Novel strategy to stimulate the food wastes anaerobic fermentation performance by eggshell wastes conditioning and the underlying mechanisms

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 CO2 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.