Application of bioelectrochemical systems to regulate and accelerate the anaerobic digestion processes

Electrochemical optimization of bioelectrochemically improved anaerobic digestion for agricultural digestates' valorisation to biomethane

Bioelectrochemically improved anaerobic digestion (AD-BES) represents an upgrading strategy for existing biogas plants, consisting of the integration of bioelectrodes within the AD reactor. For this study, a series of laboratory-scale AD-BES reactors were operated, valorising agricultural digestates through the production of biogas. The reactors were inoculated and started-up with three different digestates, leading to significant differences in the microbial community developed on the bioelectrodes. After the start-up was completed, the AD-BES were all fed with a unique digestate, to evaluate the stability of the bioelectrodes' biofilm performances against variations of the organic feedstock. In terms of methane (CH4) production rate, the presence of bioelectrodes allowed between 25 and 82% improvement, compared with control AD reactors. The application of an optimal voltage of 0.3 V resulted in an additional 40% improvement in CH4 production rate, but only when the biofilm was previously acclimated to the fed digestate. Comprehensive microbial characterization revealed that fed digestate significantly influences the composition and homogenization of microbial communities within AD-BES reactors, with applied voltage showing only a secondary effect. Even when reactors were transitioned to a uniform digestate feeding, resulting in closely similar microbial profiles, variations in CH4 production persisted, underscoring the lasting impact of initial microbial conditioning. A critical observation was the differentiation in archaeal colonization on bioelectrodes at 0.3 V, the voltage yielding the highest CH4 conversion. These insights suggest that while the microbial community structure depends on fed digestate, operational efficiency and methanogenic potential are intricately linked to both initial microbial establishment and the specific electrochemical conditions applied to AD-BES reactors.

Response of fulvic acid linking to redox characteristics on methane and short-chain fatty acids in anaerobic digestion of chicken manure

Fulvic acids (FAs) is formed during the bioconversion of organic matter (OM) to biogas during anaerobic digestion (AD) and has a complex structure and redox function. However, the evolutionary mechanisms of FAs during AD and its interactions with acid and methane production have not been sufficiently investigated, especially at different stages of AD. Intermittent AD experiments by chicken manure and rice husk showed significant structural changes and reduced aromatization of FAs (e.g., O-H stretch6, 14.10-0%; SR, 0.22-0.60). The electron donating capacity (EDC) [9.76-45.39 μmole-/(g C)] and electron accepting capacity (EAC) [2.55-5.20 μmole-/(g C)] of FAs showed a tendency of decreasing and then increasing, and FAs had a stronger electron transfer capacity (ETC) in the methanogenic stage. Correlation analysis showed that the EDC of FAs was influenced by their own structure (C-O stretch2, C-H bend1, C-H bend4, and N-H bend) and also had an inhibitory effect on propionic production, which further inhibited acetic production. The EAC of FAs was affected by molecular weight and had a promoting effect on methane production. Structural equation modelling identified three possible pathways for AD. The C-O stretch2 structure of FAs alone inhibits the production of propionic. In addition, pH can directly affect the EDC of FAs. This study provides a theoretical basis for the structural and functional evolution of FAs in AD of chicken manure on the mechanism of methane production.

Paradigm shift in Nutrient-Energy-Water centered sustainable wastewater treatment system through synergy of bioelectrochemical system and anaerobic digestion

With advancements in research and the necessity of improving the performance of bioelectrochemical system (BES), coupling anaerobic digestion (AD) with BES is crucial for energy gain from wastewater and bioremediation. Hybridization of BES-AD concept opens new avenues for pollutant degradation, carbon capture and nutrient-resource recovery from wastewater. The strength of merging BES-AD lies in synergy, and this approach was employed to differentiate fads from strategies with the potential for full-scale implementation and making it an energy-positive system. The integration of BES and AD system increases the overall performance and complexity of combined system and the cost of operation. From a technical standpoint, the primary determinants of BES-AD feasibility for field applications are the scalability and economic viability. High potential market for such integrated system attract industrial partners for more industrial trials and investment before commercialization. However, BES-AD with high energy efficacy and negative economics demands performance boost.

Implementation of cytochrome c proteins and carbon nanotubes hybrids in bioelectrodes towards bioelectrochemical systems applications

Multiheme cytochrome c (Cyt c) can function as a redox protein on electrode to accomplish bioelectrocatalysis. However, the direct electron transfer (DET) between the redox site of Cyt c and electrode is low due to the large coupling distance. A close proximity or a connection pathway from the deeply buried active site to the protein surface can be established by modifying the electrode with carbon nanotubes (CNTs) to improve the DET. Therefore, the isolated Cyt c has been assembled or casted with CNTs by various processes to form Cyt c-CNTs bioelectrodes that can be further applied to biosensing and bioanalysis. These strategies can be transplanted to the fabrication of biofilm-CNTs based electrodes by complexing the out membrane (OM) Cyt c of natural electricigen with CNTs to realize the application of the electrochemical properties of "in vivo" Cyt c to bioelectrochemical systems (BESs). This review intends to highlight the preparation strategies of bioelectrodes that have been well studied in electrochemical biosensors and improving approaches of the DET from the CNTs surface to Cyt c in their hybrids. The efficient fabrication processes of the biofilm-CNTs based electrodes that can be considered as "in vivo" Cyt c-CNTs based electrodes for BES designs are also summarized, aiming to provide an inspiration source and a reference to the related studies of BES downstream.

Scale-up of the bioelectrochemical system: Strategic perspectives and normalization of performance indices

Electrochemists and ecological engineers find environmental bioelectrochemistry appealing; however, there is a big gap between expectations and actual progress in bioelectrochemical system (BES). Implementing such technology opens new opportunities for novel electrochemical reactions for resource recovery and effective wastewater treatment. Loopholes of BES exist in its scaling-up applications, and numerous attempts toward practical applications (200, 1000, and 1500 L) are key successive indicators toward its commercialization. This review emphasized the critical rethinking of standardization of performance indices i.e. current generation (A/m²), net energy recovery (kWh/kg·COD), product/resource yield (mM), and economic feasibility ($/kWh) to make fair comparison with the existing treatment system. Therefore, directional perspectives, including modularity, energy-cost balance, energy and resource recovery, have been proposed for the sustainable market of BES. The current state of the art and up-gradation in resource recovery and contaminant removal warrants a systematic rethinking of functional worth and niches of BES for practical applications.

Regulation and augmentation of anaerobic digestion processes via the use of bioelectrochemical systems

Anaerobic digestion (AD) is a biological process that can be used to treat a wide range of carbon-rich wastes and producerenewable, green energy. To maximize energy recovery from various resources while controlling inhibitory chemicals, notwithstanding AD's efficiency, many limitations must be addressed. As a result, bioelectrochemical systems (BESs) have emerged as a hybrid technology, extensively studied to remediate AD inhibitory chemicals, increase AD operating efficacy, and make the process economically viable via integration approaches. Biogas and residual intermediatory metabolites such as volatile fatty acids are upgraded to value-added chemicals and fuels with the help of the BES as a pre-treatment step, within AD or after the AD process. It may also be used directly to generate power. To overcome the constraints of AD in lab-scale applications, this article summarizes BES technology and operations and endorses ways to scale up BES-AD systems in the future.