Low-alkaline fermentation for efficient short-chain fatty acids production from waste activated sludge by enhancing endogenous free ammonia

Alkaline fermentation at pH 10 is an effective method of producing short-chain fatty acids from waste activated sludge, but requires high cost of alkali. In this study, low-alkaline fermentation at pH 9 with enhanced endogenous free ammonia was proposed to improve sludge hydrolysis and short-chain fatty acids production for the first time. The results showed that the release of 210.6 ± 5.9 mg/L of endogenous free ammonia was sufficient for sludge hydrolysis, with a soluble chemical oxygen demand production of 6303.8 ± 45.9 mg/L within 24 h. Following the increase in sludge hydrolysis, ammonia stripping was employed to further improve acidogenesis by eliminating free ammonia inhibition. The final short-chain fatty acids production was 431.4 ± 1.7 mg chemical oxygen demand/g volatile suspended solid, which was even higher than that of regular alkaline fermentation. Furthermore, the theoretical availability of the produced short-chain fatty acids as a carbon source for denitrification was improved by 56.7%. A schematic of a wastewater treatment process employing low-alkaline fermentation of sludge considering carbon neutrality and nutrients recovery was proposed.