Carbon-based conductive materials accelerated methane production in anaerobic digestion of waste fat, oil and grease

Seaweed valorization as anaerobic co-substrate with fat, oil, and grease: Biomethane potential and microbial dynamics

The present study explored the anaerobic co-digestion (AcD) of seaweed Gracilaria vermiculophylla with fat, oil, and grease (FOG) at 75, 50, and 25 % w/w of volatile solids (VS). Mono-digestion of FOG and SW led to a methane production of 133 and 109 mL/(g.d) with 40 days lag-phase, lower than 235 mL/(g.d) of AcD at FOG-50:SW-50 with reduced lag-phase of 20 days. The palmitic and oleic acid reduction was 95 % in the reactors FOG-50:SW-50, followed by FOG-25:SW-75, which was 84 %, as compared to FOG mono-digestion (47 %). Relative abundance of Firmicutes, Chloroflexi, and Bacteroidetes were enriched during AcD. The relative abundance of Methanosaeta was enhanced (40-90 %) in FOG-50:SW-50 compared with FOG-100:SW-0 as the reduction in Methanosaeta was replaced by Methanoculleus (30 %) and RuMen-M2 (10 %). The present study offers essential perspectives for the AcD of FOG with SW, showcasing the benefits of SW as a co-substrate for improved methane recovery from FOG.

Shewanella oneidensis and Methanosarcina barkerii augmentation and conductive material effects on long-term anaerobic digestion performance

This study explores the use of conductive material in scaling up anaerobic digestion for enhanced biogas production. Focusing on Direct Interspecies Electron Transfer (DIET), the research employs a syntrophic DIET-able consortium formed by Shewanella oneidensis and Methanosarcina barkerii in 3.8-L experiments utilizing reticulated vitreous carbon (RVC) as conductive material. In short-term tests with acetate the syntrophic co-culture with RVC resulted in 86% higher maximum velocity of methane production, while in long term with real feed 13% increased rate was observed: the addition of 1.77 (S/m)*m2 RVC resulted in a faster methane production of 2.39 mL/gVS*h compared to 2.08 mL/gVS*h of the reference. The experimental conditions of syntrophic inoculum and RVC as conductive material gave a benefit in terms of process rate compared to the reference, considering the inoculum fate, Methanosarcina barkerii was among the dominant taxa at the end of the experiment, while Shewanella oneidensis was outcompeted. Among the methanogenesis production pathways, an increase of hydrogenotrophic methanogenesis has been observed in presence of conductive material. Further research is needed to understand the role of RVC in sulfur compounds production. Utilization of RVC to augment methane production yielded interesting results for real-scale application. As an added carrier, RVC remains unaltered and can be readily recuperated and reused multiple times.

Microbial activity of the inoculum determines the impact of activated carbon, magnetite and zeolite on methane production

The conversion of organic matter to methane through anaerobic digestion (AD) process can be enhanced by different materials. However, literature reports show inconsistent results on the effect of materials in different AD systems. In this study, we evaluated the influence of the inoculum's activity on methane production (MP) efficiency in the presence of different materials (activated carbon (AC), magnetite (Mag), and zeolite (Zeo)). The inocula included pure cultures of methanogens, syntrophic cocultures, and complex microbial communities, and the kinetic parameters assessed were the lag phase duration and methane production rates (MPR). The results showed that the microbial activity of the inocula is an important factor determining materials' effect on MP kinetics. AC, Mag, and Zeo significantly enhanced the MP profiles of less active microbial communities or low-active microorganisms by decreasing lag phases duration up to 85 %, consequently increasing MPR up to 15 times. Contrarily, these materials did not affect highly active microbial communities or pure cultures, as MP profiles tend to be similar with and without materials. These results indicate that from an applied point of view, the addition of materials to anaerobic bioreactors should be considered only when the methanogenic activity of the sludge is low or compromised.

Microbiome regulation for sustainable wastewater treatment

Sustainable wastewater treatment is essential for attaining clean water and sanitation, aligning with UN Sustainable Development Goals. Wastewater treatment plants (WWTPs) have utilized environmental microbiomes in biological treatment processes in this effort for over a century. However, the inherent complexity and redundancy of microbial communities, and emerging chemical and biological contaminants, challenge the biotechnology applications. Over the past decades, understanding and utilization of microbial energy metabolism and interaction relationships have revolutionized the biological system. In this review, we discuss how microbiome regulation strategies are being used to generate actionable performance for low-carbon pollutant removal and resource recovery in WWTPs. The engineering application cases also highlight the real feasibility and promising prospects of the microbiome regulation approaches. In conclusion, we recommend identifying environmental risks associated with chemical and biological contaminants transformation as a prerequisite. We propose the integration of gene editing and enzyme design to precisely regulate microbiomes for the synergistic control of both chemical and biological risks. Additionally, the development of integrated technologies and engineering equipment is crucial in addressing the ongoing water crisis. This review advocates for the innovation of conventional wastewater treatment biotechnology to ensure sustainable wastewater treatment.

Effects of carbon-based conductive materials on semi-continuous anaerobic co-digestion of organic fraction of municipal solid waste and waste activated sludge

Organic fraction of municipal solid waste (OFMSW) and waste activated sludge (WAS) are the most produced organic waste streams in urban centres. Their anaerobic co-digestion (AcoD) allows to generate methane (CH4) and digestate employable as renewable energy source and soil amendment, respectively, fully in accordance with circular bioeconomy principles. However, the widespread adoption of such technology is limited by relatively low CH4 yields that fail to bridge the gap between benefits and costs. Among strategies to boost AcoD of OFMSW and WAS, use of conductive materials (CMs) to promote interspecies electron transfer has gained increasing attention. This paper presents one of the few experimental attempts of investigating the effects of four different carbon(C)-based CMs (i.e., granular activated carbon - GAC, graphite - GR, graphene oxide - GO, and carbon nanotubes - CNTs) separately added in semi-continuous AcoD of OFMSW and thickened WAS. The presence of C-based CMs has been observed to improve CH4 yield of the control process. Specifically, after 63 days of operation (concentrations of GAC and GR of 10.0 g/L and of GO and CNTs of 0.2 g/L), 0.186 NL/gVS, 0.191 NL/gVS, 0.203 NL/gVS, and 0.195 NL/gVS of CH4 were produced in reactors supplemented with GAC, GR, GO, and CNTs, respectively, compared to 0.177 NL/gVS produced in the control process. Likewise, at the end of the test (i.e., after 105 days at concentrations of C-based CMs half of the initial ones), CH4 yields were 0.193 NL/gVS, 0.201 NL/gVS, 0.211 NL/gVS, and 0.206 NL/gVS in reactors supplemented with GAC, GR, GO, and CNTs, respectively, compared to 0.186 NL/gVS of the control process. Especially with regard to GR, GO, and CNTs, results obtained in the present study represent a significant advance of the knowledge on the effects of such C-based CMs to realistic and scalable AD process conditions respect to previous literature.

Environmental impact of sewage sludge co-digestion with food waste and fat-oil-grease: Integrating plant-wide modeling with life cycle assessment approach

Anaerobic co-digestion of fat-oil-grease (FOG) and food waste (FW) with sewage sludge (SS) in wastewater treatment plants is a method used to increase biogas production. In this study, digestion scenarios were compared using plant-wide modeling and life cycle assessment: Scenario-0 (mono-digestion of waste-activated sludge (WAS)), Scenario-1 (co-digestion of WAS with FOG), and Scenario-2 (co-digestion of WAS with FW). Scenario-0, with the highest energy use and landfilling of FOG/FW, has the worst environmental impact. Scenario-1 and Scenario-2 minimize the environmental load by energy recovery and avoiding landfilling of organic waste. Scenario-wise, the change in greenhouse gas (GHG) emissions from treatment was negligible. However, due to the impact of landfilling, GHG emissions in Scenario-0 were 21% and 30% higher than in Scenario-1 and 2, respectively. The environmental benefit of anaerobic co-digestion of FOG/FW with SS is not only in the contribution to energy production but also in the recycling of organic waste.

Direct interspecies electron transfer mechanisms of a biochar-amended anaerobic digestion: a review

This paper explores the mechanisms of biochar that facilitate direct interspecies electron transfer (DIET) among syntrophic microorganisms leading to improved anaerobic digestion. Properties such as specific surface area (SSA), cation exchange capacity (CEC), presence of functional groups (FG), and electrical conductivity (EC) were found favorable for increased methane production, reduction of lag phase, and adsorption of inhibitors. It is revealed that these properties can be modified and are greatly affected by the synthesizing temperature, biomass types, and residence time. Additionally, suitable biochar concentration has to be observed since dosage beyond the optimal range can create inhibitions. High organic loading rate (OLR), pH shocks, quick accumulation and relatively low degradation of VFAs, and the presence of heavy metals and toxins are the major inhibitors identified. Summaries of microbial community analysis show fermentative bacteria and methanogens that are known to participate in DIET. These are Methanosaeta, Methanobacterium, Methanospirillum, and Methanosarcina for the archaeal community; whereas, Firmicutes, Proteobacteria, Synergistetes, Spirochetes, and Bacteroidetes are relatively for bacterial analyses. However, the number of defined cocultures promoting DIET is very limited, and there is still a large percentage of unknown bacteria that are believed to support DIET. Moreover, the instantaneous growth of participating microorganisms has to be validated throughout the process.

Conductive materials enhance microbial salt-tolerance in anaerobic digestion of food waste: Microbial response and metagenomics analysis

Previous studies have shown that high salinity environments can inhibit anaerobic digestion (AD) of food waste (FW). Finding ways to alleviate salt inhibition is important for the disposal of the growing amount of FW. We selected three common conductive materials (powdered activated carbon, magnetite, and graphite) to understand their performance and individual mechanisms that relieve salinity inhibition. Digester performances and related enzyme parameters were compared. Our data revealed that under normal and low salinity stress conditions, the anaerobic digester ran steady without significant inhibitions. Further, the presence of conductive materials promoted conversion rate of methanogenesis. This promotion effect was highest from magnetite > powdered activated carbon (PAC) > graphite. At 1.5% salinity, PAC and magnetite are beneficial in maintaining high methane production efficiency while control and the graphite added digester acidified and failed rapidly. Additionally, metagenomics and binning were used to analyze the metabolic capacity of the microorganisms. Some species enriched by PAC and magnetite possessed higher cation transport capacities and were to accumulate compatible solutes. PAC and magnetite promoted direct interspecies electron transference (DIET) and syntrophic oxidation of butyrate and propionate. Also, the microorganisms had more energy available to cope with salt inhibition in the PAC and magnetite added digesters. Our data imply that the promotion of Na+/H+ antiporter, K+ uptake, and osmoprotectant synthesis or transport by conductive materials may be crucial for their proliferation in highly stressful environments. These findings will help to understand the mechanisms of alleviate salt inhibition by conductive materials and help to recover methane from high-salinity FW.

Granular activated carbon stimulates biogas production in pilot-scale anaerobic digester treating agro-industrial wastewater

This work examines the continuous addition (5 g/L) of conductive granular activated carbon (GAC) in an integrated pilot-scale unit containing an anaerobic digester (180 L) and an aerobic submerged membrane bioreactor (1600 L) connected in series for the treatment of agro-industrial wastewater. Biogas production increased by 32 % after the addition of GAC. Methanosaeta was the dominant methanogen in the digester, and its relative abundance increased after the addition of GAC. The final effluent after post-treatment with the aerobic membrane bioreactor had a total solids content <0.01 g/L and a chemical oxygen demand between 120 and 150 mg/L. A simple cost analysis showed that GAC addition is potentially profitable, but alternatives ways of retaining the GAC in the system need to be found. Overall, this study provides useful scientific data for the possible application of GAC in full-scale biogas projects.

Effect of Lipid Type on the Acidogenic Performance of Food Waste

Due to its high lipid content and intricate constitution, food waste poses a considerable challenge for biotreatment. This research aims to investigate the potential influence of diverse lipid species on anaerobic fermentation, induced by the varying dietary patterns observed in distinct regions. The investigation involved incorporating 5% (w/w) of beef tallow, mutton fat, soybean oil, peanut oil, and rapeseed oil, separately, into simulated food waste, and subjected it to batch mode acidogenic fermentation. The inclusion of unsaturated fatty acids resulted in a redirection of the metabolic pathway from the lactic acid type to the ethanol, acetic acid, and butyric acid types. The succession of the acidogenic metabolic pathway was highly correlated with the lipid types; beef tallow, mutton fat, soybean oil, and peanut oil delayed the metabolic process by 1, 2, 3, and 8 d, respectively, whereas rapeseed oil accelerated it by 2 d. The lipids contained within the food waste did not facilitate the buildup of soluble substances, resulting in a decrease of 14.0~59.7%. Notwithstanding, valeric acid was exclusively generated during the beef tallow and peanut oil treatments, whereas the production of lactic acid in peanut oil showed a 35.9% increase in comparison to the control.

Mechanisms, performance, and the impact on microbial structure of direct interspecies electron transfer for enhancing anaerobic digestion-A review

Direct interspecies electron transfer (DIET) has been received tremendous attention, recently, due to the advantages of accelerating methane production via organics reduction during anaerobic digestion (AD) process. DIET-based syntrophic relationships not only occurred with the existence of pili and some proteins in the microorganism, but also can be conducted by conductive materials. Therefore, more researches into understanding and strengthening DIET-based syntrophy have been conducted with the aim of improving methanogenesis kinetics and further enhance methane productivity in AD systems. This study summarized the mechanisms, application and microbial structures of typical conductive materials (carbon-based materials and iron-based materials) during AD reactors operation. Meanwhile, detail analysis of studies on DIET (from substrates, dosage and effectiveness) via conductive materials was also presented in the study. Moreover, the challenges of applying conductive materials in boosting methane production were also proposed, which was supposed to provide a deep insight in DIET for full scale application.

Effect of granular activated carbon adsorption and size of microbial aggregates in inoculum on stimulating direct interspecies electron transfer during anaerobic digestion of fat, oil, and grease

To investigate the effect of granular activated carbon (GAC) adsorption and size of microbial aggregates in inoculum on stimulating direct interspecies electron transfer (DIET) during anaerobic digestion of fat, oil, and grease (FOG), seed sludge was divided into two inocula (big (>0.85 mm)/small (0.15-0.85 mm)) for FOG digestion with/without GAC. More long-chain fatty acids (LCFAs) were adsorbed on GAC in the reactor with small aggregates than that with big aggregates, corresponding to 57 % and 10 % decreased methane production, respectively. Adsorption of unsaturated LCFAs (e.g., oleic acid) on GAC was found to reduce LCFA bioavailability, hinder DIET via GAC, and change community structure. Compared to pre-adsorption of oleic acid on GAC, pre-attachment of microbes on GAC resulted in 5.6-fold higher methane yield for oleic acid digestion. Together, competition of LCFA adsorption between GAC and microbial aggregates is essential for enhanced methane recovery from FOG digestion via GAC-induced DIET.

Enhancement from Anaerobic Digestion of Food Waste by Conductive Materials: Performance and Mechanism

Conductive materials (CM) have recently attracted research interest in the anaerobic digestion of food waste to achieve reduction and resource utilization. Fe-metal organic frameworks (Fe-MOF) and Ketjen Black (KB), the conductive materials (CMs), were added for the enhancement of food waste digestion. This study therefore, is intended to fill in this knowledge gap and clarify the underlying mechanism of CM-promoted performance. Batch experiments revealed that the optimal additions of Fe-MOF and KB were 0.5 g·L^-1 and 0.2 g·L^-1, respectively. The biogas production increased by 27.50% and 29.45% compared with the blank group, and the removal efficiency of volatile solids (VS), total solids (TS), and chemical oxygen demand (COD) increased by 18.28%, 40.52%, and 15.31%. The lag period was shortened from 3.042 to 2.006 and 1.544 days, respectively. Mechanism studies revealed that Fe-MOF and KB were beneficial to food waste digestion, and the functional groups of Fe-MOF and KB increased the buffer capacity of the system to pH and ammonia nitrogen. The physicochemical properties of Fe-MOF and KB promote the activity of the electron transfer system (ETS); the ETS activity was about 2 times the 11.32 mg·(g·h)^-1 of the blank group. Zeta potential and electrical conductivity were beneficial to the establishment of intermicrobial direct interspecies electron transfer (DIET).

Enhanced direct interspecies electron transfer and methane production during anaerobic digestion of fat, oil, and grease by coupling carbon-based conductive materials and exogenous hydrogen

To investigate the combination of carbon-based conductive materials and exogenous hydrogen (EH2) on methane recovery from fat, oil, and grease (FOG), granular activated carbon (GAC) and carbon cloth (CC) were chosen to collaborate with EH2, resulting in increased methane production by 59 % and 84 %, respectively. Further digestion of long chain fatty acids (LCFAs) confirms that enhanced direct interspecies electron transfer (DIET) was achieved in the reactors with GAC/CC + EH2 than those with GAC/CC only. Other evidences (such as increased microbial population and rapid degradation of volatile fatty acids) were found to support the role of GAC/CC + EH2 in promotion of DIET. Significant change of microbial community was observed using GAC/CC + EH2, which was mainly attributed to the enrichment of electrogenic species (such as Spirochaetaceae, Syntrophomonas palmitatica, and Methanosaeta), leading to some changes in metabolic pathways during acidogenesis and methanogenesis. Together, enhanced DIET was achieved by GAC/CC + EH2, thus improving the methane recovery from FOG.

Enhancing methane production of co-digested food waste with granular activated carbon coated with nano zero-valent iron in an anaerobic digester

Anaerobic digestion (AD) possesses dual benefits of waste treatment and energy generation. The use of conductive additives in AD matrix has potential to improve process yield. Hence, the study aimed to investigate a thermophilic AD (TAD) inserted by granular activated carbon coated with nano zero-valent iron (GAC/nZVI) in the matrix and was operated for mono-digestion and co-digestion of cow manure with food wastes (rice and bread) to check the bioprocess improvement. The results were compared with the control TAD without conductive additives. Biogas production increased by 11 folds upon using GAC/nZVI addition compared to the control TAD. Moreover, the addition of GAC/nZVI increased the methane in biogas by 20.7 folds compared to control one. With GAC/nZVI, the maximum COD removal of 78.29% and 85.21% were noticed for co-digestion and mono digestion, respectively. Such improvement of TAD performance was due to easy bacterial communication and electron exchange through the conductive particles.

Enhanced methane production in anaerobic digestion: A critical review on regulation based on electron transfer

Anaerobic digestion (AD) is a potential bioprocess for waste biomass utilization and energy conservation. Various iron/carbon-based CMs (e.g., magnetite, biochar, granular activated carbon (GAC), graphite and zero valent iron (ZVI)) have been supplemented in anaerobic digestors to improve AD performance. Generally, the supplementation of CMs has shown to improve methane production, shorten lag phase and alleviate environmental stress because they could serve as electron conduits and promote direct interspecies electron transfer (DIET). However, the CMs dosage varied greatly in previous studies and CMs wash out remains a challenge for its application in full-scale plants. Future work is recommended to standardize the CMs dosage and recover/reuse the CMs. Moreover, additional evidence is required to verify the electrotrophs involved in DIET.

Anode modification of sediment microbial fuel cells (SMFC) towards bioremediating mariculture wastewater

Remediation of mariculture wastewater is of great practical importance. In this study, sediment microbial fuel cells (SMFCs) were adopted and carbon felt anodes were modified to enhance COD and ammonia removal in mariculture system. The results showed that the SMFC anode with 5 % (w/w) graphene oxide (GO) coating performed best in pollutants removal and electricity generation. The maximum power density approached 132 mW/m², nearly 4.5 times higher than the unmodified anode. The removal efficiency of COD and ammonia reached 82.1 % and 95.8 % respectively, both improved compared with the control and chemical modification. The modified anode effectively enriched the electrogenic Sulfurovum and Lactobacillus and thus led to a significant improvement in the electrochemical performance of SMFC. This study demonstrates the successful application of SMFCs with GO modified anodes in the in-situ removing pollutants and SMFCs present obvious remediation potential on the contaminated mariculture inhabitant.

Carbon-based conductive materials enhance biomethane recovery from organic wastes: A review of the impacts on anaerobic treatment

Amongst the most important sustainable waste management strategies, anaerobic biotechnology has had a central role over the past century in the management of high-pollution load sources, such as food, agricultural and municipal wastes. During anaerobic digestion (AD), valuable by-products such as digestate and biogas are produced. Biogas (mainly composed of methane) is generated through a series of reactions between bacteria and archaea. Enhancement of AD process with higher methane yield, accelerated methane production rate, and shorter start-up time is possible via tapping into a novel methanogenic pathway discovered a decade ago. This fundamentally new concept that is a substitute to interspecies hydrogen transfer is called direct interspecies electron transfer (DIET). DIET, a thermodynamically more feasible way of electron transfer, has been proven to occur between bacteria and methanogens. It is well-documented that amendment of carbon-based conductive materials (CCMs) can stimulate DIET via serving as an electrical conduit between microorganisms. Therefore, different types of CCMs such as biochar and activated carbon have been amended to a variety of AD reactors and enhancement of process performance was reported. In this review, a comparative analysis is presented for enhancement of AD performance in relation to major CCM related factors; electrical conductivity, redox properties, particle size and dosage. Additionally, the impacts of AD operational conditions such as organic loading rate and temperature on CCM amended reactors were discussed. Further, the changes in microbial communities of CCM amended reactors were reviewed and future perspectives along with challenges for CCM application in AD have been provided.

Saponification with calcium has different impacts on anaerobic digestion of saturated/unsaturated long chain fatty acids

Little is known about the influence of the saturation degree of long chain fatty acids (LCFAs) on the bio-methane potential of calcium-LCFAs salts. In this study, palmitic acid and oleic acid were chosen as the model compounds to investigate the impact of saponification between calcium and saturated/unsaturated LCFAs on the methane recovery from LCFAs in anaerobic digestion. A 2.2-fold enhancement of methane yield was obtained due to the formation of calcium palmitate, which was primarily attributed to the enhanced bio-aggregation and significant change of microbial community. However, saponification between calcium and oleic acid decreased the methane recovery from oleic acid digestion. Only partial saponification with excess oleic acid led to 4% increment of methane production. The low bio-accessibility of calcium oleate and the little change of microbial community may be responsible for the small difference of methane recovery due to the formation of calcium oleate.

Distribution patterns of functional microbial community in anaerobic digesters under different operational circumstances: A review

Anaerobic digestion (AD) processes are promising to effectively recover resources from organic wastes or wastewater. As a microbial-driven process, the functional anaerobic species played critical roles in AD. However, the lack of effective understanding of the correlations of varying microbial communities with different operational factors hinders the microbial regulation to improve the AD performance. In this paper, the main anaerobic functional microorganisms involved in different stages of AD processes were first demonstrated. Then, the response of anaerobic microbial community to different operating parameters, exogenous interfering substances and digestion substrates, as well as the digestion efficiency, were discussed. Finally, the research gaps and future directions on the understanding of functional microorganisms in AD were proposed. This review provides insightful knowledge of distribution patterns of functional microbial community in anaerobic digesters, and gives critical guidance to regulate and enrich specific functional microorganisms to accumulate certain AD products.

Promotion of direct interspecies electron transfer and potential impact of conductive materials in anaerobic digestion and its downstream processing - a critical review

Addition of conductive materials (CMs) has been reported to facilitate direct interspecies electron transfer (DIET) and improved anaerobic digestion (AD) performance. This review summarises the benefits and outlines remaining research challenges of the addition of CMs with a focus on the downstream processing of AD. CM addition may alter biogas quality, digestate dewaterability, biosolids volume, and centrate quality. Better biogas quality has been observed due to the adsorption of H₂S to metallic CMs. The addition of CMs results in an increase in solid content of the digestate and thus an additional requirement for sludge dewatering and handling and the final biosolids volume for disposal. This review highlights the need for more research at pilot scale to validate the benefits of CM addition and to evaluate CM selection, doses, material costs, and the impact on downstream processes. The lack of research on the impact of CMs on the downstream process of AD is highlighted.

Anaerobic digestion beyond biogas

Anaerobic digestion (AD) is a matured technology for waste (water) remediation/stabilization and bioenergy generation in the form of biogas. AD technology has several inherent benefits ranging from generating renewable energy, remediating waste (water), and reducing greenhouse gas emission to improving health/hygiene and the overall socio-economic status of rural communities in developing nations. In recent years, there has been a paradigm shift in applications of AD technology beyond biogas. This special issue (SI) entitled, "Anaerobic Digestion Beyond Biogas (ADBB-2021)," was conceptualized to incorporate some of the recent advances in AD in which the emphasis is beyond biogas, such as anaerobic biorefinery, chain elongation, treatment of micropollutants, toxicity and system stability, digestate as biofertilizer, bio-electrochemical systems, innovative bioreactors, carbon sequestration, biogas upgrading, microbiomes, waste (water) remediation, residues/waste pre-treatment, promoter addition, and modeling, process control, and automation, among others. This VSI: ADBB-2021 contains 53 manuscripts (14 critical reviews and 39 research). The key findings of each manuscript are briefly summarized here, which can serve as a valuable resource for AD researchers to learn of major advances in AD technology and identify future research directions.