Influence of sodium hypochlorite/ultrasonic pretreatment on sewage sludge and subsequent hydrothermal liquefaction: Study on reaction mechanism and properties of bio-oil

Subcritical hydrothermal liquefaction (HTL) is an efficient approach for bio-oil production from wet sewage sludge (SS). Before HTL, pretreatment of wet SS by sodium hypochlorite (NaClO) or combination of NaClO and ultrasonic treatment was implemented with different NaClO dosage or ultrasonic temperature for the first time. The results demonstrated that TOC and content of protein in EPS of SS increased after two pretreatment methods. Meanwhile, the bio-oil yield was improved and the maximum of 39.4 wt% was obtained from the bio-oil under combined pretreatment (9.39 mg NaClO/g dried SS and ultrasounds at 80 degrees C) when HTL was performed at 300 degrees C for 15 min. Additionally, the maximum energy recovery reached 58.0% with the highest carbon recovery (61.9%) and the highest hydrogen recovery (39.0%) in bio-oils obtained from HTL of combined pretreated SS. GC-MS and NMR analysis showed that the chemical compositions of bio-oil were not affected significantly by pretreatment methods but relative percentage of each compound was changed. The esters formation was favored after combined pretreatment and increased to 16.3%, while the contents of acids and N-containing compounds decreased by 8.2% and 4.0%, respectively. TGA of bio-oils revealed that both pretreatments could promote the formation of light oil components such as diesel. Finally, the main possible reaction pathways for HTL of SS have been elucidated. Overall, this research provided a theoretical foundation for utilization of wet SS from sewage treatment plant directly for bioenergy production.