Storm water filtration unit in the city of Helsinki

The filtration unit is located in Taivallahti area near the city center of Helsinki, on the southern part of the Helsinki peninsula. The area has few permeable spaces, such as parks and other urban greenery, and the building density is higher than elsewhere in the city. The filtration unit is located at an outfall of a stormwater pipe that gathers stormwater from heavily trafficked roads. The pilot unit is underground and thus it won’t take space from other use purposes. However, it would be possible to use the filtration units on very different sites in the city as a part of the urban environment and re-use the purified water as an recreational element. In addition, filter unit can be integrated with waterfront structures (e.g. marine piers) thus it provides many architectural opportunities.

To meet the challenge of lack of space for treatment of contaminated storm water in densely built areas, City of Helsinki is testing a new nature-based solution in Taivallahti area, close to the city centre. The filtration unit is underground and located at an outfall of a storm water pipe which gathers stormwater from heavily trafficked roads. It has a modular structure with a body made of concrete and inside it has both coarse and fine filter departments made of recycled plastic lumber. Depending on the rain event, the filtration unit captures part of the stormwater which would otherwise be discharged to the sea without treatment. The filtration unit reduces the concentrations of solid substances, litter and different harmful substances such as microplastics.

Components installed in the solution

The purification process is based on sedimentation and filtration. According to the preliminary measurements, 10 m long filter unit with extra detention basin is able to filter total volume of a 17 mm rainfall from ~1-hectare catchment area (60 min design storm with 3 year return period). The filtration unit has coarse and fine filter departments. Storm water flows from inlet into stabilization basin, which is meant to balance water level so that water column flows evenly through the filter grid into the primary coarse filter. In addition, stabilization basin also acts as a sedimentation basin. Water leaches from coarse filter into fine filter through under-sluice at the beginning of the rain event. During the peak flow, water accumulates on top of the fine filter and the flow speed increases because of the higher hydraulic head. Different filter materials can be used. In this pilot, expanded clay is used as the coarse filter. This filter removes larger particles from stormwater. During the first study period in year 2019, two different materials were tested as the fine filter: biochar and filter sand. In future studies, it is possible to test four different fine filter media simultaneously.

The pilot has a length of 8 meters. When scaling solution in different sites, length is the primary parameter. In the pilot version the body is made of concrete, but it can be made of other materials too, like wood. Furthermore, same hydraulic principle can be applied, when designing open biofilters such as bioswales.

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