Controlling and optimizing the volatile organic compound and organic contaminant removal by improved water management in plastic recycling plants

Water-based washing is a critical step in the mechanical recycling of flexible post-consumer polyolefin packaging, essential for achieving high-quality recyclates. Despite its importance, the process remains poorly understood and entails a substantial water consumption. This study aims to optimize the trade-off between water usage and recyclate quality, focusing on organic contaminants (i.e. chemical oxygen demand (COD), total nitrogen (TN), total phosphorus (TP)) and volatile organic compounds (VOCs). Lab-scale experiments were conducted to: (1) characterize various plastic input streams and identify correlations among key parameters, (2) assess the influence of input stream variability and solid-to-liquid (SL) ratios on washing efficiency, (3) investigate water reuse effects on both recyclate quality and contaminant removal kinetics, and (4) test alternative wastewater treatment methods. A kinetic model was adopted, calibrated, and implemented in Aspen Plus® using experimental data. Findings revealed no correlation between COD and VOC loads on plastic films. Washing experiments demonstrated a dynamic equilibrium between the contaminant load on plastics and the final concentration in the washing water. Given the inherent heterogeneity of plastic waste, real-time monitoring of water quality and treatment performance is essential to consistently ensure recyclate quality. Additionally, VOC-removal was found to be more sensitive to water reuse and quality than the removal of organic contaminants. Based on these insights, a modified washing line incorporating a third polishing wash with a separate water cycle is proposed. This configuration improved removal efficiencies for VOCs (25%), COD (12%), TN (19%), and TP (9%) compared to conventional washing systems.