Improved azo dye decolorization in an advanced integrated system of bioelectrochemical module with surrounding electrode deployment and anaerobic sludge reactor

Putting the electro-bugs to work: A systematic review of 22 years of advances in bio-electrochemical systems and the parameters governing their performance

Wastewater treatment using bioelectrochemical systems (BESs) can be considered as a technology finding application in versatile areas such as for renewable energy production and simultaneous reducing environmental problems, biosensors, and bioelectrosynthesis. This review paper reports and critically discusses the challenges, and advances in bio-electrochemical studies in the 21st century. To sum and critically analyze the strides of the last 20+ years on the topic, this study first provides a comprehensive analysis on the structure, performance, and application of BESs, which include Microbial Fuel Cells (MFCs), Microbial Electrolysis Cells (MECs) and Microbial Desalination Cells (MDCs). We focus on the effect of various parameters, such as electroactive microbial community structure, electrode material, configuration of bioreactors, anode unit volume, membrane type, initial COD, co-substrates and the nature of the input wastewater in treatment process and the amount of energy and fuel production, with the purpose of showcasing the modes of operation as a guide for future studies. The results of this review show that the BES have great potential in reducing environmental pollution, purifying saltwater, and producing energy and fuel. At a larger scale, it aspires to facilitate the path of achieving sustainable development and practical application of BES in real-world scenarios.

Microbial fuel cells a state-of-the-art technology for wastewater treatment and bioelectricity generation

Wastewater treatment and electricity generation have been the major concerns for the last few years. The scarcity of fossil fuels has led to the development of unconventional energy resources that are pollution-free. Microbial fuel cell (MFC) is an environmental and eco-friendly technology that harvests energy through the oxidation of organic substrates and transform into the electric current with the aid of microorganisms as catalysts. This review presents power output and colour removal values by designing various configurations of MFCs and highlights the importance of materials for the fabrication of anode and cathode electrodes playing vital roles in the formation of biofilm and redox reactions taking place in both chambers. The electron transfer mechanism from microbes towards the electrode surface and the generation of electric current are also highlighted. The effect of various parameters affecting the cell performance such as type and amount of substrate, pH and temperature maintained within the chambers have also been discussed. Although this technology presents many advantages, it still needs to be used in combination with other processes to enhance power output.

Bioelectrochemical systems with a cathode of stainless-steel electrode for treatment of refractory wastewater: Influence of electrode material on system performance and microbial community

The large-scale application of the bioelectrochemical system (BES) is limited by the cost-effective electrode materials. In this study, five kinds of stainless-steel materials were used as the cathode of the BES coupled with anaerobic digestion (BES-AD) for the treatment of diluted N, N-dimethylacetamide (DMAC) wastewater. Compared with a carbon-cloth cathode, BES-AD with a stainless-steel cathode had more engineering due to its low cost, although the operating efficiencies were slightly inferior. Stainless-steel mesh with a 100 µm aperture (SSM-100 μm) was the most cost-effective electrode and the implanted BES exhibited better COD removal efficiency, electrochemical performance and biodegradability. Analysis of microbial community revealed the synergetic effect between exoelectrogen and fermentative bacteria had been strengthened in the SSM-100 μm cathode biofilm. Function analysis of the microbial community based on PICRUSt predicted metagenomes revealed that the metabolic pathways of xenobiotics biodegradation and metabolism in the SSM-100 μm cathode were stimulated.

Direct micro-electric stimulation alters phenanthrene-degrading metabolic activities of Pseudomonas sp. strain DGYH-12 in modified bioelectrochemical system

Bioelectrochemical systems (BESs) have great potential for treating wastewater containing polycyclic aromatic hydrocarbons (PAHs); however, detailed data on cell physiological activities in PAH biodegradation pathways stimulated by BESs are still lacking. In this paper, a novel BES device was assembled to promote the growth of Pseudomonas sp. DGYH-12 in phenanthrene (PHE) degradation. The results showed that in the micro-electric field (0.2 V), cell growth rate and PHE degradation efficiency were 22% and 27.2% higher than biological control without electric stimulation (BC), respectively. The extracellular polymeric substance (EPS) concentration in BES (39.38 mg L^-1) was higher than control (33.36 mg L^-1); moreover, the membrane permeability and ATPase activities were also enhanced and there existing phthalic acid and salicylic acid metabolic pathways in the strain. The degradation genes nahAc, pcaH, and xylE expression levels were upregulated by micro-electric stimulation. This is the first study to analyze the physiological and metabolic effect of micro-electric stimulation on a PHE-degrading strain in detail and systematically.

Deciphering the Anode-Enhanced Azo Dye Degradation in Anaerobic Baffled Reactors Integrating With Microbial Fuel Cells

Microbial anode respiration in microbial fuel cells (MFCs) can enhance the degradations of many electron acceptor-type contaminants which are presumed to be competitive to anode respiration. The mechanisms underlying those counterintuitive processes are important for MFCs application but are unclear. This study integrated MFCs with anaerobic baffled reactor (ABR), termed MFC-ABR, to enhance the reduction of azo dye acid orange-7 (AO-7). Compare with ABR, MFC-ABR enhanced the degradation of AO-7, especially at high AO-7 concentration (800 mg/L). Acute toxicity test suggested a higher detoxication efficiency in MFC-ABR. Higher microbial viability, dehydrogenase activity and larger sludge granule size were also observed in MFC-ABR. MFC-ABR significantly enriched and reshaped the microbial communities relative to ABR. Bacteria with respiratory versatility, e.g., Pseudomonas, Geobacter, and Shewanella, were significantly enriched. Functional prediction showed that six metabolism functions (manganese-, iron-, fumarate- and nitrate-respiration, oil bioremediation and chemoheterotrophy) were significantly stimulated while methanogenesis, sulfate-respiration, hydrogen-oxidation were suppressed in MFC-ABR relative to ABR. The results provided important information for understanding the role of microbial anode respiration in contaminated environments.

Effect of biochar on bio-electrochemical dye degradation and energy production

The effect of coconut shell biochar on dye degradation in a microbial fuel cell (MFC) was investigated in the present study. Two different doses of biochar (0.5 g and 1 g) and one control without bio-char were studied. The highest COD removal efficiency was about 77.7% (0.5 g biochar), maximum current (1.07 mA) and voltage (722 mV) were obtained with 1 g biochar. Biofilm optical microscopy characterization revealed the micro colonies intricate plate-like structures. High adsorbent dosage might provide a high surface area for biofilm to generate electricity. BET results of coconut shell biochar showed the maximum surface area of 0.9669 m²/g and macroporosity (0.0032 cm³/g). The overall results highlighted the possibility of using biochar as an additive in MFC for efficient dye degradation.

Periphyton biofilms: A novel and natural biological system for the effective removal of sulphonated azo dye methyl orange by synergistic mechanism

Due to their large scale use, azo dyes are adversely affecting aquatic fauna and flora as well as humans. The persistent nature of sulphonated azo dyes makes them potential ecotoxic hazards. The aim of the present study was to employ a proficient, locally available biomaterial, viz. periphyton (i.e. epiphyton, epilithon or metaphyton), for removal of the azo dye, methyl orange (MO). Results showed that the periphytic biofilms are capable of completely removing comparatively high concentrations (up to 500 mg L^-1) of MO from wastewater. The removal of MO occurs by a synergistic mechanism involving bioadsorption and biodegradation processes. The adsorption of MO by periphyton can be described by pseudo-second order kinetics. Elovich and intraparticle diffusion models as well as Langmuir equations fit well to the MO adsorption process. FTIR analysis of MO and its metabolites demonstrated biotransformation into simpler compounds within 72 h. GC-MS/MS analysis showed the conversion of MO into simpler compounds such as phenol, ethyl acetate and acetyl acetate. The results indicated that periphyton is a promising biomaterial for the complete removal of MO from wastewater and that the treatment process has the potential for in situ removal of MO at contaminated sites.

Microbial fuel cells as pollutant treatment units: Research updates

Microbial fuel cells (MFC) are a device that can convert chemical energy in influent substances to electricity via biological pathways. Based on the consent that MFC technology should be applied as a waste/wastewater treatment unit rather than a renewable energy source, this mini-review discussed recent R&D efforts on MFC technologies for pollutant treatments and highlighted the challenges and research and development needs. Owing to the low power density levels achievable by larger-scale MFC, the MFC should be used as a device other than energy source such as being a pollutant treatment unit.

Process and kinetics of azo dye decolourization in bioelectrochemical systems: effect of several key factors

This study explored the influence of several key factors on the process and kinetics of azo dye decolourization in bioelectrochemical systems (BESs), including cathode potential, dissolved oxygen (DO) concentration of catholyte and biofilm formed on the cathode. The results show that azo dye methyl orange (MO) decolourization in the BES could be well described with the pseudo first-order kinetics. The MO decolourization efficiency increased from 0 to 94.90 ± 0.01% and correspondingly the reaction rate constant increased from 0 to 0.503 ± 0.001 h⁻¹ with the decrease in cathodic electrode potential from −0.2 to −0.8 V vs Ag/AgCl. On the contrary, DO concentration of the catholyte had a negative impact on MO decolourization in the BES. When DO concentration increased from zero to 5.80 mg L⁻¹, the MO decolourization efficiency decreased from 87.19 ± 4.73% to 27.77 ± 0.06% and correspondingly the reaction rate constant reduced from 0.207 ± 0.042 to 0.033 ± 0.007 h⁻¹. Additionally, the results suggest that the biofilm formed on the cathode could led to an adverse rather than a positive effect on azo dye decolourization in the BES in terms of efficiency and kinetics.

p-Nitrophenol degradation and microbial community structure in a biocathode bioelectrochemical system

The biocathode bioelectrochemical system (bioc-BES) was used forp-nitrophenol (PNP) degradation with sodium bicarbonate as the carbon source.

Optimization of working cathode position in sleeve-type bioelectrochemical system with inner chamber/outer chamber for azo dye treatment

In this study, the optimization of working cathode position in sleeve-type bioelectrochemical system (BES) was evaluated with inner/outer chamber for azo dye decolorization. Results showed that the working position in outer chamber performed better with decolorization efficiencies of 97.8 ± 2.1% (7h) and 94.0 ± 2.3% (16 h) than that in inner chamber as the volume ratio Vcathode:Vanode=1:1 and 3:1, respectively. The current and electrochemical impedance spectroscopy (EIS) analysis indicated that the proton/electron transfer and anolyte diffusion could be improved using outer chamber as working position. The decolorization with increased volume ratio could be further improved through the strategy of increasing substrate concentration, which would provide enough electrons and decrease diffusion resistance, further improving the whole performance with increased outer cathode volume. It has the great potential in sleeve-type configuration application and would create more challenges for process optimization and maintenance.

A horizontal plug-flow baffled bioelectrocatalyzed reactor for the reductive decolorization of Alizarin Yellow R

An application-oriented membrane-free, continuous plug-flow baffled bioelectrocatalyzed reactor (PFB-BER), was designed and testified for the decolorization of Alizarin Yellow R. Decolorization efficiency (DE) with an external power source of 0.5 V was higher than without electrolysis, i.e. 93.4% versus 73.6% (HRT of 24 h). Product formation efficiencies of p-phenylenediamine and 5-aminosalicylic acid were above 95% and 50%, respectively. When HRT decreased to 8 h and 4 h, DE reduced to 69.9% and 44.9%, respectively. An additional electrode assembly improved DE to 96.4% (HRT of 8 h) and 80% (HRT of 4 h), while energy consumption (HRT of 4 h) was lower than that of HRT of 12 h with single electrode assembly under comparable DE. The PFB-BER with higher removal capacity, lower internal resistance and energy consumption provides a new solution to treat the high loading azo dye-containing wastewaters.

Optimized matching modes of bioelectrochemical module and anaerobic sludge in the integrated system for azo dye treatment

In this work, three matching modes (relative positions, catholyte flow sequences, and flow regimes) of bioelectrochemical module and anaerobic sludge were evaluated and optimized for azo dye treatment in the integrated system with embedding modular bioelectrochemical system into anaerobic sludge reactor. Results showed that it was favorable to operate this integrated system under the condition of 1/4 cathode soaking into sludge with spiral distributor in down-flow direction. Current, electrochemical impedance spectroscopy and pH clearly demonstrated the important role of 1/4 soaking in electron/proton transfer. The down-flow direction flowed through electrode zone and then sludge zone could benefit to the efficient use of cathode and improve AO7 treatment. Furthermore, the positive effect of spiral catholyte distributor might be due to its promoting role in mixing and creating a spiral flow channel around the cathode electrode-microbes-solution interface. These results exhibited great potential for matching modular bioelectrochemical system with anaerobic treatment process.

A lab-scale anoxic/oxic-bioelectrochemical reactor for leachate treatments

A membraneless, liter-scale bioelectrochemical reactor with both bioanode and biocathode was established for landfill leachate treatment. Anoxic/oxic (A/O) zones at anode compartment and cathode compartment, respectively, were connected with a reflux to facilitate nitrogen removal. With raw landfill leachate of 17,500-22,600 mg L(-1) chemical oxygen demand (COD) and 1170-1490 mg L(-1) NH4(+)-N, the tested reactor removed 89.1±1.6% of chemical oxygen demand and 99.2±0.1% of NH4(+)-N at 3.0 kg COD m(-3) d(-1). The corresponding maximum power density was 2.71±0.09 W m(-3), with internal resistance of 46.7±1.6 Ω and open circuit voltage of 727±7 mV. The species of Pseudomonas, Desulfovibrio, Bacillus, Enterococcus, Pelospora, Dehalobacter dominated the anodic community, while those of methylotrophs, Rhodobacter, Verrucomicrobiaceae, Geobacter, Flavobacterium, Thauera, Desulfovibrio and Aeromonas dominated the cathodic community. The proposed A/O bioelectrochemical reactor is a prototype for practical treatment of landfill leachate at affordable costs.