Constructed Wetland-microbial Fuel Cell Research Articles

Studies on Cr (VI) removal by constructed wetland integrated microbial fuel cell: Effect of electrodes

Heavy metal contamination significantly threatens human health and the environment, even at low concentrations. Among these metals, hexavalent chromium (Cr (VI)), primarily originating from mining activities, stands out as a significant concern. The Constructed Wetland-Microbial Fuel Cell (CW-MFC) represents an engineered solution inspired by natural wetlands. This study presents a novel approach to utilizing cement-graphite (CG) electrodes in CW-MFC to reduce Cr (VI). In this study, Cr (VI) spiked synthetic wastewater (25 mg/L to 100 mg/L) was treated using CG-CW-MFC (Cement-Graphite) and G-CW-MFC (Graphite) systems, at a constant hydraulic retention time of 48 hours. The results indicated that both CG-CW-MFC and G-CW-MFC exhibited a removal efficiency of 99% for Cr (VI). COD removal rates for CG-CW-MFC and G-CW-MFC ranged from 89% to 94% and 82.5–91.35%, respectively. Meanwhile, CG-CW-MFC showed stable voltage generation at 100 mg/L of Cr (VI) (Average voltage 191.45 mV), while G-CW-MFC showed a steep reduction in voltage generation with an average voltage of 1.48 mV. Thus, the study concludes that, in contrast to the G-CW-MFC electrode, the CG electrode can generate voltage under highly hazardous conditions because of the porous structure of CG. The findings of this study highlight the potential of CG electrodes in CW-MFC systems designed to remediate wastewater contaminated with Cr (VI).

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.

In Situ Utilization of Electron-Enhanced Degradation of Azo Dyes in a Constructed Wetland–Microbial Fuel Cell Coupling System

In this study, a constructed wetland was coupled with a microbial fuel cell to establish a coupled system known as the constructed wetland–microbial fuel cell (CW–MFC), utilized for the treatment of X-3B azo dye wastewater at varying concentrations. Experimental results indicated that the anodic region made the primary contributions to the discoloration of azo dyes and COD removal, with a contribution rate of 60.9–75.8% for COD removal and 57.8–83.0% for the effectiveness of discoloration. Additionally, the role of plants in the constructed wetland area could achieve the removal of small molecular substances and further discoloration. In comparison to open-circuit conditions, under closed-circuit conditions the CW–MFC effectively degraded X-3B azo dye wastewater. Under an external resistance of 2000 Ω, a maximum COD removal rate of 60.0% and a maximum discoloration rate of 85.8% were achieved for X-3B azo dye at a concentration of 100 mg/L. Improvements in the treatment efficiency of X-3B dye wastewater were achieved by altering the external resistance. Under an external resistance of 100 Ω and an influent concentration of X-3B of 800 mg/L, the COD removal rate reached 78.6%, and the decolorization rate reached 85.2%. At this point, the CW–MFC exhibited a maximum power density of 0.024 W/m3 and an internal resistance of 99.5 Ω. Spectral analysis and GC–MS results demonstrated the effective degradation of azo dyes within the system, indicating azo bond cleavage and the generation of numerous small molecular substances. Microbial analysis revealed the enrichment of electrogenic microorganisms under low external resistance conditions, where Geobacter and Trichococcus were dominant bacterial genera under an external resistance of 100 Ω, playing crucial roles in power generation and azo dye degradation within the system.

Evaluation of electrode separators and the external resistance in electrochemically assisted constructed wetlands

A proton exchange membrane increases the electrical performance of a microbial fuel cell (MFC). New inexpensive materials should be sought, especially in a constructed wetland microbial fuel cell (CW-MFC). Here, in a laboratory-scale system of five CW-MFCs, wet clay, wet earth or mud, and non-woven cloth were used as inexpensive separators with long-term stability. The five CW-MFCs were planted with Typha latifolia, fed with synthetic wastewater, and packed with natural porous material. Graphite felt was used as electrodes and the experimental system had a hydraulic residence time of 3 days, operating under shade and natural conditions of temperature and light. Electrodes were connected to current collectors (copper wire) and to an external resistance, with a change every 20 days, starting in open-circuit and following with 20000, 18000, 15000, 10000, 5600, 1000, 560, and 10 Ω. These laboratory-scale CW-MFCs reduced concentrations of nitrates, ammonium ion, and sulfates without inhibiting electricity production. Microbiological analyses indicated that anaerobic, facultative, aerobic, and denitrifying bacteria may have caused these reductions. The reactor with the live plant and with the wet earth or mud separator achieved the highest production of electricity (22.6 mW/m2), and may be worth further attention.

Constructed wetland – Microbial fuel cell (CW-MFC) packed with suspended fillers to enhance denitrification with Acorus calamus biomass addition

There is an urgent need to meet strict nitrogen discharge limitation for secondary effluent from wastewater treatment plants, but the nitrogen removal is restricted due to the lack of carbon sources. To address the issues, high-density polyethylene (HDPE) was utilized in the constructed wetland-microbial fuel cell (CW-MFC) for the treatment of low carbon wastewater. Meanwhile, wetland plant biomass was efficiently reclaimed, leading to higher nitrogen removal and bioelectricity generation. CW-MFC with biomass added in the anode attained the maximum total nitrogen (TN) removal of 97.55 % and power density of 2.22 mW/m2 with hydraulic retention time (HRT) of 2d, which were much better than those in CW, CW-MFC with no biomass and CW-MFC with biomass added in the cathode. Owing to the HDPE fillers and Acorus calamus biomass addition in CW-MFC, more than 88 % chemical oxygen demand (COD) and 70 % TN were removed with HRT of 1.5d. In addition, metabolisms related to glycolysis, cellulose degradation and nitrogen were increased in CW-MFC, contributing to the enrichment of denitrifying, electroactive and heterotrophic nitrification-aerobic denitrification (HN-AD) bacteria. This study verifies the feasibility of application of HDPE as the biocarriers in CW-MFC and provides mechanistic insights for the enhancement of nitrogen removal in low carbon wastewater treatment with plant biomass addition.

Treatment of formaldehyde-containing wastewater and power generation by constructed wetland–microbial fuel cells enhanced by formaldehyde-degrading bacteria

In this study, three formaldehyde-degrading bacteria, Paenibacillus, Hyphomicrobium, and Ensifer were isolated from activated sludge. Three strains of formaldehyde-degrading bacteria were enriched and added to a constructed wetland–microbial fuel cells (CW-MFC) for bioaugmentation. Three laboratory-scale devices were built: constructed wetland (CW), enhanced CW-MFC (EG), and non-enhanced CW-MFC (CG). During the 70 days of operation, a comprehensive study was conducted to investigate the impact of bioaugmentation on the performance of CW-MFC in treating formaldehyde wastewater, considering water quality, power generation, and microbial communities. The formaldehyde removal rate by the EG (98.8 % ± 0.6 %) was significantly higher than that of the CG (91.0 % ± 0.6 %) (P < 0.05). The average voltage of the EG (167.3 ± 4.8 mV) was higher than the CG (122.5 ± 2.9 mV). Bioaugmentation of a constructed wetland–microbial fuel cell can significantly improve its ability to treat formaldehyde-containing wastewater and generate electricity.

Performance optimization for Pb(II) -containing wastewater treatment in constructed wetland-microbial fuel cell triggered by biomass dosage and Pb(II) level.

Three identical sets of constructed wetland-microbial fuel cells (CW-MFCs) fabricated with biomass carbon source addition were constructed and underwent the short- and long-term experiments. For this, the efficacy of biomass dosage and Pb(II) concentration towards Pb(II) removal and concurrent bioelectricity production of CW-MFCs were systematically explored. From the perspective of integrated capabilities and economic benefits, the solid biomass carbon sources equivalent to 500mg/L COD was regarded as the optimal dosage, and the corresponding device was labeled as CW-MFC-2. For the short-term experiment, the closed-circuit CW-MFC-2 produced maximum output voltages and power densities in a range of 386-657mV and 1.55 × 103-6.31 × 103 mW/m2 with the increasing Pb(II) level, respectively. Also, Pb(II) removal up to 94.4-99.6% was obtained in CW-MFC-2. With respect to long-term experiment, Pb(II) removal, the maximum output voltage, and power density of CW-MFC-2 ranged from 98.7 to 99.2%, 322 to 387mV, and 3.28 × 102 to 2.26 × 103 mW/m2 upon 200mg/L Pb(II) level, respectively. The migration results confirmed the potential of substrate and biomass for Pb(II) adsorption and fixation. For the cathode, Pb(II) was fixed and removed via binding to O. This study enlarges our knowledge of effective modulation of CW-MFCs for the treatment of high-level Pb(II)-containing wastewater and bioelectricity generation via adopting desirable biomass dosage.

Critical evaluation of electroactive wetlands: traditional and modern advances.

In the field of sustainable wastewater management, electroactive wetlands (EW), or constructed wetland-microbial fuel cells (CW-MFC), are an emerging technology. With the growing problem of untreated wastewater, the emphasis must shift to decentralisation of wastewater treatment infrastructure, and CW-MFC can be an excellent choice. This review provides a chronologically organized account of the design and configuration of CW-MFCs developed between 2010 and 2023. The research on CW-MFC has mainly focused on material, positioning and number of electrodes; use of electroconductive media and filler materials; flow regime; algal-based CW-MFC and multistage setups. Compared to traditional constructed wetlands (CW) and microbial fuel cells (MFC), CW-MFCs have a number of advantages, including better treatment efficiency, faster organic matter utilisation, lower capital and land requirements and a smaller carbon footprint. However, there are some limitations as well, such as upscaling and viable electricity generation, which are covered in more detail in the article. Moreover, the economics of this technology is also evaluated. The microbiology of a CW-MFC and its influence on its performance are also elaborated. Recent advancements in this field in terms of design, configuration and performance are discussed. Finally, the knowledge gaps that must be addressed before this technique can be successfully implemented on a large scale are highlighted, along with specific recommendations. This article aims to advocate for EWs as an ideal decentralised wastewater treatment technique, while also shedding light on the areas that still need to be worked on.

Integrated Constructed Wetland–Microbial Fuel Cell Systems Using Activated Carbon: Structure-Activity Relationship of Activated Carbon, Removal Performance of Organics and Nitrogen

This study designed integrated constructed wetland–microbial fuel cell (CW–MFC) systems using activated carbon (AC) as both CW substrates and MFC anodes and investigated the structure-activity relationship of six kinds of commercial columnar AC, as well as the organics and nitrogen removal, microbial activity and diversity of CW–MFCs. Results showed that the nitrogen adsorption by AC tended to be a linear process in which physical adsorption played a leading role and micropores made great contributions. A higher specific surface area, developed mesopores, and oxygen functionalities were conducive to the capacitance properties of AC, while a higher specific surface area and developed micropores were conducive to reduce material resistance and improve ion permeability. Coconut-shell-based AC had both excellent nitrogen adsorption capacity and electrochemical properties, making it ideal as both CW substrates and MFC anodes for CW–MFCs. The electricity generation, coulombic efficiency, internal resistance, and organics and nitrogen removal of CW–MFCs were positively correlated with the total depth of AC anodes. The total depth of AC anodes can be determined based on the influent organics/nitrogen loadings and organics/nitrogen removal load of AC, and a relatively smaller depth of a single AC anode (5 cm) was recommended. The MFC effectively improved the enzymatic activity (by 10.33% dehydrogenase, 8.72% catalase, and 7.35% ammonia monooxygenase), nitrification/denitrification intensity (by 9.53%/6.68%), and microbial diversity (by 1.64–4.07%) of AC (MFC anodes) in CW–MFCs, while the depth of a single AC anode barely influenced the microbial activity and diversity. MFCs increased COD and NH3-N removal in CW–MFCs by 11.60% and 3.4%, respectively. The increased total adsorption capacity of AC with the increase of its total depth narrowed the difference in COD removal resulting from the promotion of MFCs on organics degradation. MFCs increased TN removal in CW–MFCs by 5.29% through promoting denitrification in cathodes and enhancing NH3-N assimilation in anodes. The phyla of EAB (Proteobacteria, Bacteroidetes, Firmicutes, and Acidobacteria) and genera of EAB (Citrobacter, Geobacter, and Pseudomonas) accounted for 85–86% and 15.58–16.64% of the microbial community on AC anodes in CW–MFCs, respectively.

Assessment of microbial population in integrated CW-MFC system and investigation of organics and fecal coliform removal pathway

The current study is focused on understanding the operational mechanism of an integrated constructed wetland-microbial fuel cell (CW-MFC) reactor emphasizing fecal coliform (FC) removal. Few studies are available in the literature investigating the inherent mechanisms of pathogen inactivation in a CW-MFC system. Raw domestic wastewater was treated in three vertical reactors, one planted constructed wetland (R1), one planted CW-MFC (R2), and one unplanted CW-MFC (R3). Spatial analysis of treated effluents showed a considerable amount of organics and fecal coliform removal at the vicinity of the anode in R2. Assessment of the microbial population inside all the reactors revealed that EABs (Firmicutes, Bacteroidetes, and Actinobacteria) were more abundant in R2 compared to R1 and R3. During the activity study, biomass obtained from R2 showed a maximum substrate utilization rate of 1.27 mg COD mgVSS−1 d−1. Kinetic batch studies were carried out for FC removal in all the reactors, and the maximum first order FC removal rate was obtained at the anode of R2 as 2.13 d−1 when operated in closed circuit mode. This value was much higher than the natural die-off rate of FCs in raw wastewater which was 1.16 d−1. Simultaneous bioelectricity monitoring inferred that voltage generation can be correlated to faster FC inactivation, which was probably due to EABs outcompeting other exogenous microbes in a preferable anaerobic environment with the presence of an anode. Reactor R2 was found to be functioning as a symbiotic bio-electrochemical mesocosm.

Variation of graphene/titanium dioxide concentration as electrocatalyst for an oxygen reduction reaction in constructed wetland-microbial fuel cells

Constructed wetlands-microbial fuel cells (CW-MFC) are an innovative technology used for simultaneous bioelectricity generation and wastewater treatment. This is possible due to the installation of macrophytes in an electrode configuration, in which electroactive microorganisms use organic substrates as biofuel. One way to improve the electrochemical performance of CW-MFCs is through the impregnation of cathodic electrocatalysts. Therefore, in this study the bioelectricity production capacity of CW-MFCs was evaluated from the oxygen reduction reaction (ORR). For this study, the concentrations 0 (CW-MFC1), 0.5 (CW-MFC2), and 1 mg/cm2 (CW-MFC3) of graphene/titanium dioxide (G/TiO2) as electrocatalyst on the cathodes were evaluated. Using the Koutecky-Levich analysis, it was determined that the ORR transfer mechanism arises via a 4-electron pathway. The electrokinetic parameters of Tafel slope, charge transfer coefficient, and exchange current density determined the efficiency of the ORR, registering 92 mV/dec, 0.93 (α), and 2.30 x10-3 mA/cm2, respectively for CW-MFC3. The highest electrochemical performance was obtained at a concentration of 1 mg/cm2 (CW-MFC3) of G/TiO2, generating 144 mW/m2 of power density, 157 Ω of internal resistance, −150 mV of anodic potential, and 383 mV of cathodic potential. The surface modification carried out on the cathodes resulted in a catalytic increase in the ORR.

Study on the Removal Characteristics of IBP and DCF in Wastewater by CW-MFC with Different Co-Substrates

Pharmaceuticals and Personal Care Products (PPCPs) constitute a group of compounds that are challenging to break down and potentially pose risks to both ecosystems and human health when they accumulate in water bodies. This study established and operated small-scale constructed wetland–microbial fuel cells (CW-MFCs) continuously for 60 days, maintaining a hydraulic retention time (HRT) of 3 days. The research aimed to assess the treatment efficiency of wastewater containing Ibuprofen (IBP) and Diclofenac (DCF) using different co-substrates (glucose, sucrose, and sodium acetate) and to analyze the impact of these co-substrates on the composition of bacterial communities within the CW-MFC. After 60 days of operation, CW-MFC achieved removal rates of 89.29% for IBP and 84.10% for DCF. The elimination of IBP was primarily dependent on co-metabolic degradation processes occurring in both the anode and cathode, while DCF removal relied on anodic co-metabolism. Additionally, various co-substrates have an influence on the bacterial community diversity of the anode and cathodes. The possible bacterial groups involved in PPCP degradation were identified. In summary, Glu was identified as a more suitable co-substrate for CW-MFC in the removal of IBP and DCF, while SA as a co-substrate favored the induction and enrichment of EAB in the anodes. These findings offer valuable insights into the potential of CW-MFC for mitigating emerging contaminants.

Iron-sulfur mediated antibiotic resistance genes attenuation and autotrophic denitrification mechanisms in bioelectrochemical systems

Aiming at the problems of antibiotic resistance genes (ARGs) transfer, mobile gene elements (MGEs) and nitrogen removal in livestock wastewater, pyrite was introduced into the bioelectrochemical constructed wetland, mainly focusing on the mechanism of ARGs transfer and autotrophic denitrification in bioelectrochemical system. The results indicated that pyrite promoted the degradation of antibiotics while inhibiting the efflux of ARGs and MGEs in effluent. The removal rate of antibiotics in pyrite-mediated constructed wetland coupled with microbial fuel cell (PCW-MFC) was 26.84 % and 21.20 % higher than constructed wetland-microbial fuel cell (CW-MFC) and pyrite-mediated constructed wetland (PCW). Metagenomic analysis revealed that the abundance of ARGs in effluent and anodic microorganism in PCW-MFC was effectively controlled by inhibiting the expression of genes encoding antibiotic efflux (macB, tetA, smeS). The expression levels of periplasmic nitrate reduction genes (napA, napB) and encoding cytochrome c nitrite reductase small subunit genes (nrfH) in PCW-MFC were much higher than CW-MFC. The total expression of ARGs in CW-MFC anode biofilms was 35.77 % higher than PCW-MFC, indicating that the system can effectively reduce the spread of ARGs and abate environmental risks. The findings can provide theoretical reference for the control of ARGs and efficient nitrogen removal in livestock wastewater.

A double-edged sword: Constructed wetland-microbial fuel cells promote organics removal via entrapment

Recently, constructed wetland-microbial fuel cells (CW-MFCs) are found to enhance the organics removal via the connection of the external circuit. Yet, it is unclear why the energy output is unmatched with the enhancement of the organics removal. This study compared the dynamic changes of the organics in a CW-MFC microcosm operated under the close circuit and open circuit. As a result, the close circuit facilitated the organics removal by 9 % before the proportional discharge of carbon metabolites. This suggested that organics entrapment should account for the huge loss of carbon recovery; and closing the external circuit could further promote the organics entrapment. Besides, polyhydroxybutyrate was found accumulated in the MFC culture experiment, evidencing that the fed-batch mode of operation could result in a feast-famine pattern of microbial metabolism. Despite the fast organics entrapment during the first hours, prolonging the operation time would lead to continuous carbon gas emission, along with the substantially elevated coulombic efficiency. Together, these results explained the substantial COD removal enhancement with low electricity yield, and cautioned the safe use of the MFC integration to spare the system from overaccumulation of organics.

Accelerated Pb(II) removal and concurrent bioelectricity production via constructed wetland-microbial fuel cell: Structural orthogonal optimization and microbial response mechanism

In this study, a novel constructed wetland-microbial fuel cell (CW-MFC) filled with biomass carbon sources was established to elucidate the impacts of orthogonal configuration of multiple constraints towards wastewater purification and bioelectricity generation under Pb(II) stress. As a result, the optimal structural layouts of CW-MFC were granular activated carbon, Iris pseudacorus and 20 cm of electrode spacing, respectively. The homologous system yielded a favorable Pb(II) removal efficiency of 99.05 %, a maximum output voltage of 394.3 mV and power density of 4468.40 mW/m2, respectively. The migration results highlighted the major contribution of substrate and biomass to Pb(II) adsorption and fixation. Likewise, Pb(II) was immobilized on the cathode via potential binding to O. A visible enrichment of electroactive bacteria (Clostridium, Geobacter, Pseudomonas) and metal-resistant bacteria (Novosphingobium, Aeromonas) was noted. Also, microorganisms modulated the intracellular transport, chemotactic and antioxidant systems by engaging in the metabolic processes of carbohydrate, glutathione, pyruvate, etc. The microbial analysis signified that the genera enriched in disparate regions of CW-MFC-6 regulated the metabolic processes to facilitate their activities and resistance to Pb(II) stress, thereby synergistically enhancing the integrated capability of closed-circuit device. This study was dedicated to elucidating the microbial response mechanism of structural orthogonal optimization of CW-MFC, and providing a theoretical guidance for an effective regulation of Pb(II)-containing wastewater treatment.

Impact of separate concentrations of polyethylene microplastics on the ability of pollutants removal during the operation of constructed wetland-microbial fuel cell

Microplastics (MPs) in water pose a great threat to the ecological environment, but the impact of MPs on constructed wetland microbial fuel cells (CW-MFCs) has not been studied, so in order to fill the research gap and enrich the research in the field of microplastics, a 360-day experiment was designed to determine the operating status of CW-MFCs at different concentrations (0, 10, 100 and 1000 μg/L) polyethylene microplastics (PE-MPs) at different times, focusing on the changes of the CW-MFCs' ability to handle pollutants, power production performance and microbial composition. The results showed that with the accumulation of PE-MPs, the removal effect of COD and TP did not change significantly, and that the removal rate was maintained at around 90% and 77.9% respectively, within 120 d of operation. What's more, the denitrification efficiency increased (from 4.1% to 19.6%), but with the passage of time, it decreased significantly (from 7.16% to 31.9%) at the end of the experiment, while oxygen mass transfer rate was significantly increased. Further analysis showed that the accumulation of PE-MPs did not affect the current power density significantly with the changes of time and concentration, but the accumulation of PE-MPs would inhibit the exogenous electrical biofilm and increase the internal resistance, thereby affecting the electrochemical performance of the system. In addition, the results of microbial PCA showed that the composition and the activity of the microorganisms were changed under the action of PE-MPs, that the microbial community in CW-MFC showed a dose effect on the input of PE-MPs, and that the relative abundance of nitrifying bacteria with time was significantly affected by PE-MPs concentration. The relative abundance of denitrifying bacteria decreased over time, but PE-MPs promoted the reproduction of denitrifying bacteria, which was consistent with the changes in nitrification and denitrification rates. The removal modes of EP-MPs by CW-MFC include the adsorption and the electrochemical degradation, with two isothermal adsorption models of Langmuir and Freundlich being constructed in the experiment, and the electrochemical degradation process of EP-MPs being simulated. In summary, the results show that the accumulation of PE-MPs can induce a series of changes in substrate, microbial species and activity of CW-MFCs, which in turn affects the pollutant removal efficiency and power generation performance during its operation.

Performance optimization of two-stage constructed wetland-microbial fuel cell system for the treatment of high-concentration wastewater.

Constructed wetland-microbial fuel cell (CW-MFC) has attracted much attention because of its dual functions of wastewater treatment and energy recovery. However, its performance in treating high-concentration wastewater is degraded by the decreased dissolved oxygen at the cathode and insufficient electron acceptors. In this study, two CW-MFC systems with cathodic aeration were connected in series to investigate the effects of aeration rate and hydraulic retention time (HRT) on the removal of pollutants and the performance of electricity production in high-concentration wastewater. Results showed that aeration enhanced NH4+-N and TP removal by 45.0-49.8% and 11.5-18.0%, compared with the unaerated condition, respectively. Meanwhile, no significant change regarding COD removal was observed. Aeration enhances the output voltage and power density of the system, especially the first stage CW-MFC, which improved the power production performance by 1 to 2 orders-of-magnitude. Increasing HRT improves the system's pollutant treatment efficiency and power generation performance for high-concentration wastewater. Still, the extension of HRT to 2days will not contribute much to improving the removal efficiency. Under optimized conditions, the maximum total removal rates of COD, NH4+-N, and TP for the two-stage tandem CW-MFC system were 99.3 ± 0.2%, 92.4 ± 1.6%, and 79.5 ± 3.4%, respectively. Meanwhile, the maximum output voltage and maximum power density of the first-stage CW-MFC were 405mV and 138.0mW/m3, respectively. In contrast, the maximum output voltage and maximum power density of the second stage are 105mV and 14.7mW/m3, respectively.

Influence of Electrodes Configuration on Hydraulic Characteristics of Constructed Wetland–Microbial Fuel Cell Systems Using Graphite Rods and Plates as Electrodes

Constructed wetland–microbial fuel cell coupling systems (CW–MFCs) have received significant academic interest in the last decade mainly due to the promotion of MFCs in relation to pollutants’ degradation in CWs. Firstly, we investigated the effect of hydraulic retention time (HRT) and electrode configuration on the flow field characteristics of CW–MFCs using graphite rods and plates as electrodes, as well as the optimization of electrode configuration using computational fluid dynamics (CFD) numerical simulation. The results showed that: (1) the apparent HRT was the most influential and decisive factor, with a contribution of over 90% for the average HRT of CW–MFCs; (2) anode spacing was the most influential factor for the hydraulic performance of CW–MFCs, with contributions of over 50% for water flow divergence and hydraulic efficiency (λ) and over 45% for effective volume ratio (e); (3) anode size was significant for e and λ, with a contribution of over 20%; (4) cathode position and cathode size had no statistically significant effect on the hydraulic performance of CW–MFCs. It was mainly through the blocking of water flows, flows around, compressing water flow channels and boundary layer separation that the MFC electrodes influenced the hydraulic characteristics of the flow field in CW–MFCs. Optimizing the flow field by optimizing the electrode configuration helped to facilitate electricity generation and pollutants’ removal in CW–MFCs. This study offers a scientific reference for improving the hydraulic performance of CW–MFCs, and it also provides a new research perspective for improving the wastewater treatment and electricity production performance of CW–MFCs.

Transformation from biofiltration unit to hybrid constructed wetland-microbial fuel cell: Improvement of wastewater treatment performance and energy recovery

This study aimed to compare the performance of biofiltration, constructed wetland, and constructed wetland microbial fuel cell (CW-MFC). The transformation from a biofiltration unit to a hybrid CW-MFC was demonstrated with the advantages of improvement of wastewater treatment while generating electricity simultaneously. The introduction of plants to the upper region of the bioreactor enhanced the DO level by 0.8 mg/L, ammonium removal by 5 %, and COD removal by 1 %. The integration of electrodes and external circuits stimulated the degradation rate of organic matter in the anodic region (1 % without aeration and 3 % with aeration) and produced 5.13 mW/m3 of maximum power density. Artificial aeration improved the nitrification efficiency by 38 % and further removed the residual COD to an efficiency of 99 %. The maximum power density was also increased by 3.2 times (16.71 mW/m3) with the aid of aeration. In treating higher organic loading wastewater (3M), the maximum power density showed a significant increment to 78.01 mW/m3 (4.6-fold) and the COD removal efficiency was 98 %. The ohmic overpotential dominated the proportion of total loss (67-91 %), which could be ascribed to the low ionic conductivity. The reduction in activation and concentration loss contributed to the lower internal resistance with the additional aeration and higher organic loading. Overall, the transformation from biofiltration to a hybrid CW-MFC system is worthwhile since the systems quite resemble while CW-MFC could improve the wastewater treatment as well as recover energy from the treated wastewater.

Effect of External Aeration on Cr (VI) Reduction in the Leersia hexandra Swartz Constructed Wetland-Microbial Fuel Cell System

Cr (VI) is hazardous to humans and our environment. Leersia hexandra Swartz (L. hexandra) is the first wet chromium hyperaccumulator found in China. This study constructed the L. hexandra constructed wetland-microbial fuel cell (CW-MFC) system to treat Cr (VI) wastewater. It also determined the effects of different dissolved oxygen (DO) concentrations on power generation, pollutant removal, and Cr (VI) reduction. Cathode aeration promoted the voltage output and pollutant removal of the L. hexandra CW-MFC when the DO concentration was 4.5 mg·L−1: the highest voltage was 520 mV, the chemical oxygen demand (COD) removal rate was 93.73%, and the Cr (VI) removal rate was 97.77%. Moreover, the increase in the DO concentration improved the absorption of heavy metal Cr by the substrate and L. hexandra, and promoted the transformation from Cr (VI) to Cr (III). Chromium mostly exists as a residue with low toxicity and low mobility in L. hexandra and the substrate. This proves that the increased DO concentration promotes the redox reaction in the system and plants, reducing Cr (VI) to Cr (III). At the same time, the key micro-organism Geobacter that enhances the performance of the system and Cr (VI) reduction was found. The research results can provide a reference for the subsequent CW-MFC treatment of actual Cr-containing wastewater.