Microplastics in German paper mills' wastewater and process water treatment plants: Investigation of sources, removal rates, and emissions

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.