Analysis of a bio-electrochemical reactor containing carbon fiber textiles for the anaerobic digestion of tomato plant residues

Improving agricultural sustainability - A review of strategies to valorize tomato plant residues (TPR)

Considerations for the modification of agricultural practices and waste management to improve environmental sustainability remain a subject of great importance. Prioritization of intensive mass food production to meet the demand of an increasing human population has introduced a multitude of environmental issues due to, among other factors, the large volumes of waste output. Tomato production in greenhouses, for example, generates tonnes of bio-waste per hectare each harvest including green tomato plant residues (i.e., stems, leaves, branches). Giving value to these green tomato plant residues collected during the growing cycle and after harvest has not proven straightforward despite a massive yearly release of tonnes of carbon dioxide from stems and leaves disposed on landfills. This paper aims to summarize current research in tomato plant residue valorization and to identify considerations for future valorization strategies. Peer reviewed articles, scientific books and governmental, economic and statistical reports on the topic of tomato plant residues were collected and analyzed. Focuses included traditional valorization approaches, bio-refinement strategies and conversion of fiber-rich residues into high value packaging materials. Initiatives for sustainable agriculture, their market relevance, and the strengths and weaknesses of using tomato plant residues in these valorization approaches are discussed. Overall, it was concluded that valorization of tomato plant residues would be a highly integrative endeavor that would require coordination from multiple levels in the agricultural production chain.

Rice husk-intensified cathode driving bioelectrochemical reactor for remediating nitrate-contaminated groundwater

To achieve economical and eco-friendly denitrification, rice husk-intensified cathode driving bioelectrochemical reactor (RCBER) was constructed with rice husk as solid-phase carbon source and microbial carrier. Results demonstrated that the application of current improved the utilization of rice husk and enhanced the denitrification, and the quenching of anodic hydroxyl radicals by rice husk also improved the microbial resistance to current. The highest nitrate removal rate as 0.34 mg-N/(L∙d), higher economic benefits, i.e., current efficiency as 31.6% and energy consumption as 2.43 kWh/g NO₃^--N, and the highest environmental benefit, i.e., hydrogenotrophic denitrification contribution as 37.9%, were obtained at 200 mA/m². The best performance at 200 mA/m² was related to its better microenvironment, such as lower accumulation of anodic by-products and higher bioavailability of rice husks, as well as higher microbial metabolic activity, such as stable extracellular polymeric substance, the maximum electron transport system activity as 11.63 ± 0.14 μg O₂·g^-1·min^-1·mg protein^-1 and the highest activity of nitrate reductase (3.15-fold that of control check). The application of current realized the coexistence of heterotrophic and hydrogenotrophic denitrifiers, and multiple functional bacteria such as anaerobic denitrifiers Flavobacterium, aerobic denitrifiers Comamonas, hydrogenotrophic denitrifiers Thermomonas and electron transfer-related Enterobacter coexisted at 200 mA/m², thereby improving RCBER's adaptability to the complex microenvironment. This study provides the theoretical basis for realizing a win-win situation of environmental pollution remediation and agricultural waste disposal.

Non-invasive wearable chemical sensors in real-life applications

Over the past decade, non-invasive wearable chemical sensors have gained tremendous attention in the field of personal health monitoring and medical diagnosis. These sensors provide non-invasive, real-time, and continuous monitoring of targeted biomarkers with more simplicity than the conventional diagnostic approaches. This review primarily describes the substrate materials used for sensor fabrication, sample collection and handling, and analytical detection techniques that are utilized to detect biomarkers in different biofluids. Common substrates including paper, textile, and hydrogel for wearable sensor fabrication are discussed. Principles and applications of colorimetric and electrochemical detection in wearable chemical sensors are illustrated. Data transmission systems enabling wireless communication between the sensor and output devices are also discussed. Finally, examples of different designs of wearable chemical sensors including tattoos, garments, and accessories are shown. Successful development of non-invasive wearable chemical sensors will effectively help users to manage their personal health, predict the potential diseases, and eventually improve the overall quality of life.

Enhanced methane production and organic matter removal from tequila vinasses by anaerobic digestion assisted via bioelectrochemical power-to-gas

In this study, showed a strategy to generate methane and remove organic matter removal from tequila vinasses through of anaerobic digestion assisted via bioelectrochemical power to-gas. Specific methanogenic activity (SMA) assays in batch mode were tested and a single-stage bioelectrochemical upflow anaerobic sludge blanket reactor (UASB) was evaluated to generate methane during tequila vinasses treatment. The results showed that the methane production in the bioelectrochemical UASB reactor applied at low voltage of 0.5 V and under HRT of 7 d was higher than the in the conventional UASB reactor. The specific methane production rate in bioelectrochemical UASB reactor was up to 2.9 NL CH₄/L d, with a maximum methane yield of 0.32 NL CH₄/g COD_removed. Similar COD removals were observed in the bioelectrochemical UASB reactor and conventional reactors (92-93%). High carbon dioxide reduction and hydrogen production were observed in the bioelectrochemical UASB reactor.

Towards the practical application of bioelectrochemical anaerobic digestion (BEAD): Insights into electrode materials, reactor configurations, and process designs

Anaerobic digestion (AD) is one of the most widely adopted bioenergy recovery technologies globally. Despite the wide adoption, AD has been challenged by the unstable performances caused by imbalanced substrate and/or electron availability among different reaction steps. Bioelectrochemical anaerobic digestion (BEAD) is a promising concept that has demonstrated potential for balancing the electron transfer rates and enhancing the methane yield in AD during shocks. While great progress has been made, a wide range of, and sometimes inconsistent engineering and technical strategies were attempted to improve BEAD. To consolidate past efforts and guide future development, a comprehensive review of the fundamental bioprocesses in BEAD is provided herein, followed by a critical evaluation of the engineering and technical optimizations attempted thus far. Further, a few novel directions and strategies that can enhance the performance and practicality of BEAD are proposed for future research to consider. This review and outlook aim to provide a fundamental understanding of BEAD and inspire new research ideas in AD and BEAD in a mechanism-informed fashion.

Recyclable magnetite-enhanced electromethanogenesis for biomethane production from wastewater

Improving the yield and methane content of biogas is of great concern for wastewater treatment by anaerobic digestion. Herein we developed a nanomagnetite-enhanced electromethanogenesis (EM_nano) process for the first time, the sustainable utilization of which improved the biomethane production rate from dairy wastewater. The maximum CH₄ production rate in the EM_nano process is 2.3 ± 0.3-fold higher than it is in the conventional methanogenesis (CM) process, and it is accompanied by an almost delay-free start-up. The technical-economic evaluation revealed that an 82.1 ± 5.0% greater net benefit was obtained in the third generation of the EM_nano process compared with the CM process. The improved methanogenesis was attributed to the formation of dense planktonic cell co-aggregates that are triggered by nanomagnetite, which facilitated the interspecies electron exchange during acetoclastic methanogenesis. Simultaneously, a cathode biofilm with high viability and catalytic activity was also formed in the EM_nano process that decreased the biofilm resistance and facilitated the electron transfer during electromethanogenesis. This study is a worthwhile attempt to combine recyclable conductive materials with an electromethanogenesis process for wastewater treatment, and it effectively achieves energy recovery with high stability and cost-effectiveness.

Progress towards catalyzing electro-methanogenesis in anaerobic digestion process: Fundamentals, process optimization, design and scale-up considerations

Electro-methanogenesis represents an emerging bio-methane production pathway that can be achieved through integrating microbial electrolysis cell (MEC) with conventional anaerobic digester (AD). Since 2009, a significant number of publications have reported superior methane productivity and kinetics from MEC-AD integrated systems. The overall objective of this review is to communicate the recent advances towards promoting electro-methanogenesis in the anaerobic digestion process. Firstly, the electro-methanogenesis pathways and functional roles of key microbial members are summarized. Secondly, various extrinsic process parameters, such as applied voltage/potential, pH, and temperature are discussed with emphasis on process optimization. Moreover, available methods for the inoculation and start-up of MEC-AD process are critically reviewed. Finally, system design and scale-up considerations, such as the selection of electrode materials, surface area and surface chemistry of electrode materials, and electrode spacing are summarized.

Contribution of electrodes and electric current to process stability and methane production during the electro-fermentation of food waste

Electro-fermentation is used as an alternative to conventional anaerobic digestion to enhance system stability and methane production from food waste. In particular, the contributions of electrode materials and an electric current are analyzed separately. The results showed that the introduction of electrodes (conductive carbon brushes without applied voltage) rapidly decreased the average concentration of volatile fatty acids (VFAs) from 6617 mg/L to 174 mg/L quickly, accelerated stabilization of digestion system, and improved methane production by 13.5%. When low voltage was supplied, the VFAs concentration declined to 129 mg/L, and methane production increased by 26.3%. Electric current stimulated the growth of hydrogenotrophic methanogens, but acetotrophic Methanosaeta still made up 27.6-61.9% of archaeal community. Geobacter occurred at the cathode with a low abundance. The energy contained in incremental methane was 4.55 times the consumption of electric energy, indicating the enhanced methanogenesis was mainly attributed to the improved digestion environment.

Changes of Bacterial Communities in an Anaerobic Digestion and a Bio-Electrochemical Anaerobic Digestion Reactors According to Organic Load

Bacterial communities change in bulk solution of anaerobic digestion (AD) and bio-electrochemical anaerobic digestion reactors (BEAD) were monitored at each organic loading rate (OLR) to investigate the effect of voltage supply on bacterial species change in bulk solution. Chemical oxygen demand (COD) degradation and methane production from AD and BEAD reactors were also analyzed by gradually increasing food waste OLR. The BEAD reactor maintained stable COD removal and methane production at 6.0 kg/m3·d. The maximum OLR of AD reactor for optimal operation was 4.0 kg/m3·d. pH and alkalinity decline and volatile fatty acid (VFA) accumulation, which are the problem in high load anaerobic digestion of readily decomposable food wastes, were again the major factors destroying the optimal operation condition of the AD reactor at 6.0 kg/m3·d. Contrarily, the electrochemically activated dense communities of exoelectrogenic bacteria and VFA-oxidizing bacteria prevented VFAs from accumulating inside the BEAD reactor. This maintained stable pH and alkalinity conditions, ultimately contributing to stable methane production.

Effects of a novel auxiliary bio-electrochemical reactor on methane production from highly concentrated food waste in an anaerobic digestion reactor

In this study, the effects of indirect voltage supply to an anaerobic digestion (AD) reactor on methane production and the removal of chemical oxygen demand (COD) were studied at different organic loading rates (OLRs) of food waste by the circulation from an auxiliary bio-electrochemical reactor (ABER) with stainless steel (STS304) electrodes. The effects of the indirect voltage on microbial communities in the AD reactor were also investigated. In a bio-electrochemical anaerobic digestion (BEAD) reactor with direct voltage, it was possible to achieve stable COD removal and methane production even at a higher OLR of 10.0 kg/(m³·d). However, in the AD reactor, the COD removal efficiency and methane production decreased sharply at an OLR of 6.0 kg/(m³·d) due to the accumulation of volatile fatty acids (VFAs) and decreases in the pH and alkalinity. The supply of indirect voltage through the ABER increased the community of exoelectrogenic bacteria and hydrogenotrophic methanogens in the AD + ABER bulk solution. As a result, rapid oxidation of the accumulated VFAs occurred, and methane production increased in the new AD + ABER system. The results confirm that an indirect voltage supply to the new AD + ABER system can have effects similar to those of a direct voltage supply to the BEAD reactor, and the findings are expected to provide useful information for the development and application of BEAD technology for commercialization.

Long-term evaluation of methane production in a bio-electrochemical anaerobic digestion reactor according to the organic loading rate

In this study, the effects of differing organic loading rates (OLRs) on methane production were evaluated via long-term operation of a bio-electrochemical anaerobic digestion (BEAD) reactor and an anaerobic digestion (AD) reactor. In the AD reactor, the maximum OLR was 4 kg/m³·d whereas the electro-active microbial community in bulk and on the biofilm of the BEAD reactor produced methane even at 10 kg/m³·d. The BEAD reactor rapidly removed volatile fatty acids (VFAs) and reduced H⁺ to H₂ at high OLRs, thereby preventing VFA accumulation and pH decrease. After the steady state was achieved, both the AD and BEAD reactors exhibited similar organic matter removal and methane production at a low OLR. Thus, a BEAD reactor can stably produce methane under conditions of high OLR and severe OLR fluctuation and can even shorten the stabilization period over the long term.

Graphite-assisted electro-fermentation methanogenesis: Spectroelectrochemical and microbial community analyses of cathode biofilms

The stimulatory effect of conductive particles on anaerobic digestion has been demonstrated in recent years. However, it is yet to be determined whether and how conductive particles affect methanogenesis via electro-fermentation (electro-fermentation methanogenesis). In this study, it was demonstrated, for the first time, that conductive graphite boosted the methane production yield by 54.3% and increased the maximum methane production rate by 72.2% during electro-fermentation methanogenesis. Graphite significantly affected the composition of cathode biofilms, with more live and large aggregates being observed. Spectroelectrochemical analyses further showed that the kinds and intensities of biocatalytic active sites and redox groups on the cathode biofilms increased during graphite-assisted electro-fermentation methanogenesis. Particularly, c-type cytochromes, humic acid-like substances, and humic substances improved the long-range electron transport to methanogens such as Methanobacterium and Methanosarcina. The results have implications for the improvement of electro-fermentation process and the use of conductive materials for biofuel recovery.

Response of enhanced sludge methanogenesis by red mud to temperature: Spectroscopic and electrochemical elucidation of endogenous redox mediators

Adding conductive materials can promote methanogenesis via facilitating electron exchange between syntrophic bacteria and methanogenic archaea. However, little is known about how temperature would interact with such an addition and thus affect the compositions and characteristics of endogenous redox mediators (ERMs). In particular, it is of strong interest to understand how the temperature variation would affect the improvement on methanogenesis induced by ERMs with conductive materials. Herein, we have investigated the response of sludge methanogenesis to temperature variation (from 15 to 35 °C) and spectroscopically detected the ERMs induced by conductive red mud. It was demonstrated that the increasing temperature enhanced the stimulating effect of conductive red mud on methane accumulation, and the methane production potential showed a linear relationship with redox parameters such as areal capacitance (C_a), free charges (R₂) and electron exchange capacity (EEC). 2DCOS spectra further indicated that ν_(C-O) and δ_(O-H) in humic acids, β-turn type III amide I ν_s(C=O) in Cytochrome c, and δ_(C-H) in amines and lipids became the main redox groups in ERMs at 35 °C with the addition of red mud. The model revealed that the contribution of ERMs to the CO₂ reduction to CH₄ increased from 35.2 ± 1.4% to 58.6 ± 1.5% when the temperature increased from 15 to 35 °C. Our finding that conductive materials stimulated the formation and electroactivity of ERMs with the increasing temperature during anaerobic digestion can have important implications for the improvement of engineered methanogenic processes.

Electrical-biological hybrid system for CO2 reduction

Here we have developed an electrochemical-biological hybrid system to fix CO₂. Natural biological CO₂ fixation processes are relatively slow. To increase the speed of fixation we applied electrocatalysts to reduce CO₂ to formate. We chose a user-friendly organism, Escherichia coli, as host. Overall, the newly constructed CO₂ and formate fixation pathway converts two formate and one CO₂ to one pyruvate via glycine and L-serine in E. coli. First, one formate and one CO₂ are converted to one glycine. Second, L-serine is produced from one glycine and one formate. Lastly, L-serine is converted to pyruvate. E. coli's genetic tractability allowed us to balance various parameters of the pathway. The carbon flux of the pathway was sufficient to compensate L-serine auxotrophy in the strain. In total, we integrated both electrocatalysis and biological systems into a single pot to support E. coli growth with CO₂ and electricity. Results show promise for using this hybrid system for chemical production from CO₂ and electricity.