Efficient hydrothermal pretreatment of mixed PHA/PLA and subsequent continuous anaerobic fermentation into VFA

Abstract

Recycling biobased biodegradable plastics such as polyhydroxyalkanoates (PHA) and polylactic acid (PLA) is promising for developing a circular economy. This study presents the development of a combined technology to convert PHA and PLA into C₂–C₆ monocarboxylates via batch hydrothermal pretreatment (HTP, 150 °C) and continuous open-culture fermentation. HTP experiments revealed that supplementing PHA with carboxylic acids or PLA strongly enhanced depolymerization, reaching up to 91 ± 4% dissolution compared to 46 ± 8% without supplements. In sequential fermentation, acetate and n-butyrate were the dominant products. Co-fermentation of PHA hydrolysates (∼10 g L⁻¹ 3-hydroxybutyrate) and PLA hydrolysates (5 g L⁻¹ lactate) yielded acetate (4.4 g L⁻¹), n-butyrate (8.0 g L⁻¹), and n-caproate (0.3 g L⁻¹), while reducing alkali demand for pH control relative to PHA-only hydrolysates. Overall, ∼90% of the bioplastics’ chemical oxygen demand (COD) was recovered as carboxylates, with a n-butyrate COD selectivity of 71%. Ethanol supplementation (7 g L⁻¹) further increased n-butyrate production to 10.1 g L⁻¹, although ethanol was not fully consumed. Microbial bacteria community analysis identified Clostridium tyrobutyricum as the dominant species, likely driving n-butyrate production and capable of utilizing crotonate, 3-hydroxybutyrate, and lactate. This study demonstrates, for the first time, the feasibility of continuous open-culture fermentation for valorizing mixed bioplastic hydrolysates into carboxylates.