Investigation and fate of microplastics in wastewater and sludge filter cake from a wastewater treatment plant in China

Microplastics (MPs) have been widely detected in wastewater treatment plants (WWTPs) due to their small particle size, wide distribution, and difficulty in removal. Previous studies, however, mostly focused on MPs in wastewater, thereby neglecting sludge. To comprehensively understand the changes of MPs in WWTPs, we investigated the quantity and characteristics of MPs in wastewater and sludge of a WWTP in Harbin, a typical inland city in China, and calculated the MPs removal rate. The results showed that there were 126.0 ± 14.0 particles/L MPs in the influent and 30.6 ± 7.8 particles/L in the effluent, about 75.7% MPs were removed and transferred to the sludge during this WWTP. The abundance of MPs in dewatered sludge and sludge filter cake was 36.3 ± 5.7 and 46.3 ± 6.2 particles/g (dry sludge), the sludge disposal scale of this WWTP can reach 1300 tons/day, which was equivalent to about 7.74 × 1012 microplastic particles accumulated in sludge per year. These sludges were used as fertilizers in the soil, which will cause secondary pollution of MPs. Raman spectroscopic analysis showed that about 89.5% of particles were plastic polymers, such as polyesters, polyamide (PA), polyethylene terephthalate (PET) and polyethylene (PE), which suggested that MPs may be derived from laundry and personal care products. Therefore, we recommend that more work should be devoted to how to control the release of MPs at the source and the reuse of sludge after treatment by WWTPs.