Effect of acid and basic pretreatment on the performance of hydrothermal liquefaction of sewage sludge and olive oil pomace as real waste biofeedstocks

INTRODUCTION: According to the Global Waste Management Outlook of 2024 [1], 2.1 billion tons of municipal solid waste were produced in 2023, and only 13% of the controlled fraction was disposed of using the waste-to-energy approach.Hydrothermal liquefaction (HTL) is a promising thermochemical method that transforms wet bioresources into biocrude (BC) without expensive pre-drying [2]. To our knowledge, the existing literature contains just one study that examined the impact of hydrolytic pretreatment on the quality of BC produced through HTL of microalgae [3]. Furthermore, the literature indicates that acidic and alkaline additions enhance biofeedstock degradation during pretreatments, as well as improve the production and quality of BC in HTL processes. This work provides a first comparative examination of acid and alkaline pretreatment utilizing two kinds of real waste biofeedstocks, sewage sludge (SS) and olive oil pomace (OOP) residues, for BC production by HTL. The objective was to do an empirical systematic investigation to elucidate the impact of specific pretreatments on the distribution of the organic and inorganic fractions of the feed. RESULTS Following the HTL of pretreated SS, we were able to get larger yields of BC that were richer in aliphatic chains, along with higher values of H/C ratios and overall energy recovery. It is worth noting that pretreating SS with KOH resulted in the minimization of chemical species containing oxygen and nitrogen from the feedstock. Additionally, the H/C ratio of the BC obtained after HTL increased from 1.79 to 1.93 at 300°C and from 1.67 to 1.85 at 350°C. After HTL at 350 °C, SS that had been pretreated with HCOOH showed a significant increase in both the yield of BC and the amount of energy recovered. This can be due to the favorable impact that acid pretreatment has on increasing the organic content of the matrix. A densification of chemical energy in the generated BC is made possible, according to the data that was collected, by pretreatment with HCOOH and by ensuring that the kinetic severity of HTL is appropriate. CONCLUSIONS Hydrolytic pretreated SS as HTL feedstock increased BC yield from 18% to 23 and 34%; increased H/C ratios led to higher HHV values; and reduced relative intensity of the olefinic and aromatic C=C stretching bands, suggesting that the pretreatment may increase the aliphatic character of BC generated from hydrolysis. OOP pretreated feedstocks had lower BC yields than native ones and no significant HHV improvement. These findings show that pretreatments improve BC yield more for heterogeneous and mixed biofeedstocks like SS than for homogeneous matrices as OOP. ACKNOWLEDGEMENTS PRIN Refuel (PRJ-0900) Progetti di Rilevante Interesse Nazionale 2022 funded by Ministero dell’Università e della Ricerca is acknowledged for supporting and funding this work.