Enhancing Microplastics Recovery from Complex Sludge Samples Using COD-Guided Pretreatment

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 quantifying and identifying MPs, obscuring our understanding of this pollution’s extent. Therefore, this study aims to develop a chemical oxygen demand (COD) based repeatable method for MPs 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. 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 using fluorescence microscopy. Moreover, quality assurance and quality control strategies including blank preparation and spike-and-recovery test procedures are followed. The protocol ensures a minimum 80% recovery rate of various MPs from both waste activated sludge (WAS) and wastewater samples, aligning with recommended standards. Crucially, the method preserves the chemical identity of MPs. Application of the protocol revealed that urban WWTP WAS had 475 MPs/g TS; industrial influent and effluents wastewater had 73 and 26 MPs/L; and industrial secondary and dewatered sludge had 114 and 132 MPs/g TS, consistent with the literature. This demonstrates the method’s robustness by revealing MPs reduction within the WWTP process and sludge accumulation as treatment progresses. Graphical Abstract