Hydrothermal liquefaction of high ash containing sewage sludge at sub and supercritical conditions

With the rapid growth in population and urbanization, sustainable disposal of sewage sludge has become a prominent problem worldwide. Therefore, an adequate treatment is required to reduce the environmental impacts created from traditional methods such as incineration, landfill, etc. In this context, sewage sludge was liquefied hydrothermally under sub-supercritical conditions with and without catalyst (K2CO3). The effect of temperature and alkali catalyst on product distribution was investigated. Obtained results showed that the temperature had a negligible influence, whereas catalyst slightly improved the bio-crude yield and quality for both sub-supercritical conditions (350 and 400 °C). Bio-crude contained N-containing compounds, ketones, phenols, acids, and long-chain hydrocarbons. Carbon and nitrogen recoveries revealed that 58–67% of the carbon went into bio-crude, while the majority of the nitrogen was transferred to the aqueous phase. ICP-AES analysis indicated that approximately 80% of the heavy metals were concentrated in the solid phase. The leaching action of citric acid with sewage sludge not only removed 40% of ash content but also reduced 38% of the fat content.