The use of surrogate standards as a QA/QC tool for routine analysis of microplastics in sewage sludge

The efficient retention of microplastic particles (MP) during wastewater treatment results in their accumulation in the sewage sludge. Thus, sewage sludge represents a key matrix for understanding MP flows between engineered and natural systems. Building on previous reports, we present a sample preparation protocol optimized for digested sewage sludge. The key steps include acid digestion supported by Fenton reagents, enzymatic digestion, and density separation using sodium polytungstate (density of 1.9 gcm^-3). We use colored polyethylene (PE) spheres as surrogate standards to assess sample specific recoveries in terms of number and size, based on visible light (vis) microscopy and focal plane array - micro-Fourier transform - infrared (FPA-μ-FT-IR) imaging. The FT-IR spectra of common MP were identical before and after the digestion procedures and morphological changes were observed for polylactide fibers only. Average recovery rates for PE spheres, polypropylene fibers and polyethylene terephthalate fragments extracted from spiked digested sewage sludge and determined using (automated) vis microscopy ranged from 80% to 100%. Similar recovery rates of around 80% were also obtained for PE spheres based on FPA-μ-FT-IR measurements. The median diameters of red and blue PE spheres in dry state and recovered from spiked deionized water and from extracts of spiked digested sewage sludge determined using vis microscopy ranged between 46 μm and 67 μm. These diameters were similar to 54 μm and 61 μm obtained from the FPA-μ-FT-IR measurements of the corresponding deionized water samples and digested sludge extracts and in line with data from the producer (53 μm-63 μm). Using our digestion protocol in combination with surrogate standards, we measured MP number concentrations of around 10,000 #/g in dried, digested sewage sludge, in agreement with recent results from other studies.