Biobased additives for asphalt applications produced from the hydrothermal liquefaction of sewage sludge

Abstract

Sewage sludge from wastewater treatment plants is a large source of organic waste with suboptimal disposal solutions available. Current common handling solutions include disposing of it as fertilizer on arable land, or direct discharge in the sea. This work investigates the valorization of sewage sludge into biocrude oils using hydrothermal liquefaction (HTL). Biocrude oils are bitumen-like materials with potential applications as green additives for asphalt binder, one of the most used materials in infrastructure. Here, we study the links for sewage sludge between feedstock (digested versus non-digested sludge), HTL conditions (temperature, biomass loading to the reactor and reaction time) and yields of biocrude oil. Our data suggests that non-digested sewage sludge leads to higher biocrude oil yields (30–40 wt%) at temperatures of 300–320 °C and biomass loadings of 20 wt%. Furthermore, we use density functional theory (DFT) calculations to study the reactivity and clustering mechanisms of asphaltenes – a key molecular component of asphalt binder, and largely responsible for its mechanical performance. Biobased asphaltenes are present in biocrude oil, and our aim was to understand their differences with fossil asphaltenes derived from petroleum. Our data suggests that biobased asphaltenes are similar to petroleum-based ones in terms of thermodynamic stability and π-π stacking, despite the higher content in polar chemical functionalities in biobased asphaltenes. Overall, the chemical features and intermolecular interactions indicate that biocrude oils produced from sewage sludge via HTL are promising candidates for application as asphalt additives.