Polylactic acid (PLA) was developed as a biodegradable alternative to fossil-based plastics. However, practical disposal 

method for PLA has not yet been established. To address this, the present study proposes hydrothermal liquefaction (HTL) 

as a strategy for valorizing PLA into energy resources (biocrude). Spent coffee grounds (SCG), which are rich in lipids, 

were employed as a co-feedstock to enhance biocrude quality. The optimal operating conditions for maximizing biocrude 

yield were explored by adjusting various operating parameters, including the PLA-to-SCG mass ratio, a reaction temperature,

a reaction time, a water-to-feedstock (W/F) ratio, and an initial pressure. To further boost biocrude yield, acetone was 

introduced as a co-solvent. Utilizing a solvent mixture with a higher proportion of acetone than water (on a volume basis) 

enhanced biocrude yield, reaching a maximum value of 83.9 wt%. Comprehensive characterization through Fourier transform

infrared spectroscopy, ultimate analysis, a Van Krevelen diagrammatic assessment, and gas chromatography-mass 

spectrometry analyses revealed the underlying reaction mechanism of the co-HTL process, demonstrating that acetone 

reduces the polarity of the reaction medium, thereby shifting the partitioning of PLA-derived organic acids from the 

aqueous product into the biocrude. Consequently, the atomic O/C ratio decreased while the higher heating value (HHV) 

increased, indicating an improvement in biocrude quality. These observations highlight that the introduction of acetone as 

a co-solvent during the co-HTL of PLA and SCG is an effective strategy for simultaneously maximizing biocrude yield 

and improving its overall quality.