In Helsinki, population of naturally breeding sea trout has been restored in many streams. Contaminated urban storm waters threaten these populations, and thus, special attention needs to be paid to quality of storm waters discharged into these streams.
City of Helsinki wanted to tackle one special hot spot causing threat to the trout population in Haaganpuro stream. A small industry area discharging its storm waters directly into Haaganpuro stream has caused several dangerous situations, in which leakages of oil and harmful substances have entered the stream. As part of the project “Kaupunkivesistöt kuntoon” or “Improving City Waters” financed by the Finnish Ministry of the Environment City of Helsinki built a desilting basin and a biochar filtration pilot in a branch stream running form the industry area.
The filtration system is based on the ability of biochar to retain water, nutrients and contaminants. Due to its porosity, durability and its large specific surface area, biochar is well suited for treating storm water. The structure of the filtration system consists of a shallow basin with the filtration layer on the bottom. The filtration layer is a mixture of biochar and crushed aggregate in order to optimize the permeability and filtration. On top of the filtration basin there is a dense willow thicket which evaporates the water and absorbs nutrients and contaminants. The filtration basin is able to take up to 200 m3 of water, which corresponds to the rainfall of 3 mm in this densely built and mainly impermeable catchment area. All storm water is primarily led to the filtration basin, however, there is a bypass entering another sedimentation basin where the overflow will be led if the level of water in the sedimentation basin rises too high. For emergency situations, there is a sluice gate in the gallery which can be used to stop the discharge of water into the Haaganpuro stream, and facilitate oil spill response measures in the event of damage.
Storm water filtration site in action
With the biochar filtration solution, it was possible to reduce the amount of oil, nutrients and heavy metals entering the Haaganpuro stream, and at the same time, slow down the water flow. The suspended solids were effectively removed by filtration. The decrease of suspended solids probably partly explains the decrease of many substances typically bound in suspended matter. Especially concentrations of dissolved phosphorus were very low after the filtration. Furthermore, concentrations of many metals typical in urban storm waters, like cadmium, cobalt, chromium, nickel and copper were decreased in the filtration through biochar. Oil that has been a special problem in this location, markedly decreased by water retention and biochar filtration.
Illustrative cross-section of the strom water filtration site
In the future, similar structures hopefully will be constructed in other urban areas to improve storm water quality. Biochar not only cleans strom water, but in this form, coal is very persistent and long-lasting, so the use of biochar in various structures also helps mitigate climate change.