Effects of typical sludge treatment on microplastics in China—Characteristics, abundance and micro-morphological evidence

Microplastics (MPs) are emerging pollutants that are enriched in sludge. They enter soil through sludge soil amendment, landfill, and discard, which will cause inescapable environmental pollution risks. Sludge treatment technology commonly used in China include anaerobic digestion (AD), thermal drying (TD), thermal hydrolysis (TH) and aerobic composting (AC). In this study, characteristics of MPs in sewage sludge from four representative large cities in China (Zhengzhou, Chongqing, Guangzhou, and Guilin) were analyzed. Effects of four representative sludge treatment technology on sludge MPs were also studied. In addition, the amount of MPs input to soil from sludge in China was estimated. The abundance range of sludge MPs of representative cities in China was 1448-11,125 n∙kg DW. Previous studies indicate that this abundance range is low among other domestic cities and is close to that of European countries. MPs were predominantly fiber-shaped, accounting for 46.66%; 56.5% MPs were white and transparent, and 62.5% were polypropylene and polyethylene. The abundance of MPs in the sludge increased after TH, indicating that MPs broke into smaller particles. However, the other three treatment methods had no significant influence on the abundance of MPs. Scanning electron microscopy analysis showed that the micro-morphology of sludge MPs surface were rougher after AD, and MPs cracked following TD and TH. Furthermore, broken edges were more blurred after TH, and surfaces of MPs were damaged and eroded after AC. The input quantities of MPs in sludge to soil was deduced to be 10 particles per year. These results are important for controlling the potential risk of sludge MPs in China.