Continuous volatile fatty acid production from waste activated sludge hydrolyzed at pH 12

• Excess activated sludge was hydrolyzed at pH 12 for 66% dissolution achieved.
• Continuous fermented produced maximum productivity of 365 mg VFAs g−1 VSS.
• Produced VFAs were majorly acetate and propionate.
• Bacteroidia, ε-proteobacteria and Clostridia respond to fermentation processes.
• Fermented liquor was applied to denitrifying phosphorus removal process.

This study adopted rapid alkaline treatment at pH 12 to hydrolyze 66% of total chemical oxygen demands. Then the hydrolyzed liquor was fermented in a continuous-flow stirred reactor to produce volatile fatty acids (VFAs) at 8-h hydraulic retention time and at 35 °C. The maximum VFA productivity reached 365 mg VFAs g−1 volatile suspended solids in a 45-d operation, with most produced VFAs being acetate and propionate, principally produced by protein degradation. The Bacteroidia, ε-proteobacteria and the Clostridia were identified to be the classes correlating with the fermentation processes. The fermented liquor was applied to denitrifying phosphorus removal process as alternative carbon source after excess phosphorus and nitrogen being recycled via struvite precipitation. Fermented liquors from alkaline hydrolysis–acid fermentation on waste activated sludge are a potential renewable resource for applications that need organic carbons.