Microbial Electrochemical Technologies for Wastewater Treatment: Principles and Evolution from Microbial Fuel Cells to Bioelectrochemical-Based Constructed Wetlands

Innovative application of basalt fibers as biological carrier in constructed wetland-microbial fuel cell for improvement of performance under perfluorooctanoic acid exposure

Basalt fiber (BF) was filled in constructed wetland-microbial fuel cell (CW-MFC) as bio-carrier for enhancement of operation performance under perfluorooctanoic acid (PFOA) exposure. In this study, although PFOA caused significant decline of ammonium removal by 7.5-7.7 %, slight promotion on nitrogen and phosphorus removal was observed with BF filling, compared to control. PFOA removal also increased by 1.7-3.4 % in BF filling group. Besides, improved electrochemical performance was discovered with BF filling, in which the highest power density increased by 86.6 % than control, even under PFOA stress. Enhanced stability and performance of CW-MFC resulted from stimulation of functional bacteria on electrodes like Dechloromonas, Thauera, Zoogloea, Gemmobacter, and Pseudomonas, which were further enriched on BF carrier. Higher abundance of nitrogen metabolism and related genes on electrodes and BF carrier was also discovered with BF filling. This study offered new findings on application of BF in CW-MFC systems with PFOA exposure.

Urban wastewater oxidation by bioelectrochemical systems: To what extent does the inoculum matter?

Bioelectrochemical systems offer an environmental-friendly alternative to activated sludge for future wastewater treatment but have not yet reached technological maturity. This study aims to assess the long-term influence of the inoculation strategy on real urban wastewater treatment by bioelectrochemical systems, focusing on both process performances and biofilm assembly dynamics. Four inoculation strategies were investigated in triplicates during six consecutive batches to treat primary clarifier effluent at lab scale. At the studied anodic potential (0.05 vs SHE), no long-term impact of the inoculation strategy on the performances was observed. Indeed, after three batches, electrochemical (88.0 ± 3.9 % coulombic efficiencies) and treatment performances (30.8 ± 3.9 % COD removals) converged for all inoculation strategies. Consistently, the microbial compositions of the different biofilms converged, with selection being the main assembly process. For larger scale bioelectrochemical reactors, the use of wastewater as both substrate and inoculum would be the most convenient choice, since the other inoculation strategies only displayed short-term effects.

Advances from conventional to biochar enhanced biotreatment of dyeing wastewater: A critical review

DW (Dyeing wastewater) contains a large amount of dye organic compounds. A considerable proportion of dye itself or its intermediate products generated during wastewater treatment process exhibits CMR (Carcinogenic/Mutagenic/Toxic to Reproduction) toxicity. Compared with physicochemical methods, biological treatment is advantageous in terms of operating costs and greenhouse gas emissions, and has become the indispensable mainstream technology for DW treatment. This article reviews the adsorption and degradation mechanisms of dye organic compounds in wastewater and analyzed different biological processes, ranging from traditional methods to processes enhanced by biochar (BC). For traditional biological processes, microbial characteristics and communities were discussed, as well as the removal efficiency of different bioreactors. BC has adsorption and redox electron mediated effects, and coupling with biological treatment can further enhance the process of biosorption and degradation. Although BC coupled biological treatment shows promising dye removal, further research is still needed to optimize the treatment process, especially in terms of technical and economic competitiveness.

Unraveling the contemporary use of microbial fuel cell in pesticide degradation and simultaneous electricity generation: a review

Pesticide is an inevitable substance used worldwide to kill pests, but their indiscriminate use has posed serious complications to health and the environment. Various physical, chemical, and biological methods are employed for pesticide treatment, but this paper deals with microbial fuel cell (MFC) as a futuristic technology for pesticide degradation with electricity production. In MFC, organic compounds are utilized as the carbon source for electricity production and the generation of electrons which can be replaced with pollutants such as dyes, antibiotics, and pesticides as carbon sources. However, MFC is been widely studied for a decade for electricity production, but its implementation in pesticide degradation is less known. We fill this void by depicting a real picture of the global pesticide scenario with an eagle eye view of the bioremediation techniques implemented for pesticide treatment with phytoremediation and rhizoremediation as effective techniques for efficient pesticide removal. The enormous literature survey has revealed that not many researchers have ventured into this new arena of MFC employed for pesticide degradation. Based on the Scopus database, an increase in annual trend from 2014 to 2023 is observed for MFC-implemented pesticide remediation. However, a novel MFC to date for effective remediation of pesticides with simultaneous electricity generation is discussed for the first time. Furthermore, the limitation of MFC technology and the implementation of MFC and rhizoremediation as a clubbed system which is the least applied can be seen as promising and futuristic approaches to enhance pesticide degradation by bacteria and electricity as a by-product.

Energy generation from bioelectrochemical techniques: Concepts, reactor configurations and modeling approaches

The process industries play a significant role in boosting the economy of any nation. However, poor management in several industries has been posing worrisome threats to an environment that was previously immaculate. As a result, the untreated waste and wastewater discarded by many industries contain abundant organic matter and other toxic chemicals. It is more likely that they disrupt the proper functioning of the water bodies by perturbing the sustenance of many species of flora and fauna occupying the different trophic levels. The simultaneous threats to human health and the environment, as well as the global energy problem, have encouraged a number of nations to work on the development of renewable energy sources. Hence, bioelectrochemical systems (BESs) have attracted the attention of several stakeholders throughout the world on many counts. The bioelectricity generated from BESs has been recognized as a clean fuel. Besides, this technology has advantages such as the direct conversion of substrate to electricity, and efficient operation at ambient and even low temperatures. An overview of the BESs, its important operating parameters, bioremediation of industrial waste and wastewaters, biodegradation kinetics, and artificial neural network (ANN) modeling to describe substrate removal/elimination and energy production of the BESs are discussed. When considering the potential for use in the industrial sector, certain technical issues of BES design and the principal microorganisms/biocatalysts involved in the degradation of waste are also highlighted in this review.

Methane emission reduction oriented extracellular electron transfer and bioremediation of sediment microbial fuel cell: A review

Sediment is the internal and external source of water environment pollution, so sediment remediation is the premise of water body purification. Sediment microbial fuel cell (SMFC) can remove the organic pollutants in sediment by electroactive microorganisms, compete with methanogens for electrons, and realize resource recycling, methane emission inhibiting and energy recovering. Due to these characteristics, SMFC have attracted wide attention for sediment remediation. In this paper, we comprehensively summarized the recent advances of SMFC in the following areas: (1) The advantages and disadvantages of current applied sediment remediation technologies; (2) The basic principles and influencing factors of SMFC; (3) The application of SMFC for pollutant removal, phosphorus transformation and remote monitoring and power supply; (4) Enhancement strategies for SMFC in sediments remediation such as SMFC coupled with constructed wetland, aquatic plant and iron-based reaction. Finally, we have summarized the drawback of SMFC and discuss the future development directions of applying SMFC for sediment bioremediation.

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.

Role of bio-electrochemical technology for enzyme activity stimulation in high-consumption pharmaceuticals biodegradation

Active pharmaceutical ingredients (APIs) and their intermediate residues have recently been considered a serious concern. Among technologies, bio-electrochemical technologies (BETs) have stimulated the production of bio-electrical energy. This review aims to examine the benefit and mechanism of BETs on the degradation of high-consumption pharmaceutical compounds, including antibiotic, anti-inflammatory, and analgesic drugs, and the stimulation of enzymes induced in a bioreactor. Moreover, intermediates and the proposed pathways of pharmaceutical compound biodegradation in BETs are to be explained in this review. According to studies performed exclusively, the benefit of BETs is using bio-electroactive microbes to mineralize recalcitrant pharmaceutical contaminants by promoting enzyme activity and energy. Since BETs use the electron transfer chain between bio-anode/-cathode and pharmaceuticals, the enzyme activity is essential in the oxidation and reduction of phenolic rings of drugs and the ineffective detoxification of effluent from the treatment plant. This study is suggested a vital and influential role of BETs in mineralizing and enzyme induction in bioreactors. Eventually, a content of future developments or outlooks of BETs are propounded to improve the pharmaceutical industries' wastewater problems.

Are integrated bioelectrochemical technologies feasible for wastewater management?

The need for sustainable technological solutions for wastewater management at different scales has led to the emergence of several promising integrated bioelectrochemical technologies in the past decade. A thorough assessment of these technologies is imperative to understand their practical implementation feasibility and to identify the key challenges to prioritise the research and development work. Our multicriteria-based assessment reveals that the integrated technologies are efficient for wastewater treatment in terms of normalised land footprint [(0.31-1.39 m²/population equivalent (PE))] - and energy consumption (0.18-1.49 kWH/m³) as compared to the conventional biotechnologies, and suggests that they have potential for real-world application. Specifying the boundaries according to their treatment capabilities and scale-up potential besides niche application sites or geographical locations is required to expedite their transition to the real-world wastewater management sector.

Improving the performance of bioelectrochemical sulfate removal by applying flow mode

Treatment of wastewater contaminated with high sulfate concentrations is an environmental imperative lacking a sustainable and environmental friendly technological solution. Microbial electrochemical technology (MET) represents a promising approach for sulfate reduction. In MET, a cathode is introduced as inexhaustible electron source for promoting sulfate reduction via direct or mediated electron transfer. So far, this is mainly studied in batch mode representing straightforward and easy-to-use systems, but their practical implementation seems unlikely, as treatment capacities are limited. Here, we investigated bioelectrochemical sulfate reduction in flow mode and achieved removal efficiencies (E_sulfate , 89.2 ± 0.4%) being comparable to batch experiments, while sulfate removal rates (R_sulfate , 3.1 ± 0.2 mmol L^-1 ) and Coulombic efficiencies (CE, 85.2 ± 17.7%) were significantly increased. Different temperatures and hydraulic retention times (HRT) were applied and the best performance was achieved at HRT 3.5 days and 30°C. Microbial community analysis based on amplicon sequencing demonstrated that sulfate reduction was mainly performed by prokaryotes belonging to the genera Desulfomicrobium, Desulfovibrio, and Desulfococcus, indicating that hydrogenotrophic and heterotrophic sulfate reduction occurred by utilizing cathodically produced H₂ or acetate produced by homoacetogens (Acetobacterium). The advantage of flow operation for bioelectrochemical sulfate reduction is likely based on higher absolute biomass, stable pH, and selection of sulfate reducers with a higher sulfide tolerance, and improved ratio between sulfate-reducing prokaryotes and homoacetogens.

Bacterial community structure of electrogenic biofilm developed on modified graphite anode in microbial fuel cell

Formation of electrogenic microbial biofilm on the electrode is critical for harvesting electrical power from wastewater in microbial biofuel cells (MFCs). Although the knowledge of bacterial community structures in the biofilm is vital for the rational design of MFC electrodes, an in-depth study on the subject is still awaiting. Herein, we attempt to address this issue by creating electrogenic biofilm on modified graphite anodes assembled in an air-cathode MFC. The modification was performed with reduced graphene oxide (rGO), polyaniline (PANI), and carbon nanotube (CNTs) separately. To accelerate the growth of the biofilm, soybean-potato composite (plant) powder was blended with these conductive materials during the fabrication of the anodes. The MFC fabricated with PANI-based anode delivered the current density of 324.2 mA cm-2, followed by CNTs (248.75 mA cm-2), rGO (193 mA cm-2), and blank (without coating) (151 mA cm-2) graphite electrodes. Likewise, the PANI-based anode supported a robust biofilm growth containing maximum bacterial cell densities with diverse shapes and sizes of the cells and broad metabolic functionality. The alpha diversity of the biofilm developed over the anode coated with PANI was the loftiest operational taxonomic unit (2058 OUT) and Shannon index (7.56), as disclosed from the high-throughput 16S rRNA sequence analysis. Further, within these taxonomic units, exoelectrogenic phyla comprising Proteobacteria, Firmicutes, and Bacteroidetes were maximum with their corresponding level (%) 45.5, 36.2, and 9.8. The relative abundance of Gammaproteobacteria, Clostridia, and Bacilli at the class level, while Pseudomonas, Clostridium, Enterococcus, and Bifidobacterium at the genus level were comparatively higher in the PANI-based anode.

Electrochemical Methods for Water Purification, Ion Separations, and Energy Conversion

Agricultural development, extensive industrialization, and rapid growth of the global population have inadvertently been accompanied by environmental pollution. Water pollution is exacerbated by the decreasing ability of traditional treatment methods to comply with tightening environmental standards. This review provides a comprehensive description of the principles and applications of electrochemical methods for water purification, ion separations, and energy conversion. Electrochemical methods have attractive features such as compact size, chemical selectivity, broad applicability, and reduced generation of secondary waste. Perhaps the greatest advantage of electrochemical methods, however, is that they remove contaminants directly from the water, while other technologies extract the water from the contaminants, which enables efficient removal of trace pollutants. The review begins with an overview of conventional electrochemical methods, which drive chemical or physical transformations via Faradaic reactions at electrodes, and proceeds to a detailed examination of the two primary mechanisms by which contaminants are separated in nondestructive electrochemical processes, namely electrokinetics and electrosorption. In these sections, special attention is given to emerging methods, such as shock electrodialysis and Faradaic electrosorption. Given the importance of generating clean, renewable energy, which may sometimes be combined with water purification, the review also discusses inverse methods of electrochemical energy conversion based on reverse electrosorption, electrowetting, and electrokinetic phenomena. The review concludes with a discussion of technology comparisons, remaining challenges, and potential innovations for the field such as process intensification and technoeconomic optimization.

Microbiomics for enhancing electron transfer in an electrochemical system

In microbial electrochemical systems, microorganisms catalyze chemical reactions converting chemical energy present in organic and inorganic molecules into electrical energy. The concept of microbial electrochemistry has been gaining tremendous attention for the past two decades, mainly due to its numerous applications. This technology offers a wide range of applications in areas such as the environment, industries, and sensors. The biocatalysts governing the reactions could be cell secretion, cell component, or a whole cell. The electroactive bacteria can interact with insoluble materials such as electrodes for exchanging electrons through colonization and biofilm formation. Though biofilm formation is one of the major modes for extracellular electron transfer with the electrode, there are other few mechanisms through which the process can occur. Apart from biofilm formation electron exchange can take place through flavins, cytochromes, cell surface appendages, and other metabolites. The present article targets the various mechanisms of electron exchange for microbiome-induced electron transfer activity, proteins, and secretory molecules involved in the electron transfer. This review also focuses on various proteomics and genetics strategies implemented and developed to enhance the exo-electron transfer process in electroactive bacteria. Recent progress and reports on synthetic biology and genetic engineering in exploring the direct and indirect electron transfer phenomenon have also been emphasized.

Advancements in Bioelectricity Generation Through Nanomaterial-Modified Anode Electrodes in Microbial Fuel Cells

The use of nanotechnology in bioelectrochemical systems to recover bioelectricity and metals from waste appears to be a potentially appealing alternative to existing established procedures. This trend exactly characterizes the current renewable energy production technology. Hence, this review focuses on the improvement of the anode electrode by using different functional metal oxide-conducting polymer nanocomposites to enhance microbial fuel cell (MFC) performance. Enhancement of interfacial bioelectrocatalysis between electroactive microorganisms and hierarchical porous nanocomposite materials could enhance cost-effective bioanode materials with superior bioelectrocatalytic activity for MFCs. In this review, improvement in efficiency of MFCs by using iron oxide- and manganese oxide-based polypyrrole hybrid composites as model anode modifiers was discussed. The review also extended to discussing and covering the principles, components, power density, current density, and removal efficiencies of biofuel cell systems. In addition, this research review demonstrates the application of MFCs for renewable energy generation, wastewater treatment, and metal recovery. This is due to having their own unique working principle under mild conditions and using renewable biodegradable organic matter as a direct fuel source.

A Short Overview of Biological Fuel Cells

This short review summarizes the improvements on biological fuel cells (BioFCs) with or without ionomer separation membrane. After a general introduction about the main challenges of modern energy management, BioFCs are presented including microbial fuel cells (MFCs) and enzymatic fuel cells (EFCs). The benefits of BioFCs include the capability to derive energy from waste-water and organic matter, the possibility to use bacteria or enzymes to replace expensive catalysts such as platinum, the high selectivity of the electrode reactions that allow working with less complicated systems, without the need for high purification, and the lower environmental impact. In comparison with classical FCs and given their lower electrochemical performances, BioFCs have, up to now, only found niche applications with low power needs, but they could become a green solution in the perspective of sustainable development and the circular economy. Ion exchange membranes for utilization in BioFCs are discussed in the final section of the review: they include perfluorinated proton exchange membranes but also aromatic polymers grafted with proton or anion exchange groups.

Bioelectrochemical systems-based metal recovery: Resource, conservation and recycling of metallic industrial effluents

Metals represent a large proportion of industrial effluents, which due to their high hazardous nature and toxicity are responsible to create environmental pollution that can pose significant threat to the global flora and fauna. Strict ecological rules compromise sustainable recovery of metals from industrial effluents by replacing unsustainable and energy-consuming physical and chemical techniques. Innovative technologies based on the bioelectrochemical systems (BES) are a rapidly developing research field with proven encouraging outcomes for many industrial commodities, considering the worthy options for recovering metals from industrial effluents. BES technology platform has redox capabilities with small energy-intensive processes. The positive stigma of BES in metals recovery is addressed in this review by demonstrating the significance of BES over the current physical and chemical techniques. The mechanisms of action of BES towards metal recovery have been postulated with the schematic representation. Operational limitations in BES-based metal recovery such as biocathode and metal toxicity are deeply discussed based on the available literature results. Eventually, a progressive inspection towards a BES-based metal recovery platform with possibilities of integration with other modern technologies is foreseen to meet the real-time challenges of viable industrial commercialization.

Performance evaluation of the integrated hydroponics-microbial electrochemical technology (iHydroMET) for decentralized domestic wastewater treatment

Here we report on the performance of the integrated drip hydroponics-microbial electrochemical technology (iHydroMET) for decentralized management of domestic wastewater at the household level. The study focused on optimizing the iHydroMET reactor components, followed by its performance evaluation for domestic wastewater treatment at different feed volumes. Based on the reactor components optimization work, granular activated charcoal:cocopeat (20:80) combination for bed matrix, 75% immersed cathode in effluent configuration, and Catharanthus roseus plant were selected for further experiments. The iHydroMET system with the optimized reactor components achieved efficient removal of organic matter (up to 93%) and turbidity (up to 98%) but minimal total nitrogen (<24%) and total phosphorous (<8%) removal after 24 h with 10 L of feed volume. It also removed the contaminants of emerging concern, such as sterols, at >95% efficiency. The UV-treated effluent (<2 MPN/100 mL) with considerable concentrations of N (∼34 mg/L) and P (∼5 mg/L) nutrients qualifies the standards for agricultural use and landscape irrigation purposes and contribute to lowering the burden on freshwater usage. The system also produced a power density of 30.3 mW/m². Cultivation of evergreen C. roseus, a high aesthetic value ornamental and medicinal plant, further adds to ecological and environmental benefits of the iHydroMET technology. Further modifications in system operation like creating a saturation zone in the reactor units might improve the electric output and result in sufficient removal of nutrients, making the use of effluent for flushing and other purposes possible in households.

Influence of applied potential on treatment performance and clogging behaviour of hybrid constructed wetland-microbial electrochemical technologies

A two-stage hybrid Constructed Wetland (CW) integrated with a microbial fuel cell (MFC), and microbial electrolysis cell (MEC) has been assessed for treatment performance and clogging assessment and further compared with CW. The CW-MEC was operated with applied potential to the working electrode and compared with the performance of naturally adapted redox potential of the CW-MFC system. A complex synthetic municipal wastewater was used during the study, which was composed of trace metals, organics, inorganics, and dye. The study demonstrated that providing a constant potential to the working electrode in CW-MEC has resulted in high treatment performance and reduced sludge generation. The maximum chemical oxygen demand (COD), ammonium (NH₄⁺), and phosphate (PO₄^3-) removal achieved during treatment by CW-MEC at 24 h hydraulic retention time was 89 ± 6%, 72 ± 6% and 93 ± 2%, respectively. ICP-MS results indicated that trace metal removals were also higher in CW-MEC than in CW alone (p < 0.05). At the end of the experiment, significant volumetric change (total volume of the microcosm) occurred in CW (1.3 L), which indicates high sludge generation, whereas it was lesser in CW-MEC (0.3 L) and in CW-MFC (0.5 L). Further, Energy Dispersive X-ray (EDX) spectroscopy results indicated low levels of metal precipitation in the CW-MEC system. Based on the Shannon diversity index, the CW-MEC was assessed to be characterised by high species richness and diversity. The observations from this study indicate that the applied potential at the working electrode has a significant impact on treatment performance and clogging behaviour of the system.

Enabling Water Reuse by Treatment of Reverse Osmosis Concentrate: The Promise of Constructed Wetlands

As more cities experience water stress, the use of reverse osmosis (RO) membranes for wastewater treatment and reuse will expand. The concentrated waste stream resulting from RO treatment can pose chronic ecotoxicity risks if discharged to surface waters or shallow coastal ecosystems. Most existing RO concentrate treatment technologies are cost prohibitive, but constructed wetlands hold promise as a viable multibenefit solution because they have the potential to provide simultaneous treatment of nutrients, metals, and trace organic contaminants at a relatively low cost. They also are popular with the public. A handful of water-stressed cities have already begun experimenting with constructed wetlands for RO concentrate treatment. However, further research is needed to reduce the land area needed for treatment and increase the reliability of constructed wetland systems.

Interrelation between sulphur and conductive materials and its impact on ammonium and organic pollutants removal in electroactive wetlands

This investigation is the first of its kind to evaluate the interrelation of sulphate (SO₄^2-) with conductive materials as well as their individual and synergetic effects on the removal of ammonium and organic pollutants in electroactive wetlands, also known as constructed wetland (CW) - microbial fuel cell (MFC). The role of MFC components in CW was investigated to treat the sulphate containing wastewater under a long-term operation without any toxicity build-up in the system. A comparative study was also performed between CW-MFC and CW, where sulphate containing wastewater (S-replete) and without sulphate wastewater (S-deplete) was assessed. The S-replete showed high NH₄⁺ removal than the S-deplete, and the requesnce of removal was: CW-MFC-replete>CW-MFC-deplete>CW-replete>CW-deplete. The chemical oxygen demand (COD) removal was high in the case of CW-MFC-replete, and the sequence of removal was CW-MFC-replete>CW-MFC-deplete>CW-deplete>CW-replete. X-ray photon spectroscopic study indicates 0.84% sulphur accumulation in CW-MFC-replete and 2.49% in CW-replete, indicating high sulphur precipitation in CW without the MFC component. The high relative abundance of class Deltaproteobacteria (7.3%) in CW-MFC-replete along with increased microbial diversity (Shannon index=3.5) rationalise the symbiosis of sulphate reducing/oxidising microbes and its impact on the treatment performance and electrochemical activity.

Identifying potential inhibitors of biofilm-antagonistic proteins to promote biofilm formation: a virtual screening and molecular dynamics simulations approach

Microbial biofilms play a critical role in environmental biotechnology and associated applications. Biofilm production can be enhanced by inhibiting the function of proteins that negatively regulate their formation. With this objective, an in silico approach was adopted to identify competitive inhibitors of eight biofilm-antagonistic proteins, namely AbrB and SinR (from Bacillus subtilis) and AmrZ, PDE (EAL), PslG, RetS, ShrA and TpbA (from Pseudomonas aeruginosa). Fifteen inhibitors that structurally resembled the natural ligand of each protein were shortlisted using ligand-based and structure-based virtual screening. The top four inhibitors obtained from molecular docking using Autodock Vina were further docked using SwissDock and DOCK 6.9 to obtain a consensus hit for each protein based on different scoring functions. Further analysis of the protein-ligand complexes revealed that these top inhibitors formed significant non-covalent interactions with their respective protein binding sites. The eight protein-ligand complexes were then subjected to molecular dynamics simulations for 30 ns using GROMACS. RMSD and radius of gyration values of 0.1-0.4 nm and 1.0-3.5 nm, respectively, along with hydrogen bond formation throughout the trajectory indicated that all the complexes remained stable, compact and intact during the simulation period. Binding energy values between -20 and -77 kJ/mol obtained from MM-PBSA calculations further confirmed the high affinities of the eight inhibitors for their respective receptors. The outcome of this study holds great promise to enhance biofilms that are central to biotechnological processes associated with microbial electrochemical technologies, wastewater treatment, bioremediation and the industrial production of value-added products.

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.

Maximizing Benefits to Nature and Society in Techno-Ecological Innovation for Water

Nature-based solutions (NbS) build upon the proven contribution of well-managed and diverse ecosystems to enhance resilience of human societies. They include alternatives to techno-industrial solutions that aim to enhance social-ecological integration by providing simultaneous benefits to nature (such as biodiversity protection and green/blue space) and society (such as ecosystem services and climate resiliency). Yet, many NbS exhibit aspects of a technological or engineered ecosystem integrated into nature; this techno-ecological coupling has not been widely considered. In this work, our aim is to investigate this coupling through a high-level and cross-disciplinary analysis of NbS for water security (quantity, quality, and/or water-related risk) across the spectrums of naturalness, biota scale, and benefits to nature and society. Within the limitations of our conceptual analysis, we highlight the clear gap between “nature” and “nature-based” for most NbS. We present a preliminary framework for advancing innovation efforts in NbS towards maximizing benefits to both nature and society, and offer examples in biophysical innovation and innovation to maximize techno-ecological synergies (TES).

Sustainable, Decentralized Sanitation and Reuse with Hybrid Nature-Based Systems

Nature (ecosystem) based processes for wastewater treatment include constructed wetlands (CWs), waste stabilization ponds, vegetated drainage ditches, buffer zones, instream or bankside river techniques, and mixotrophic systems, where light and CO2 are utilized, in addition to organic carbon compounds, by algal cultures. Algae-based systems can simultaneously remove organic matter, N, and P and may offer substantial energetic advantages compared to traditional biological treatment systems, require small spatial footprint, and contribute to biofuels production and CO2 emissions mitigation. Bioelectrochemical systems (BES) such as microbial fuel cells (MFCs) present characteristics compatible with the use in isolated realities for water and wastewater treatment with contextual energy recovery and may be combined with other nature-based process technologies to achieve good treatment and energy efficiencies. Despite that their application in real-scale plants has not been assessed yet, the most probable outcome will be the in situ/on site treatment (or pretreatment) of wastes for small “in house” plants not connected to the sewerage network. This paper focuses on the current practices and perspectives of hybrid nature-based systems, such as constructed wetlands and microalgae integrated phytoremediation plants, and their possible integration with microbial electrochemical technologies to increase recovery possibilities from wastes and positively contribute to a green economy approach.

Multi-Criteria Evaluation and Sensitivity Analysis for the Optimal Location of Constructed Wetlands (METland) at Oceanic and Mediterranean Areas

METland is a new variety of Constructed Wetland (CW) for treating wastewater where gravel is replaced by a biocompatible electroconductive material to stimulate the metabolism of electroactive bacteria. The system requires a remarkably low land footprint (0.4 m2/pe) compared to conventional CW, due to the high pollutant removal rate exhibited by such microorganisms. In order to predict the optimal locations for METland, a methodology based on Multi-Criteria Evaluation (MCE) techniques applied to Geographical Information Systems (GIS) has been proposed. Seven criteria were evaluated and weighted in the context of Analytical Hierarchy Process (AHP). Finally, a Global Sensitivity Analysis (GSA) was performed using the Sobol method for resource optimization. The model was tested in two locations, oceanic and Mediterranean, to prove its feasibility in different geographical, demographic and climate conditions. The GSA revealed as conclusion the most influential factors in the model: (i) land use, (ii) distance to population centers, and (iii) distance to river beds. Interestingly, the model could predict best suitable locations by reducing the number of analyzed factors to just such three key factors (responsible for 78% of the output variance). The proposed methodology will help decision-making stakeholders in implementing nature-based solutions, including constructed wetlands, for treating wastewater in rural areas.

Assessment of Voltage Influence in Carbon Dioxide Fixation Process by a Photo-Bioelectrochemical System under Photoheterotrophy

Bioelectrochemical systems are a promising technology capable of reducing CO₂ emissions, a renewable carbon source, using electroactive microorganisms for this purpose. Purple Phototrophic Bacteria (PPB) use their versatile metabolism to uptake external electrons from an electrode to fix CO₂. In this work, the effect of the voltage (from −0.2 to −0.8 V vs. Ag/AgCl) on the metabolic CO₂ fixation of a mixed culture of PPB under photoheterotrophic conditions during the oxidation of a biodegradable carbon source is demonstrated. The minimum voltage to fix CO₂ was between −0.2 and −0.4 V. The Calvin–Benson–Bassham (CBB) cycle is the main electron sink at these voltages. However, lower voltages caused the decrease in the current intensity, reaching a minimum at −0.8 V (−4.75 mA). There was also a significant relationship between the soluble carbon uptake in terms of chemical oxygen demand and the electron consumption for the experiments performed at −0.6 and −0.8 V. These results indicate that the CBB cycle is not the only electron sink and some photoheterotrophic metabolic pathways are also being affected under electrochemical conditions. This behavior has not been tested before in photoheterotrophic conditions and paves the way for the future development of photobioelectrochemical systems under heterotrophic conditions.

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

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

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

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

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

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

Novel bioelectrochemical strategies for domesticating the electron flow in constructed wetlands

Constructed wetlands are an effective biofilter-based technology for treating wastewater in a sustainable way; however, their main disadvantage is a large area footprint. To cope with this limitation a new generation of constructed wetlands, the METlands®, have been recently reported. METlands® replace gravel with a granular electrically conductive material to enhance the oxidative metabolisms of electroactive bacteria by facilitating the flux of electron through the material and, consequently, increase bioremediation rates. In this work we evaluated the performance of a new electron sink (e-sink) device with the purpose of controlling and enhancing the electrochemical consumption of electrons from microbial metabolism without energy consumption. The e-sink device was integrated inside the biofilter bed and was tested using different electron acceptors with high redox potentials, like oxygen and hypochlorite. Interestingly, the presence of the e-sink allowed novel redox gradients to form inside the METland® and, consequently, a new electron flow was demonstrated by measuring both the electric potential and current density profiles of the bed. Three independent biofilters were constructed and operated under flooded conditions. Ec-coke and electroconductive biochar (ec-biochar) were used as electrically conductive bed materials, while gravel was used as an inert control. Furthermore, e-sink integration inside the electrically conductive bed outperformed METlands® for removing pollutants, already much more efficient than standard gravel biofilters. COD removal was increased from 90% in METland® to 95% in the e-sink METland® as compared to 75% for the control, while total nitrogen removal was enhanced from 64% in METland® to 71% in e-sink METland® as compared to 55% for the control. Our results indicate that increasing the electrochemical availability of electron acceptors by using the e-sink will be a suitable method for controlling the electron flow inside the filter bed and can be integrated in full scale METlands® for achieving high removal rates.

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.

Community level physiological profiling of microbial electrochemical-based constructed wetlands

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

Enhanced chromium(VI) treatment in electroactive constructed wetlands: Influence of conductive material

A constructed wetland (CW) microcosm based on conductive graphite gravel was investigated for hexavalent chromium (Cr(VI)) treatment from synthetic wastewater. Its performance was evaluated and compared with a traditional gravel-based CW microcosm. The microcosms were operated at varying initial Cr(VI) concentrations (5-20 mg/L) and hydraulic retention times (HRT) (3-7.5 h). Near complete treatment (99.9 ± 0.06 %) was achieved in the graphite-based microcosm throughout the experiment. The performance was consistently high throughout with 42.9 % improvement in Cr (VI) treatment compared to a traditional gravel microcosm. Scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDX) analysis indicated that chromium was adsorbed to microbial biofilms. Moreover, microbial diversity profiling suggested that the microbial population in both microcosms differed in diversity and communities. The results suggest that the use of conductive materials in CW significantly enhances the treatment of Cr(VI) and more importantly, allows microbial activity even at high levels of Cr(VI) in the CW.

Microbial electrochemistry for bioremediation

Lack of suitable electron donors or acceptors is in many cases the key reason for pollutants to persist in the environment. Externally supplementation of electron donors or acceptors is often difficult to control and/or involves chemical additions with limited lifespan, residue formation or other adverse side effects. Microbial electrochemistry has evolved very fast in the past years - this field relates to the study of electrochemical interactions between microorganisms and solid-state electron donors or acceptors. Current can be supplied in such so-called bioelectrochemical systems (BESs) at low voltage to provide or extract electrons in a very precise manner. A plethora of metabolisms can be linked to electrical current now, from metals reductions to denitrification and dechlorination. In this perspective, we provide an overview of the emerging applications of BES and derived technologies towards the bioremediation field and outline how this approach can be game changing.

Exoelectrogenic Anaerobic Granular Sludge for Simultaneous Electricity Generation and Wastewater Treatment

A thick and electroactive biofilm is the key to the successful development of microbial electrochemical systems and technologies (METs). In this study, intact anaerobic granular sludge (AGS), which is a spherical and dense microbial association, was successfully demonstrated as a novel and efficient biocatalyst in METs such as microbial fuel cells. Three different strategies were explored to shift the microbial composition of AGS from methanogenic to exoelectrogenic microbes, including varying the external resistance and organic loading and manipulating the anode potential. Among all the strategies, only with positive anode potential, AGS was successfully shifted from methanogenic to exoelectrogenic conditions, as indicated by the significantly high current response (10.32 A/m²) and 100% removal of organic carbon from wastewater. Moreover, the AGS bioanode showed no significant decrease in current generation and organic removal at pH 5, indicating good tolerance of AGS to acidic conditions. Finally, 16S rRNA sequencing revealed the enrichment of exoelectrogens and inhibition of methanogens in the microbial community of AGS after anode potential control. This study provides a proof of concept for extracting electrical energy from organic wastes by exoelectrogenic AGS along with simultaneous wastewater treatment and meanwhile opens up a new paradigm to create an efficient and cost-effective exoelectrogenic biocatalyst for boosting the industrial application of METs.

A 3D porous NCNT sponge anode modified with chitosan and Polyaniline for high-performance microbial fuel cell

A microbial fuel cell (MFC) is a potential bio-electrochemical technology that utilizes microorganisms to convert chemical energy into electrical energy. The low power output of MFCs remain the bottleneck for their practical applications. In this paper, a novel, biocompatible and bioelectrocatalytic composite chitosan-nitrogen doped carbon nanotubes-polyaniline (CS-NCNT-PANI) was prepared in situ on the 3D porous NCNT/sponge and applied to an MFC anode. The PANI was grafted on the CS-NCNT backbone to synthesize the ternary composite. This bioanode not only increased the active surface area and capacity but also facilitated bacterial adhesion and enrichment of microbes. Compared with the NCNT/sponge electrode, the charge transfer impedance of the ternary composite bioanode decreased from 14.07 Ω to 2.25 Ω, and the maximum power density increased from 1.4 W·m^-3 to 4.2 W·m^-3; meanwhile, during the chronoamperometric experiment with a charge-discharge time of 60-60 min, the cumulative charge of the composite bioanode was 18,865.8 C·m^-2, which is much higher than that of the NCNT/S anode (3625.3 C·m^-2). High-throughput sequencing technology revealed that the ternary composite bioanode had good biocompatibility and high diversity. Therefore, this synthesized ternary composite is a promising candidate as a capacitive and biocompatible anode material in MFC.

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

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