Yue Liu, Wangmi Chen, Yan Hao, Qi Zhou, Yufang Wei, Beidou Xi
Owing to its high organic content, good digestibility, and low moisture, up-concentrated magnetic sludge (UCMS) generated by enhanced magnetic-driven up-concentration is an ideal substance for direct conversion into short-chain fatty acids (SCFAs), but fermentation is hindered by the dense floc structure of UCMS. This study investigated the feasibility of alkali thermal hydrolysis and the underlying mechanisms which enhances soluble chemical oxygen demand (SCOD) release, SCFAs efficiency, and methanogenesis. Results showed that SCOD release increased with pH and fermentation temperature. Acetic, propionic, and isovaleric acids were the predominant SCFAs components across all fermenters. The enzymatic activities of acetate kinase, oxaloacetate transcarboxylase, and butyrate kinase significantly improved after alkaline thermal hydrolysis. The optimal pH and fermentation temperature were 11 and 70°C, respectively, which yielded a maximum SCFAs concentration of 13,139 mg COD/L and a maximum SCFAs yield of 27.3%, respectively. Tryptophan-, tyrosine-, and humic acid-like substances were the dominant components of dissolved organic matters (DOMs). Microbial analysis revealed a pronounced enrichment of bacterial populations associated with hydrolysis and acidogenesis. Notably, the highest methane production, 250 mL/g volatile solids (VS), was obtained from hydrolyzed UCMS at an inoculum-to-substrate ratio (ISR) of 2.0. Mechanistically, iron species within the system facilitated potential direct interspecies electron transfer (DIET). High temperatures favored Fe(III)/Fe(II) redox cycling over pili as pathway mediators, accompanied by enhanced cellular Fe3+ uptake and improved electron transfer potential. This study therefore offers a highly efficient strategy for advancing the application of alkaline thermal hydrolysis in resource recovery from UCMS, with mechanistic insights centered on iron-mediated DIET.