Hydrothermal treatment and biorefinery of sewage sludge for waste reduction and production of fungal hyphae fibers and volatile fatty acids

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Sewage sludge poses a serious environmental problem for large cities, and large amounts of materials in sludge, including organics, should be recovered rather than wasted. To achieve both sludge reduction and organic resource recovery, this study developed a new biorefinery technology that includes the hydrothermal treatment of sludge followed by fungal fermentation to form hyphae fibers and acidogenic fermentation to produce volatile fatty acids (VFAs). Hydrothermal treatment at 160 °C or 180 °C allows solubilization of approximately 40% of solid organics in sludge into the supernatant, resulting in significant sludge reduction and dewaterability improvement. The fermentation of the fungus Aspergillus niger converted waste organics in the hydrolyzed sludge liquor, with a chemical oxygen demand (COD) concentration of around 15,000 mg/L, into hyphae fibers, resulting in fungal biomass production of more than 1.7 g/L. After the recovery of hyphae fibers, the residual sludge liquor was further fermented to produce VFAs to a concentration of about 2500 mg COD/L. The genera Petrimonas, Arcobacter, and Aminobacterium played important roles in acidogenic fermentation. The stepped biorefinery strategy can transform approximately 35% of waste organics in the hydrothermal sludge liquor into valuable fibrous materials and VFAs for effective resource recovery.