise the microbial community and improve the tightness and stability of the module. The enhanced difference might also account for the changes in the SMX degradation pathway. 4-aminobenzenesulfonic acid (TP174), 3-amino-5-methylisoxazole (TP99) and 5-methylisoxazole (TP84) were all common products in the three reactors, whereas TP99 underwent further ring-opening in MFCCW and TP174 underwent further hydrolysis in ECCW. This study provided an important reference for the targeted regulation of plants and microorganisms in constructed wetlands via different bioelectrochemistry to enhance characteristic pollutants degradation.

Bioenergy generation and nitrogen removal in a novel ecological-microbial fuel cell

A novel ecological-microbial fuel cell (E-MFC) was constructed based on the mutualistic symbiosis relationship among wetland plants Ipomoea aquatic, benthic fauna Tubifex tubifex (T. tubifex) and microorganisms. The maximum power densities of sediment MFC (S-MFC), wetland plant MFC (WP-MFC) and E-MFC were 6.80 mW/m², 10.60 mW/m² and 15.59 mW/m², respectively. Ipomoea aquatic roots secreted organic matter as electricigens' fuel for electricity generation, while T. tubifex decomposed decaying leaves and roots into soluble organic matter and plant nutrients, forming a co-dependent and mutually beneficial system, which was conducive to bioelectricity production. The E-MFC obtained the highest nitrogen removal, and the removal efficiencies of NH₄⁺-N and NO₃^--N were 90.4% and 96.5%, respectively. Hydraulic retention time (HRT), cathodic aeration and T. tubifex abundance had significant effects on E-MFC power generation. The performeance boost of E-MFC was closely related to anodic microbial community change caused by the introduction of T. tubifex.

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.

Organic matter removal and nitrogen transformation by a constructed wetland-microbial fuel cell system with simultaneous bioelectricity generation

Microbial fuel cells integrated into constructed wetlands have been previously studied. Nevertheless, their application as a suitable treatment for wastewater is still in the developmental stage. In this context, the aim of this study was to evaluate organic matter removal and nitrogen transformation by a microbial fuel cell integrated into a constructed wetland (CWMFC). To accomplish this, three experimental systems were operated under batch-mode conditions over 170 days: i) one was planted with Schoenoplectus californicus (P-CWMFC); ii) another was unplanted (NP-CWMFC); and iii) the third system did not have any electrodes (CW) and was used as a control. Chemical oxygen demand (COD) removal efficiency ranged between 74-87%, 69-81% and 62-72% for the P-CWMFC, NP-CWMFC and CW systems, respectively, with organic loading rates (OLR) ranging from 4.8 to 7.9 g COD/m² d. NH₄⁺-N removal efficiency exceeded 98%, 90% and 83% for P-CWMFC, NP-CWMFC and CW, respectively. Wastewater treatment performance was improved due to anaerobic oxidation that occurred on the anodes. Organic matter removal was 18% higher in closed-circuit mode than in open-circuit mode in both integrated systems (P-CWMFC and NP-CWMFC), and these differences were significant (p < 0.05). With respect to the performance of microbial fuel cells, the maximum power density (8.6 mW/m²) was achieved at an organic loading rate of 7.9 g COD/m² d with an internal resistance and coulombic efficiency of 251 Ω and 2.4%, respectively. The results obtained in this work can provide positive impacts on CW development by enhancing anaerobic degradation without forced aeration.

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.

A comprehensive review on emerging constructed wetland coupled microbial fuel cell technology: Potential applications and challenges

Constructed wetlands (CWs) integrated with bioelectrochemical systems (BESs) are being intensively researched with the names like constructed wetland-microbial fuel cell (CW-MFC), electro-wetlands, electroactive wetlands, and microbial electrochemical technologies-based constructed wetland since the last decade. The implantation of BES in CW facilitates the tuning of redox activities and electron flow balance in aerobic and anaerobic zones in the CW bed matrix, thereby alleviating the limitation associated with electron acceptor availability and increasing its operational controllability. The benefits of CW-MFC include high treatment efficiency, electricity generation, and recalcitrant pollutant abatement. This article presents CW-MFC technology's journey since its emergence to date, encompassing the research done so far, including the basic principle and functioning, bio-electrocatalysts as its machinery, influential factors for microbial interactions, and operational parameters controlling different processes. A few key challenges and potential applications are also discussed for the CW-MFC systems.

Constructed wetlands integrated with microbial fuel cells for COD and nitrogen removal affected by plant and circuit operation mode

Organic matter and NH₄⁺-N are two major pollutants in domestic sewage. This study evaluated the influence of plant and circuit operation mode on the performance of constructed wetlands integrated with microbial fuel cells (CW-MFCs) and investigated the removal mechanisms of organic matter and nitrogen. Better chemical oxygen demand (COD) removal was achieved in closed-circuit CW-MFCs regardless of planting or not, with average removal efficiencies of 83.19-86.28% (closed-circuit CW-MFCs) and 76.54-83.19% (open-circuit CW-MFCs), respectively. More than 70% organic matter was removed in the anaerobic region of all CW-MFCs. In addition, the planted CW-MFCs outperformed the unplanted CW-MFCs in ammonium, nitrate, and total nitrogen removal irrespective of circuit connection or not, for example, the NH₄⁺-N removal efficiencies of 95.91-96.82% were achieved in planted CW-MFCs compared with 56.54-59.95% achieved by unplanted CW-MFCs. Besides, 33.14-55.69% of NH₄⁺-N was removed in the anaerobic region. Throughout the experiment, the average voltages of planted and unplanted CW-MFCs were 264 mV and 108 mV, with the corresponding maximum voltage output of 544 mV and 321 mV, respectively. Furthermore, planted CW-MFCs, simultaneously producing a peak power density of 92.05 mW m^-3 with a coulombic efficiency of 0.50%, exhibited better than unplanted CW-MFCs (3.29 mW m^-3 and 0.21%, respectively) in bioelectricity generation characteristics. Graphical abstract.

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.

Operation mechanism of constructed wetland-microbial fuel cells for wastewater treatment and electricity generation: A review

Constructed wetland-microbial fuel cells (CWL-MFCs) are eco-friendly and sustainable technology, simultaneously implementing contaminant removal and electricity production. According to intensive research over the last five years, this review on the operation mechanism was conducted for in-depth understanding and application guidance of CWL-MFCs. The electrochemical mechanism based on anodic oxidation and cathodic reduction is the core for improved treatment in CWL-MFCs compared to CWLs. As the dominant bacterial community, the abundance and gene-expression patterns of electro-active bacteria responds to electrode potentials and contaminant loadings, further affecting operational efficiency of CWL-MFCs. Plants benefit COD and N removal by supplying oxygen for aerobic degradation and rhizosphere secretions for microorganisms. Multi-electrode configuration, carbon-based electrodes and rich porous substrates affect transfer resistance and bacterial communities. The possibilities of CWL-MFCs targeting at recalcitrant contaminants like flame retardants and interchain interactions among effect components need systematic research.

Electrical current generation from a continuous flow macrophyte biocathode sediment microbial fuel cell (mSMFC) during the degradation of pollutants in urban river sediment

A new type of sediment microbial fuel cell (SMFC) with floating macrophyte Limnobium laevigatum, Pistia stratiotes, or Lemna minor L. biocathode was constructed and assessed in three phases at different hydraulic retention time (HRT) for electrical current generation during the degradation of urban river sediment. The results showed a highest voltage output of 0.88 ± 0.1 V, maximum power density of 80.22 mW m^-3, highest columbic efficiency of 15.3%, normalized energy recovery of 0.030 kWh m^-3, and normalized energy production of 0.005 kWh m^-3 in the Lemna minor L. SMFC during phase 3 at HRT of 48 h, respectively. Highest removal efficiencies of total chemical oxygen demand of 80%, nitrite of 99%, ammonia of 93%, and phosphorus of 94% were achieved in Lemna minor L. system, and 99% of nitrate removal and 99% of sulfate removal were achieved in Pistia stratiotes and Limnobium laevigatum system during the SMFC operation, respectively. Pistia stratiotes exhibited the highest growth in terms of biomass and tap root system of 29.35 g and 12.2 cm to produce the maximum dissolved oxygen of 16.85 ± 0.2 mg L^-1 compared with other macrophytes. The predominant bacterial phylum Proteobacteria of 62.86% and genus Exiguobacterium of 17.48% were identified in Limnobium laevigatum system, while the class Gammaproteobacteria of 28.77% was observed in the control SMFC. The integration of technologies with the continuous flow operation shows promising prospect in the remediation of polluted urban river sediments along with the generation of electrical current.

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.

Treatment of typical antibiotics in constructed wetlands integrated with microbial fuel cells: Roles of plant and circuit operation mode

This study evaluated the removal efficiencies of sulfamethoxazole (SMX), tetracycline (TC) and their common co-existing contaminants, i.e., chemical oxygen demand (COD) and nitrogen in constructed wetlands integrated with microbial fuel cells (MFC-CWs), as affected by plant, circuit operation mode and influent antibiotic loads. The results demonstrated that MFC-CWs with plant and circuit connection exhibited the best performance in SMX and TC removal. The removal percentages for SMX and TC were 99.70-100% and 99.66-99.85% at HRT of 1 d, respectively, in MFC-CWs with plant and circuit connection when the influent SMX and TC concentrations were 5-100 μg L^-1 and 5-50 μg L^-1. The removal efficiencies of both SMX and TC were mainly enhanced by the circuit connection, compared to the plants. The presence of plant and circuit connection also accelerated the accumulation of SMX and TC in electrode layers, and the residues of both antibiotics in the anode layer were higher than in the cathode layer. Besides, closed-circuit MFC-CWs showed better COD removal performance than open-circuit MFC-CWs, irrespective of the increasing influent COD and antibiotic concentrations. The NH₄⁺-N removal in MFC-CWs was mainly promoted by the presence of plants and decreased with increasing influent antibiotic concentrations. Additionally, the bioelectricity generation of planted MFC-CWs was better than in unplanted systems. The coulombic efficiencies in both planted and unplanted MFC-CWs decreased with increasing influent antibiotic concentrations. In summary, MFC-CWs with plant and circuit connection have potential for the treatment of wastewater containing SMX and TC.

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.

Community level physiological profiling of microbial electrochemical-based constructed wetlands

The performance of constructed wetlands (CW) can be enhanced through the use of microbial electrochemical technologies like METland systems. Given its novelty, uncertainties exist regarding processes responsible for the pollutant removal and microbial activity within the systems. Genetic characterization of microbial communities of METlands is desirable, but it is a time and resource consuming. An alternative, is the functional analysis based on community-level physiological profile (CLPP), which allows to evaluate the diversity of microbial communities based on the carbon consumption patterns and derived indexes (average well color development - AWCD -, richness, and diversity). This study aimed to characterize the microbial community function of laboratory-scale METlands using the CLPP method. It encompassed the analysis of planted and non-planted set-ups of two carbon-based electroconductive materials (Coke-A and Coke-LSN) colonized with electroactive biofilms, and compared to Sand-filled columns. Variations in the microbial metabolic activity were found to depend on the characteristics of the material rather than to the presence of plants. Coke-A systems showed lower values of AWCD, richness, and diversity than Sand and Coke-LSN systems. This suggests that Coke-A systems provided more favorable conditions for the development of relatively homogeneous microbial biofilms. Additionally, typical parameters of water quality were measured and correlations between utilization of carbon sources and removal of pollutants were established. The results provide useful insight into the spatial dynamics of the microbial activity of METland systems.

Accumulation of sulfonamide resistance genes and bacterial community function prediction in microbial fuel cell-constructed wetland treating pharmaceutical wastewater

Microbial fuel cell constructed wetlands (CW-MFCs) with different circuit operation conditions and hydraulic retention time (HRT) were constructed to evaluate their ability to remove and accumulate pharmaceutical and personal care products (PPCPs) (sulfadiazine (SDZ), carbamazepine (CBZ), naproxen (NPX) and ibuprofen (IBP)) during four months running process. The abundance level of corresponding sulfonamide antibiotic resistance genes (ARGs) was also investigated. The results showed that closed circuit operation of CW-MFC contributed to the decrease in mass loading of COD, NH₄⁺-N, PPCPs, and wastewater toxicity in the effluent. Additionally, closed circuit operation with low HRT contributed to enhancing selected PPCP mass accumulation on electrodes by electro-adsorption, and thus the higher sulfonamide ARG abundance was detected in the electrodes and effluent. Moreover, the composition of bacteria was greatly influenced by the mass accumulation of PPCPs revealed by redundancy analysis results. Procrustes analysis results further demonstrated that bacterial community contributed greatly to the ARGs profiles. Therefore, ARGs with their host bacteria revealed by network analysis were partially deposited on electrode substrates, and thus ARGs were effectively accumulated on electrodes. Function analysis of the bacterial community from PICRUSt predicted metagenomes revealed that closed circuit mode enhanced the abundances of the function genes of metabolic and the multiple ARGs, suggesting that closed circuit operation exhibited positive effects on metabolic process and ARG accumulation in CW-MFC system.

Effects of Concentration Variations on the Performance and Microbial Community in Microbial Fuel Cell Using Swine Wastewater

The variation of substrate concentration in anode chamber directly affects the power generation efficiency and decontamination performance of microbial fuel cell (MFC). In this study, three concentrations of swine wastewater with 800 mg/L, 1600 mg/L and 2500 mg/L were selected as substrates, and the performance of MFC and response characteristics of anode microbial community were investigated. The results show that the concentration of a selected substrate is positively correlated with the output voltage of MFC and chemical oxygen demand (COD) removal rate. The microbial community diversity in the anode chamber and the performance of battery can be significantly affected when concentration changes in different ways, which helps to selectively cultivate the adaptable dominant bacteria to enhance the stability and decontamination performance of MFC. The community structure of anodic biofilm is mainly composed of Proteobacteria, Bacteroidetes, Firmicutes, Chloroflexi and Spirochaetae. These findings are meaningful to improve the treatment effects of swine wastewater and can help to find out the mechanism of varying concentration that influences the production of microorganisms in MFC.

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

Plants constitute a major element of constructed wetlands (CWs). In this study, a coupled system comprising an integrated vertical flow CW (IVCW) and a microbial fuel cell (MFC) for swine wastewater treatment was developed to research the effects of macrophytes commonly employed in CWs, Canna indica, Acorus calamus, and Ipomoea aquatica, on decontamination and electricity production in the system. Because of the different root types and amounts of oxygen released by the roots, the rates of chemical oxygen demand (COD) and ammonium nitrogen (NH₄⁺-N) removal from the swine wastewater differed as well. In the unplanted, Canna indica, Acorus calamus, and Ipomoea aquatica systems, the COD removal rates were 80.20%, 88.07%, 84.70%, and 82.20%, respectively, and the NH₄⁺-N removal rates were 49.96%, 75.02%, 70.25%, and 68.47%, respectively. The decontamination capability of the Canna indica system was better than those of the other systems. The average output voltages were 520±42, 715±20, 660±27, and 752±26mV for the unplanted, Canna indica, Acorus calamus, and Ipomoea aquatica systems, respectively, and the maximum power densities were 0.2230, 0.4136, 0.3614, and 0.4964W/m³, respectively. Ipomoea aquatica had the largest effect on bioelectricity generation promotion. In addition, electrochemically active bacteria, Geobacter and Desulfuromonas, were detected in the anodic biofilm by high-throughput sequencing analysis, and Comamonas (Proteobacteria), which is widely found in MFCs, was also detected in the anodic biofilm. These results confirmed the important role of plants in IVCW-MFCs.

Enhanced bioremediation of heavy metals and bioelectricity generation in a macrophyte-integrated cathode sediment microbial fuel cell (mSMFC)

Sediment microbial fuel cell (SMFC) and constructed wetlands with macrophytes have been independently employed for the removal of heavy metals from polluted aquatic ecosystems. Nonetheless, the coupling of macrophytes at the cathode of SMFCs for efficient and synchronous heavy metal removal and bioelectricity generation from polluted river sediment has not been fully explored. Therefore, a novel macrophyte biocathode SMFC (mSMFC) was proposed, developed, and evaluated for heavy metals/organics removal as well as bioelectricity generation in an urban polluted river. With macrophyte-integrated cathode, higher heavy metal removals of Pb 99.58%, Cd 98.46%, Hg 95.78%, Cr 92.60%, As 89.18%, and Zn 82.28% from the sediments were exhibited after 120 days' operation. Total chemical oxygen demand, total suspended solids, and loss on ignition reached 73.27%, 44.42 ± 4.4%, and 5.87 ± 0.4%, respectively. A maximum voltage output of 0.353 V, power density of 74.16 mW/m³, columbic efficiency of 19.1%, normalized energy recovery of 0.028 kWh/m³, and net energy production of 0.015 kWh/m³ were observed in the Lemna minor L. SMFC. Heavy metal and organic removal pathways included electrochemical reduction, precipitation and recovery, bioaccumulation by macrophyte from the surface water, and bioelectrochemical reduction in the sediment. This study established that mSMFC proved as an efficient system for the remediation of heavy metals Pb, Cd, Hg, Cr, As, and Zn, and TCOD in polluted rivers along with bioelectricity generation.

Electroactive biofilm-based constructed wetland (EABB-CW): A mesocosm-scale test of an innovative setup for wastewater treatment

Constructed wetlands (CWs) performance enhancement can be done with intensification strategies. A recent strategy still in study is the coupling with Microbial Electrochemical Technologies (MET). An alternative system using electro-conductive biofilters instead of electrodes and circuits used in MET, resulted in the development of a Microbial Electrochemical-based CW (METland). This system relies on electroactive bacteria (EAB) metabolism to transfer electrons to an electro-conductive material, thus boosting substrate consumption, and diminishing electron availability for biomass build-up and methane generation. In previous studies this biofilters have shown an improvement in biodegradation rates in comparison with subsurface flow CW. However, this set-up is still in development, hence there are uncertainties regarding the dynamics involve in the removal of pollutants. Considering that, this work aimed at establishing the capacity and removal kinetics of organic matter and nutrients in an Electroactive Biofilm-Based CW (EABB-CW). Two electro-conductive materials were tested (PK-A and PK-LSN) in planted and non-planted mesocosms and compared with sand. The systems were operated in a continuous upflow mode for 32 weeks and fed with real wastewater. The electro-conductive systems reached removal efficiencies up to 88% for BOD₅, 90% for COD, 46% for NH₄-N, and 86% for PO₄-P. Organic matter removal in electro-conductive systems was possible even at loading rates 10-fold higher than recommended for horizontal flow CWs. First-order area-based removal constants (k), calculated for organic matter and nutrients are higher than values typically reported for saturated CW and in certain cases comparable with vertical flow CW. The organic removal was correlated with electron current densities measures, as indicator of the presence of EAB. The tested EABB-CW profiles as a promising CW type for the removal of organic matter and PO₄-P with margin for modifications to improve nitrogen removal. Future studies with pilot/real scale systems are proposed to validate the findings of this study.

Bioenergy generation and degradation pathway of phenanthrene and anthracene in a constructed wetland-microbial fuel cell with an anode amended with nZVI

The frequent occurrence of polycyclic aromatic hydrocarbons (PAHs) in aquatic environments is of great concern because of their teratogenicity, toxicity, carcinogenicity, and mutagenicity to plants, animals and human beings. In this study the bioelectricity generation, biodegradation, phytoextraction and substrate adsorption of phenanthrene and anthracene in a constructed wetland-microbial fuel cell (CW-MFC) were investigated with an anode electrode amended with or without biochar-nZVI. During a 182-day operation period, the average removal efficiency for phenanthrene and anthracene ranged from 88.5% to 96.4%. The concentration of phenanthrene in roots, stems and laminas of T. orientalis was 14.9, 3.9 and 2.3 ng g^-1 respectively, while that of anthracene was 22.2, 3.1 and 1.3 ng g^-1, respectively. In addition, the application of nZVI was conducive to bioelectricity generation and organic compound degradation in the CW-MFC reactor. The distribution of the bacterial community indicated that the relative abundance of Bacillus, Paludibacter, Desulfovibrio and Lactococcus with a degradation capability for refractory organics was significantly increased. Especially the genus Bacillus for excreting catalase became more abundant. The results of our study indicate how to promote bioelectricity generation and biodegradation of refractory organic compounds in a CW-MFC by improving the culture conditions for bacteria.

Azo dye as part of co-substrate in a biofilm electrode reactor-microbial fuel cell coupled system and an analysis of the relevant microorganisms

In general, refractory organics were hardly used as co-substrate in bioelectrochemical system. This study established a coupled bioelectrochemical system composed of a biofilm electrode reactor and a microbial fuel cell for using the azo dye X-3B as part of co-substrate. The two units degraded the azo dye X-3B stepwise while using it as part of co-substrate. Our results indicated that the removal efficiency of X-3B increased 28.5% using the coupled system compared with a control system. Moreover, the addition of the co-substrate glucose, which was necessary for MFC electricity generation, was reduced on the premise of stable removal efficiency in the coupled system to prevent resource waste due to using X-3B as part of co-substrate. The intermediate products of X-3B degradation were further explored using gas chromatography-mass spectrometry and a X-3B degradation pathway was proposed at the same time. Microbial communities were analyzed, illustrating that the mechanism of X-3B degradation was dependent on bioelectrochemistry rather than on microbial degradation.

Potential methane production and molecular characterization of bacterial and archaeal communities in a horizontal subsurface flow constructed wetland under cold and warm seasons

Organic matter removal in a horizontal subsurface flow constructed wetland (HSSF) treating wastewater is associated with the presence of bacteria and archaea. These organisms perform anaerobic microbial processes such as methanogenesis, which can lead to methane emissions. The aim of this study was to evaluate methane production and characterize the bacterial and archaeal communities found in HSSFs treating secondary urban wastewater during cold and warm seasons. The pilot system used in this study corresponds to four HSSFs, two planted with Phragmites australis (HSSF-Phr) and two planted with Schoenoplectus californicus (HSSF-Sch), the monitoring was carried out for 1335 days. Removal efficiencies for organic matter (biological and chemical oxygen demand) and total and volatile suspended solids were evaluated in each HSSF. Moreover, biomass from each HSSF was sampled during warm and cold season, and methane productions determined by Specific Methanogenic Activity assays_(maximum) (SMA_m). In the same samples, the quantification and identification of bacteria and archaea were performed. The results showed that the degradation of organic matter (53-67% BOD₅ and 51-62% COD) and suspended solids (85-93%) was not influenced by seasonal conditions or plant species. Potential methane production from HSSF-Sch was between 20 and 51% higher than from HSSF-Phr. Moreover, potential methane production during warm season was 3.4-42% higher than during cold season. The quantification of microorganisms in HSSFs, determined greater development of bacteria (38%) and archaea (50-57%) during the warm season. In addition, the species Schoenoplectus californicus has a larger number of bacteria (4-48%) and archaea (34-43%) than Phragmites australis. The identification of microorganisms evidenced the sequences associated with bacteria belong mainly to Firmicutes (42%), Proteobacteria (33%) and Bacteroidetes (25%). The archaea were represented primarily by Methanosarcinales, specifically Methanosaeta (75%) and Methanosarcina (16%). The community structure of the methanogenic archaea in HSSFs did not change throughout the seasons or plant species.

Electricity production and the analysis of the anode microbial community in a constructed wetland-microbial fuel cell

The objective of this study is to assess bioelectricity generation, pollutant removal (COD, ammonium, nitrate) and the bacterial communities on anodes in constructed wetlands coupled with microbial fuel cells (CW-MFCs), through feeding the systems with three different types of synthetic wastewater (system 1: normal wastewater; system 2: ammonium-free wastewater; system 3: nitrate-free wastewater). Three CW-MFCs were operated with different wastewater concentrations and hydraulic retention times (HRTs) over a long time period (6 months). The results indicate that the maximum open circuit voltage (775.63 mV) and maximum power density (0.628 W m⁻³) were observed in system 3 (period 3), and that bioenergy production was inhibited in system 2, when feeding with ammonium-free wastewater continuously. COD removal rates in the three systems were similar during each period and ranged from 82.2 ± 6.8% to 98.3 ± 2.2%. Ammonium removal occurred at the air cathode of the CW-MFCs through nitrification, and a higher level of ammonium removal was found in system 1 (period 3) compared with the others. Meanwhile, denitrification occurred at the anaerobic anode of the CW-MFCs, and a large amount of nitrate was removed effectively. The highest nitrate removal rate was 98.8 ± 0.5% in system 2 (period 3). Additionally, four genera related to electricity generation were detected at the anode: Geothrix; Desulfovibrio; Desulfobulbus; and Geobacter. The relative abundances of Desulfovibrio, Desulfobulbus and Geothrix gradually increased during the three periods in system 3, which might be beneficial for bioelectricity generation. Further investigations are needed to optimize the CW-MFC performance and explain the mechanism behind the pollutant degradation and electron motion in the CW-MFCs.

The objective of this study is to assess bioelectricity generation, pollutant removal and the bacterial communities on anodes in constructed wetlands coupled with microbial fuel cells, through feeding the systems with three different types of synthetic wastewater.

Anodic potentials, electricity generation and bacterial community as affected by plant roots in sediment microbial fuel cell: Effects of anode locations

A planted sediment microbial fuel cell (PSMFC) is a promising new technology for harvesting energy and remediating a contaminated geo-environment. In this study, the effects of roots (of Acorus tatarinowii) on oxygen profiles in sediment, power generation, and anodic bacterial community were investigated in PSMFCs and unplanted SMFCs with different anode locations to roots. The presence of plant did not improve the electricity generation when roots were placed on the surface of an anode because a high amount of oxygen loss from roots increased the redox potential at anode and made aerobic bacteria co-exit and compete with electrochemically active bacteria in substance utilization. It was suggested to place the anode under the roots with a proper distance, where the PSMFCs made use of root-derived organics, avoiding the negative effects of oxygen loss. Oxygen loss could control the diurnal rhythm of power generation in the PSMFCs.