Microplastics in wastewater and the role of local wastewater treatment stations in controlling microplastic pollution: a case study from Vietnam

Wastewater has been identified as one of the main contributors to microplastic (MP) pollution in aquatic environments. Hence, this study investigates the presence, characteristics of MPs in wastewater sample types (industrial, domestic, and medical wastewater), and also the removal efficacy of MPs by local wastewater treatment stations. Overall, industrial wastewater showed a higher MP abundance level at 60,881 ± 48,154 items/m, compared to domestic and medical wastewater with values of 31,494 ± 10,142 items/m and 35,453 ± 13,186 items/m, respectively. Fiber and fragment were the main shapes observed among the MPs found in all wastewater samples, and the dominant form was microfiber, ranging from 63 to 97.5% of total MPs. The performance of local wastewater treatment stations showed varied efficiencies in MP removal, ranging between 15.8 ± 5 and 90.2 ± 1.3%. Domestic wastewater treatment stations showed lower MP removal effectiveness, at 43.9 ± 13.1%, while treatment stations receiving industrial and medical wastewater achieved 59.5 ± 20.7 and 69.6 ± 22.1% of removal efficiencies, respectively. As estimated, 2.9 × 10 microplastic items could be emitted to the water bodies around Hanoi every day, which MPs originated from domestic wastewater accounted for 80.3% due to its high discharge volume and inadequate treatment capacity. Optimization of the septic tank system operation and the sewage sludge treatment processes could prevent secondary contamination of MPs, while an additional primary sedimentation step could improve the overall MP elimination efficacy of the studied treatment stations. The results from this study suggested that more in-depth investigations were required for a proper understanding of the migration routes of MPs from different anthropogenic activities to wastewater.