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Crawfish shell wastes (CSW) are abundant with valuable substrates, which possess great resource potentials. This work has revealed the feasibility of CSW reutilization for effective volatile fatty acids (VFAs) generation with waste active sludge (WAS) via anaerobic fermentation. The maximum VFAs generation reached 17963 mg COD/L when the CSW/WAS mass ratio was 3.25:1, while it was 784 mg COD/L in the control reactor. Meanwhile, each gram of CSW could be transformed into 264 mg COD equivalent VFAs. The feed of CSW brought in massive organic substrates, which significantly promoted the solubilization, hydrolysis and acidification processes in the CSW/WAS co-fermentation systems simultaneously. The metabolism of carbohydrates took priority over proteins in contributing to the VFAs promotion. More importantly, the functional anaerobes (i.e. , Paludicola sp. and Macellibacteroides sp.), which participated in the main substrates decomposition with the VFAs as the primary metabolites, were evidently enriched in the presence of CSW. The critical hydrolases and acid-forming enzymes relevant to VFAs biosynthesis were all enhanced. Moreover, the key genes encoding for the main substrates metabolisms (i.e., chitin) and VFAs production were remarkably up-regulated. The evident promotion of microbial activities and metabolic functions was mainly ascribed to the favorable fermentation conditions (i.e., pH, ORP and C/N) induced by CSW conditioning. This study proposed an innovative strategy to recover high-value products from CSW, and in-depth insights of microbial traits and genetic functions in CSW/WAS co-fermentation systems. [Display omitted] • Each gram of CSW can be transformed into 264 mg COD equivalent VFAs. • The solubilization, hydrolysis and acidification steps are synchronously promoted. • The VFAs promotion is mainly attributed to the carbohydrates metabolism. • The functional anaerobes involved in VFAs generation are enriched. • The critical metabolic activities (i.e. enzymes and genes) are enhanced. [ABSTRACT FROM AUTHOR]