A critical review on dry anaerobic digestion of organic waste: Characteristics, operational conditions, and improvement strategies

The development and utilization of bioenergy from various biomass can effectively reduce our dependence on fossil fuels and mitigate greenhouse gas emissions. The dry anaerobic digestion (AD) process is a promising technology for the treatment and stabilization of organic wastes (e.g., agricultural residues, livestock waste) with high total solid contents (15%–40%), while simultaneously recovering energy through biogas production (mainly methane). In the past few years, it has attracted significant attention because it possesses several advantages over wet AD, including higher volumetric methane yield, reduced digester size, less digested residue, and greater ease in handling the low-moisture digestate. This technology, however, still faces challenges owing to its excessive solids content. Additionally, dry AD is greatly different from wet AD in terms of technical operation, reactor design, and process performance. It is therefore essential to comprehensively understand dry AD and its characteristics, to analyze its operational factors, and to evaluate the optimization methods and techniques in order to develop practical applications. This paper presents a critical overview of the fundamental and engineering aspects of dry AD, and reviews specific characteristics of dry AD. Moreover, the operational conditions affecting the process stability of dry AD are discussed, as well as the strategies for improving its performance. Finally, challenges and prospects for its future application in treating high-solid organic waste are discussed. Future research on continuous/semi-continuous operations and scaling up of the lab-scale dry AD systems should be conducted.