The revised Directive on Urban Wastewater Treatment introduces stricter requirements for managing discharges from combined sewer overflows (CSOs) and urban surface runoff, reflecting their significant impact on the ecological status of water bodies. This thesis examines the implications of these new provisions and evaluates the magnitude of pollutant loads associated with CSOs in the Ljubljana sewerage system. The results of the analysis showed that total annual organic load of the system for the years 2019 and 2020, amounting to 8272.1 t BOD₅/year. A detailed case study was then conducted on two representative CSOs. At CSO R39, estimated annual loads reached 33.8 t BOD₅, 70.2 t COD, and 33.4 t TSS in 2019, and 20.3 t BOD₅, 37.3 t COD, and 15.1 t TSS in 2020. By contrast, CSO R40 displayed substantially lower and more stable loads: 1.2–1.3 t BOD₅, 2.4–2.5 t COD, and 1.1–1.4 t TSS. Based on the calculations, different measures have been proposed to reduce impacts on environment, combining traditional hydraulic engineering solutions, such as retention structures, with blue–green infrastructure measures, including green roofs, infiltration trenches, and rain gardens. While the analysis suggests that CSO R39 may constitute a more critical source of pollution due to its large catchment, the findings are based on a limited dataset and should provide an orientational estimate of the situation.
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