Sewage sludge and olive oil pomace residues as real waste models to feed hydrothermal liquefaction: effect of acid and basic pretreatments

We have studied the possibility of using acid and basic pretreatment with HCOOH and KOH to produce high-quality biocrude (BC) from HTL of sewage sludge (SS) and olive oil pomace (OOP) residues as models of waste. Pretreatments are more effective in improving the BC yield and quality obtained from heterogeneous and mixed SS, whereas they are less effective with more homogeneous matrices, such as OOP. In fact, after the HTL of pretreated SS, we obtained higher yields of BC enriched in aliphatic chains with higher values of H/C ratios and energy recovery. In particular, KOH-pretreatment of SS removed oxygen and nitrogen-bearing chemical species from the feedstock and increased the H/C of the BC obtained after HTL from 1.79 to 1.93 at 300°C and from 1.67 to 1.85 at 350°C. A notable increase in BC yield and energy recovery was observed with HCOOH-pretreated SS after HTL at 350°C, attributed to the beneficial impact of acid pretreatment in enhancing the organic content of the matrix. Collected data indicate that HCOOH-pretreatment and proper kinetic severity of HTL allow a densification of chemical energy in the produced BC. • Pretreatment effectively increases the BC yield from SS. • Pretreated SS produces higher yields of BC enriched with aliphatic chains and ER. • KOH pretreatment of SS increased the H/C of the BC at both reaction temperatures. • FA pretreatment and optimal KSF induce densification of chemical energy in the BC.