The elimination of microplastic particles (MPP) and microplastic fibers (MPF) was investigated in the final treatment stage (sand filtration) in two municipal wastewater treatment plants (WWTP) and the final treatment stage (sand filtration) at a process WWTP of a manufacturer of polyvinyl chloride (PVC). At each sampling site, three samples on three different days were taken (before/after sand filtration). The samples were filtered through a 10 μm stainless steel cartridge filter utilizing a stainless steel centrifugal pump. Microplastics (MP) were separated from the wastewater matrix by oxidative treatment and density separation and analyzed by Raman microspectroscopy. Due to precautionary measures, procedural blanks were very low with a mean number of 4.3 ± 2.7 MPP and 0.88 ± 0.56 MPF within eight blank samples. The municipal WWTPs were able to eliminate 99.2% ± 0.29% and 99.4% ± 0.15% of MP in the sand filtration stage. The sand filtration of a PVC manufacturer eliminated 99.2%–99.9%.

The packaging material of bottled water is suspected to be a source of microplastics (MP). In this preliminary study, the screw cap system was examined for its MP release potential. Therefore, the concentration of MP (≥10 μm) was determined in reusable polyethylene terephthalate (PET) bottles with polypropylene (PP) caps and polyethylene (PE) seals after opening and closing once and 11 times. The entire contents of the bottles were filtered onto a silicon filter after opening. The inside of the bottles and caps was rinsed with filtered water. The total Si filter area was analyzed by Raman microspectroscopy. After one opening, 131 ± 25 microplastic particles (MPP) per liter were detected. After 11 openings and closings, 242 ± 64 MPP/L were detected. The increase is caused by a significant increase in the number of PP particles from 100 ± 27 to 185 ± 52 MPP/L. The concentration of PE and PET particles did not change significantly. Regardless of polymer type and the number of openings and closings, 80% of the MPP were identified in the smallest size class investigated (10–50 μm). Using a specially developed method, the concentration of MPP was determined before the screw cap system was opened and did not differ from the procedural blank.

This study is one of the first to investigate the recovery rate of high- and low-density microplastic particles (polyvinyl chloride and polypropylene) from wastewater treatment plant effluents or comparable technical facilities under nearly realistic experimental conditions. For this purpose, a method of continuous dosing of microplastic particles into an experimental flume for open-channel flow was developed. Subsequently, 12 samples were taken using volume-reduced sampling and the entire sample purification process including oxidative treatment (with hydrogen peroxide and sodium hypochlorite), density separation (with sodium polytungstate), and subsampling was carried out. Detection was conducted using automatic particle recognition and µ-Ramanspectroscopy. An average recovery rate of 27 ± 10% was determined for polypropylene microplastic particles (d = 53 ± 29 µm), decreasing with the particle size, and 78 ± 14% for polyvinyl chloride microplastic particles (d = 151 ± 37 µm). The results suggest that microplastic emissions from wastewater treatment plants are underestimated, particularly because the recovery rate of small microplastic particles < 50 µm is only 9%.

Although the paper industry processes polymeric materials and discharges large amounts of wastewater, no research on microplastics in the wastewater from paper mills has been published to date. This study is the first to investigate this issue. The wastewater treatment plants of twelve representatively selected German paper mills were investigated using an analysis protocol based on µ-Raman spectroscopy. The results show that treated process water from surface waters is negligible as a source of microplastics (MPs) ≥ 20 µm. The microplastics concentrations in untreated wastewater range from 10⁶ to 10⁸ (MPs ≥ 20 µm)/m³. Sources of microplastics in wastewater include recovered paper, functional polymers, and coating colors, among others. The most frequently detected polymers are polyethylene and polystyrene. In four cases, moving bed biofilm reactors were identified as a source of microplastics. The microplastics concentration in treated wastewater ranges from 10² to 10⁴ (MPs ≥ 20 µm)/m³. Hence, the removal rate of the wastewater treatment plants exceeds 99 %. Mechanical treatment and the activated sludge process have the highest removal rates of all treatment stages. The loads emitted into surface waters range from 10⁶ to 10⁸ (MPs ≥ 20 µm)/h, comparable to municipal wastewater treatment plants with a population equivalent of over 10,000 inhabitants. Compared with other wastewater-related emissions (the total emissions of municipal wastewater treatment plants, or combined sewer overflow), the contribution of paper mills to microplastics in the aquatic environment is low. The results of the removal efficiency can be transferred to other branches of industry and municipal wastewater treatment plants.