Techno-economic and life cycle assessment of an integrated hydrothermal carbonization system for sewage sludge

The need of more effective solutions for sewage sludge management is twofold: i) to alleviate the overall operating costs for wastewater treatment and ii) to ensure proper utilization/disposal considering environmental legislation. A comprehensive solution integrating hydrothermal carbonization and anaerobic digestion to treat sewage sludge was analyzed and compared with that of standalone anaerobic digestion. In order to evaluate its performance, a process simulation model in Aspen Plus® served as the main data source of the required input-output inventories to perform both techno-economic and life cycle assessment, which follow conventional methodological standards. It was observed that the integrated strategy generally reduces the environmental impacts compared to the standalone configuration due to the recovery of a hydrochar that could replace fossil fuels. In contrast, the standalone option only recovers a digestate that can generate even a higher impact than its counterfactual alternative. The integration of hydrothermal treatments showed an increase of 14% in the gross energy efficiency of the anaerobic digestion. However, economic concerns make the approach to require further optimization, since the estimated cost for the HTC option were 42% higher in comparison to the conventional anaerobic digestion alternative. Although there is a changing regulatory framework around sewage sludge, especially on nutrient recovery, hydrothermal treatments have been proven as a potentially sustainable route for sewage sludge treatment.