Quantification of microplastic mass and removal rates at wastewater treatment plants applying Focal Plane Array (FPA)-based Fourier Transform Infrared (FT-IR) imaging

Abstract

This paper presents a method for microplastic (MP) mass quantification using a Focal Plane Array-based Fourier Transform Infrared imaging technique. It discusses the issue that particle number is not a conserved base quantity and hence less suited than mass to compare independent studies on MP in the environment. It concludes that MP mass should be included when quantifying MP pollution in the environment, supplementing the conventional approach of reporting particle numbers. Applying mass as the unit of MP measurement, the paper presents data showing that Danish wastewater treatment plants discharge around 3 t/year of MP in the size range 10-500 μm. This value corresponds to an annual per capita emission from these plants of 0.56 g MP/(capita year). The distribution of polymer types by mass and particle number differed because the size of MP particles of the different material types varied.

Microplastic concentrations, size distribution, and polymer types in the surface waters of a northern European lake

“…However, when concentrations of different size fractions are compared, particle number per volume may not be the best unit for reporting. Because particles degrade continuously, it is not a conserved quantity (Simon, Alst, & Vollertsen, ). Although a filter with smaller pores collects higher number of particles, the overall plastic mass, which is a conserved quantity, also matters.…”

Removal of >10 µm Microplastic Particles from Treated Wastewater by a Disc Filter

“…It assigns the material with the highest correlation score to each pixel and re-constructs the particle based on the scores of adjacent pixels. MPhunter also estimates the size and mass of the identified MPs as described in [19]. In short, the major dimension, defined as the particle's longest axis, was measured directly, while the minor dimension was estimated based on the particle's area and an assumption of elliptical particle shape.…”

“…Applying light microscopy in their study as an identification technique could have resulted in underestimating MP concentrations, as this approach is prone to higher uncertainty as particle size decreases [27]. The identification method used in the present study (imaging-µFTIR) can identify the material composition of particles down to the size of 10 µm more precisely and it is widely recognized as a suitable technique for analysis of small MPs [19,23,[28][29][30].…”

“…MPs accounted for a minor fraction of the SS reaching the receiving water body as the concentration of MPs related to SS was 6.35 × 10 −5 mg MP (mg SS) −1 in BF wastewater and 1.0 × 10 −4 -3.8 × 10 −5 mg MP (mg SS) −1 in AF wastewater. Furthermore, our study has shown that the studied WWTP cannot be considered as a significant contributor to MP pollution as the approximated annual MP load by treated wastewater on the aquatic environment in Denmark is 3 t [19], and this treatment plant manages about 0.5% of all Danish wastewater.…”

“…That is because WWTPs collect vast volumes of wastewater of various origin such as industrial plants and households, along with stormwater runoff, which potentially contains considerable amounts of MPs [13,14]. Although research has shown that WWTPs remove 96-98% of the inflowing MPs [17][18][19], the limited percentage of MP particles discharged to receiving water bodies adds up to a substantial amount, owing to the large flux of wastewater [14,15]. For instance, annual wastewater production in Denmark alone is approximately 700 million m 3 year −1 [20].…”