Functional biochar as sustainable precursors to boost the anaerobic digestion of waste activated sludge from a circular economy perspective: a review

Abstract Waste activated sludge (WAS) is attracting attention for its energy and resource potential. Anaerobic digestion (AD) can be used to efficiently recover energy resources and stabilize pollutants in WAS, with biochar emerging as an ideal additive to boost this process. However, the low abundance of surface functional groups and small pore structure of raw biochar may limit its performance in the AD of WAS. These limitations can be overcome by using functional biochar. In this review, the preparation and modification methods for functional biochar and its role in the AD of WAS are systematically summarized and discussed. Notably, the role of functional biochar in electron transfer, buffering effects, microbial immobilization, alleviation of inhibitory effects, and functional microbial modulation are analyzed. Additionally, the impacts of biochar on digestate utilization, pollutant removal, and carbon fixation are discussed. Special attention is given to functional biochar for its ability to promote circular economy and carbon neutrality. Finally, future perspectives on the use of functional biochar in the AD of WAS are presented. The aim of this review is to provide insights into the application of functional biochar in the AD of WAS from the perspective of performance improvement, resource utilization, and circular economy. Graphical Abstract