Biofiltration in Maunulanpuisto Park in Helsinki, Finland

Biofiltration units are engineered structures that mimic the natural hydrological process of filtration. Most of the water is detained in the structure and further infiltrated in the underlying soil or drained but also evapotranspiration occurs depending on the conditions.

Components installed in the solution

Biofiltration systems generally consist of a top layer, filtration media, storage layer and a drainage layer. Usually in connection to larger biofiltration areas the water is lead through a sedimentation basin in order to separate particles from the water before the filtration. To avoid clogging of the system, the biofiltration unit itself should not be used as a sediment trap but pretreatment of the stormwater should be included to the system. In the biofiltration, stormwater runoff is first filtered through the vegetation and then vertically through the soil filter media. Therefore, the permeability of the upper layer of the system should be sufficiently high to allow infiltration. Filtered water is either infiltrated on site or collected in under-drains located at the base of the system and then lead to receiving waters or the stormwater network. Treatment in biofiltration is based on various processes including fine filtration, adsorption, chemical reactions and biological uptake. Ponding of the water at the surface occurs once the soil pore space capacity of the media is exceeded. The treatment media is generally porous soil with a topping layer of hardwood mulch and vegetation which also prevents the system from erosion. As rapid infiltration of water is desired, the soil has typically a high sand content but to promote pollutant attenuation also low levels of silt and clay are usually required.

Operational mode

In 2015 a biofiltration area was realised in Maunulanpuisto Park in Helsinki, in order to purify rainwater and meltwater and improve water quality in the Haaganpuro brook, where salmon is breeding. The water quality is improved through sedimentation of suspended particles in a sedimentation basin prior to the biofiltration. Rainwater and meltwater collected in drains from a lively trafficked drainage area are channelled to a sedimentation basin, in which a large part of the solids sinks to the bottom. The water then continues its journey to the biofiltration area where it is allowed to pool, giving vegetation time to bind nutrients and heavy metals. After this, the water slowly permeates the filtrating sandy layers under the vegetation, during which harmful substances are absorbed by microbiological activity. Finally, the purified water is fed into the ditch via a drainpipe and onwards into the Haaganpuro brook. The sedimentation basins and the biofiltration area need a check of functioning annually and the basins need to be dredge to remove sedimentated material approximately every 5th year. In addition the biofiltration layers need to be renewal every 10th year and the vegetation is cut around August/September.

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