Environmental performance of pulp and paper mill biosludge utilization by hydrothermal and biochemical treatment

Abstract

Sludge is the largest solid waste stream generated in pulp and paper mills. Biosludge, which accounts for 30% of the total amount of sludge generated, is difficult to handle mainly due to a moisture content over 90%. While the most common end-of-life treatment for biosludge in Finland is combustion, this option is not efficient in recovering the energy and nutrients contained in biosludge. This calls for alternative treatment methods. Therefore, this study carried out a comparative environmental life cycle assessment (LCA) of four different treatments: hydrothermal carbonization (HTC) to produce hydrochar for energy or nutrient recovery, anaerobic digestion (AD) combined with HTC, separate hydrolysis and fermentation (SHF). Combustion of biosludge was considered as the baseline scenario. The LCA results show that HTC and AD technologies improve the environmental performance of biosludge handling in four out of seven impact categories. HTC performance could be improved by optimising the valorisation of the filtrate. SHF results in considerably higher environmental impacts than combustion and other alternative technologies in several impact categories, mainly because of enzyme production. However, SHF of biosludge is still under development, leading to high uncertainties on the data.