Establishment and application of standard analysis methods for microplastic samples: Urban sewage and sewage sludge as a source of microplastics in the environment

The widespread use of plastics has led to the ubiquitous presence of microplastics (MPs) in the environment, posing risks to ecosystems and human health. Wastewater treatment plants (WWTPs), which often fail to completely remove MPs during treatment, have become a significant source of pollution. However, inconsistencies in sampling, pretreatment, and identification methods hinder comparative studies. This study developed a standardized method for MP analysis in WWTP water and sludge samples. Metal filters and ultrasound-assisted transfer improved desorption efficiency, while NaI flotation achieved nearly complete MP recovery. A two-step digestion method combining Fenton reagent and cellulase effectively removed organic matter (weight loss of 54.21 ± 2.00%) while maintaining 100% MP recovery. By tailoring the method to variables such as treatment processes, water volume, and pollution sources, a "gold standard" approach was designed to evaluate the environmental abundance of MPs in various WWTPs. Application of this method revealed MP concentrations of 2530-18,240 MP/L in influent and 650-1700 MP/L in effluent, with an estimated daily discharge of 1.42 × 10 MP/d into the environment. Primary sedimentation and skimming removed 57.07% of MPs, with secondary and advanced treatments enhancing removal. MPs primarily transferred to sludge, averaging 38.6-104.5 MP/g (dry weight). The most abundant MPs in influent were PU, PET, and PTFE, while PA, PU, and PET dominated in effluent. MPs smaller than 0.5 mm accounted for 98%, with regular particles increasing in effluent. This efficient method establishes a "gold standard" for MP analysis in WWTPs.