Life cycle assessment of biochar and hydrochar derived from sewage sludge: Material or energy utilization?

In this work, alternative process chains for sewage sludge treatment by hydrothermal carbonization (HTC) and pyrolysis and the subsequent utilization of biochar and hydrochar were investigated by means of a comparative life cycle assessment (LCA). The mono-incineration of sewage sludge was defined as the benchmark process, being the main valorization route in several countries. The results revealed an advantage for HTC compared to pyrolysis and mono-incineration in most environmental impact categories, thanks to the higher char yield and the lower energy request. The global warming potential (GWP) of using hydrochar ranged from −71.4 to 7.7 g CO2-eq. kg-1 sewage sludge. However, the direct material application of the hydrochar in agriculture showed an increased toxicity potential, so that an energy utilization appears to be more environmentally friendly. For the pyrolysis route, a slightly higher energy demand and thus a higher environmental impact was determined, whereby GWP was between −11.7 to 9.1 g CO2-eq. kg-1 sewage sludge. The direct material application of the biochar in agriculture also showed low toxicity potentials while achieving nutrient recycling for phosphorus and a long-term carbon sequestration potential. Overall, ecological advantages were demonstrated for both thermochemical processes, making pyrolysis and HTC promising alternatives for sustainable sludge management.