A Selective Thermal-Alkaline Hydrolysis Method for the Separation of Compostable and Conventional Plastics in Compost

Compostable plastics, unlike conventional plastics, can be treated as biowaste and are a valid component of the organic fraction of municipal solid waste. However, distinguishing between plastics and compostable plastics remains a significant challenge, particularly in composting facilities, due to the lack of suitable analytical methods. This distinction is crucial for verifying the complete degradation of compostable plastics during composting and for accurately assessing their organic recycling rates. In this context, this study aimed to develop an analytical method to distinguish plastics and compostable plastics residues in compost, based on their different susceptibility to thermal-alkaline hydrolysis. First, optimal conditions to selectively hydrolyze starch-based and polylactic acid-based compostable plastics without affecting plastics were established (80 °C, 5% w/v NaOH, 2 h). Subsequently, after isolating impurities (e.g., lithoids, metals, glass, and plastic-like materials) through hydrogen peroxide bleaching, the method was employed to distinguish plastics and compostable plastics residues in compost samples, validating the results comparing the method with ATR FT-IR. Recovery tests confirmed method’s reliability across a 0–1% w/w concentration range, with a recovery efficiency of 98%. Beyond assessing composting performance and compost quality, this method may also serve as foundation for updating official methods referenced in national and European fertilizer regulations.