Improving Medium-Chain Fatty Acid Production from Anaerobic Fermentation of Waste Activated Sludge Using Free Ammonia

The utilization of waste activated sludge (WAS) to recover energy in the form of medium-chain fatty acids (MCFAs) is increasingly attractive due to the high energy density of products. However, the anaerobic fermentation process of WAS would be restricted by the poor degradability of WAS. Herein, we propose an effective approach to improving MCFA production from anaerobic fermentation of WAS using free ammonia (FA) pretreatment. The maximum MCFA production was achieved at 8.3 g of COD/L when the pretreated FA concentration was 255 mg/L, increased by 4.8-fold compared with the control (1.4 g COD/L). In addition, FA pretreatment shortened the lag phase of MCFAs production and promoted the WAS degradation compared to the control without FA pretreatment. Mechanism analysis showed that FA improved the solubilization process, which should be attributed to FA pretreatment altering the WAS properties, that is, smaller particle size and lower hydrophobicity. Moreover, FA-pretreated sludge as feedstock also promoted hydrolysis, acidification, and chain elongation processes for MCFA production, indicating that the activity of related microorganisms has been improved, which was also strongly confirmed by the microbial analysis results. On the basis of these results, a "circle economy" hypothesis that combines FA pretreatment strategy with wastewater treatment plants (WWTPs) by using easily recovered FA from anaerobic fermentation liquor was finally proposed with potential environmental and economic benefits.