Circulation of fibrous microplastic (microfiber) in sewage and sewage sludge treatment processes

The fate of microplastics (MPs) in the sewage treatment process has been investigated worldwide, and novel results have been reported; few studies have also clarified the fate of MPs in the sewage sludge treatment process. Although most MPs in sewage are transferred to sludge, some flow back from the sludge treatment process to the sewage treatment process. Therefore, throughout the sewage treatment plant, the removal rate of MPs may increase via a countermeasure during the sludge treatment process. In this study, samples obtained from sewage and sewage sludge treatments were used to degrade organic matter with hydrogen peroxide. Water sample particles were trapped on Ni filters with 20-pm-sized pores, dried at room temperature and then the MPs were detected and identified by FTIR microscopy. Note that sludge samples were treated with hydrogen peroxide and separated by specific gravity using NaI solution. The concentration of MPs per unit volume was then calculated and the MPs load was estimated using flow rates of water and sludge. Subsequently, we clarified the fate of MPs with sizes of 20 pm or greater in a sewage treatment plant. When the MPs load in the influent sewage is 100%, 12% of the MPs were found to return to the sewage treatment process via the sidestream of the sludge treatment process. Per this observation, it was made evident that MPs are in fact circulating throughout the sewage and sludge treatment processes. MPs in the sidestream mainly consisted of the effluent from the sludge concentration process, and most MPs were thought to be fibrous polyethylene terephthalate (fibrous MPs, i.e. microfibers [MFs]). The results show that MFs circulate throughout the sewage and sludge treatment processes, and for effectively increasing the removal rate of MPs, the removal of MFs proves correspondingly effective during the sludge treatment process. (c) 2021 Elsevier B.V. All rights reserved.

Automated summary

The fate of microplastics (MPs) in sewage treatment reveals that some MPs from sludge can flow back to sewage, contributing to the circulation of MPs between sewage and sludge treatment processes. Fibrous polyethylene terephthalate microplastics (microfibers) were found to be the main MPs in the sludge sidestream, highlighting the importance of their removal during sludge treatment for effectively increasing the overall removal rate of MPs.