Ying Xu

The introduction of exogenous materials offers a highly promising technical approach for enhancing the performance of sludge anaerobic digestion (AD). This review systematically presents the mechanisms and application effects of carbon-based materials, metal-based materials, inorganic mineral materials, and metal–carbon composites in enhancing sludge AD. In-depth mechanism analysis revealed that these materials primarily enhance both methane yields and system stability through multiple synergistic mechanisms, such as mediating interspecies electron and proton transfer, promoting biofilm formation as microbial carriers, and adsorbing inhibitors while enhancing the buffering capacity of the system. However, a critical analysis indicated that the inherent limitations of various materials in engineering applications, particularly in terms of stability and recyclability, are the fundamental obstacles hindering the engineering implementation of this technology. Based on the above analysis, this review further proposes that the field is shifting from a sole pursuit of efficiency enhancement to a paradigm that balances efficiency and sustainability. Developing new recyclable and circular material systems with both high efficiency and excellent recycling potential is crucial for promoting the development of sludge AD technology towards a green, low-carbon, and resource-cycling direction, and also provides key scientific and technological support for the circular economy transformation of the sludge treatment system.

Dark fermentation of food waste for biohydrogen production can simultaneously achieve waste resource utilization and clean energy production. However, the widespread application of this technology remains constrained by challenges such as low substrate hydrolysis efficiency and suboptimal metabolic performance of functional microorganisms. This study evaluated the synergistic enhancement of biohydrogen production from food waste through dark fermentation by integrating thermal–alkaline (TA) pretreatment with varying concentrations (50, 100, 150, and 200 mg/L) of nickel–cobalt oxide nanoparticles (NiCo₂O₄ NPs), and the underlying mechanisms involved were systematically elucidated. The results demonstrated that individual TA pretreatment (pH 11, 70 °C, 1 h) and TA coupled with NiCo₂O₄ NPs (100 mg/L) significantly (p < 0.01) enhanced the cumulative biohydrogen yields of the food waste dark fermentation by 20.89% and 35.76%, respectively. Mechanism research revealed that TA pretreatment effectively facilitated the dissolution and hydrolysis of macro-molecular organics such as polysaccharides and proteins, thereby enhancing the bio-accessibility of fermentation substrates. The introduction of NiCo₂O₄ NPs further intensified the microbial biohydrogen-producing metabolism by augmenting enzymatic activity and enriching functional bacteria. NiCo₂O₄ NPs significantly (p < 0.001) enhanced the overall activity of hydrogenase by 95.10% compared to the control group (CG) by providing the cofactor of hydrogenase and accelerating electron transfer. Additionally, this synergistic strategy significantly (p < 0.01) increased the activities of hydrolases (e.g., protease and α-glucosidase), as well as key enzymes in acetate-type and butyrate-type fermentation pathways (e.g., acetate kinase and butyrate kinase), and enriched the biohydrogen-producing microbial community centered on Clostridium_sensu_stricto_1. This study systematically elucidated the synergistic strategy of TA pretreatment and NiCo₂O₄ NPs, which achieved dual-pathway reinforcement from substrate degradability to microbial metabolic activity. The findings are expected to provide theoretical support for developing efficient biohydrogen production technology from perishable organic solid waste.

Anaerobic digestion (AD) is a sustainable and widely adopted technology for the treatment of organic solid wastes (OSWs). However, AD efficiency varies significantly across different substrates, primarily due to differences in the microbial community and metabolic pathways. This review provides a comprehensive summary of the AD processes for four types of typical OSWs (i.e., sewage sludge, food waste, livestock manure, and straw), with an emphasis on their universal characteristics across global contexts, focusing mainly on the electron transfer mechanisms, essential microbial communities, and key metabolic pathways. Special attention was given to the mechanisms by which substrate-specific structural differences influence anaerobic digestion efficiency, with a focused analysis and discussion on how different components affect microbial communities and metabolic pathways. This study concluded that the hydrogenotrophic methanogenesis pathway, TCA cycle, and the Wood–Ljungdahl pathway serve as critical breakthrough points for enhancing methane production potential. This research not only provides a theoretical foundation for optimizing AD efficiency, but also offers crucial scientific insights for resource recovery and energy utilization of OSWs, making significant contributions to advancing sustainable waste management practices.

Cation exchange resin (CER) has been reported to promote sludge fermentation. However, previous studies have typically focused on the effects of CER on sludge properties to enhance fermentation, and the role of CER as a biocarrier for anaerobic consortia during fermentation has been overlooked. Thus, in this study, the potential of gel-type and macro-reticular type CERs to serve as biocarriers in fermentation was investigated. A significant number of anaerobes appeared to be attached to the surfaces of CER during 2-day fermentation. However, an extended fermentation time negatively affected the attachment of anaerobic consortia, suggesting that CER may be a suitable carrier for short-term fermentation processes, such as biohydrogen fermentation. Electrochemical analyses revealed that the electron transfer capacities of CER with attached anaerobes were enhanced after both 2-day and 28-day fermentation periods, with the macro-reticular type CER exhibiting higher electron transfer capacity than the gel-type CER. Fermentation experiments using mixing model substrates with macro-reticular and gel-type CERs with attached anaerobes showed that the macro-reticular type CER was more beneficial for biohydrogen fermentation than the gel-type CER. Further analyses of microbial communities revealed that hydrogen-producing bacteria (i.e., Caloramator, unclassified_f_Caloramatoraceace, and Sporanaerobacter) were more likely to adhere to the macro-reticular type CERs. This outcome confirmed that macro-reticular type CERs have significant potential as a carrier for anaerobic consortia to promote the generation of hydrogen and volatile fatty acids. These findings are expected to provide a reference for using materials as biocarriers to enhance the biohydrogen fermentation of sludge.