Iron-enhanced sludge anaerobic fermentation for short-chain fatty acids production: A comprehensive review

Iron-assisted anaerobic fermentation (AF) has emerged as a promising strategy for valorizing waste activated sludge (WAS) by enhancing the production of short-chain fatty acids (SCFAs), which serve as versatile precursors for various biobased and industrial applications. With its potential to significantly contribute to carbon neutrality goals, this strategy offers hope for a more sustainable future. Iron-based strategies-such as zero-valent iron (ZVI) pretreatment, ferrate oxidation, and iron-based advanced oxidation processes (AOPs)-promote sludge hydrolysis and acidogenesis by disrupting extracellular polymer substances and microbial cells, enriching fermentative populations, and activating hydrolytic and acidogenic enzymes, thereby significantly improving SCFA yields. However, a comprehensive review of iron-assisted AF for SCFA production remains limited, which impedes a deeper understanding of the multifaceted roles of iron in influencing microbial populations, microbial ecological interactions, enzymatic catalysis, and electron transfer mechanisms. This review systematically summarizes recent advances in iron-assisted sludge AF, including ZVI, iron oxides, ferrate pretreatment, and iron-based AOPs for enhancing SCFA production. Additionally, it comprehensively examines the regulatory mechanisms by which iron influences microbial community structures, microbial interactions, enzymatic expression, and microbial electron transfer processes. The research gaps, challenges of practical applications, and future perspectives are further proposed and discussed. This work provides a comprehensive foundation for advancing sustainable sludge valorization through iron-enhanced AF strategies.