The Viimsi catchment is made up of different types of areas; some are mostly pervious (green), while others are impervious (grey) and some are a mix of the two. The possible hydraulic load as well as the types of constituents found in stormwater runoff are determined by the specific conditions of sub-areas within the catchment. The green areas, including parks and patches of green are linked with mainly organic constituents, such as organic matter and suspended solids and developed, grey areas are linked with mainly inorganic constituents, such as oil, heavy metals, microplastics etc. The contaminated runoff is collected from the catchment into the main collector, which is a network of pipelines and ditches, and then discharged into the outfall connected to the Baltic Sea. The discharge of untreated stormwater has adverse effects on the aquatic ecosystems.