Synergistic Co-Fermentation of Ulva and Waste Activated Sludge for Sustainable Resource Recovery: Mechanistic Insights From Dissolved Organic Matter Transformation and Multi-Omics Analysis.

Anaerobic fermentation for volatile fatty acids (VFAs) production represents the sustainable and resource-recovery strategy for waste activated sludge (WAS) treatment, yet its efficiency is often hampered by low substrate C/N ratio and limited hydrolysis. Ulva, a macroalga responsible for extensive "green tides" that burden coastal ecosystems and economies, emerges as an ideal co-substrate for WAS anaerobic fermentation due to its high C/N ratio and biodegradability. This study established a novel Ulva-WAS co-fermentation system to boost VFAs production and explained the mechanism of acidogenic enhancement and methanogenesis suppression through integrated analyses of VFAs generation, dissolved organic matter (DOM) molecular transformation, and microbial metabolic activity. Co-fermentation enhanced VFAs production (1.67-2.9-fold increase of maximum VFAs accumulation) while stimulating microbial activity and accelerating hydrolysis of macromolecular components into polysaccharides and amino acids. Within the carbohydrate-supplemented environment, DOM transformation was dominated by microbial acidogenic processes, including glycolysis and amino acid metabolism. Microbial community succession and metabolic profiling confirmed that the high-C/N, carbohydrate-rich fermentation environment drove deterministic assembly of communities toward the function of acidogenesis, which in turn further promoted VFAs production. Concurrently, reactive oxygen species (ROS) stress induced by VFAs accumulation selectively inhibited methanogenic activity. This work provides an innovative strategy for WAS treatment and offers profound insights into the interplay between DOM dynamics and microbial metabolism in co-fermentation systems.