Microplastics in wastewater – new results related to the efficiency of wastewater treatment plants along the Danube [93]. The previous study of the PA4 water quality area on the treatment of sewage sludge in the Danube region was published in 2020, enjoying considerable international interest, and a successful professional workshop on the topic of the study was also organized in the framework of the European Green Week 2021 (EU Green Week).

Related to the topic of wastewater treatment and at the same time highlighting the problem of microplastic pollution, which is now very significant, a gap-filling study was prepared in 2021 entitled Microplastics in wastewater – Wastewater analysis in Hungarian sewage treatment plants along the Danube [93]. Microplastics entering the environment are a growing concern as they pose a threat to both aquatic species and humans. The problem is not only the accumulation of plastics in the environment, but that they also contribute to the spread of micropollutants in the environment. Studies indicated [94] that Wastewater Treatment Plants (WWTPs) play an important role in releasing microplastics to the environment. Therefore, effective detection of the microplastics and understanding their occurrence and fate in Wastewater Treatment Plants are of great importance in controlling the phenomenon. In this review, the current status on the detection, occurrence, and removal of microplastics in wastewater treatment plants is comprehensively discussed. Specifically, the different methods used for collecting microplastics from both wastewater and sewage sludge, as well as their pretreatment and characterization methods were investigated and analyzed. The key aspects regarding microplastics occurrence in Wastewater Treatment Plants, such as concentrations, total discharges, materials, shapes and sizes were summarized and compared. The removal of microplastics in different treatment stages and their retention in sewage sludge were explored. The development of potential microplastics-targeted treatment technologies was also presented. Although previous research in microplastics have undoubtedly improved our level of understanding, it is clear that much remains to be learned about microplastics in Wastewater Treatment Plants, as many unanswered questions and thereby concerns still remain; some of these important future research areas are outlined. The key challenges seem to be to harmonize detection methods as well as microplastic mitigation from wastewater and sewage sludge.

Urban wastewater treatment plants (WWTPs) are one of the most important anthropogenic sources of contaminants of emerging concern (CEC), which can have toxic and harmful effects on aquatic organisms and, consequently, on human beings [95]. Unfortunately, wastewater treatment plants are not designed to remove CECs, and secondary (such as the standard activated sludge process) and tertiary (such as filtration and disinfection) treatments are ineffective in removing CEC entering the treatment plant. Therefore, several advanced treatment methods have been investigated to remove CECs from wastewater, including conventional (namely, activated carbon (AC) adsorption, ozonation, and membrane filtration) and new, such as advanced oxidation processes (AOP) processes/technologies. Rizzo et al. summarizes the efforts of a group of international experts (NEREUS COST Action ES1403), who have been discussing the latest and the best available technologies for the advanced treatment of urban wastewater for three years. The article provides a critical review of the documents available in the scientific literature on complex (ozonation, AC, and membrane filtration) and new advanced treatment methods (mainly AOPs) to analyze: i) their effectiveness in removing CEC from wastewater, ii) their advantages and disadvantages, iii) possible obstacles to the application of AOPs, iv) medium and long term perspectives for the application of technological limitations and heterogeneous processes, and v) technical and economic comparison of different processes/technologies.