Integration of anaerobic digestion and hydrothermal liquefaction for sewage sludge management
Creators: Tatla, Harveen Kaur
Publisher: name: University of Alberta Library
Year: 2024

<p>Given the volume of sewage sludge produced worldwide, efficient waste management and energy recovery techniques are essential. Anaerobic digestion (AD) and hydrothermal liquefaction (HTL) are viable avenues for the valorization of sewage sludge. Their combination produces a synergistic framework that reduces intrinsic constraints, leading to improved productivity and all-encompassing sludge usage. Energy recovery from either standalone AD followed by HTL or standalone HTL followed by AD has been the subject of numerous research; however, a systematic comparison of energy recovery from both integrated sequences under the same operating circumstances and feedstock has not yet been comprehensively examined. This research gap hinders the thorough understanding of the optimal approach for sewage sludge valorization processes and their advancement in a circular bioeconomy. In order to close this gap, this thesis includes a comparative analysis of energy recovery from both integrated schemes under identical operating conditions using sewage sludge as a feedstock. Additionally, it also explores the techno-economic assessment (TEA) for a large-scale integrated AD-HTL system to evaluate economic feasibility and viability of this integrated approach.</p>

<p>Firstly, the energy recovery efficiency of two sequencing configurations: HTL followed by AD and AD followed by HTL, at varying HTL operating conditions of 250, 300, and 350 °C for 30 and 60 minutes each was systemically evaluated. Our results demonstrated that the HTL-AD sequence yields higher energy recovery in the form of biocrude, with a concentration of fatty acids due to the high lipid content in primary sludge. On the other hand, the AD-HTL sequence recovers more energy in the form of biomethane, due to the easily degradable nature of primary sludge in regard to lower nitrogen content. Energy recovery for the HTL-AD sequence ranges from 47.2% to 84.5%, while the AD-HTL sequence ranges from 57.2% to 77.3%. The HTL-AD system recovers the highest energy at 300 °C for 60 minutes, whereas at other operating conditions, the AD-HTL system achieves higher energy recovery than the HTL-AD system.</p>

<p>Secondly, a techno-economic assessment aimed at identifying the performance targets necessary for achieving economic viability was conducted. Three proposed configurations with a processing capacity of 1.1 million tons of primary sludge annually were assessed. Our results concluded that AD- HTL system computed the lowest LCOE of $11.4/ GJ, due to additional revenue stream generated from biocrude and hydrochar. The integration of HTL process for the degradation of digestate emphasis on importance of generation of additional revenue streams for achieving economic viability. Sensitivity analysis showcased that the incorporation of low- cost electricity or adoption of renewable energy sources, is crucial for achieving economic and environmental goals. Additionally, optimizing the AD and HTL process can be beneficial to achieve economic targets.</p>