An advanced process for thermal treatment of municipal sewage sludge

Sewage sludge, a by-product of the wastewater treatment process, should not be considered as a waste, but as a potential resource for renewable energy and nutrient recovery. However, sewage sludge contains harmful contaminants (e.g. pathogens, pharmaceutical residuals, and microplastics), and if the sludge – composted or digested – is utilized directly in agriculture, these contaminants may leak into the environment. To tackle this issue, an advanced combustion scheme is proposed. In this process, mechanically dewatered sludge (20–25 wt% dry solids con-tent) from a nearby wastewater treatment plant is first dried at 110 °C in a circulating mass dryer and then combusted at 850 °C in a circulating mass reactor. Thereby, all the unwanted organic compounds are eliminated, and the ash – relatively high in nutrients and sufficiently low in heavy metals – can be further processed into fertilizers and used in forestry or farming. The process is self-sufficient in terms of energy; the excess heat extracted from the process can be utilized, e.g., as district heat. This paper describes the process in detail and reviews initial experiences from the first commercial plant in Rovaniemi, Finland. Also, the potential of the technology for efficient and sustainable handling of municipal sewage sludge is discussed.