Residual biodegradable microplastics derived from compostable packaging under large-scale industrial composting fully degrade in soil

• Optimized method for quantifying biodegradable microplastics in compost. • Analytical workflow validated on full-scale industrial compost samples. • Near-complete biodegradation confirmed for certified compostable packaging • Residual compostable microplastics show complete mineralization in soil. The increasing use of compostable plastics in organic waste management calls for reliable analytical tools to verify their complete biodegradation and environmental safety. This study builds upon a previous full-scale composting experiment that monitored the disintegration of certified compostable packaging under industrial composting conditions. A dedicated analytical protocol was optimized and validated to extract, identify, and quantify biodegradable microplastics (BMPs) remaining in the compost. The method combines preconcentration by fractionation, deagglomeration in hot water, density separation with CaCl 2 , mild oxidative digestion with H 2 O 2 , and identification by Fourier-transform infrared (FTIR) microscopy. Fragments larger than 35 µm for PBAT and larger than 27 µm for PLA and PHBV were detected. In full-scale industrial composting, the protocol revealed near-complete biodegradation of PLA (98.4 %) and PBAT (92.6 %), whereas PHBV showed partial degradation (49.9 %) due to the structural constraints of the coffee capsule format. Residual BMPs were further assessed under controlled soil conditions, where continued mineralization of PBAT and PHBV confirmed their environmental degradability and low persistence. These findings bridge the gap between disintegration and mineralization, providing quantitative evidence of biodegradation from compost to soil and supporting the environmental compatibility of compostable packaging materials.