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• Eggshell conditioning enhanced the VFAs production from FW anaerobic fermentation. • FW solubilization, hydrolysis and acidification were synchronously enhanced. • Eggshell conditioning greatly improved the environmental conditions (i.e pH, ORP). • The key enzymes involved in carbohydrates and energy metabolisms were improved. • The functional genes distributed in fatty acid biosynthesis pathway were enhanced. A novel strategy of utilizing eggshell wastes to condition food wastes (FW) anaerobic fermentation for efficient volatile fatty acids (VFAs) production was proposed. The VFAs production, especially acetic and butyric acids, was significantly accelerated and enhanced by eggshell conditioning. The enhanced effects were dose-dependent and improved by approximate 13-folds with optimal dose when compared with that in control. Eggshell conditioning synchronously enhanced the FW solubilization, hydrolysis and acidification steps, resulting in high VFAs production. Further analysis found that the environmental conditions (i.e., pH, ORP, trace elements, etc.) were significantly improved by eggshell conditioning. Hence, the microbial activities (i.e., hydrolases, acidogenic enzymes and ATP) and functional fermentative bacteria related with VFAs generation were enhanced and enriched. The massive accumulation of H 2 and CO 2 in eggshell conditioning reactors stimulated the homoacetogenesis, which also contributed to the VFAs promotion. Metatranscriptomic analysis indicated that 3 carbohydrate-active enzymes (Glycoside hydrolases, carbohydrate esterases and carbohydrate binding modules), which participate in carbohydrates and energy metabolisms, were improved. Moreover, 10 kinds of synthetase, hydrolase and oxidoreductase genes, and 4 kinds of acetyl-CoA carboxylase genes that distributed throughout the fatty acid biosynthesis pathway (especially the acetic and butyric acids generation) were all enhanced. It was the highly expressed genes and metabolic enzymes that contributed to high microbial activities and subsequent efficient VFAs production in eggshell conditioning reactors. Besides, the improvement of dewaterability and phosphorus fixation in fermentation residues further increased the added-value of conditioning FW fermentation by eggshell wastes. [ABSTRACT FROM AUTHOR]

[Display omitted] • PCMX promote WAS solubilization, hydrolysis and acidification but inhibit methanogenesis. • PCMX interact with EPS and cause the release of organics (especially proteins). • The hydrolytic-acidifying bacteria are enriched while the methanogens are reduced. • Microbial metabolic profiles related with VFAs biosynthesis (i.e. , critical enzymatic and genetic traits) are enhanced. • Anaerobes adapt the PCMX stimulus by regulating two-component and quorum sensing system. Para-chloro- meta -xylenol (PCMX), as a massively used antimicrobial agent, exhibited dose-dependent effects on the sludge fermentation for volatile fatty acids (VFAs). The presence of PMCX at environmental levels (10–20 mg/g TSS) promoted the VFAs production by 2.5–3.0 folds but caused evident inhibition at high level (100 mg/g TSS). PCMX synchronously promoted the sludge solubilization, hydrolysis, and acidification but inhibited methanogenesis. Mechanistic investigations revealed that PCMX could interact with the extracellular polymeric substances (EPS) in sludge and cause the release of biodegradable substrates (especially proteins) by altering the WAS structures. Moreover, the functional fermentative bacteria (i.e. , Firmicutes and Chloroflexi) were enriched while those VFAs consumers (i.e. , Methanothrix and Methanomassiliicoccus) were reduced with low level of PCMX exposure. The microbial enzymatic activities (i.e. , protease) and critical genes relevant to the substrates transport (i.e. , glu C and lhp N) and metabolisms (especially the proteins, i.e. , pck A), and VFAs biosynthesis (i.e. , fab G) were all upregulated with the low PCMX stimulus. The microbial response and adaptation to PCMX toxicity were mainly attributed to the regulation of two-component and quorum sensing systems. Overall, PCMX contributed to the VFAs accumulation during sludge fermentation by mainly interacting and disrupting the sludge structures, and improving the microbial metabolic function traits. This work provided new insights into exogenous pollutants' interactions, potential influences, and reaction mechanisms in the complicated sludge matrix. [ABSTRACT FROM AUTHOR]

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]