Startup performance of microbial electrolysis cell assisted anaerobic digester (MEC-AD) with pre-acclimated activated carbon

Enhancing degradation of organic matter in microbial electrolytic cells coupled with anaerobic digestion (MEC-AD) systems by carbon-based materials

Currently, little information is available on relative contributions among biochar (BC), activated carbon (AC), magnetic BC (MBC), and magnetic AC (MAC) to enhance the effectiveness of a microbial electrolytic cells coupled with anaerobic digestion (MEC-AD) system and the impact of carbon-based materials on microbial community. In this study, six anaerobic reactors were constructed to demonstrate the effects of different carbon-based materials on organic matter elimination in the MEC-AD system. Remarkably, the reactor containing MBC exhibited a significant increase in organic removal, achieving 95.0 % chemical oxygen demand (COD) eradication. Additionally, the MBC-added MEC-AD reactor yields acetic acid at a rate 2.9 times higher than that of the BC-added reactor. Electrical stimulation enriched electro-producing bacteria such as Pseudomonas (18.1 %) and Gordonia (6.8 %), which were further promoted by the addition of MBC, indicating that the microbial communities cultivated with the MBC could provide the necessary microbiome for the MEC.

Development of a rapid startup method of direct electron transfer-dominant methanogenic microbial electrosynthesis

The rapid startup of carbon dioxide reduction-methanogenic microbial electrosynthesis is crucial for its industrial application, and the development of cathode biofilm is the key to its industrialization. Based on the new discovery that biofilm formed by placing graphite felt in an anaerobic reactor was electroactive, with strong direct electron transfer and methanogenesis ability (24.52 mL/L/d), a new startup method was developed. The startup time was shortened by at least 20 days and charge transfer resistance was reduced by 4.45-10.78 times than common startup methods (inoculating cathode effluent or granular sludge into the cathode chamber). The new method enriched electroactive bacteria. Methanobacterium and Methanosaeta accounted for 62.04% and 34.96%, respectively. The common methods inoculating cathode effluent or granular sludge enriched hydrogenotrophic microorganisms (>95%) or Methanosaeta (54.10%) due to the local environments of cathode. This new rapid and easy startup method may support the scale-up of microbial electrosynthesis.

Enhancement of methane production from waste activated sludge using hybrid microbial electrolysis cells-anaerobic digestion (MEC-AD) process - A review

Hybrid microbial electrolysis cells-anaerobic digestion (MEC-AD) was proved to increase methane productivity and methane yield of waste activated sludge (WAS) by establishing direct interspecies electron transfer method and enriching functional microorganisms. This review first summarized the pretreatment methods of WAS for MEC-AD and then reviewed the reactor configurations, operation parameters, and the economic benefit of MEC-AD. Furthermore, the enhancement mechanisms of MEC-AD were reviewed based on the analysis of thermodynamics and microbial community. It was found that the decrease of hydrogen partial pressure due to the hydrogenotrophic methanogens enriched in cathodic biofilm and direct interspecies electron transfer between exoelectrogens and anode were the core mechanisms for improving acidogenesis, acetogenesis, and methanogenesis. Finally, the potentially technological issues that need to be addressed to increase energy efficiency in large-scale MEC-AD processes were discussed.

A review of recent advances in electrode materials for emerging bioelectrochemical systems: From biofilm-bearing anodes to specialized cathodes

Bioelectrochemical systems (BES), mainly microbial fuel cells (MEC) and microbial electrolysis cells (MFC), are unique biosystems that use electroactive bacteria (EAB) to produce electrons in the form of electric energy for different applications. BES have attracted increasing attention as a sustainable, low-cost, and neutral-carbon option for energy production, wastewater treatment, and biosynthesis. Complex interactions between EAB and the electrode materials play a crucial role in system performance and scalability. The electron transfer processes from the EAB to the anode surface or from the cathode surface to the EAB have been the object of numerous investigations in BES, and the development of new materials to maximize energy production and overall performance has been a hot topic in the last years. The present review paper discusses the advances on innovative electrode materials for emerging BES, which include MEC coupled to anaerobic digestion (MEC-AD), Microbial Desalination Cells (MDC), plant-MFC (P-MFC), constructed wetlands-MFC (CW-MFC), and microbial electro-Fenton (BEF). Detailed insights on innovative electrode modification strategies to improve the electrode transfer kinetics on each emerging BES are provided. The effect of materials on microbial population is also discussed in this review. Furthermore, the challenges and opportunities for materials scientists and engineers working in BES are presented at the end of this work aiming at scaling up and industrialization of such versatile systems.

Recent Developments in Microbial Electrolysis Cell-Based Biohydrogen Production Utilizing Wastewater as a Feedstock

Carbon constraints, as well as the growing hazard of greenhouse gas emissions, have accelerated research into all possible renewable energy and fuel sources. Microbial electrolysis cells (MECs), a novel technology able to convert soluble organic matter into energy such as hydrogen gas, represent the most recent breakthrough. While research into energy recovery from wastewater using microbial electrolysis cells is fascinating and a carbon-neutral technology that is still mostly limited to lab-scale applications, much more work on improving the function of microbial electrolysis cells would be required to expand their use in many of these applications. The present limiting issues for effective scaling up of the manufacturing process include the high manufacturing costs of microbial electrolysis cells, their high internal resistance and methanogenesis, and membrane/cathode biofouling. This paper examines the evolution of microbial electrolysis cell technology in terms of hydrogen yield, operational aspects that impact total hydrogen output in optimization studies, and important information on the efficiency of the processes. Moreover, life-cycle assessment of MEC technology in comparison to other technologies has been discussed. According to the results, MEC is at technology readiness level (TRL) 5, which means that it is ready for industrial development, and, according to the techno-economics, it may be commercialized soon due to its carbon-neutral qualities.

On-site CO2 bio-sequestration in anaerobic digestion: Current status and prospects

The advantages of anaerobic digestion (AD) technology in organic solid waste treatment for bioenergy recovery are evidenced in worldwide. Recently, more attention has been paid to on-site biogas research, as well as biogenic CO₂ sequestration from AD plant, to promote "carbon neutral". Single-phase and two-phase AD system can be incorporated with various CO₂ bioconversion technologies through H₂ mediated CO₂ bioconversion (in-situ and ex-situ biogas upgrading), or other emerging strategies for CO₂ fixation without exogenous H₂ injection; these include in-situ direct interspecies electron transfer reinforcement, electromethanogenesis, and off-gas reutilization. The existing and potential scenarios for on-site CO₂ bio-sequestration within the AD framework are reviewed from the perspectives of metabolic pathways, functional microorganisms, the limitations on reaction kinetics. This review concluded that on-site CO₂ bio-sequestration is a promising solution to reduce greenhouse gas emissions and increase renewable energy recovery.

Microbial Electrolysis Cells for Decentralised Wastewater Treatment: The Next Steps

Traditional wastewater treatment methods have become aged and inefficient, meaning alternative methods are essential to protect the environment and ensure water and energy security worldwide. The use of microbial electrolysis cells (MEC) for wastewater treatment provides an innovative alternative, working towards circular wastewater treatment for energy production. This study evaluates the factors hindering industrial adoption of this technology and proposes the next steps for further research and development. Existing pilot-scale investigations are studied to critically assess the main limitations, focusing on the electrode material, feedstock, system design and inoculation and what steps need to be taken for industrial adoption of the technology. It was found that high strength influents lead to an increase in energy production, improving economic viability; however, large variations in waste streams indicated that a homogenous solution to wastewater treatment is unlikely with changes to the MEC system specific to different waste streams. The current capital cost of implementing MECs is high and reducing the cost of the electrodes should be a priority. Previous pilot-scale studies have predominantly used carbon-based materials. Significant reductions in relative performance are observed when electrodes increase in size. Inoculation time was found to be a significant barrier to quick operational performance. Economic analysis of the technology indicated that MECs offer an attractive option for wastewater treatment, namely greater energy production and improved treatment efficiency. However, a significant reduction in capital cost is necessary to make this economically viable. MEC based systems should offer improvements in system reliability, reduced downtime, improved treatment rates and improved energy return. Discussion of the merits of H2 or CH4 production indicates that an initial focus on methane production could provide a stepping-stone in the adoption of this technology while the hydrogen market matures.

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.

Treatment of anthraquinone dye textile wastewater using anaerobic dynamic membrane bioreactor: Performance and microbial dynamics

The performance and microbial community structure of anaerobic dynamic membrane bioreactor (AnDMBR) treating textile wastewater was investigated. The reactor showed excellent soluble COD and color removal of 98.5% and >97.5%, respectively. Dynamic membrane layer grown over the 3D printed dynamic membrane support showed decent rejection for high molecular weight compounds (>20 kDa); and the total suspended solid rejection by the dynamic layer was >98.8%. Gel permeation chromatography analysis of extracellular polymeric substance (EPS) and effluent samples revealed EPS accounted for more than 76.7% of low molecular weight fractions (<20 kDa) that end up in the effluent. Higher applied flux facilitated the rapid formation dynamic layer which enabled a satisfactory effluent quality. Microbial community analysis revealed that during the operation the archaeal community was relatively stable while obvious changes took place in the bacterial community. Introduction of dye Remazol Brilliant Blue R (RBBR) to the AnDMBR increased the abundances of phyla of Proteobacteria and Spirochaetae whereas fractions of Firmicutes and Euryarchaeota decreased obviously. Furthermore, relative stable abundances of phyla Aminicenantes, Bacteroidetes, Thermotogae and Chloroflexi among the top six phyla detected in the system ensured a healthy anaerobic degradation environment for RBBR wastewater treatment.

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.