Impediments to bioaccessibility in the anaerobic digestion of waste activated Sludge: An in-depth review of challenges and influencing factors

Waste-activated sludge (WAS) is a by-product of biological wastewater treatment with significant biomass resource potential. Anaerobic digestion (AD) is recognized as an environmentally friendly disposal method; however, its efficiency is often hindered by the complex physical and chemical structure of WAS. This paper reviews the key factors influencing the AD of WAS, focusing on its bioaccessibility as a substrate. Section 1 outlines the importance of AD for WAS and current challenges. Section 2 discusses the rheological and geometric properties of WAS and their effects on bioaccessibility. Section 3 explores the role of surface thermodynamics in WAS stability, proposing surface-modified pretreatment methods to improve AD efficiency. Section 4 examines how the hierarchical structure of EPS impacts sludge stability, while Section 5 addresses challenges posed by abiotic components such as heavy metals, microplastics, and underexplored inorganic particles. Based on the above discussion, a comprehensive analysis of factors affecting WAS bioaccessibility is presented, advocates for further research into these factors and suggests machine learning models to predict sludge bioaccessibility under different conditions. Accordingly, this study contributes to the understanding of WAS microstability and offers insights for improving pretreatment strategies to enhance bioaccessibility. • Front-end exploration of causes of inefficiency in WAS anaerobic digestion. • Factors affecting bioaccessibility of organic matters in WAS fully reviewed. • Surface free energy effects on WAS bioaccessibility are innovatively stated. • Systematic statement of multifaceted interactions of organic matter in WAS. • Inert fractions advocated for inclusion in studies affecting WAS bioaccessibility.