Exploring the potential of sewage sludge for gasification and resource recovery: A review

The production of sewage sludge, a by-product of wastewater treatment plants, is steadily increasing. With its hazardous properties but valuable resources and materials, sewage sludge requires innovative management strategies to tackle environmental challenges and create opportunities for energy recovery and circular resource utilization. The most common applications currently include incineration, agricultural use and composting. The trend towards thermal treatment is steadily increasing, especially in Europe. State-of-the-art is to incinerate sewage sludge, which offers the benefits of volume reduction and the destruction of toxins, pathogens as well as the potential for recovery of the critical resource phosphorus from the generated ash. The latter is becoming increasingly important, particularly due to emerging regulations within the EU. Gasification as a thermal treatment method is also gaining traction as a suitable approach. The thermochemical process offers the same benefits as incineration but additionally produces gas (synthesis gas), a versatile fuel for energy generation and a precursor for the chemical industry. Sewage sludge gasification has yet to be adopted on a large scale due to gaps in research, technological development and systemic requirements as well as financial constraints. This review provides a comprehensive overview of legislation related to sewage sludge management and feedstock characteristics, including ash chemistry. Furthermore, it explores the fundamentals of gasification, focusing on optimizing parameters for sewage sludge by summarizing findings from studies on the effects of feedstock parameters, process designs and operating conditions on efficiency and output. It concludes with an outlook on large-scale implementation to maximize waste valorization through gasification.