Characterization of polylactic acid degradation in rotary drum composting systems: from laboratory respirometry to industrial-scale performance

Abstract

This thesis characterizes polylactic acid (PLA) degradation in Rotary Drum Composting (RDC) systems through laboratory respirometry and industrial-scale composting. Laboratory respirometry was conducted in 2 L vessels containing ~5 g of PLA in 275 g of PLA-adapted compost sourced from a local operating RDC. At 58 °C, complete mineralization was achieved within 45 days. At 30 °C, there was negligible mineralization within 81 days regardless of biostimulation or bioaugmentation. RDC trials, the first characterization of PLA degradation in rotating in-vessel composters, showed rapid physical degradation of PLA. Most films degraded beyond recoverable mass within two to four weeks under intermittent thermophilic exposure. Outcomes were bimodal; some films were completely degraded, while others remained intact over six weeks despite identical residence time. This was hypothesized to result from spatial thermal heterogeneity within the vessel, which was supported by per-bag temperature sensor data and was reduced but not eliminated through increased vessel rotation. As abiotic hydrolysis and molecular weight reduction are the rate-limiting steps in PLA degradation, UV-C pretreatment was investigated and achieved in hours the degree of molecular weight reduction that required weeks under thermophilic composting, though without a corresponding reduction in PLA mass. Across composting experiments, cumulative thermal exposure was the strongest predictor of PLA degradation, and residence time alone was not a reliable metric in heterogeneous composting environments. Future work should prioritize per-sample thermal tracking in full-length RDC trials with continuous rotation and integration of UV-C pretreatment with composting to determine whether accelerated molecular weight reduction translates to faster mineralization.