Improving domestic wastewater treatment efficiency with constructed wetland microbial fuel cells: Influence of anode material and external resistance

Biochar and zero-valent iron alleviated sulfamethoxazole and tetracycline co-stress on the long-term system performance of bioretention cells: Insights into microbial community, antibiotic resistance genes and functional genes

Antibiotics and antibiotic resistance genes (ARGs), known as emerging contaminants, have raised widespread concern due to their potential environmental and human health risks. In this study, a conventional bioretention cell (C-BRC) and three modified bioretention cells with biochar (BC-BRC), microbial fuel cell coupled/biochar (EBC-BRC) and zero-valent iron/biochar (Fe/BC-BRC) were established and two antibiotics, namely sulfamethoxazole (SMX) and tetracycline (TC), were introduced into the systems in order to thoroughly investigate the co-stress associated with the long-term removal of pollutants, dynamics of microbial community, ARGs and functional genes in wastewater treatment. The results demonstrated that the SMX and TC co-stress significantly inhibited the removal of total nitrogen (TN) (C-BRC: 37.46%; BC-BRC: 41.64%; EBC-BRC: 55.60%) and total phosphorous (TP) (C-BRC: 53.11%; BC-BRC: 55.36%; EBC-BRC: 62.87%) in C-BRC, BC-BRC and EBC-BRC, respectively, while Fe/BC-BRC exhibited profoundly stable and high removal efficiencies (TN: 89.33%; TP: 98.36%). Remarkably, greater than 99% removals of SMX and TC were achieved in three modified BRCs compared with C-BRC (SMX: 30.86 %; TC: 59.29%). The decreasing absolute abundances of denitrifying bacteria and the low denitrification functional genes (nirK: 2.80 × 105-5.97 × 105 copies/g; nirS: 7.22 × 105-1.69 × 106 copies/g) were responsible for the lower TN removals in C-BRC, BC-BRC and EBC-BRC. The amendment of Fe/BC successfully detoxified SMX and TC to functional bacteria. Furthermore, the co-stress of antibiotics stimulated the propagation of ARGs (sulI, sulII, tetA and tetC) in substrates of all BRCs and only Fe/BC-BRC effectively reduced all the ARGs in effluent by an order of magnitude. The findings contribute to developing robust ecological wastewater treatment technologies to simultaneously remove nutrients and multiple antibiotics.

Performance evaluation of three constructed wetland-microbial fuel cell systems: wastewater treatment efficiency and electricity generation potential

Constructed wetlands (CWs) have proven to be effective and environmentally friendly for removing pollutants, while microbial fuel cells (MFCs) offer the potential for electricity generation. Thus, this study evaluated the performance of three CW-MFC systems (zigzag, single-column, and triple-column continuous) for domestic wastewater treatment and electricity generation. Results showed that parallel connection of CW-MFCs significantly improved power generation compared to series connection. Additionally, using three copper wires to connect carbon fiber felt electrodes demonstrated superior pollutant capture capabilities compared to a single copper wire. During the 14-day testing period, the single-column system achieved the highest power density of 5.55 mW m-2, followed closely by the triple-column continuous system at 4.77 mW m-2. In contrast, the zigzag system exhibited a lower power density of 2.49 mW m-2. Interestingly, the implementation of facultative anaerobic conditions in the anode, along with the application of a plastic bag cover, facilitated the maintenance of anaerobic conditions in both the single-column and triple-column continuous systems. This resulted in increased power density and reduced internal resistance. In contrast, the zigzag system, with its larger surface area, aeration, and circulation, exhibited higher internal resistance and lower current dissipation. Despite its inferior electricity generation performance, the zigzag system demonstrated higher efficiency removal of chemical oxygen demand (COD), nitrate (NO3-), and phosphate (PO43-) than the single-column system. This can be attributed to the extended contact time, resulting in enhanced pollutant removal. Overall, the multi-column continuous system shows promise as a viable approach for simultaneous domestic wastewater treatment and electricity production, offering potential benefits for sustainable wastewater management.

Organic media-based two-stage traditional and electrode-integrated tidal flow wetlands to treat landfill leachate: Influence of aeration strategy and plants

Landfill leachate treatment employing normal and electrode-integrated constructed wetlands is difficult due to the presence of significant amounts of organic compounds, which frequently impede the progression of microbial-based aerobic pollutant removal pathways. As a result, this study examines the effect of supplementary air availability via intermittent and continuous aeration strategies in improving organic, nutrient, and coliform removals of the unplanted, planted (normal and electrode-integrated) two-stage tidal flow constructed wetlands designed to treat landfill leachate. The constructed wetlands were filled with coal and biochar media and planted with Canna indica. Mean chemical oxygen demand (COD), total nitrogen (TN), total phosphorus (TP), and coliform removal percentages of the externally aerated two-stage unplanted, only planted, planted-microbial fuel cell integrated constructed wetland systems ranged between 96 and 99%, 82 and 93%, 91 and 98%, 86 and 96%, respectively, throughout the experimental campaign. External aeration inhibited the development of a dominant anaerobic environment within the media of the wetland systems and improved overall pollutant removal. The electrode-integrated planted tidal flow wetlands produced better effluent quality than the unplanted or only planted tidal flow systems without electrode assistance. The first stages of the three wetland systems achieved an additional 5-7% COD, 7-12% TN, and 15-22% coliform removal during the continuous aeration period compared to the corresponding performance of the intermittent aeration phase. The pollutant removal performance of the second-stage wetlands decreased during the continuous aeration phase. The media composition supported electrochemically active and inactive microbial-based pollutant removal routes and the chemical adsorption of pollutants. Nitrogen and phosphorus accumulation percentage in plant tissues was low, i.e., 0.4-2.2% and 0.04-0.8%, respectively. During the continuous aeration period, the electrode-integrated tidal flow constructed wetlands achieved higher power density production, i.e., between 859 and 1432 mW (mW)/meter³(m³). This study demonstrates that external aeration might improve pollutant removal performance of the normal, electrodes integrated tidal flow-based constructed wetlands when employed for high organic-strength wastewater treatment such as landfill leachate.

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.

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

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

Role played by the physical structure of carbon anode materials in MFC biosensor for BOD measurement

Microbial fuel cell (MFC) has been extensively studied as a biosensor for determining biochemical oxygen demand (BOD). The method for quantifying BOD by employing coulombic yield (Q) of a bio-electrochemical degradation process obtained from MFC biosensors is referred to as BOD_Q. The physical structures of anode materials greatly affect the sensitivity and accuracy of the biosensor. In this work, the effects of carbon cloth (CC) and carbon felt (CF) as anode substrate materials on the BOD_Q determination efficiencies were studied. The CF-MFC biosensor showed higher BOD_Q response than that of the CC-MFC within 25-400 mg L^-1 BOD concentration range, and the test value was very close to the theoretical BOD. The difference is resulting from higher coulombic efficiency (CE) of CF-MFC (64.89-65.38 %) than CC-MFC (55.58-63.51 %). It should be noted that for water samples with low BOD concentrations the physical structures of anode materials play a leading role in CE. For synthetic wastewaters with 25 mg L^-1 BOD, the CE of CF-MFC (65.38 %) was 17.63 % higher than that of CC-MFC (55.58 %). In contrast to the densely woven CC coated with thick biofilm, CF with loose carbon fiber and thin biofilm makes it good for organic diffusion and electron transportation, thus contributing to higher and more stable CE. These results indicate that the CF-MFC is more suitable for determining BOD_Q values over a wide concentration range. This work provides a useful strategy for selecting desirable MFC's anode material as the BOD biosensor. MFC biosensors with high-porosity biological anodes can obtain more accurate BOD test values.

Microbial fuel cell: A green eco-friendly agent for tannery wastewater treatment and simultaneous bioelectricity/power generation

This review paper emphasised on the origin of hexavalent chromium toxicity in tannery wastewater and its remediation using novel Microbial Fuel Cell (MFC) technology, including electroactive bacteria, which are known as exoelectrogens, to simultaneously treat wastewater and its action in the production of bioenergy and the mechanism of Cr⁶⁺ reduction. Also, there are various parameters like electrode, pH, mode of operation, time of operation, and type of exchange membrane used for promising results shown in enhancing MFC production and remediation of Cr⁶⁺. Destructive anthropological activities, such as leather making and electroplating industries are key sources of hexavalent chromium contamination in aquatic repositories. When Cr⁶⁺ enters the food chain and enters the human body, it has the potential to cause cancer. MFC is a green innovation that generates energy economically through the reduction of toxic Cr⁶⁺ to less toxic Cr³⁺. The organic substrates utilized at the anode of MFC act as electrons (e^-) donors. This review also highlighted the utilization of cheap substrates to make MFCs more economically suitable and the energy production at minimum cost.

Recent progress in microbial fuel cells using substrates from diverse sources

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

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

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

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

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

Enhancement of nitrogen removal and energy recovery from low C/N ratio sewage by multi-electrode electrochemical technology and tidal flow via siphon aeration

In view of the difficulty in denitrification of low C/N ratio wastewater, electrochemical technology with multiple electrodes and tidal flow method via siphon aeration were used to enhance the denitrification process. At the same time, because of the low phosphorus removal efficiency in traditional activated sludge process, the constructed wetland and microbial fuel cell (CW-MFC) reactor with dewatered alum sludge (DAS) as substrate were constructed. In addition, the REDOX conditions of the reactor were changed by siphon, which significantly improved the removal efficiency of N and P and the energy recovery capacity of the reactor. In the 172 d, the Tidal Flow Constructed Wetland-Microbial Fuel Cell (TF CW-MFC) had the highest removal efficiency of COD and total nitrogen (TN), which were 97.4% and 83.4%, respectively. Although the removal rate of total phosphorus (TP) by TF CW-MFC was lower than artificial aeration, it can still reached 89.0%. The removal effect of aromatic protein substances in water was also significant. The amount of electrons generated by the artificial aeration anode and the amount of oxygen generated by the cathode were not enough to match. The voltage of TF CW-MFC was significantly higher than artificial aeration, around 350 mV, and the maximum power density was 98.16 mW m^-3. In addition, MFC had an inhibitory effect on CW methane emissions. The analysis of the microbial community structure showed that most of the dominant bacteria of TF CW-MFC belonged to the Proteobacteria, Actinobacteria and Chloroflexi. These results showed that the TF CW-MFC technology as a zero-energy oxygen supply mode had high efficiency in the treatment of low C/N ratio wastewater and also had the environmental effect of reducing methane emissions. This study suggests that this green wastewater treatment technology has potential application value.

Microbial Electrochemically Assisted Treatment Wetlands: Current Flow Density as a Performance Indicator in Real-Scale Systems in Mediterranean and Northern European Locations

A METland is an innovative treatment wetland (TW) that relies on the stimulation of electroactive bacteria (EAB) to enhance the degradation of pollutants. The METland is designed in a short-circuit mode (in the absence of an external circuit) using an electroconductive bed capable of accepting electrons from the microbial metabolism of pollutants. Although METlands are proven to be highly efficient in removing organic pollutants, the study of in situ EAB activity in full-scale systems is a challenge due to the absence of a two-electrode configuration. For the first time, four independent full-scale METland systems were tested for the removal of organic pollutants and nutrients, establishing a correlation with the electroactive response generated by the presence of EAB. The removal efficiency of the systems was enhanced by plants and mixed oxic-anoxic conditions, with an average removal of 56 g of chemical oxygen demand (COD) mbed material -3 day-1 and 2 g of total nitrogen (TN) mbed material -3 day-1 for Ørby 2 (partially saturated system). The estimated electron current density (J) provides evidence of the presence of EAB and its relationship with the removal of organic matter. The tested METland systems reached the max. values of 188.14 mA m-2 (planted system; IMDEA 1), 223.84 mA m-2 (non-planted system; IMDEA 2), 125.96 mA m-2 (full saturated system; Ørby 1), and 123.01 mA m-2 (partially saturated system; Ørby 2). These electron flow values were remarkable for systems that were not designed for energy harvesting and unequivocally show how electrons circulate even in the absence of a two-electrode system. The relation between organic load rate (OLR) at the inlet and coulombic efficiency (CE; %) showed a decreasing trend, with values ranging from 8.8 to 53% (OLR from 2.0 to 16.4 g COD m-2 day-1) for IMDEA systems and from 0.8 to 2.5% (OLR from 41.9 to 45.6 g COD m-2 day-1) for Ørby systems. This pattern denotes that the treatment of complex mixtures such as real wastewater with high and variable OLR should not necessarily result in high CE values. METland technology was validated as an innovative and efficient solution for treating wastewater for decentralized locations.

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

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

Sustainable strategy on microbial fuel cell to treat the wastewater for the production of green energy

Microbial fuel cell (MFC) is one of the promising alternative energy systems where the catalytic conversion of chemical energy into electrical energy takes places with the help of microorganisms. The basic configuration of MFC consists of three major components such as electrodes (anode and cathode), catalyst (microorganism) and proton transport/exchange membrane (PEM). MFC classified into four types based on the substrate utilized for the catalytic energy conversion process such as Liquid-phase MFC, Solid-phase MFC, Plant-MFC and Algae-MFC. The core performance of MFC is organic substrate oxidation and electron transfer. Microorganisms and electrodes are the key factors that decide the efficiency of MFC system for electricity generation. Microorganism catalysis degradation of organic matters and assist the electron transfer to anode surface, the conductivity of anode material decides the rate of electron transport to cathode through external circuit where electrons are reduced with hydrogen and form water with oxygen. Not limited to electricity generation, MFC also has diverse applications in different sectors including wastewater treatment, biofuel (biohydrogen) production and used as biosensor for detection of biological oxygen demand (BOD) of wastewater and different contaminants concentration in water. This review explains different types of MFC systems and their core performance towards energy conversion and waste management. Also provides an insight on different factors that significantly affect the MFC performance and different aspects of application of MFC systems in various sectors. The challenges of MFC system design, operations and implementation in pilot scale level and the direction for future research are also described in the present review.

Development of a Bio-Digital Interface Powered by Microbial Fuel Cells

This paper reports the first relatable bio-digital interface powered by microbial fuel cells (MFCs) that was developed to inform the public and introduce the concept of using live microbes as waste processors within our homes and cities. An innovative design for the MFC and peripherals system was built as a digital data generator and bioreactor, with a custom-built energy-harvesting controller that was connected to the system to enable efficient system operation using adaptive dynamic cell reconfiguration and transmit data for the bio-digital interface. This system has accomplished multiple (parallel) tasks such as electricity generation, wastewater treatment and autonomous operation. Moreover, the controller demonstrated that microbial behaviour and consequent system operation can benefit from smart algorithms. In addition to these technical achievements, the bio-digital interface is a site for the production of digital art that aims to gain acceptance from a wider-interest community and potential audiences by showcasing the capabilities of living microorganisms in the context of green technologies.

From Waste to Watts: Updates on Key Applications of Microbial Fuel Cells in Wastewater Treatment and Energy Production

Due to fossil fuel depletion and the rapid growth of industry, it is critical to develop environmentally friendly and long-term alternative energy technologies. Microbial fuel cells (MFCs) are a powerful platform for extracting energy from various sources and converting it to electricity. As no intermediate steps are required to harness the electricity from the organic substrate’s stored chemical energy, MFC technology offers a sustainable alternative source of energy production. The generation of electricity from the organic substances contained in waste using MFC technology could provide a cost-effective solution to the issue of environmental pollution and energy shortages in the near future. Thus, technical advancements in bioelectricity production from wastewater are becoming commercially viable. Due to practical limitations, and although promising prospects have been reported in recent investigations, MFCs are incapable of upscaling and of high-energy production. In this review paper, intensive research has been conducted on MFCs’ applications in the treatment of wastewater. Several types of waste have been extensively studied, including municipal or domestic waste, industrial waste, brewery wastewater, and urine waste. Furthermore, the applications of MFCs in the removal of nutrients (nitrogen and sulphates) and precious metals from wastewater were also intensively reviewed. As a result, the efficacy of various MFCs in achieving sustainable power generation from wastewater has been critically addressed in this study.

Constructed wetland substrates: A review on development, function mechanisms, and application in contaminants removal

Constructed wetlands (CWs) are economical, efficient, and sustainable wastewater treatment method. Substrates in CWs inextricably link with the other key components and significantly influence the performance and sustainability of CWs. Gradually, CWs have been applied to treat more complex contaminants from different fields, thus has brought forward new demand on substrates for enhancing the performance and sustainability of CWs. Various materials have been used as substrates in CWs, and their individual characteristics and application advantages have been extensively studied in recent years. Therefore, this review summarizes the development, function mechanisms (e.g., filtration, adsorption, electron supply, supporting plant growth and microbial reproduction), categories, and applications of substrates in CWs. The interaction mechanisms of substrates with contaminants/plants/microorganisms are comprehensively described, and the characteristics and advantages of different substrate categories (e.g., Natural mineral materials, chemical products, biomass materials, industrial and municipal by-products, modified functional materials, and novel materials) are critically evaluated. Meanwhile, the influences of substrate layer arrangement and synergism on contaminants removal are firstly systematically reviewed. Furthermore, further research about substrates (e.g., clogging, life cycle assessment/management, internal relationship between components) should be systematically carried out for improving efficiency and sustainability of CWs.

Microbial activity enhancement in constructed wetlands operated as bioelectrochemical systems

Treatment wetlands (TW) operated as bioelectrochemical systems (BES-TW) provide a higher degree of treatment than conventional TW. Yet, the fundamental processes or mechanisms for the envisaged better performance of BES-TW over conventional TW remains poorly understood. This work aimed to determine to which extent microbial activity enhancement could be the reason behind this treatment performance increase. To this purpose, pilot-scale horizontal sub-surface flow BES-TW operated under three different configurations were continuously fed with real urban wastewater. BES-TW were evaluated for COD and ammonia removal efficiency, and two techniques of microbial activity assessment were applied. Configurations, tested in duplicate, were: control TWs without electrodes (C-TW), TWs operated as microbial fuel cells (MFC-TW), and TWs operated as microbial electrolysis cells (MEC-TW). Microbial activity was assessed by measuring the enzymatic activity (EA) (FDA hydrolysis technique) and the aerobic activity (AA) (estimated through respirometry). Results showed that BES-TW outperformed C-TW in terms of both microbial activity enhancement and contaminants removal efficiency, especially in the case of MEC-TW. More precisely, this configuration showed an average improvement of 17%, and 56% in COD removal and EA efficiencies, respectively, compared to C-TW. Regarding AA activity, although MEC-TW seemed to outperform the rest of the configurations, differences were not statistically significant. This work demonstrates that TWs operated as BES increase the overall enzymatic activity of the treatment bed and this, in turn, is the leading cause to a higher degree of treatment performance.

Life cycle and economic assessment of enhanced ecological floating beds applied water purification

The ecological floating beds (EFB) are widely used in water quality restoration because of its low cost, high efficiency, and green characteristics. However, there is a potential impact of the EFB on the environment while water purification is not in progress. In this study, the life cycle assessment (LCA) and life cycle cost (LCC) methods were used to evaluate the overall environment of mixed-fill and biofilm enhanced EFB. The results show that the total environmental impact of the mixed-fill ecological floating beds (MEFB) is greater than that of the biofilm ecological floating beds (BEFB). In the raw material acquisition and operational stages, the environmental impact of the MEFB is smaller than that of the BEFB, while the environmental impact of the MEFB during the construction phase is much greater than that of the BEFB. The environmental impact of the construction stage of the MEFB accounts for 98.3% of the environmental impact of the entire life cycle. The operational stage of the MEFB was eco-friendly with regard to eutrophication potential, photochemical oxidation potential, ozone layer depletion potential, human toxicity potential, freshwater aquatic eco-toxicity potential, and terrestrial eco-toxicity potential environmental impact, and these effects of the operational stage of the MEFB account for 45.5% of the total environmental impact. The impact of the BEFB on the environment during raw material acquisition, construction, and operation accounts for 46.7%, 37.7%, and 15.6%, respectively, of the entire life cycle impact. Both two EFB technologies, the capital cost was the main expenditure with LCC, accounting for 60.4% and 52.9% of the MEFB and BEFB, respectively.

Integration of CW-MFC and anaerobic granular sludge to explore the intensified ammonification-nitrification-denitrification processes for nitrogen removal

The integration of constructed wetland-microbial fuel cell (CW-MFC) and anaerobic granular sludge (AGS) is an important way to promote its ammonification efficiency and decrease the land use scale. This study explored the integration of CW-MFC and AGS for nitrogen removal via the intensified ammonification-nitrification-denitrification processes with initial NH₃-N, NO₃-N, Org-N and total nitrogen (TN) concentrations of 10.5, 13.8, 21.4, and 45.7 mg L^-1 in wastewater. Two reactors with AGS inoculated with a separated area (R1) and directly inoculated into gravel substrate (R2) were designed, respectively. Results showed that chemical oxygen demand (COD) removal efficiency could reach 85% in R1 and 81% in R2, and the conversion of Org-N to NH₃-N and NO₃-N to gaseous nitrogen were 80% and 90%, respectively. Although the conversion efficiency of NH₃-N to NO₂-N/NO₃-N via nitrification process was only 18%, it could reach 45%, 94%, and 98% with the aeration rates of 50-, 100-, and 200-mL min^-1. According to microstructural property and microbial community analyses, the separation gravel substrate and AGS areas in R1 availed for stable particle size of AGS, archaeal diversity, and metabolic activity even with a 1.5 times daily wastewater treatment capacity than that of R2. Overall, although the intensified ammonification-nitrification-denitrification processes for nitrogen removal could be achieved with supplementary aeration, further investigation is still needed to explore other substrate materials and high CW-MFC/AGS volume ratio for intensified nitrification process in CW-MFC associated with AGS.

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.

Microbial Fuel Cell for Energy Production, Nutrient Removal and Recovery from Wastewater: A Review

The world is facing serious threats from the depletion of non-renewable energy resources, freshwater shortages and food scarcity. As the world population grows, the demand for fresh water, energy, and food will increase, and the need for treating and recycling wastewater will rise. In the past decade, wastewater has been recognized as a resource as it primarily consists of water, energy-latent organics and nutrients. Microbial fuel cells (MFC) have attracted considerable attention due to their versatility in their applications in wastewater treatment, power generation, toxic pollutant removal, environmental monitoring sensors, and more. This article provides a review of MFC technologies applied to the removal and/or recovery of nutrients (such as P and N), organics (COD), and bioenergy (as electricity) from various wastewaters. This review aims to provide the current perspective on MFCs, focusing on the recent advancements in the areas of nutrient removal and/or recovery with simultaneous power generation.

Constructed wetlands operated as bioelectrochemical systems for the removal of organic micropollutants

The removal of organic micropollutants (OMPs) has been investigated in constructed wetlands (CWs) operated as bioelectrochemical systems (BES). The operation of CWs as BES (CW-BES), either in the form of microbial fuel cells (MFC) or microbial electrolysis cells (MEC), has only been investigated in recent years. The presented experiment used CW meso-scale systems applying a realistic horizontal flow regime and continuous feeding of real urban wastewater spiked with four OMPs (pharmaceuticals), namely carbamazepine (CBZ), diclofenac (DCF), ibuprofen (IBU) and naproxen (NPX). The study evaluated the removal efficiency of conventional CW systems (CW-control) as well as CW systems operated as closed-circuit MFCs (CW-MFCs) and MECs (CW-MECs). Although a few positive trends were identified for the CW-BES compared to the CW-control (higher average CBZ, DCF and NPX removal by 10-17% in CW-MEC and 5% in CW-MFC), these proved to be not statistically significantly different. Mesoscale experiments with real wastewater could thus not confirm earlier positive effects of CW-BES found under strictly controlled laboratory conditions with synthetic wastewaters.

Floating treatment wetlands integrated with microbial fuel cell for the treatment of urban wastewaters and bioenergy generation

The objective of the present study was to develop a combined system composed of anaerobic biofilter (AF) and floating treatment wetlands (FTW) coupled with microbial fuel cells (MFC) in the buoyant support for treating wastewater from a university campus and generate bioelectricity. The raw wastewater was pumped to a 1450 L tank, operated in batch flow and filled with plastic conduits. The second treatment stage was composed of a 1000 L FTW box with a 200 L plastic drum inside (acting as settler in the entrance) and vegetated with mixed ornamental plants species floating in a polyurethane support fed once a week with 700 L of wastewater. In the plant roots, graphite rods were placed to act as cathodes, while on the bottom of the box 40 graphite sticks inside a plastic hose with a stainless-steel cable acting as the anode chamber. Open circuit voltages were daily measured for 6 weeks, and later as closed circuit with the connection of 1000 Ω resistors. Plant harvestings were conducted, in which biomass production and plant uptake from each of the species were measured. On average, system was efficient in reducing BOD₅ (55.1%), COD (71.4%), turbidity (90.9%) and total coliforms (99.9%), but presented low efficiencies regarding total N (8.4%) and total P (11.4%). Concerning bioenergy generation, voltage peaks and maximum power density were observed on the feeding day, reaching 225 mV and 0.93 mW/m², respectively, and in general decaying over the 7 days. In addition, plant species with larger root development presented higher voltage values than plants with the smaller root systems, possible because of oxygen release. Therefore, the combined system presented potential of treating wastewater and generating energy by integrating FTW and MFC, but further studies should investigate the FTW-MFC combination in order to improve its treatment performance and maximize energy generation.

Advancement in Benthic Microbial Fuel Cells toward Sustainable Bioremediation and Renewable Energy Production

Anthropogenic activities are largely responsible for the vast amounts of pollutants such as polycyclic aromatic hydrocarbons, cyanides, phenols, metal derivatives, sulphides, and other chemicals in wastewater. The excess benzene, toluene and xylene (BTX) can cause severe toxicity to living organisms in wastewater. A novel approach to mitigate this problem is the benthic microbial fuel cell (BMFC) setup to produce renewable energy and bio-remediate wastewater aromatic hydrocarbons. Several mechanisms of electrogens have been utilized for the bioremediation of BTX through BMFCs. In the future, BMFCs may be significant for chemical and petrochemical industry wastewater treatment. The distinct factors are considered to evaluate the performance of BMFCs, such as pollutant removal efficiency, power density, and current density, which are discussed by using operating parameters such as, pH, temperature and internal resistance. To further upgrade the BMFC technology, this review summarizes prototype electrode materials, the bioremediation of BTX, and their applications.

Removal of sulfamethoxazole and tetracycline in constructed wetlands integrated with microbial fuel cells influenced by influent and operational conditions

Constructed wetlands integrated with microbial fuel cells (MFC-CWs) have been recently developed and tested for removing antibiotics. However, the effects of carbon source availability, electron transfer flux and cathode conditions on antibiotics removal in MFC-CWs through co-metabolism remained unclear. In this study, four experiments were conducted in MFC-CW microcosms to investigate the influence of carbon source species and concentrations, external resistance and aeration duration on sulfamethoxazole (SMX) and tetracycline (TC) removal and bioelectricity generation performance. MFC-CWs supplied with glucose as carbon source outperformed other carbon sources, and moderate influent glucose concentration (200 mg L^-1) resulted in the best removal of both SMX and TC. Highest removal percentages of SMX (99.4%) and TC (97.8%) were obtained in MFC-CWs with the external resistance of 700 Ω compared to other external resistance treatments. SMX and TC removal percentages in MFC-CWs were improved by 4.98% and 4.34%, respectively, by increasing the aeration duration to 12 h compared to no aeration. For bioelectricity generation performance, glucose outperformed sodium acetate, sucrose and starch, with the highest voltages of 386 ± 20 mV, maximum power density (MPD) of 123.43 mW m^-3, and coulombic efficiency (CE) of 0.273%. Increasing carbon source concentrations from 100 to 400 mg L^-1, significantly (p < 0.05) increased the voltage and MPD, but decreased the internal resistance and CE. The highest MPD was obtained when the external resistance (700 Ω) was close to the internal resistance (600.11 Ω). Aeration not only improved the voltage and MPD, but also reduced the internal resistance. This study demonstrates that carbon source species and concentrations, external resistances and aeration duration, all play vital roles in regulating SMX and TC removal in MFC-CWs.

The influential role of external electrical load in microbial fuel cells and related improvement strategies: A review

The scope of the currentreviewis to discuss and evaluate the role of the external electrical load/resistor (EEL) on the overall behavior and functional properties of microbial fuel cells (MFCs). In this work, a comprehensive analysis is made by considering various levels of MFC architecture, such as electric and energy harvesting efficiency, anode electrode potential shifts, electro-active biofilm formation, cell metabolism and extracellular electron transfer mechanisms, as a function of the EEL and its control strategies. It is outlined that taking the regulation of EEL into account at MFC optimization is highly beneficial, and in order to support this step, in this review, a variety of guidelines are collected and analyzed.

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.

Performance of mediator-less double chamber microbial fuel cell-based biosensor for measuring biological chemical oxygen

Recently, the microbial fuel cell-based biosensor has been considered as an attractive technology for measuring wastewater quality such as biochemical oxygen demand (BOD). In this study, a mediator-less double compartment MFC based biosensor utilizing carbon felt as an anode electrode and inoculated with mixed culture was developed to improve the real application of a rapid BOD detection. This study aims to: (i) establish the effect of the operating conditions (i.e., pH, external resistance, fuel feeding rate) on MFC performance; (ii) investigate the correlation between biochemical oxygen demand (BOD) and signal output, and (iii) evaluate the operational stability of the biosensor. The presented result reveals that the maximum current and power production was obtained while 100 mM NaCl and 50 mM Phosphate buffer saline was used as a catholyte solution, neutral pH condition of media and fuel feeding rate at 0.3 mL min^-1. Notably, a wider range of BOD concentration up to 300 mg L ^-1 can be obtained with the voltage output (R² > 0.9901). Stable and steady power was produced by running MFC in 30 days when cells operated at 1000 Ω external resistance. Our research has some competition with the previous double chamber MFC in the upper limit of BOD detection. This results might help to increase the real application of MFC based BOD biosensor in real-time measurement.

A review on the contribution of electron flow in electroactive wetlands: Electricity generation and enhanced wastewater treatment

In less than a decade, bioelectrochemical systems/microbial fuel cell integrated constructed wetlands (electroactive wetlands) have gained a considerable amount of attention due to enhanced wastewater treatment and electricity generation. The enhancement in treatment has majorly emanated from the electron transfer or flow, particularly in anaerobic regions. However, the chemistry associated with electron transfer is complex to understand in electroactive wetlands. The electroactive wetlands accommodate diverse microbial community in which each microbe set their own potential to further participate in electron transfer. The conductive materials/electrodes in electroactive wetlands also contain some potential, due to which, several conflicts occur between microbes and electrode, and results in inadequate electron transfer or involvement of some other reaction mechanisms. Still, there is a considerable research gap in understanding of electron transfer between electrode-anode and cathode in electroactive wetlands. Additionally, the interaction of microbes with the electrodes and understanding of mass transfer is also essential to further understand the electron recovery. This review mainly deals with the electron transfer mechanism and its role in pollutant removal and electricity generation in electroactive wetlands.

Study of the Effect of Activated Carbon Cathode Configuration on the Performance of a Membrane-Less Microbial Fuel Cell

In this paper, the effect of cathode configuration on the performance of a membrane-less microbial fuel cell (MFC) was evaluated using three different arrangements: an activated carbon bed exposed to air (MFCE), a wetland immersed in an activated carbon bed (MFCW) and a cathode connected to an aeration tower featuring a water recirculation device (MFCT). To evaluate the MFC performance, the efficiency of the organic matter removal, the generated voltage, the power density and the internal resistance of the systems were properly assessed. The experimental results showed that while the COD removal efficiency was in all cases over 60% (after 40 days), the MFCT arrangement showed the best performance since the average removal value was 82%, compared to close to 70% for MFCE and MFCW. Statistical analysis of the COD removal efficiency confirmed that the performance of MCFT is substantially better than that of MFCE and MFCW. In regard to the other parameters surveyed, no significant influence of the different cathode arrangements explored could be found.

Numerical Models of Subsurface Flow Constructed Wetlands: Review and Future Development

Numerical model as a simulation tool was used to describe the pollutants transformation and degradation process in constructed wetlands (CWs). It can help provide insight into the “black box” and increase the understanding of the complex processes in CWs. In the last few decades, several process-based numerical models were developed to depict the pollutants removal processes in CWs, which include biochemical model, hydraulic model, reactive-transport model, plants model, clogging model, and coupling model combining two or more sub-models. However, there was a long way to go before fully understanding the decontamination mechanisms of CWs. On the one hand, single or a composite model coupling a small number of sub-models cannot fully reveal the decontamination processes. On the other hand, a comprehensive model including all sub-models of current cognition involves numerous parameters, most of which are interaction and cannot quantitatively determined, thus making the model complex and leading to diffuse interaction. Therefore, in order to describe the reaction processes in CWs more accurately, it is expected that all parameters should be quantified as far as possible in the future model. This study aims to provide a review of the numerical models of CWs and to reveal mechanism of decontamination. Based on the advantages and disadvantages of existing models, the study presented the improvement method and future research direction: (1) new detection/monitoring technique or computing method to quantitatively assess the parameters in CWs models, (2) correcting the simulation errors caused by the assumption of Activated Sludge Models (ASMs) and developing a complete biofilm reaction sub-model, (3) simplification of the comprehensive model, and (4) need of emerging pollutants modeling.

Characteristics of microbial eukaryotic community recovery in eutrophic water by using ecological floating beds

Ecological floating beds can rapidly remove nutrients (nitrogen and phosphorus) from eutrophic water, but we still know little about whether this process can simultaneously recover microbial eukaryotic communities. To fill this gap, planktonic microbial eukaryotic communities were investigated using 18S rRNA high-throughput gene sequencing during nutrient removal by floating beds of Canna indica L. We found that nutrient concentrations were high in both the control and treatment groups during period 1 (days 0-5) but rapidly decreased in the treatment group during period 2 (days 6-9) and period 3 (days 10-18). However, the microbial eukaryotic species richness and community compositions were similar between the control and treatment groups during periods 1 and 2 but showed small differences during period 3. The microbial eukaryotic co-occurrence networks between the control and treatment groups also showed similar degree centrality and interconnected eukaryotic members. We found that some abundant fungi species significantly responded to nutrient variations, but a large number of abundant ciliates were insensitive to nutrient removal. Our findings suggest that ecological floating beds can rapidly remove nutrients in eutrophic waters but that it is difficult to quickly and simultaneously improve microbial eukaryotic communities. This result reveals the critical influence of nutrient pollution on aquatic ecosystems and therefore on long-term and comprehensive aquatic habitat restoration, as aquatic macrophyte recoveries should be conducted after nutrient controls have been implemented.

Toward Optimization of Wood Industry Wastewater Treatment in Microbial Fuel Cells—Mixed Wastewaters Approach

Microbial fuel cell (MFC) has the potential to become a promising sustainable technology of wastewater treatment. Usually, the investigations on MFCs are aimed at maximized power production in the system. In this article, we focused on the optimization of wood industry wastewater treatment in MFC, in combination with municipal wastewater as a source of microorganisms. We investigated the influence of different external resistance (2000 Ω, 1000 Ω, 500 Ω, and 100 Ω) on power density and wastewater treatment efficiency (chemical oxygen demand (COD) removal) in 1-month MFC operation time. We found that the highest COD removal was for MFCs under R = 1000 Ω after 22 days of MFC operation, while the highest current density was obtained for the lowest applied resistance. The results imply that wastewater treatment parameters such as resistance and time of MFC operation should be a subject of optimization for each specific type of wastewater used, in order to maximize either wastewater treatment efficiency or power production in MFC. Thus, optimization of power production and COD removal efficiency in MFCs need to be run separately as different resistances are required for maximizing these two parameters. When COD removal efficiency is a subject of optimization, there is no universal value of external resistance, but it should be set to the specific wastewater characteristics.

Electrode dependent anaerobic ammonium oxidation in microbial fuel cell integrated hybrid constructed wetlands: A new process

This study provides a new approach of electrode dependent anaerobic ammonium oxidation (electroanammox) in microbial fuel cell (MFC) integrated hybrid constructed wetlands (CWs). The study was carried out in three CWs, each with a horizontal flow (HF) followed by a vertical upflow (VUF). Two of the CWs were integrated with MFC, one was operated in closed circuit (CL) mode and the other in open circuit (OP) mode to determine the influence of electron transfer through an external electrical circuit. The initial nitrogen and carbon concentration were 40 mg/l and 880 mg/l respectively. The total nitrogen (TN), NH₄⁺-N, TOC and COD removal achieved in CW-MFC-CL were 90.0 ± 1.15%, 94.4 ± 0.75%, 64.8 ± 3.0% and up to 99.5 ± 3.4%, respectively. The TN and NH₄⁺-N removal in CW-MFC-CL was 20.0% and 13.6% higher than normal CW. Maximum current density achieved in CW-MFC-HF was of 75 mA/m³ and in CW-MFC-VUF was 156 mA/m³. Furthermore, the study revealed that even at low microbiological biomass, an MFC integrated CW operating in closed circuit gave higher removal of NH₄⁺-N and COD than the normal CW and open circuit CW-MFC. Microbiological analysis shows the presence of already known nitrifier and denitrifer which indicates their role in electrode dependent nitrogen removal.

The configuration, purification effect and mechanism of intensified constructed wetland for wastewater treatment from the aspect of nitrogen removal: A review

Constructed wetland (CW) for wastewater treatment has attracted increasing attention. In this review, the system configuration optimization, purification effect and general mechanisms of nitrogen removal in CW are systematically summarized and discussed. Ammonia oxidation is a crucial and primary process for total nitrogen (TN) removal in domestic or livestock wastewater treatment. Aeration, waterdrop influent and tidal operation are three main methods to strengthen the oxygen supplement and nitrification process in CW. Aeration significantly increases the ammonia removal rate (almost 100%), followed by the removal of chemical oxygen demand (COD) and TN. Solid carbon source, iron and anode material can be filled as electron donor for the denitrification process. The co-adjustment of oxygen and carbon/electron donor can form different conditions for different nitrogen removal pathways (e.g. the simultaneous nitrification-denitrification, the partial nitrification-denitrification and the anammox process), and achieve the optimal removal of nitrogen.

Increasing of electricity production from Echinodosus cordifolius-microbial fuel cell by inoculating Bacillus thuringiensis

The wetland-microbial fuel cell (MFC) is a novel electricity generating technology. However, these systems can generate only limited electric energy. Since nitrification is a key mechanism driving electrical power in wetland-MFC systems, an effective nitrifying bacteria, Bacillus thuringiensis, was used to inoculate a wetland-MFC to enhance the maximum power density of the system. B. thuringiensis effectively enhanced the maximum power density, producing about 20-35 mW m^-2 of maximum power density. Interestingly, over the first 120 days of operation, the wetland-MFC system with only B. thuringiensis generated more power than a system containing an Echinodosus cordifolius plant in addition to B. thuringiensis, because E. cordifolius can took up nitrate (NO₃^-) and phosphate (PO₄^3-) in system's solution. Nitrate and PO₄^3- act as important anions driving electric current in the system. After 120 days of operation though, the combined E. cordifolius and B. thuringiensis system maintained 20-35 mW m^-2 maximum power density and the maximum power density of the system only inoculated with B. thuringiensis decreased continuously. Gene (16S rRNA) copy numbers for B. thuringiensis showed that when E. cordifolius was presented, the bacterium was able to continue growing after 120 days of operation. B. thuringiensis did not grow as well after 120 days in the system that did not contain a plant. This study presents a strategy for enhancing electric power output from a wetland-MFC by inoculating the system with B. thuringiensis and maintaining the bacterium's population with the support of an E. cordifolius plant. The result clearly show that B. thuringiensis can enhance electric power generation in the presence of the plant and the system can self-sustain for longer than 180 days of operation while producing 20-35 mW m^-2 maximum power density.

Wetlands for wastewater treatment

This article presents an update on the research and practical demonstration of wetland treatment technologies for wastewater treatment. Applications of wetlands in wastewater treatment (as an advanced treatment unit or a decentralized system) and stormwater management or treatment for nutrient and pollutant removal (metals, industrial and emerging pollutants including pharmaceutical compounds and pathogens) are highlighted. A summary of studies involving the effects of vegetation, wetland design and operation, and configurations for efficient treatment of various municipal and industrial wastewaters is also included. PRACTITIONER POINTS: Provides an update on current research and development of wetland technologies for wastewater treatment. Effects of vegetation, pathogens removal, heavy metals and emerging pollutants removal are included. Wetland design and operation is a key factor to improve water quality of wetland effluent.

Electricity production enhancement in a constructed wetland-microbial fuel cell system for treating saline wastewater

The use of constructed wetlands in combination with microbial fuel cells (CW-MFC) to treat saline wastewater may enhance electricity production by increasing the ionic strength, reducing internal resistance and stimulating microbes to accelerate electron transfer. In this study, salinity did not significantly inhibit the removal of TP and COD, but TN and NH₄⁺-N removal efficiencies during saline wastewater treatment (ST) were significantly lower than during non-saline wastewater treatment (NT). However, salinity significantly increased the power density (16.4 mW m^-2 in ST and 3.9 mW m^-2 in NT, a 4-fold enhancement) by increasing the electron transfer rate and reducing internal resistance (140.29 Ω in ST and 415.21 Ω in NT). The peptides in extracellular polymeric substances (EPS) acted as electron shuttles to promote the migration of electrons and protons in ST. From start-up to stable operation, though the microorganisms in ST were reduced in diversity relative to NT, the proportion of electrochemically active bacteria (EAB), such as Ochrobactrum, significantly increased (p < 0.05) and gradually predominated in the microbial community.

Eco-electrogenic treatment of dyestuff wastewater using constructed wetland-microbial fuel cell system with an evaluation of electrode-enriched microbial community structures

This work studied eco-electrogenic treatment of real dyestuff wastewater along with characterization of electrode-enriched microbial community structures in Fimbristylis dichotoma planted closed-circuit constructed wetland-microbial fuel cell (CW-MFC) system. The CW-MFC-2 (experimental system) achieved 82.2 ± 1.7% ADMI removal and 70 ± 2% COD reduction; that were found to be 9% and 7.4% higher than the standalone constructed wetland (CW) system (bioremediation control) respectively. Likewise, the CW-MFC-2 system achieved maximum power density of 198.8 mW/m², which was 85.6 ± 2.47% higher than the CW-MFC-1 system (eco-electricity control). Quantitative reverse transcription PCR (qRT-PCR) assays revealed significant down-regulation of hepatic oxidative stress response biomarker genes in Oreochromis niloticus exposed to CW-MFC-2 system treated dyestuff wastewater as compared with untreated wastewater. The biofilms associated with the anode and cathode of the CW-MFC-2 system exhibited selective enrichment of electrochemically active and dye degrading microbial communities.

Real-Time Monitoring of Micro-Electricity Generation Through the Voltage Across a Storage Capacitor Charged by a Simple Microbial Fuel Cell Reactor with Fast Fourier Transform

The pattern of micro-electricity production of simple two-chamber microbial fuel cells (MFC) was monitored in this study. Piggery wastewater and anaerobic sludge served as fuel and inocula for the MFC, respectively. The output power, including voltage and current generation, of triplicate MFCs was measured using an on-line monitoring system. The maximum voltage obtained among the triplicates was 0.663 V. We also found that removal efficiency of chemical oxygen demand (COD) and biochemical oxygen demand (BOD) in the piggery wastewater was 94.99 and 98.63%, respectively. Moreover, analytical results of Fast Fourier Transform (FFT) demonstrated that the output current comprised alternating current (AC) and direct current (DC) components, ranging from mA to μA.

Contaminants removal and bacterial activity enhancement along the flow path of constructed wetland microbial fuel cells

Microbial fuel cells implemented in constructed wetlands (CW-MFCs), albeit a relatively new technology still under study, have shown to improve treatment efficiency of urban wastewater. So far the vast majority of CW-MFC systems investigated were designed as lab-scale systems working under rather unrealistic hydraulic conditions using synthetic wastewater. The main objective of this work was to quantify CW-MFCs performance operated under different conditions in a more realistic setup using meso-scale systems with horizontal flow fed with real urban wastewater. Operational conditions tested were organic loading rate (4.9 ± 1.6, 6.7 ± 1.4 and 13.6 ± 3.2 g COD/m²·day) and hydraulic regime (continuous vs. intermittent feeding) as well as different electrical connections: CW control (conventional CW without electrodes), open-circuit CW-MFC (external circuit between anode and cathode not connected) and closed-circuit CW-MFC (external circuit connected). Eight horizontal subsurface flow CWs were operated for about four months. Each wetland consisted of a PVC reservoir of 0.193 m² filled with 4/8 mm granitic riverine gravel (wetted depth 25 cm). All wetlands had intermediate sampling points for gravel and interstitial liquid sampling. The CW-MFCs were designed as three MFCs incorporated one after the other along the flow path of the CWs. Anodes consisted of gravel with an incorporated current collector (stainless steel mesh) and the cathode consisted of a graphite felt layer. Electrodes of closed-circuit CW-MFC systems were connected externally over a 220 Ω resistance. Results showed no significant differences between tested organic loading rates, hydraulic regimes or electrical connections, however, on average, systems operated in closed-circuit CW-MFC mode under continuous flow outperformed the other experimental conditions. Closed-circuit CW-MFC compared to conventional CW control systems showed around 5% and 22% higher COD and ammonium removal, respectively. Correspondingly, overall bacteria activity, as measured by the fluorescein diacetate technique, was higher (4% to 34%) in closed-circuit systems when compared to CW control systems.

Effect of climate change on humic substances and associated impacts on the quality of surface water and groundwater: A review

Humic substances (HS), a highly transformed part of non-living natural organic matter (NOM), comprise up to 70% of the soil organic matter (SOM), 50-80% of dissolved organic matter (DOM) in surface water, and 25% of DOM in groundwater. They considerably contribute to climate change (CC) by generating greenhouse gases (GHG). On the other hand, CC affects HS, their structure and reactivity. HS important role in global warming has been recognized and extensively studied. However, much less attention has been paid so far to effects on the freshwater quality, which may result from the climate induced impact on HS, and HS interactions with contaminants in soil, surface water and groundwater. It is expected that an increased temperature and enhanced biodegradation of SOM will lead to an increase in the production of DOM, while the flooding and runoff will export it from soil to rivers, lakes, and groundwater. Microbial growth will be stimulated and biodegradation of pollutants in water can be enhanced. However, there may be also negative effects, including an inhibition of solar disinfection in brown lakes. The CC induced desorption from soil and sediments, as well as re-mobilization of metals and organic pollutants are anticipated. In-situ treatment of surface water and groundwater may be affected. Quality of the source freshwater is expected to deteriorate and drinking water production may become more expensive. Many of the possible effects of CC described in this article have yet to be explored and understood. Enormous potential for interesting, multidisciplinary studies in the important research areas has been presented.

Challenges in the application of microbial fuel cells to wastewater treatment and energy production: A mini review

Wastewater is now considered to be a vital reusable source of water reuse and saving energy. However, current wastewater has multiple limitations such as high energy costs, large quantities of residuals being generated and lacking in potential resources. Recently, great attention has been paid to microbial fuel cells (MFCs) due to their mild operating conditions where a variety of biodegradable substrates can serve as fuel. MFCs can be used in wastewater treatment facilities to break down organic matter, and they have also been analysed for application as a biosensor such as a sensor for biological oxygen which demands monitoring. MFCs represent an innovation technology solution that is simple and rapid. Despite the advantages of this technology, there are still practical barriers to consider including low electricity production, current instability, high internal resistance and costly materials used. Thus, many problems must be overcome and doing this requires a more detailed analysis of energy production, consumption, and application. Currently, real-world applications of MFCs are limited due to their low power density level of only several thousand mW/m². Efforts are being made to improve the performance and reduce the construction and operating costs of MFCs. This paper explores several aspects of MFCs such as anode, cathode and membrane, and in an effort to overcome the practical challenges of this system.