Development of an integrated platform for the biological valorization of syngas and biochar produced through pyrolysis of municipal sewage sludge

Municipal sewage sludge (MSS) management represents one of the most critical challenges related to wastewater management and environmental protection due to the high volumes produced globally every year (45 Mton of dry matter), the presence of hazardous organic and inorganic pollutants, high management costs, and environmental impacts. Interestingly, precious resources such as organic matter and nutrients are still present in MSS and should be recovered for multiple uses. Traditional treatment and disposal methods often fall short in fully addressing these issues, while innovative thermochemical processes (e.g. pyrolysis) are gaining interest in this field for their high versatility. Pyrolysis converts MSS into valuable products such as biochar, bio-oil, and pyrolysis gas, enabling simultaneous matter and energy recovery when these products are valorized in innovative biorefinery pathways. The main objective of this doctoral thesis is to investigate the potential of an integrated platform for the biological valorization of the products of MSS pyrolysis, with a particular focus on biochar and pyrolysis gas. To assess the influence of the pyrolysis process on the yields and composition of the products, the study of the MSS pyrolysis process was conducted experimentally by testing variable temperatures and the addition of variable zeolitic catalysts. The study demonstrated a significant influence of temperature on product yields and compositions, and an interesting capability of H-Mordenite and H-ZSM5 zeolites to influence product yields and sulfur migration in pyrolysis gas, reducing H2S concentration in pyrolysis gas of up to 47 % by volume. The thesis further investigates the valorization of MSS-derived pyrolysis gas through biological processes. The production of microbial protein (MP) from pyrolysis gas mixtures using hydrogen-oxidizing bacteria (HOB) was investigated. Results indicate the feasibility of producing MP with up to 74 % protein content, leveraging the mixed culture's resistance to high H2S and CO concentrations. This finding opens new avenues for synthesizing alternative protein sources, biopolymers, and fertilizers from MSS. Further experimental activities were conducted on the valorization of pyrolysis gas and its conversion to biomethane in a pilot-scale reactor. The biomethanation process effectively converts H2 and CO into CH4, achieving up to 90 % methane concentration by employing a Venturi-type ejector as a gas-solubilization method, and by optimizing gas retention time and H2/CO2 ratios. This method enhances energy recovery from MSS and supports the transition to renewable energy sources. The thesis also evaluates the potential of biochar produced from MSS pyrolysis. Despite the limited CO2 adsorption capabilities observed in the study, the role of biochar in the in-situ upgrading of biogas to biomethane is examined. Future research should focus on biochar activation to improve its CO2 adsorption capabilities and effectiveness in anaerobic digestion processes. The construction of this innovative valorization platform for MSS through pyrolysis could represent a strategic and game-changing approach to advancing resource recovery from MSS, mitigating environmental pollution, and contrasting climate change.