Methane yield enhancement of mesophilic and thermophilic anaerobic co-digestion of algal biomass and food waste using algal biochar: Semi-continuous operation and microbial community analysis

Effects of salinity on anaerobic digestion: Performance, microbial physiology, and community dynamics

Anaerobic digestion (AD) is widely applied to treatment and energy recovery from organic wastewater/wastes, while the efficiency of AD can be limited by salinity stress. This paper reviews the effects of salinity on AD. First of all, the effects of salinity on AD performance were compared, revealing that methane production is more susceptible to salinity stress. Secondly, the influence of salinity on microbial physiology and intracellular molecules was examined, demonstrating that salinity stress reduces the activity of key enzymes and increases the concentration of extracellular polymeric substances during AD. Thirdly, variations in microbial community structure under salinity stress were discussed, with archaeal communities showing more significant restructuring, including reduced dominance of acetoclastic methanogens. At last, strategies to mitigate salinity inhibition were presented, along with prospects for future research directions. This review provides theoretical guidance for engineering applications and strategies for enhancing AD in treating saline substrates.

Metagenomic Exploration Uncovers Several Novel 'Candidatus' Species Involved in Acetate Metabolism in High-Ammonia Thermophilic Biogas Processes

Biogas reactors operating at elevated ammonia levels are commonly susceptible to process disturbances, further augmented at thermophilic temperatures. The major cause is assumed to be linked to inhibition followed by an imbalance between different functional microbial groups, centred around the last two steps of the anaerobic digestion, involving acetogens, syntrophic acetate oxidisers (SAOB) and methanogens. Acetogens are key contributors to reactor efficiency, acting as the crucial link between the hydrolysis and fermentation steps and the final methanogenesis step. Their major product is acetate, at high ammonia levels further converted by SAOB and hydrogenotrophic methanogens to biogas. Even though these functionally different processes are well recognised, less is known about the responsible organism at elevated temperature and ammonia conditions. The main aim of this study was to garner insights into the penultimate stages in three thermophilic reactors (52°C) operated under high ammonia levels (FAN 0.7-1.0 g/L; TAN 3.6-4.4 g/L). The primary objective was to identify potential acetogens and SAOBs. Metagenomic data from the three reactors were analysed for the reductive acetyl-CoA pathway (Wood-Ljungdahl Pathway) and glycine synthase reductase pathway. The results revealed a lack of true acetogens but uncovered three potential SAOB candidates that harbour the WLP, 'Candidatus Thermodarwinisyntropha acetovorans', 'Candidatus Thermosyntrophaceticus schinkii', 'Candidatus Thermotepidanaerobacter aceticum', and a potential lipid-degrader 'Candidatus Thermosyntrophomonas ammoiaca'.

Applicability analysis of algae biochar for anaerobic membrane bioreactors in wastewater treatment: A review from a sustainability assessment perspective

The incorporation of biochar can significantly enhance the performance of anaerobic membrane bioreactors (AnMBRs), achieving up to a 95 % increase in pollutant removal efficiency and an 86 % improvement in methane production. Algae biochar, in particular, shows great promise as an effective additive in AnMBR systems because of its low cost (approximately $0.470/kg) and the abundance of raw material sources. This paper presents a comprehensive applicability analysis of algae biochar-AnMBRs from a sustainability assessment perspective, addressing technical, environmental, economic, and social dimensions. Key technical benefits include a reduction in membrane fouling by 92.1 % and an enhancement of energy recovery by 58.7 % compared to conventional AnMBRs. Following this, the paper evaluates algae biochar-AnMBRs from environmental, economic, and social viewpoints to emphasize the practical applicability and potential of this process. Finally, this review addresses the limitations related to the full-scale implementation of this technology and proposes strategic approaches to overcome these challenges. Overall, the review provides valuable insights into the practical application of algae biochar-AnMBR systems, with a strong focus on sustainability.

Pathways, challenges, and strategies for enhancing anaerobic production of short-chain and medium-chain carboxylic acids from algal slurry derived from wastewater

Algal slurry (AS) generated from microalgae-based wastewater treatment processes holds significant potential for carboxylic acids production through anaerobic digestion (AD), which have emerged as promising products due to their high energy density, great economic value, and versatile applications. A comprehensive analysis of the pathways and optimization strategies for producing short-chain (SCCAs) and medium-chain (MCCAs) carboxylic acids using AS substrates is presented in this review. It begins by introducing and comparing two types of microalgae-based wastewater treatment processes: the microalgae process and the microalgal-bacterial consortia process. Afterwards, the review systematically examines the metabolic pathways involved in SCCAs and MCCAs production using AS substrates. Moreover, pretreatment strategies for enhancing the release of organic matter are critically discussed. Ultimately, specific emphasis is placed on addressing technical challenges and discussing future perspectives. This review provides a deeper understanding of the mechanisms and strategies involved in carboxylic acids production from wastewater-generated AS.

The potential role of biochar in mitigating gaseous emissions from livestock waste - A mini-review

The livestock industry plays a significant role in the economic well-being of many parts of the world with a host of environmental challenges. Key amongst them is the management of gaseous emissions emitted from livestock manure. Mitigation of gaseous emissions from livestock operations such as odor, odorous volatile organic compounds (VOCs), ammonia (NH3), hydrogen sulfide (H2S), and greenhouse gases (GHGs) have been of research interest for the last couple of decades. Biochar, a low-cost-value byproduct of biorenewable energy and thermochemical waste processing compared with syngas and bio-oil, has been actively researched as a potential surficial treatment of manure and emissions from stored or co-composted manure. Yet, the efficacy of biochar treatment differs, partly because biochar properties vary with feedstock and thermochemical processing conditions. To date, the results from laboratory-scale trials are encouraging, but a more focused effort is needed to bring this technology closer to farm-scale applications. Therefore, this review aims to summarize and highlight current research related to mitigating gaseous emissions from manure treated with biochar. Various types of biochar, and modes of biochar applications, e.g., manure additives and co-composting, dosage, and timing, are discussed in the context of targeted gas emissions mitigation. Gaps in knowledge remain, including demonstrated larger-scale mitigation performance and verifiable technoecomics. Standardization and certification of biochar properties suitable for specific environmental management applications are recommended. The potential synergy between mitigating emissions, improving manure quality, carbon, and nitrogen cycling in animal and crop production agriculture is found. Biochar can be a comprehensive solution to gaseous emissions while also upgrading manure as a high-quality additive that could improve the sustainability of animal and crop production systems.

The effects of microalgae use as a biofertilizer on soil and plant before and after its anaerobic (co-)digestion with food waste

The increase in food waste generation has resulted in significant challenges for its sustainable management. Anaerobic digestion coupled with microalgae-based ponds for digestate treatment can be used as a low-cost eco-friendly technology approach. In this case, microalgal biomass harvested from the ponds may be valorized into bioenergy (biogas) and soil conditioner and/or biofertilizers. The aim of the present study was to evaluate the microalgal biomass produced from a food waste digestate treatment ponds as agricultural fertilizer. For this purpose, microalgal biomass was tested before and after anaerobic digestion and co-digestion with food waste, exploring its potential for valorization. The inorganic fertilizer urea and soil with no fertilization were also used as treatments. The experimental design consisted of applying the treatments in pots cultivated with hybrid grass Brachiaria cv. Sabiá and distributed in randomized blocks in a controlled greenhouse. Microalgal biomass was mainly composed by Scenedesmus sp.. The assessed parameters showed comparable results on plant growth (i.e. number of tillers, fresh and dry matter and Chlorophyll content index) for fresh and digested microalgal biomass and inorganic fertilizer. Furthermore, it was observed that fresh microalgae provided the highest Phosphorus content in the leaf (21 %). Additionally, there were increases of 9 % in Nitrogen and 12 % in organic matter in the soil after applying digested microalgae compared to the control group without any fertilization. Finally, experimental data obtained suggests that microalgae-based biofertilizer holds the potential to replace inorganic fertilizer as a nutrient source. Moreover, it contributes to the valorization of by-products from organic waste treatment.

Microbial community changes and metabolic pathways analysis during waste activated sludge and meat processing waste anaerobic co-digestion

Waste activated sludge (WAS) and meat processing waste (MPW) were acted as co-substrates in anaerobic co-digestion (AcD), and biochemical methane potential (BMP) test was carried out to investigate the methane production performances. Microbial community structure and metabolic pathways analyses were conducted by 16S rRNA high-throughput sequencing and functional prediction analysis. BMP test results indicated that AcD of 70% WAS+30% MPW and 50% WAS+50% MPW (VS/VS) could significantly improve methane yield to 371.05 mL/g VS and 599.61 mL/g VS, respectively, compared with WAS acting as sole substrate (191.87 mL/g VS). The results of microbial community analysis showed that Syntrophomonas and Petrimonas became the dominant bacteria genera, and Methanomassiliicoccus and Methanobacterium became the dominant archaea genera after MPW addition. 16S functional prediction analysis results indicated that genes expression of key enzymes involved in syntrophic acetate oxidation (SAO), hydrogenotrophic and methylotrophic methanogenesis were up-regulated, and acetoclastic methanogenesis was inhibited after MPW addition. Based on these analyses, it could be inferred that SAO combined with hydrogenotrophic and methylotrophic methanogenesis was the dominant pathway for organics degradation and methane production during AcD. These findings provided systematic insights into the microbial community changes and metabolic pathways during AcD of WAS and MPW.

Pristine and modified biochar applications as multifunctional component towards sustainable future: Recent advances and new insights

Employing biomass for environmental conservation is regarded as a successful and environmentally friendly technique since they are cost-effective, renewable, and abundant. Biochar (BC), a thermochemically converted biomass, has a considerably lower production cost than the other conventional activated carbons. This material's distinctive properties, including a high carbon content, good electrical conductivity (EC), high stability, and a large surface area, can be utilized in various research fields. BC is feasible as a renewable source for potential applications that may achieve a comprehensive economic niche. Despite being an inexpensive and environmentally sustainable product, research has indicated that pristine BC possesses restricted properties that prevent it from fulfilling the intended remediation objectives. Consequently, modifications must be made to BC to strengthen its physicochemical properties and, thereby, its efficacy in decontaminating the environment. Modified BC, an enhanced iteration of BC, has garnered considerable interest within academia. Many modification techniques have been suggested to augment BC's functionality, including its adsorption and immobilization reliability. Modified BC is overviewed in its production, functionality, applications, and regeneration. This work provides a holistic review of the recent advances in synthesizing modified BC through physical, chemical, or biological methods to achieve enhanced performance in a specific application, which has generated considerable research interest. Surface chemistry modifications require the initiation of surface functional groups, which can be accomplished through various techniques. Therefore, the fundamental objective of these modification techniques is to improve the efficacy of BC contaminant removal, typically through adjustments in its physical or chemical characteristics, including surface area or functionality. In addition, this article summarized and discussed the applications and related mechanisms of modified BC in environmental decontamination, focusing on applying it as an ideal adsorbent, soil amendment, catalyst, electrochemical device, and anaerobic digestion (AD) promoter. Current research trends, future directions, and academic demands were available in this study.

Comparative study of high-performance mesophilic and thermophilic anaerobic membrane bioreactors in the co-digestion of sewage sludge and food waste: Methanogenic performance and energy recovery potential

To support smart cities in terms of waste management and bioenergy recovery, the co-digestion of sewage sludge (SeS) and food waste (FW) was conducted by the anaerobic membrane bioreactor (AnMBR) under mesophilic and thermophilic conditions in this study. The biogas production rate of the thermophilic AnMBR (ThAnMBR) at the SeS to FW ratio of 0:100, 75:25, 50:50 and 100:0 was 2.84 ± 0.21, 2.51 ± 0.26, 1.54 ± 0.26 and 1.31 ± 0.08 L-biogas/L/d, inconspicuous compared with that of the mesophilic AnMBR (MeAnMBR) at 3.00 ± 0.25, 2.46 ± 0.30, 1.63 ± 0.23 and 1.30 ± 0.17 L-biogas/L/d, respectively. The higher hydrolysis ratio and the poorer rejection efficiencies of the membrane under thermophilic conditions, resulting that the permeate COD, carbohydrate and protein of the ThAnMBR was higher than that of the MeAnMBR. The lost COD that might be converted into biogas was discharged with the permeate in the ThAnMBR, which was partly responsible for the inconspicuous methanogenic performance. Furthermore, the results of energy recovery potential assessment showed that the energy return on investment (EROI) of the MeAnMBR was 4.54, 3.81, 2.69 and 2.22 at the four SeS ratios, which was higher than that of the ThAnMBR at 3.29, 2.97, 2.02 and 1.80, respectively, indicating the advantage of the MeAnMBR over the ThAnMBR in energy recovery potential. The outcomes of this study will help to choose a more favorable temperature to co-digest SeS and FW to support the construction of smart cities.

A review of microbial responses to biochar addition in anaerobic digestion system: Community, cellular and genetic level findings

The biochar is a well-developed porous material with various excellent properties, that has been proven with excellent ability in anaerobic digestion (AD) efficiency promotion. Current research is usually focused on the macro effects of biochar on AD, while the systematic review about the mechanisms of biochar on microbial behavior are still lacking. This review summarizes the effects and potential mechanisms of biochar on microorganisms in AD systems, and found that biochar addition can provide habitats for microbial colonization, alleviate toxins stress, supply essential nutrients, and accelerate interspecies electron transferring. Moreover, it also improves microbial community diversity, facilitates EPS secretion, enhances functional enzyme activity, promotes functional genes expression, and inhibits the antibiotic resistance genes transformation. Future research directions including biochar targeted design, in-depth microbial mechanisms revelation, and modified model development were suggested, which could promote the widely practical application of of biochar-amended AD technology.

Advances, trends and challenges in the use of biochar as an improvement strategy in the anaerobic digestion of organic waste: a systematic analysis

A recently strategy applied to anaerobic digestion (AD) is the use of biochar (BC) obtained from the pyrolysis of different organic waste. The PRISMA protocol-based review of the most recent literature data from 2011-2022 was used in this study. The review focuses on research papers from Scopus® and Web of Knowledge®. The review protocol used permits to identify 169 articles. The review indicated a need for further research in the following challenges on the application of BC in AD: i) to increase the use of BC in developing countries, which produce large and diverse amounts of waste that are the source of production of this additive; ii) to determine the effect of BC on the AD of organic waste under psychrophilic conditions; iii) to apply tools of machine learning or robust models that allow the process optimization; iv) to perform studies that include life cycle and technical-economic analysis that allow identifying the potential of applying BC in AD in large-scale systems; v) to study the effects of BC on the agronomic characteristics of the digestate once it is applied to the soil and vi) finally, it is necessary to deepen in the effect of BC on the dynamics of nitrogen and microbial consortia that affect AD, considering the type of BC used. In the future, it is necessary to search for new solutions in terms of the transport phenomena that occurs in AD with the use of BC using robust and precise mathematical models at full-scale conditions.

Microalgae-based biochar production and applications: A comprehensive review

Biochar, a solid carbonaceous substance synthesized from the thermochemical degradation of biomass, holds significant potential in addressing global challenges such as soil degradation, environmental pollution, and climate change. Its potential as a carbon sequestration agent, together with its versatile applications in soil amendments, pollutant adsorption, and biofuel production, has garnered attention. On the other hand, microalgae, with their outstanding photosynthetic efficiency, adaptability, and ability to accumulate carbohydrates and lipids, have demonstrated potential as emerging feedstock for biochar production. However, despite the significant potential of microalgal biochar, our current understanding of its various aspects, such as the influence of parameters, chemical modifications, and applications, remains limited. Therefore, this review aims to provide a comprehensive analysis of microalgae-based biochar, covering topics such as production techniques, pollutant removal, catalytic applications, soil amendments, and synthesis of carbon quantum dots to bridge the existing knowledge gap in this field.

Metagenomic approach reveals the mechanism of calcium oxide improving kitchen waste dry anaerobic digestion

In light of the characteristics of excessive acidification and low biogas yield during kitchen waste (KW) dry digestion, the impact of the calcium oxide (CaO) on KW mesophilic dry digestion was investigated, and the enhanced mechanism was revealed through metagenomic approach. The results showed that CaO increased the biogas production, when the CaO dosage was 0.07 g/g (based on total solid), the biogas production reached 656.84 mL/g suspended solids (VS), approximately 8.38 times of that in the control. CaO promoted the leaching and hydrolysis of key organic matter in KW. CaO effectively promoted the conversion of volatile fatty acid (VFA) and mitigated over-acidification. Macrogenome analysis revealed that CaO increased the microbial diversity in KW dry digestion and upregulated the abundance of genes related to amino acid and carbohydrates metabolism. This study provides an effective strategy with potential economic benefits to improve the bioconversion efficiency of organic matter in KW.

Net zero emission in circular bioeconomy from microalgae biochar production: A renewed possibility

The rapid expansion of industrialization and continuous population growth have caused a steady increase in energy consumption. Despite using renewable energy, such as bioethanol, to replace fossil fuels had been strongly promoted, however the outcomes were underwhelming, resulting in excessive greenhouse gases (GHG) emissions. Microalgal biochar, as a carbon-rich material produced from the pyrolysis of biomass, provides a promising solution for achieving net zero emission. By utilizing microalgal biochar, these GHG emissions can be captured and stored efficiently. It also enhances soil fertility, improves water retention, and conduct bioremediation in agriculture and environmental remediation field. Moreover, incorporating microalgal biochar into a zero-waste biorefinery could boost the employ of biomass feedstocks effectively to produce valuable bioproducts while minimizing waste. This contributes to sustainability and aligns with the concepts of a circular bioeconomy. In addition, some challenges like commercialization and standardization will be addressed in the future.

Enhancement of anaerobic digestion of high salinity food waste by magnetite and potassium ions: Digestor performance, microbial and metabolomic analyses

The study investigated the effectiveness of magnetite and potassium ions (K+) in enhancing anaerobic digestion of high salinity food waste. Results indicated that both magnetite and K+ improved anaerobic digestion in high-salt environments, and their combination yielded even better results. The combination of magnetite and K+ promoted microorganism activity, and resulted in increased abundance of DMER64, Halobacteria and Methanosaeta. Metabolomic analysis revealed that magnetite mainly influenced quorum sensing, while K+ mainly stimulated the synthesis of compatible solutes, aiding in maintaining osmotic balance. The combined additives regulated pathways such as ATP binding cassette transport, methane metabolism, and inhibitory substance metabolism, enabling cells to resist environmental stress and maintain normal metabolic activity. Overall, this study demonstrated the potential of magnetite and K+ to enhance food waste anaerobic digestion in high salt conditions and provided valuable insights into the molecular mechanism.

Biochar symbiosis in anaerobic digestion to enhance biogas production: A comprehensive review

In recent years, anaerobic digestion (AD) has gained popularity as a practical method for generating clean energy and efficiently managing organic waste. However, the effectiveness of the reactor is compromised by the accumulation of ammonia, acids, and nutrients, leading to inhibition and instability. Because of its adaptability, biochar (BC) has sparked a substantial interest in biogas production and can be created by charring biomass and waste materials. Adding BC to the AD process could yield the following benefits: mitigating toxic inhibition, reducing the duration of the methanogenic lag phase, immobilising functional bacteria, and enhancing the rate of electron transfer between methanogenic and acetogenic microorganisms. Nonetheless, there remains to be more comprehensive knowledge regarding the multifaceted function of BC and its intricate mechanisms in the generation of biogas in AD. The research summarises scattered information from the literature on BC production from various feedstocks and factors affecting its characteristics. Additionally, a comprehensive analysis of the utilisation of BC as an additive within AD is presented here, emphasising how BC characteristics impact AD processes and how they effectively engage key challenges.

Effects of cold isostatic press pretreatment of rice straw on microstructure and efficiency of anaerobic digestion for methane production

In this study, rice straw was pretreated using by cold isostatic pressure to disrupt its microstructure and improve the performance of anaerobic digestion, and the optimal process parameters were optimized using the response surface methodology. The results showed that cold isostatic pressure pretreatment under optimal conditions (pressure of 400 MPa and holding time of 9 min) was effective in disrupting the structure of rice straw and improving its biodegradability. The cumulative methane production of the cold isostatic pressure pretreatment group increased by 76% compared to the untreated group. In addition, microbial community analysis showed that the relative abundance of Firmicutes, Halobacterota, DMER64 and Methanosaeta was higher in groups pretreated with cold isostatic pressure than in untreated groups. This study demonstrated the potential of pretreatment of rice straw with cold isostatic pressure to increase methane production during anaerobic digestion.

Chitosan-Fe3O4 composites enhance anaerobic digestion of liquor wastewater under acidic stress

Acid stress in the anaerobic digestion process of liquor wastewater leads to low anaerobic treatment efficiency. Herein, chitosan-Fe₃O₄ was prepared, and its effects on anaerobic digestion processes under acid stress were studied. Results showed that chitosan-Fe₃O₄ increased the methanogenesis rate of anaerobic digestion of acidic liquor wastewater by 1.5-2.3 times and accelerated the restoration of acidified anaerobic systems. The analysis of sludge characteristics showed that chitosan-Fe₃O₄ promoted the secretion of proteins and humic substances in extracellular polymeric substances and increased the electron transfer activity of the system by 71.4%. Microbial community analysis indicated that chitosan-Fe₃O₄ enriched the abundance of Peptoclostridium, and Methanosaeta participated in direct interspecies electron transfer. Chitosan-Fe₃O₄ could promote the direct interspecies electron transfer pathway to maintain stable methanogenesis. These methods and results regarding the use of chitosan-Fe₃O₄ could be referred to for improving the efficiency of anaerobic digestion of high concentration organic wastewater under acid inhibition.

Synergetic effects of anaerobic co-digestion of food waste and algae on biogas production

Anaerobic co-digestion of food waste and algae was assessed to offset the drawbacks of anaerobic mono-digestion of each substrate. Batch test results indicated that a food waste and algae mixture ratio of 8:2 facilitated the highest CH₄ yield (334 mL CH₄/g COD_Input). This ratio was applied to the anaerobic co-digestion reactor, resulting in a CH₄ yield that was twice that of the anaerobic mono-digestion reactors, thereby facilitating high operational stability. In contrast to the anaerobic mono-digestion, anaerobic co-digestion resulted in stable CH₄ production by overcoming volatile fatty acid accumulation and a decreased pH, even under a high organic loading rate (3 kg COD/m³∙d). Furthermore, a comparative metagenomic analysis revealed that the abundance of volatile fatty acid-oxidizing bacteria and hydrogenotrophic and methylotrophic methanogens was significantly increased in the anaerobic co-digestion reactor. These findings indicate that the anaerobic co-digestion of food waste and algae significantly improves CH₄ production and process stability.

Enhanced methane production from the anaerobic co-digestion of food waste plus fruit and vegetable waste

Food waste (FW) and fruit, vegetable waste (FVW) are important components of municipal solid waste, yet the performance and related mechanisms of anaerobic co-digestion of FW and FVW for methane production have been rarely investigated. In order to get a deeper understanding of the mechanisms involved, the mesophilic FW and FVW anaerobic co-digestion in different proportions was investigated. The experimental results showed that when the ratio of FW and FVW was 1/1 (in terms of volatile suspended solid), the maximum biomethane yield of 269.9 mL/g TCOD from the codigested substrate is significantly higher than that in FW or FVW anaerobic digestion alone. FW and FVW co-digestion promoted the dissolution and biotransformation of organic matter. When the recommended mixing ratio was applied, the maximum concentration of dissolved chemical oxygen demand (COD) was high as 11971 mg/L. FW and FVW co-digestion reduced the accumulation of volatile fatty acids (VFA) in the digestive system, thus reducing its negative impact on the methanogenesis process. FW and FVW co-digestion process synergistically enhanced microbial activity. The analysis of microbial population structure showed that when FW and FVW were co-digested at the recommended ratio, the relative abundance of Proteiniphilum increased to 26.5%, and the relative abundances of Methanosaeta and Candidatus Methanofastidiosum were also significantly increased. The results of this work provide a certain amount of theoretical basis and technical support for the co-digestion of FW and FVW.

Bioresource Upgrade for Sustainable Energy, Environment, and Biomedicine

We conceptualize bioresource upgrade for sustainable energy, environment, and biomedicine with a focus on circular economy, sustainability, and carbon neutrality using high availability and low utilization biomass (HALUB). We acme energy-efficient technologies for sustainable energy and material recovery and applications. The technologies of thermochemical conversion (TC), biochemical conversion (BC), electrochemical conversion (EC), and photochemical conversion (PTC) are summarized for HALUB. Microalgal biomass could contribute to a biofuel HHV of 35.72 MJ Kg-1 and total benefit of 749 $/ton biomass via TC. Specific surface area of biochar reached 3000 m2 g-1 via pyrolytic carbonization of waste bean dregs. Lignocellulosic biomass can be effectively converted into bio-stimulants and biofertilizers via BC with a high conversion efficiency of more than 90%. Besides, lignocellulosic biomass can contribute to a current density of 672 mA m-2 via EC. Bioresource can be 100% selectively synthesized via electrocatalysis through EC and PTC. Machine learning, techno-economic analysis, and life cycle analysis are essential to various upgrading approaches of HALUB. Sustainable biomaterials, sustainable living materials and technologies for biomedical and multifunctional applications like nano-catalysis, microfluidic and micro/nanomotors beyond are also highlighted. New techniques and systems for the complete conversion and utilization of HALUB for new energy and materials are further discussed.

Biochar addition augmented the microbial community and aided the digestion of high-loading slaughterhouse waste: Active enzymes of bacteria and archaea

The biogas production (BP), volatile fatty acids (VFAs), microbial communities, and microbes' active enzymes were studied upon the addition of biochar (0-1.5%) at 6% and 8% slaughterhouse waste (SHW) loadings. The 0.5% biochar enhanced BP by 1.5- and 1.6-folds in 6% and 8% SHW-loaded reactors, respectively. Increasing the biochar up to 1.5% caused a reduction in BP at 6% SHW. However, the BP from 8% of SHW was enhanced by 1.4-folds at 1.5% biochar. The VFAs production in all 0.5% biochar amended reactors was highly significant compared to control (p-value < 0.05). The biochar addition increased the bacterial and archaeal diversity at both 6% and 8% SHW loadings. The highest number of OTUs at 0.5% biochar were 567 and 525 in 6% and 8% SHW, respectively. Biochar prompted the Clostridium abundance and increased the lyases and transaminases involved in the degradation of lipids and protein, respectively. Biochar addition improved the Methanosaeta and Methanosphaera abundance in which the major enzymes were reductase and hydrogenase. The archaeal enzymes showed mixed acetoclastic and hydrogenotrophic methanogenesis.

Production of biochar from crop residues and its application for anaerobic digestion

Anaerobic digestion (AD) is a viable and cost-effective method for converting organic waste into usable renewable energy. The efficiency of organic waste digestion, nonetheless, is limited due to inhibition and instability. Accordingly, biochar is an effective method for improving the efficiency of AD by adsorbing inhibitors, promoting biogas generation and methane concentration, maintaining process stability, colonizing microorganisms selectively, and mitigating the inhibition of volatile fatty acids and ammonia. This paper reviews the features of crop waste-derived biochar and its application in AD systems. Four critical roles of biochar in AD systems were identified: maintaining pH stability, promoting hydrolysis, enhancing the direct interspecies electron transfer pathway, and supporting microbial development. This work also highlights that the interaction between biochar dose, amount of organic component in the substrate, and inoculum-to-substrate ratio should be the focus of future research before deploying commercial applications.

Potentiality of recovering bioresource from food waste through multi-stage Co-digestion with enzymatic pretreatment

Food waste (FW) is not only a major social, nutritional and environmental issue, but also an underutilized resource with significant energy, which has not been fully explored currently. Considering co-digestion can adjust carbon to nitrogen ratio (C/N) of the feedstock and improve the synergetic interactions among microorganisms, anaerobic co-digestion (AnCoD) is then becoming an emerging approach to achieve higher energy recovery from FW while ensuring the stability of the system. To obtain higher economic gain from such biodegradable wastes, increasing attention has been paid on optimizing the system configuration or applying enzymatic hydrolysis before digesting FW. A better understanding on the potentiality of correlating enzymatic pretreatment and AnCoD operated in various system configuration would enhance the bioresource recovery from FW and increase revenue through treating this organic waste. Specifically, the biobased chemicals outputs from FW-related co-digestion system with different configuration were firstly compared in this review. A deep discussion concerning the challenges for achieving bioresources recovery from FW co-digestion systems with enzymatic pretreatment was then given. Recommendations for future studies regarding FW co-digestion were then proposed at last.

Unblocking the rate-limiting step of the municipal sludge anaerobic digestion

Anaerobic digestion stabilizes municipal sludge through total solids reduction and biogas production. It is generally accepted that hydrolysis accounts for the rate-limiting step of municipal sludge anaerobic digestion, impacting the overall rates of solids reduction and methane production. Technically, the sludge hydrolysis rate can be enhanced by the application of thermal hydrolysis pretreatment (THP) and is also affected by the total solids concentration, temperature, and solids retention time used in the anaerobic digestion. This study systematically analyzed and compared ways to take these four factors into the consideration of modern anaerobic digestion system for achieving the maximum solid reduction. Results showed that thermophilic anaerobic digestion was superior to mesophilic anaerobic digestion in terms of solids reduction but vice versa in terms of the methane production when integrated with THP. This difference has to do with the intermediate product accumulation and inhibition when hydrolysis outpaced methanogenesis in THP-enhanced thermophilic anaerobic digestion, which can be mitigated by adjusting the solids retention time. PRACTITIONER POINTS: THP followed by TAD offers the greatest solids reduction rate. THP followed by MAD offered the greatest methane production rate. FAN inhibition appears to be an ultimate limiting factor constraining the methane production rate. In situ ammonia removal technique should be developed to further unblock the rate-limiting step.

Enhancement of Anaerobic Digestion with Nanomaterials: A Mini Review

In recent years, the number of articles reporting the addition of nanomaterials to enhance the process of anaerobic digestion has exponentially increased. The benefits of this addition can be observed from different aspects: an increase in biogas production, enrichment of methane in biogas, elimination of foaming problems, a more stable and robust operation, absence of inhibition problems, etc. In the literature, one of the current focuses of research on this topic is the mechanism responsible for this enhancement. In this sense, several hypotheses have been formulated, with the effect on the redox potential caused by nanoparticles probably being the most accepted, although supplementation with trace materials coming from nanomaterials and the changes in microbial populations have been also highlighted. The types of nanomaterials tested for the improvement of anaerobic digestion is today very diverse, although metallic and, especially, iron-based nanoparticles, are the most frequently used. In this paper, the abovementioned aspects are systematically reviewed. Another challenge that is treated is the lack of works reported in the continuous mode of operation, which hampers the commercial use of nanoparticles in full-scale anaerobic digesters.

Sustainable microalgal biomass valorization to bioenergy: Key challenges and future perspectives

The global trend is shifting toward circular economy systems. It is a sustainable environmental approach that sustains economic growth from the use of resources while minimizing environmental impacts. The multiple industrial use of microalgal biomass has received great attention due to its high content of essential nutrients and elements. Nevertheless, low biomass productivity, unbalanced carbon to nitrogen (C/N) ratio, resistant cellular constituents, and the high cost of microalgal harvesting represent the major obstacles for valorization of algal biomass. In recent years, microalgae biomass has been a candidate as a potential feedstock for different bioenergy generation processes with simultaneous treating wastewater and CO₂ capture. An overview of the appealing features and needed advancements is urgently essential for microalgae-derived bioenergy generation. The present review provides a timely outlook and evaluation of biomethane production from microalgal biomass and related challenges. Moreover, the biogas recovery potential from microalgal biomass through different pretreatments and synergistic anaerobic co-digestion (AcoD) with other biowastes are evaluated. In addition, the removal of micropollutants and heavy metals by microalgal cells via adsorption and bioaccumulation in their biomass is discussed. Herein, a comprehensive review is presented about a successive high-throughput for anaerobic digestion (AD) of the microalgal biomass in order to achieve for sustainable energy source. Lastly, the valorization of the digestate from AD of microalgae for agricultural reuse is highlighted.

Dry anaerobic digestion of ammoniated straw: Performance and microbial characteristics

This paper explores the influence of the mixing ratio of ammoniated straw to biogas residue on the stability and methane yield of dry anaerobic digestion and analyzes the structure of the microbial community with digestion time. Five reactors containing ammoniated straw and swine manure biogas residue at ratios of 5:1, 4:2, 3:3, 2:4 and 1:5 (total solids) were constructed, and neither total ammonia nitrogen nor free ammonia nitrogen was inhibited. Three reactors produced gas successfully. The reactor with a ratio of 3:3 (R3-3) yielded the best methane production, with a cumulative methane production of 115.13 mL/(g·VS_added). Analysis of the R3-3 microbial community showed that bacteria were dominant species. Archaea, mainly Methanosarcina, played an important role in anaerobic digestion and methane production. Methanobacterium, with high acid tolerance, was positively related to total volatile fatty acids (TVFA), playing a key role in preventing the acidification of the anaerobic digestion system.

Microbial succession analysis reveals the significance of restoring functional microorganisms during rescue of failed anaerobic digesters by bioaugmentation of nano-biochar-amended digestate

Nano-biochar application was investigated for anaerobic digestion of orange peel waste. The application for methane production focused on the optimization of biochar feedstock, rescue of failed digesters, and microbial succession analysis. It showed that sewage sludge (SS) derived biochar had the highest performance enhancement among the different feedstocks, which could be ascribed to the improvement of electron transfer, interspecies hydrogen transfer, and supply of trace elements. Subsequently, nano SS biochar-amended digestate was evaluated for rescuing failed digesters, and the experimental results indicated its positive roles through gradual bioaugmentation operation. The dynamic analysis of microbial succession indicated the successful application was through the mechanism of restoring partially the functional microbial communities. The major reconstruction of functional microorganisms included bacteria phyla Hydrogenispora (24.5%) and Defluviitoga (18.8%) as well as methanogenic genera of Methanosarcina (41.5%) and Methanobacterium (27.3%). These findings would contribute to rescuing failed anaerobic digesters by bioaugmentation with biochar-amended digestate.

Plastic-containing food waste conversion to biomethane, syngas, and biochar via anaerobic digestion and gasification: Focusing on reactor performance, microbial community analysis, and energy balance assessment

To manage the mixture of food waste and plastic waste, a hybrid biological and thermal system was investigated for converting plastic-containing food waste (PCFW) into renewable energy, focusing on performance evaluation, microbial community analysis, and energy balance assessment. The results showed that anaerobic digestion (AD) of food waste, polyethylene (PE)-containing food waste, polystyrene (PS)-containing food waste, and polypropylene (PP)-containing food waste generated a methane yield of 520.8, 395.6, 504.2, and 479.8 mL CH₄/gVS, respectively. CO₂ gasification of all the plastic-containing digestate produced more syngas than pure digestate gasification. Syngas from PS-digestate reached the maximum yield of 20.78 mol/kg. During the digestate-derived-biochar-amended AD of PCFW, the methane yields in the biochars-amended digesters were 6-30% higher than those of the control digesters. Bioinformatic analysis of microbial communities confirmed the significant difference between control and biochar-amended digesters in terms of bacterial and methanogenic compositions. The enhanced methane yields in biochars-amended digesters could be partially ascribed to the selective enrichment of genus Methanosarcina, leading to an improved equilibrium between hydrogenotrophic and acetoclastic methanogenesis pathways. Moreover, energy balance assessment demonstrated that the hybrid biological and thermal conversion system can be a promising technical option for the treatment of PCFW and recovery of renewable biofuels (i.e., biogas and syngas) and bioresource (i.e., biochar) on an industrial scale.

Bacterial Community Structure and Predicted Metabolic Function of Landfilled Municipal Solid Waste in China

The biodegradation behavior of municipal solid waste (MSW) depends on the diversity and metabolic function of bacterial communities, which are affected by environmental factors. However, the diversity of the bacterial communities and metabolic functions in MSW, as well as their influencing factors, remain unclear. In deep-aged MSW, the abovementioned deficiencies are more significant, and will effectively hamper landfill disposal. In this study, high-throughput sequencing was performed to examine the bacterial community structure and metabolic function from depths of 10 m to 40 m, of two large MSW landfills on the southeast coast of China. Thermotogota (1.6–32.0%), Firmicutes (44.2–77.1%), and Bacteroidota (4.0–34.3%) were the three dominant phyla among the 39 bacterial phyla identified in aged MSW samples. Bacterial genera associated with the degradation of many macromolecules, e.g., Defluviitoga, Hydrogenispora, and Lentimicrobium were abundantly detected in MSW samples, even in aged MSW. Redundancy analysis (RDA) showed that bacterial diversity in the landfills was most strongly correlated with electrical conductivity, age, and moisture content of the MSW. Tax4fun2 analysis predicted that there were abundant metabolism functions in aged MSW, especially functional enzymes (e.g., glycine dehydrogenase and cellulase) related to amino acids and cellulose degradation. This study increases our understanding of the bacterial diversity and functional characteristics in landfilled MSW.

Recent advances, current issues and future prospects of bioenergy production: A review

With the immense potential of bioenergy to drive carbon neutrality and achieve the climate targets of the Paris Agreement, this paper aims to present the recent advances in bioenergy production as well as their limitations. The novelty of this review is that it covers a comprehensive range of strategies in bioenergy production and it provides the future prospects for improvement. This paper reviewed more than 200 peer-reviewed scholarly papers mainly published between 2010 and 2021. Bioenergy is derived from biomass, which, through thermochemical and biochemical processes, is converted into various forms of biofuels. This paper reveals that bioenergy production is temperature-dependent and thermochemical processes currently have the advantage of higher efficiency over biochemical processes in terms of lower response time and higher conversion. However, biochemical processes produce more volatile organic compounds and have lower energy and temperature requirements. The combination of the two processes could fill the shortcomings of a single process. The choices of feedstock are diverse as well. In the future, it can be anticipated that continuous technological development to enhance the commercial viability of different processes, as well as approaches of ensuring their sustainability, will be among the main aspects to be studied in greater detail.

The dosage-effect of biochar on anaerobic digestion under the suppression of oily sludge: Performance variation, microbial community succession and potential detoxification mechanisms

This study investigated the dosage-effect of biochar on the suppressed mesophilic digestion of oily sludge (OS) containing naphthalene (recalcitrant compound) and starch (easily bioavailable substrate). Methanogenesis was inhibited in control with OS, where biomethane yield (63.33 mL/gVS) was obviously lower than theoretical yield (260.55 mL/gVS). With adding optimal dose of biochar (0.60 g/gVS OS), the highest CH₄ yield (138.41 mL/gVS) was 2.19 times of control. Meanwhile, the efficiencies of hydrolysis, acidogenesis and acetogenesis were significantly enhanced. However, excessive biochar (4.80 g/gVS OS) caused negative effects with methanogenic efficiency diminished by 32.5% and lag phase prolonged by 5.72 h. Dissolved organic matter (DOM) analysis showed that humic acid-like and fulvic acid-like components percentages of fluorescence regional integration were decreased because of the adsorption of biochar. In addition, biochar mediating interspecies electron transfer selectively enriched electroactive fermentation bacteria (Clostridium and Bacteroides) and acetoclastic Methanosaeta, which was responsible for promoting mesophilic digestion performance. The functional genes related to metabolism and environmental information processing were potentially activated by biochar. Above results indicate that moderate biochar application may mitigate the bio-toxicity suppression of OS, which help to provide a promising pathway for reinforcing oily wastes bio-treatment.

Effects of plastics on reactor performance and microbial communities during acidogenic fermentation of food waste for production of volatile fatty acids

The aim of this work was to study the effects of plastics (high-density polyethylene (HDPE), polystyrene (PS), polypropylene (PP), and polyethylene terephthalate (PET)) on reactor performance and microbial communities during acidogenic fermentation of food waste for the production of volatile fatty acids (VFA). The addition of HDPE and PS increased total VFA yields by 28% and 47%, respectively, whereas the addition of PP and PET decreased total VFA yields by 6% and 2%, respectively. The highest enhancing performance of PS could be ascribed to its highly porous structure that could provide immobilization effects for microbial growth. Degradation of various plastics was confirmed by FESEM results, but the degrees were limited (i.e., 3.9-8.7%). Bacterial analysis showed that the addition of various plastics altered the community diversity. Phylum Thermotogae and genus Defluviitoga dominated all the reactors. Potential HDPE- and PS-degrading microbes could belong to genus Clostridium_sensu_stricto_8, while Tepidanaerobacter_syntrophicus could be PET-degrading microbes.

Haldane-Andrews substrate inhibition kinetics for pilot scale thermophilic anaerobic degradation of sugarcane vinasse

A pilot scale anaerobic degradation of sugarcane vinasse was carried out at various hydraulic retention time (HRT) in the Anaerobic Suspended Growth Closed Bioreactor (ASGCB) under thermophilic temperature. The performance and kinetics were evaluated through the Haldane-Andrews model to investigate the substrate inhibition potential of sugarcane vinasse. All parameters show great performance between HRT 35 and 25 days: chemical oxygen demand (COD) reduction efficiency (81.6 to 86.8%), volatile fatty acids (VFA) reduction efficiency (92.4 to 98.5%), maximum methane yield (70%) and maximum biogas production (19.35 L/day). Furthermore, steady state values from various HRT were obtained in the kinetic evaluation for: r_Xmax (1.20 /day), K_s (19.95 gCOD/L), K_i (7.00 gCOD/L) and [Formula: see text] (0.33 LCH₄/gCOD _reduction). This study shows that anaerobic degradation of sugarcane vinasse through ASGCB could perform well at high HRT and provides a low degree of substrate inhibition as compared to existing studies from literature.

Timing of biochar dosage for anaerobic digestion treating municipal leachate: Altered conversion pathways of volatile fatty acids

In this study, the anaerobic digestion (AD) applications of early & late biochar dosage were compared for municipal leachate treatment, with the objective of studying the flexible use of biochar as a mitigation measure for biomethane recovery. In two experimental phases, biochar was favourable for the immediate promotion of AD performances, as revealed by Gompertz's model of reduced lag phases, higher biomethane generation rates, and increased biomethane yields. Irrespective of late biochar dosage, it could still retrieve 89% of the ultimate biomethane potential. Comparing the residual VFAs (volatile fatty acids) compositions, it was found that the fraction of long-chain VFAs accounted for 81% of total VFAs in reactor set of early biochar dosage, while it was only 38% in the reactor of late one. Parallel evidence suggested that the schedule of biochar dosage not only could affect methanogenic responses but also the VFAs conversion pathways.

Effect of organic loading rate on the anaerobic digestion of swine waste with biochar addition

The aim of this study was to investigate the impact of biochar addition on the mesophilic semi-continuous anaerobic digestion of swine waste with a focus on the effects of the organic loading rate (OLR) on biogas production, methane yield, total volatile fatty acids (TVFA), alkalinity, ammonium, volatile solids (VS) removal efficiency and process stability. Four reactors, two with amended biochar (R1 and R2) and two without biochar addition as controls (R3 and R4), were operated at OLRs in the range of 2-7 g VS/(L d), which corresponded to hydraulic retention times (HRTs) in the range of 7-2 days, respectively. The addition of biochar initially caused an increase in the generation of biogas and methane when compared to the control reactors when the process operated at OLRs of 2 and 3 g VS/(L d). This behaviour could be attributed to the presence of several trace elements (such as Fe, Co, Ni and Mn) in the biochar, which are involved in the action of acetyl-CoA synthase and methyl coenzyme M reductase to catalyse key metabolic steps, especially the methanogenic stage. The pH, alkalinity, TVFA and TVFA/Alkalinity ratio values for the effluents remained within the optimal ranges for the anaerobic digestion process. It was also found that the increase in OLR in the range of 2-5 g VS/(L d) determined a proportional increase in the VS removal rate. However, when the OLR increased up to 7 g VS/(L d), a drastic decrease in the VS removal rate was found for the control reactors. Biochar amendment contributed to a more balanced state of the anaerobic process, preventing biomass washout.

Beneficial role of biochar addition on the anaerobic digestion of food waste: A systematic and critical review of the operational parameters and mechanisms

The generation of huge amounts of food waste due to the increasing population is a serious global issue. The inadequate management of food waste and lack of proper handling approaches have created adverse negative impacts on the environment and the society. The use of traditional disposal (i.e. landfilling) and treatment (i.e. incineration and composting) methods are not considered to be efficient for managing food waste. Thus, anaerobic digestion (AD) has proven to be promising and cost-effective, as an alternative technology, for digesting and converting food waste into renewable energy and useful chemicals. However, mono-digestion of food waste suffers from process inhibition and instability which limit its efficiency. Adding biochar that has high buffering capacity and ensures optimum nutrient balance was shown to enhance biogas/methane production yields. This review reports on the physicochemical characteristics of food waste, the existing problems of food waste treatment in AD as well as the role of biochar amendments on the optimization of critical process parameters and its action mechanisms in AD, which could be a promising means of improving the AD performance. Also, this review provides insights regarding the selection of the desired/appropriate biochar characteristics, i.e. depending on the source of the feedstock and the pyrolysis temperature, and its role in enhancing biogas production and preventing the problem of process instability in the AD system. Finally, this review paper highlights the economic and environmental challenges as well as the future perspectives concerning the application of biochar amendments in AD.

Microbial community composition and metabolic functions in landfill leachate from different landfills of China

Landfill leachate usually harbors complex microbial communities responsible for the decomposition of municipal solid waste. However, the diversity and metabolic functions of the microbial communities in landfill leachate as well as the factors that influence them are still not well understood. In this study, Illumina MiSeq high-throughput sequencing was used to investigate the microbial community composition and metabolic functions in landfill leachate from 11 cities in China. The microbial diversity and structure of different leachate samples exhibited obvious differences. In general, Bacteroidetes, Firmicutes and Proteobacteria were the three dominant microbial communities among the 26 bacterial phyla identified in landfill leachate, regardless of the geographical locations. Diverse bacterial genera associated with various functions such as cellulolytic bacteria (e.g., Sphaerochaeta and Defluviitoga), acidifying bacteria (e.g., Prevotella and Trichococcus) and sulfate-reducing bacteria (e.g., Desulfuromonas and Desulfobacterium) were detected abundantly in the landfill leachate. Moreover, the archaeal community in all leachate samples was dominated by the orders Methanomicrobiales and Methanosarcinales belonging to the Euryarchaeota phylum. Notably, the archaea-specific primer pair covered more diverse archaeal communities than the universal bacteria-archaea primer pair. Seventeen archaeal genera belonging to acetoclastic, hydrogenotrophic, and methylotrophic methanogens were identified, and the composition of the dominant genera in these samples varied greatly. The canonical correlation analysis indicated that landfill age, electrical conductivity, ammonia nitrogen, and total nitrogen were significantly correlated with the microbial community structure. Based on PICRUSt2, a total of 41 metabolic pathways belonging to six metabolic pathway groups were predicted, and the KEGG pathway Metabolism was the most abundant group across all leachate samples. This study provides an important insight into the composition and functional characteristics of the microbial communities in landfill leachate.

Nitrogen-doped porous biochar derived from marine algae for efficient solid-phase microextraction of chlorobenzenes from aqueous solution

Nitrogen-doped porous biochar (NPB) with a large specific surface area, wide pore size distribution, graphitized structure, nitrogen doping, and hydrophobicity was fabricated by high-temperature modification of algal biochar with potassium carbonate. This NPB was then uniformly coated on stainless steel wire as a novel solid-phase microextraction (SPME) fiber. The extraction efficiency of NPB-coated fiber for seven chlorobenzenes (CBs) was excellent; it was 1.0-112.2 times higher than that of commercial SPME fibers. A trace determination method was developed for seven CBs in water with the optimized extraction conditions by NPB-coated fiber and gas chromatography-electron capture detector, which showed wide linear ranges (1-1000 ng L^-1), low detection limits (0.007-0.079 ng L^-1), great repeatability (2.5-6.5% for intra-day, and 3.1-6.8% for inter-day), and excellent reproducibility (3.5-6.3%, n = 5). The practicality of the developed method was evaluated using real water samples and showed great recoveries (89.55-105.19%). This study showed that low-cost biomass wastes could be converted to advanced biochar materials by a facile method, and displayed excellent performance in SPME applications.

Biochar industry to circular economy

Biochar, produced as a by-product of pyrolysis/gasification of waste biomass, shows great potential to reduce the environment impact, address the climate change issue, and establish a circular economy model. Despite the promising outlook, the research on the benefits of biochar remains highly debated. This has been attributed to the heterogeneity of biochar itself, with its inherent physical, chemical and biological properties highly influenced by production variables such as feedstock types and treating conditions. Hence, to enable meaningful comparison of results, establishment of an agreed international standard to govern the production of biochar for specific uses is necessary. In this study, we analyzed four key uses of biochar: 1) in agriculture and horticulture, 2) as construction material, 3) as activated carbon, and 4) in anaerobic digestion. Then the guidelines for the properties of biochar, especially for the concentrations of toxic heavy metals, for its environmental friendly application were proposed in the context of Singapore. The international status of the biochar industry code of practice, feedback from Singapore local industry and government agencies, as well as future perspectives for the biochar industry were explained.

A review on facilitating bio-wastes degradation and energy recovery efficiencies in anaerobic digestion systems with biochar amendment

In this review, progress in the potential mechanisms of biochar amendment for AD performance promotion was summarized. As adsorbents, biochar was beneficial for alleviating microbial toxicity, accelerating refractory substances degradation, and upgrading biogas quality. The buffering capacity of biochar balanced pH decreasing caused by volatile fatty acids accumulation. Moreover, biochar regulated microbial metabolism by boosting activities, mediating electron transfer between syntrophic partners, and enriching functional microbes. Recent studies also suggested biochar as potential useful additives for membrane fouling alleviation in anaerobic membrane bioreactors (AnMBR). By analyzing the reported performances based on different operation models or substrate types, debatable issues and associated research gaps of understanding the real role of biochar in AD were critically discussed. Accordingly, Future perspectives of developing biochar-amended AD technology for real-world applications were elucidated. Lastly, with biochar-amended AD as a core process, a novel integrated scheme was proposed towards high-efficient energy-resource recovery from various bio-wastes.