Quality assessment of post-consumer plastic bottles with joint entropy method: A case study in Beijing, China

Recycling of plastic bottles is a fundamental strategy to reduce the need of fossil resources and protect global environments. The collection of post-consumer plastic bottles (PCPBs) is a key node of the recycling of plastics due to its influences on the subsequent life cycle stages. However, there is still a lack of quantitative assessment on the quality of PCPBs, especially considering the contamination and the mixture of different plastics in the collection stage. This study provides an indicator based on the joint entropy method to evaluate the quality of PCPBs in collection stage, which includes components, polymer types and colors three levels. This indicator (Quality-value) is applied to evaluate the quality of PCPBs' collection system in Beijing city, China, which is based on an on-site survey including 27 samples gathered from 4 distribution centers and 5 collection sites. The results showed that the quality of PCPBs decreased through the collection system of Beijing, which degraded from post-consumer bottles (0.320<Q<0.325) to pre-production bottles (0.311 <Q <0.312). The type of residential area is identified as a key factor of the PCPBs' quality in collection sites. There is no obvious difference between formal and informal business models in the surveyed area. Providing three levels of quality equally weighted, colors sorting had the greatest potential to improve the quality of PCPBs compared to components and polymers sorting. We suggest that joint-entropy based quality assessment can help to manage waste collection system in an efficient way to meet the need of circular economy.