The organic output from mechanical–biological treatment plants as a source of microplastics: Mini-review on current knowledge, research methodology and future study perspectives

Plastic pollution is one of the biggest environmental problems facing the world. In recent years, microplastics, polymer particles from 1 µm to 5 mm, have been getting much attention. The presence of microplastics has already been confirmed in aquatic and marine ecosystems, soil, atmosphere, biota, and others. The organic output from mechanical–biological treatment (MBT) plants was added to the list of microplastics sources only recently. However, according to preliminary estimates, it may be the second-largest source of microplastics. This article presents an overview of already published investigations on this microplastics source and explains the supposed pathway of microplastics in the MBT plants. The main waste treatment processes that can affect the amount of microplastics in the organic output have been identified as shredding, sieving and, to a lesser extent, aerobic or anaerobic processing. This mini-review also includes methods used in the scientific literature for microplastics extraction, purification, and identification in organic-rich samples, their advantages and limitations. Particular attention is paid to the methods of identifying the small microplastics, less than 1 mm, since the methods for particles 1−5 mm have been more extensively studied. Furthermore, future research needs are highlighted.