The present work provides the first records on microplastic (MP) amounts and types in rivers and wastewater effluents entering the Aegean Sea, eastern Mediterranean. Two rivers were sampled using a manta net (mesh size, 0.33 mm): a small urban and a medium-sized river with a rural, semiurban catchment. MPs in wastewater samples were collected at two wastewater treatment plants (WWTPs) within the Athens metropolitan area after secondary treatment and from a pilot biological membrane unit (MBR), using a series of sieves. MPs in the samples were identified using stereoscopic image analysis and spectroscopic techniques. MP concentrations in the rivers were found to be variable, with as high as 27.73 items m⁻³ in the urban river. Differences in MP shape types, sizes, and polymer types reflect catchment size and usage. MP concentration in wastewater effluents was found to be 100 times higher in the secondary treatment (213 items m⁻³) than that in the pilot MBR (2.29 items m⁻³), with filaments and polymers indicative of synthetic textiles and household use. Further research is needed in order to accurately determine variability in MP concentrations and fluxes from these two pathways in the eastern Mediterranean Sea and elucidate the role of rivers in MP retention.

The presence of antibiotic-resistant (AR) E. coli, antibiotic resistance genes (ARGs), and intI1 was investigated in different treatment stages, comparing a conventional activated sludge wastewater treatment plant (WWTP) with a nature-based system in a decentralised area comprising upflow anaerobic sludge blanket reactors (UASBs), constructed wetlands (CWs), and an ultraviolet (UV) disinfection unit. Furthermore, the efficacy of UV irradiation and chlorination in the removal of AR E. coli, ARGs and intI1 from the effluent of CWs and the secondary treated effluent of the conventional WWTP was investigated. The study found that the natural system decreased AR E. coli by 3.7–4.1 log and ARGs by 0.80–2.3 log. In contrast, the conventional WWTP reduced AR E. coli by 2.5–2.8 log and ARGs by 1.9–3.2 log. CWs enhance ARB removal through longer hydraulic retention times, plant–microbe interactions, and diverse ecological processes, offering advantages over conventional WWTPs. UV treatment with doses of 70 mJ/cm² and 100 mJ/cm² effectively removed AR E. coli but increased their percentage, indicating higher resistance to UV. However, these UV doses were ineffective in removing ARGs. Chlorination at doses of 35–50 mg Cl₂/L·min effectively inactivated antibiotic-resistant bacteria (ARB) in wastewater. However, inadequate doses increased the prevalence of antibiotic-resistant strains and facilitated horizontal gene transfer via natural transformation. A high chlorine dose of 220 mg Cl₂/L·min was insufficient to remove ARGs from secondary treated and CW effluents. These results highlight the persistence of ARGs despite aggressive chlorination and high UV disinfection doses, emphasizing the need for complementary or advanced treatment methods.