Decoding Ammonia Inhibition in Waste Activated Sludge Fermentation for Valuable Liquid Products: Performance and Microbiology Insight

Waste activated sludge (WAS) from wastewater treatment represents a promising, yet underexplored, feedstock for generating valuable biorefinery products via anaerobic fermentation. However, the ubiquitous presence of ammonia in this waste stream disrupts the bioconversion, reducing WAS’s economic potential and biorefinery output. Nevertheless, in the context of WAS anaerobic fermentation with the end goal of producing valuable liquid bioenergy, no study has systematically explored the functional stability and resilience of the anaerobic fermentation system under ammonia stress. To this end, this study examined the dynamic responses of the microbial microenvironment to ammonia stress within a sludge-to-medium chain fatty acid (MCFA) fermentation system. Specifically, low ammonia (1000 mg-N/L) enhanced MCFA production by 1.77 times though optimizing the metabolic flux partitioning and microbial interactions, whereas high ammonia (4000–8000 mg-N/L) seriously inhibited MCFA formation due to cell disintegration and the biohostile conditions induced by ammonia stress. Butanol synthesis was less affected, with comparable yields obtained in both ammonia-free and inhibited systems. These findings highlight the critical role of ammonia in regulating WAS-to-liquid energy transformation, providing a systematic perspective to bridge the gap between lab-scale tests and industrial implementation for enhancing sludge recovery value.