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[Display omitted] • The roles of biochar on microbial growth and activity were systematically analyzed. • Biochar improved the microbial community diversity and population. • Biochar enhanced cellular enzyme activity and accelerated electron transfer. • Biochar promoted functional gene expression and regulated metabolic pathway. • Biochar targeted design to achieve high microbial activity in AD were suggested. 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. [ABSTRACT FROM AUTHOR]

The addition of biochar is a promising strategy to improve methane production via strengthening direct interspecies electron transfer (DIET) in anaerobic digestion (AD) field. In this study, a novel oil sludge biochar (OSBC) prepared at 500/600/700 °C was used to investigate its dosage effects on performances of sewage sludge AD. The characterization results indicated that OSBC prepared at 600 °C had highest electron accepting and donating capacity, meanwhile, it also contained moderate functional groups such as C O, which were in favor of methane production. Therefore, the highest accumulative methane yield (143.96 mL (g VS)−1) accompanying with the fast consumption of volatile fatty acids (VFAs) was achieved at dosage of 1.2 g. Additionally, the conductivity and enzyme activity (electron transport system activity and coenzyme F420) of AD with OSBC addition were also enhanced. Microbial community structures manifested that the potential syntrophic microbes including archaea Methanothrix , Methanospirillum , and bacteria Comamonas , Petrimonas and Syntrophomonas were enriched by OSBC due to its large surface area and microelement such as iron, and contributed to the improved methane production. The results suggested that OSBC exhibited excellent practical application potential in enhancing anaerobic sewage sludge treatment and biogas productivity. [Display omitted] • Oil sludge biochar prepared at 600 °C had highest electron accept/donate capacity. • Functional groups and iron of biochar were responsible for increasing CH 4 yield. • Oil sludge biochar improved the conductivity and enzyme activity of digesters. • IHT and DIET processes were promoted by oil sludge biochar from microbial perspective. [ABSTRACT FROM AUTHOR]

Strengthening direct interspecies electron transfer (DIET), via adding conductive materials, is regarded as an effective way for improving methane productivity of anaerobic digestion (AD). Therein, the supplementation of combined materials (composition of biochar and iron-based materials) has attracted increasing attention in recent years, because of their advantages of promoting organics reduction and accelerating biomass activity. However, as far as we known, there is no study comprehensively summarizing the application of this kind combined materials. Here, the combined methods of biochar and iron-based materials in AD system were introduced, and then the overall performance, potential mechanisms, and microbial contribution were summarized. Furthermore, a comparation of the combinated materials and single material (biochar, zero valent iron, or magnetite) in methane production was also evaluated to highlight the functions of combined materials. Based on these, the challenges and perspectives were proposed to point the development direction of combined materials utilization in AD field, which was hoped to provide a deep insight in engineering application. [ABSTRACT FROM AUTHOR]

• Magnetic biochar addition caused a loosed structure of protein for boosting CH 4 yield. • Fe 2 O 3 and FeO of magnetic biochar first bound with proteins via hydrogen bond. • The C=O, C N and N H of protein were subsequently destroyed by magnetic biochar. • Hydrolysis-acidification and syntrophic metabolisms were promoted by magnetic biochar. The addition of composite conductive materials is being increasingly recognized as a promising strategy to enhance anaerobic digestion (AD) performance. However, the influence of these materials on protein hydrolysis has been poorly documented. Here, a novel magnetic biochar derived from oil sludge and straw was synthesized using different iron sources and successfully applied in sludge AD. Experimental results revealed that magnetic biochar modified by Fe2+ exhibited excellent electron transfer capacity, moderate magnetization, diverse functional groups (e.g. C=O, C-O=O-), and abundant iron distribution. These characteristics significantly enhanced the hydrolysis of tryptophan-like components, leading to increased methane production (144.44 mL gVS−1 vs 79.72 mL gVS−1 in the control test). Molecular docking analysis revealed that the binding of magnetic biochar related Fe2+ and Fe3+, onto sludge proteins via hydrogen bond played a key role in promoting subsequent protein hydrolysis. Additionally, the noteworthy conservation of protein structures from α-helix and β-sheet to random coil, along with the breakdown of the amide I-associated C=O group and amide III-related C N and N H bonds following the addition of magnetic biochar, accelerated the degradation of sludge protein. Observation of variations in protease activity, coenzyme F420, electron transfer system (ETS), and conductivity within the AD systems, particularly the enrichment of Methanospirillum and Methanosaeta archaea , as well as the Petrimonas, Comamonas, and Syntrophomonas bacteria, suggested that magnetic biochar facilitated a conducive environment by improving hydrolysis-acidification and the direct interspecies electron transfer (DIET) process for acetoclastic methanogens. Moreover, metabolic pathways further proved that tryptophan metobalism and acetoclastic methanogenesis were both facilitated by magnetic biochar. This study provides an in-depth understanding of the impact of magnetic biochar on protein hydrolysis in sewage sludge AD. [Display omitted] [ABSTRACT FROM AUTHOR]

[Display omitted] • Molecular weight of protein and carbohydrate greatly affected anaerobic digestion. • Protein resulted in higher methane yield than carbohydrate, but a longer lag phase. • Bacterial communities were particularly influenced by protein-like substrates. • High molecular weight substrates favored the acetoclastic metabolic pathway. • Methanoregulaceae and Methanomassillicoccus served as biomarker methanogens. Proteins and carbohydrates are important organics in waste activated sludge, and greatly affect methane production and microbial community composition in anaerobic digestion systems. Here, a series of co-substrates with different molecular weight were applied to investigate the interactions between microbial dynamics and the molecular weight of co-substrates. Biochemical methane production assays conducted in batch co-digesters showed that feeding high molecular weight protein and carbohydrate substrates resulted in higher methane yield and production rates. Moreover, high-molecular weight co-substrates increased the microbial diversity, enriched specific microbes including Longilinea , Anaerolineaceae , Syner-01 , Methanothrix , promoted acidogenic and acetoclastic methanogenic pathways. Low-molecular weight co-substrates favored the growth of JGI-0000079-D21 , Armatimonadota , Methanosarcina , Methanolinea , and improved hydrogenotrophic methanogenic pathway. Besides, Methanoregulaceae and Methanolinea were indicators of methane yield. This study firstly revealed the complex interactions between co-substrate molecular weight and microbial communities, and demonstrated the feasibility of adjusting co-substrate molecular weight to improve methane production process. [ABSTRACT FROM AUTHOR]