Protein transformation induced by tribromophenol regulates volatile fatty acids production during sludge fermentation: Interplay between structural modification and metabolic modulation

The escalating global production of waste activated sludge (WAS) presents both environmental challenges and opportunities for resource recovery through anaerobic fermentation (AF) to volatile fatty acids (VFAs). However, the ubiquitous presence of emerging contaminants (ECs, e.g., tribromophenol (TBP)) in WAS, resulting from extensive industrial applications, raises critical concerns about potential impacts on fermentation process. This study systematically investigated dose-dependent effects of TBP on VFA production during WAS fermentation, focusing on organic matter (OM) transformation, microbial metabolism, and adaptive mechanisms. Results demonstrated that low-dose TBP (10 mg/g TSS) enhanced VFA yield by 30.4 % (from 1288 to 1680 mg COD/L), while high-dose TBP (100 mg/g TSS) inhibited production by 8.0 % (to 1185 mg COD/L). Spectroscopic analysis confirmed TBP addition significantly attenuated amide I/II band intensities (1656/1546 cm −1 ) in FTIR and reduced amino acid-associated organic C=O/C-O XPS peak areas in the solid phase, while proportionally increasing amino acids in the liquid phase. This dissolution of solid-phase proteins by TBP thereby enhanced substrate bioavailability for fermentation. The improved substrate availability stimulated the growth of Firmicutes (increased by 53.52 %) and protein-degrading genera (e.g., Proteocatella ), while upregulating key metabolic pathways including amino acid metabolism (e.g., hisC and maiA ), glycolysis (e.g., HK and apgM ), and pyruvate metabolism (e.g., porA and korA ). Moreover, low-dose TBP enhanced microbial adaptability through quorum sensing (e.g., lsrB increased by 43.9 %) and two-component systems (e.g., rstB increased by 14.0 %). However, these beneficial effects were reversed at high TBP concentrations, leading to decreased microbial activity and VFA production. This work provides a comprehensive insight into ECs impacts on WAS fermentation, highlighting the critical need for dose-dependent process control to ensure safe resource recovery.