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The utilization of biochar derived from biomass residue to enhance anaerobic digestion (AD) for bioenergy recovery offers a sustainable approach to advance sustainable energy and mitigate climate change. However, conducting comprehensive research on the optimal conditions for AD experiments with biochar addition poses a challenge due to diverse experimental objectives. Machine learning (ML) has demonstrated its effectiveness in addressing this issue. Therefore, it is essential to provide an overview of current ML-optimized energy recovery processes for biochar-enhanced AD in order to facilitate a more systematic utilization of ML tools. This review comprehensively examines the material and energy flow of biochar preparation and its impact on AD is comprehension reviewed to optimize biochar-enhanced bioenergy recovery from a production process perspective. Specifically, it summarizes the application of the ML techniques, based on artificial intelligence, for predicting biochar yield and properties of biomass residues, as well as their utilization in AD. Overall, this review offers a comprehensive analysis to address the current challenges in biochar utilization and sustainable energy recovery. In future research, it is crucial to tackle the challenges that hinder the implementation of biochar in pilot-scale reactors. It is recommended to further investigate the correlation between the physicochemical properties of biochar and the bioenergy recovery process. Additionally, enhancing the role of ML throughout the entire biochar-enhanced bioenergy recovery process holds promise for achieving economically and environmentally optimized bioenergy recovery efficiency. [ABSTRACT FROM AUTHOR]

This study focused on conversion of algal biomass to bio-oil through in-situ catalytic fast pyrolysis. In particular, novel Ni 2 P-loaded and unmodified (zeolite, activated carbon and chicken manure biochar) catalysts were evaluated to determine their effect on in-situ upgrading of algal pyrolysis products. Among the catalysts evaluated, Ni 2 P-loaded zeolite achieved the highest bio-oil hydrocarbon (34.97 %) and calorific value (32.66 MJ/kg), and the lowest O/C (0.075) and N/C (0.074) ratios. There was a 137 % increase of the aliphatic hydrocarbon fraction and a 72 % increase of the biochar specific surface area (SSA) with the loading of Ni 2 P to zeolite. Additionally, unfavourable major oxygen- and nitrogen-containing compound, phytol and tetradecanamide, decreased seven- and six-fold, respectively, compared to the non-catalytic baseline case. Ni 2 P-loaded biochar achieved a reduction in the bio-oil acidity from a pH of 4.52–6.78 and a 41 % decrease of the carboxylic acids content. Biochar produced from the zeolite formulations exhibited especially favourable sorbent quality with SSA greater than 500 m2/g. According to the proposed catalytic mechanism, the generation of n-heptadecane and hexadecanamide may be due to the decomposition of phytol and tetradecanamide, and the catalytic activity was correlated to the pore size of the catalyst (22.22 Å). Direct addition reactions were the dominant pathway in the formation of liquid hydrocarbons. These results suggest that it is feasible to develop an in-situ multi-catalyst system to achieve high-quality bio-oil production. Thus, the development of novel Ni 2 P-loaded catalysts addressed a promising approach for biomass conversion into hydrocarbon-rich products with low risk to the environment. [Display omitted] • Novel Ni 2 P-loaded catalysts were effective at reduction of heteroatom species. • Ni 2 P-loaded chicken manure biochar catalyst was effective at neutralizing high acidity in algal bio-oil. • Ni 2 P-loaded zeolite catalyst achieved the highest hydrocarbon and lowest oxygenous and nitrogenous species concentrations. • Ni 2 P-loaded catalysts achieved biochar specific surface areas greater than 500 m2/g. • A catalytic mechanism and formation pathways of bio-oil aromatic and aliphatic hydrocarbons are proposed. [ABSTRACT FROM AUTHOR]

The management and disposal of livestock manure has become one of the top environmental issues at a global scale in line with the tremendous growth of poultry industry over the past decades. In this work, a potential alternative method for the disposal of chicken manure from Singapore local hen layer farms was studied. Gasification was proposed as the green technology to convert chicken manure into clean energy. Through gasification experiments in a 10 kW fixed bed downdraft gasifier, it was found that chicken manure was indeed a compatible feedstock for gasification in the presence of wood waste. The co-gasification of 30 wt% chicken manure and 70 wt% wood waste produced syngas of comparable quality to that of gasification of pure wood waste, with a syngas lower heating value (LHV) of 5.23 MJ/Nm 3 and 4.68 MJ/Nm 3 , respectively. Furthermore, the capability of the gasification derived biochar in the removal of an emerging contaminant (artificial sweetener such as Acesulfame, Saccharin and Cyclamate) via adsorption was also conducted in the second part of this study. The results showed that the biochar was effective in the removal of the contaminant and the mechanism of adsorption of artificial sweetener by biochar was postulated to be likely via electrostatic interaction as well as specific interaction. Finally, we conducted a cost-benefit analysis for the deployment of a gasification system in a hen layer farm using a Monte Carlo simulation model. [ABSTRACT FROM AUTHOR]

Gasification is recognized as a green technology as it can harness energy from biomass in the form of syngas without causing severe environmental impacts, yet producing valuable solid residues that can be utilized in other applications. In this study, the feasibility of co-gasification of woody biomass and food waste in different proportions was investigated using a fixed-bed downdraft gasifier. Subsequently, the capability of biochar derived from gasification of woody biomass in the rehabilitation of soil from tropical secondary forests on degraded land (adinandra belukar) was also explored through a water spinach cultivation study using soil-biochar mixtures of different ratios. Gasification of a 60:40 wood waste-food waste mixture (w/w) produced syngas with the highest lower heating value (LHV) 5.29 MJ/m 3 —approximately 0.4–4.0% higher than gasification of 70:30 or 80:20 mixtures, or pure wood waste. Meanwhile, water spinach cultivated in a 2:1 soil-biochar mixture exhibited the best growth performance in terms of height (a 4-fold increment), weight (a 10-fold increment) and leaf surface area (a 5-fold increment) after 8 weeks of cultivation, owing to the high porosity, surface area, nutrient content and alkalinity of biochar. It is concluded that gasification may be an alternative technology to food waste disposal through co-gasification with woody biomass, and that gasification derived biochar is suitable for use as an amendment for the nutrient-poor, acidic soil of adinandra belukar. [ABSTRACT FROM AUTHOR]

[Display omitted] • Nano-biochar application was studied for anaerobic digestion of orange peel waste. • Sewage sludge derived biochar had the highest performance among different biochars. • Nano-biochar facilitated electron transfer, hydrogen transfer and nutrition supply. • Successfully rescued failed digester relied on restore of microbial communities. • Restored key microbes included phylum Hydrogenispora and genus Methanosarcina. 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. [ABSTRACT FROM AUTHOR]

The sustainability of anaerobic digestion (AD) is often challenged by the low energy efficiency of sulfur-contaminated waste and subsequent high carbon emission in biogas desulfurization. Recent advances in the direct interspecies electron transfer (DIET) mechanism present a unique biotechnological means to cope with relevant fundamental constraints of microbial competition. This study aims to provide a systematic investigation regarding how and to what extent can DIET-conductive materials (especially biochar) mitigate relevant AD away from the environmental burdens of hydrogen sulfide generation. Through the experimental validation of biochar applications in bioreactor trials, the profiles of intermediates pathways and electron diversion were first examined together with microbiological analysis. Instead of suppressing the notorious growth of sulfate-reducing bacteria, the results revealed an interesting phenomenon of enriched coexistence but a reduced hydrogen sulfide generation with DIET application. In a stoichiometric relationship, around 26.6% of the total electrons were migrated from hydrogen sulfide production back to methane generation. Sensitivity analysis of life cycle assessment (LCA) was applied to better understand the environmental significance of electron diversion by biochar application in the systematic context of bioenergy recovery and biogas desulfurization. The LCA results characterized the benefits and limitations of DIET-driven desulfurization for the environmental performance of each impact category, while the potential improvement through better resource circularity in the desulfurization unit was also highlighted. Overall, built on experimental validation, this study frames relevant evaluations to achieve insights into the pivotal roles of biochar application as well as the quantitative interdependencies between DIET-driven desulfurization and AD sustainability. • Biogas desulfurization created huge environmental burdens to AD sustainability. • Profile of microbial electron diversion and its environmental impacts were studied. • Competing relationship between hydrogen sulfide and methane in AD was illustrated. • DIET mechanism retrieved 26% of the total electrons from unwanted hydrogen sulfide. • Multiple roles of biochar application in the AD sustainability were investigated. [ABSTRACT FROM AUTHOR]

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 CH 4 /gVS, respectively. CO 2 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. [Display omitted] • A hybrid bio/thermal system for plastic-containing food waste conversion to energy. • Plastic/digestate gasification produced higher syngas yields than pure digestate. • Gasification biochar enhanced methane yield by 6–30% in the AD processes. • Biochar shifted methanogens toward a balance of two flexible methanogenic pathways. • Average profit of PCFW treatment via the hybrid system is 20.7–64.2 US$/ton waste. [ABSTRACT FROM AUTHOR]

[Display omitted] • Wood waste-derived biochar was used first for upgrading biogas from food waste. • The biochar was next applied to treat the food waste anaerobic digestate. • The mixture of biochar and AD solids were utilised to grow kale, lettuce and rocket. • Virgin raw biochar was not effective at absorbing carbon dioxide. • Biochar was efficacious for wastewater treatment and biofertiliser application. A wood waste-derived biochar was applied to food-waste anaerobic digestion to evaluate the feasibility of its utilisation to create a circular economy. This biochar was first purposed for the upgrading of the biogas from the said anaerobic digestion, before treating and recovering the nutrients in the solid fraction of the digestate, which was finally employed as a biofertilizer for the organic cultivation of three green leafy vegetables: kale, lettuce and rocket salad. Whilst the amount of CO 2 the biochar could absorb from the biogas was low (11.17 mg g−1), it could potentially be increased by modifying through physical and chemical methods. Virgin as well as CO 2 -laden biochar were able to remove around 31% of chemical oxygen demand, 8% of the ammonia and almost 90% of the total suspended solids from the digestate wastewater, which was better than a dewatering process via centrifugation but worse than the industry standard of a polytetrafluoroethylene membrane bioreactor. Nutrients were recovered in the solid fraction of the digestate residue filtered by the biochar, and utilised as a biofertilizer that performed similarly to a commercial complete fertilizer in terms of aerial fresh weight growth for all three vegetables cultivated. Contingent on the optimal upgrading of biogas, the concept of a circular economy based on biochar and anaerobic digestion appears to be feasible. [ABSTRACT FROM AUTHOR]

[Display omitted] • Biochar-assisted HSAD and steam gasification were coupled to treat food waste. • Over-acidification in HSAD was boosted with biochar assistance. • Steam gasification is a feasible and efficient technology to treat the digestates. • The final biochar product with a comparable amount of nutrients can be fertilizer. • Energy recovery efficiency was improved by the HSAD-steam gasification system. A food waste treating system was proposed in this study by combining biochar-assisted high-solid anaerobic digestion and subsequent steam gasification of the digestate. The effect of solid level, biochar dosage in anaerobic digestion on the properties of biogas, syngas, and final biochar products were investigated. Results showed that at a high total solid level and biochar dosage of 25 g/L and 50 g/L, the accumulative methane yield reached 110.3 mL CH 4 /g VS and 126.7 mL CH 4 /g VS, respectively. From steam gasification of different digestates under 850 °C for 15 min, a maximum of 34.92 mmol/g for the hydrogen yield and 11.44 MJ/m3 for the higher heating value could be obtained for the syngas. Furthermore, the by-product produced from steam gasification was a nutrient-enriched porous biochar, which was suitable to be used as compost. This study demonstrated a pathway for food waste treating to produce methane-enriched biogas, hydrogen-enriched syngas, and nutrient-enriched biochar. [ABSTRACT FROM AUTHOR]

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. Unlabelled Image • Waste to biochar is a sustainable partway to circular economy. • Four key uses of the Singapore biochar industry are analyzed and reviewed. • The code of practice for biochar application in Singapore is proposed. • Future perspective of the research, innovation and development for biochar industry is discussed. [ABSTRACT FROM AUTHOR]

• Algal biochar enhanced co-digestion of algal biomass and food waste for biomethane. • 75% algal biomass + 25% food waste mixture co-digestion showed a synergistic effect. • Average methane yields enhanced by 12–54% due to 15 g/L of algal biochar amendment. • Proteiniphilum enriched in mesophilic reactor like Defluviitoga in thermophilic one. • Biochar shifted methanogens towards a balance of two flexible methanogenic pathways. The impact of algal biochar addition on mesophilic and thermophilic anaerobic co-digestion of algal biomass and food waste was investigated with a focus on semi-continuous operations and functional microbial communities. Under batch co-digestion, the highest co-digestion synergy was observed for a mixture of 25% food waste and 75% algal biomass. During semi-continuous co-digestion of 25% food waste-75% algal biomass mixture, biochar amended digesters exhibited a 12–54% increase in average methane yield (275.8–394.6 mL/gVS) compared to the controls. Elevated temperature induced narrow distributions of volatile fatty acids (VFAs) by inhibiting the production of branched VFAs. Genus Proteiniphilum was selectively enriched by 3.2 folds in mesophilic digesters with biochar amendment while genus Defluviitoga was selectively enriched in thermophilic digesters due to elevated temperature. Methanogenic communities were significantly different in mesophilic and thermophilic digesters. Biochar amendment contributed to shifts in the predominant methanogens leading to a more balanced state of two methanogenic pathways. [ABSTRACT FROM AUTHOR]

• Thermophilic food waste digestion with varied particle size of biochar was examined. • All examined biochars (<50 μm to 3 cm) significantly improved methane production. • Bacteria Thermotogae and methanogens Methanothermobacter & Methanosarcina enriched. • The synergy of hydrogenotrophic and acetoclastic methanogenic pathways was achieved. • Both technical and economic feasibility of adding biochar strategy were validated. Effectiveness of biochar addition to enhance thermophilic semi-continuous anaerobic digestion (AD) of food waste for methane production was investigated with a focus on dosage and particle size of biochar, pilot-scale application and elucidation of methanogenic pathways. Optimal dosage range of biochar was determined as 7.5 to 15 g per L working volume based on lab-scale batch AD. Effects of biochar with different particle sizes at a model dosage of 15 g/L were evaluated in a semi-continuous AD experiment, results of which showed that all the examined biochars with different particle sizes (<50 μm to 3 cm) substantially enhanced the average methane yields (0.465–0.543 L/gVS) compared to control digesters which failed due to overloading (≥3.04 gVS/L/d). No significant difference in methane yields, however, was observed among digesters with different particle sizes of biochars, except for 1–3 cm. The core reason for this phenomenon was that the biochars with different particle sizes had similar properties (e.g. density, surface area and pore size) and that the floating of large particle size (1–3 cm) of biochar with a density of 847 kg/m3 was not conducive to microbial growth. Metagenomic analysis was performed to determine the predominant microbial species and to explain the main methanogenic pathways in biochar-amended digesters using 16S rRNA sequencing. In the biochar-amended digester, bacterial phylum Thermotogae containing a major genus of Defluviitoga was selectively enriched with gradual increase of organic loadings, while simultaneously enriched methanogen genera Methanothermobacter and Methanosarcina , which showed a synergy of hydrogenotrophic and acetoclastic methanogenic pathways, jointly enhanced the methane productivity. Both technical feasibility and economic feasibility of adding biochar with simple pretreatment (e.g. smash) were validated in the pilot-scale thermophilic semi-continuous AD operations. [ABSTRACT FROM AUTHOR]