COD as a Dynamic Tool for Pretreatment of Sludge Samples in Microplastic Analysis

Abstract Given the society’s continuous reliance on plastic materials, large amounts of micron-sized plastic particles (i.e., microplastics, MPs) reach wastewater treatment plants (WWTPs) every day. Despite their effective removal from influent wastewater, over 90% of MPs in WWTPs are accumulated in sludge. Yet, there is no universally accepted method for quantification and identification of MPs, which beclouds our understanding of the extent of this pollution. Therefore, this study aims to develop a chemical oxygen demand (COD) based repeatable method for MP analysis in sludge, which is a very complex, MPs-laden by product of WWTPs. The developed method is unique in that it removes the organic substances interfering with polymer analysis by monitoring the COD of sludge for the first time in literature. Upon 90% of organic matter removal, MPs are extracted from the medium by a two-step density-based separation, sieved, stained with Nile Red, and counted under Fluorescence Microscope. Moreover, quality assurance and quality control (QA/QC) strategies including blank preparation and spike and recovery test procedures are followed. The developed protocol ensures at least 80% recovery efficiency of variety of MPs from waste activated sludge samples, which meets the recommendations given in the literature.