Natural treatment of storm water on a large-scale in Lahti

Solution to which problem

Previously all storm water from the downtown area of the city of Lahti was discharged untreated to Lake Vesijärvi where it worsened the quality of water with nutrient and heavy metal loads. Suspended solids, hydrocarbons, microplastics and PAHs were also detected from the stormwater. Especially phosphorus was a problem in Vesijärvi. It was estimated that around 35 % of the external phosphorus load coming to the Enonselkä basin, adjacent to the city, originated from storm water discharge.

Technical conditions

In order to improve the quality of water in Vesijärvi, it was decided that part of the storm water from downtown would be conveyed to another part of the city, Hennala, which is located about 3 km away. This transfer of storm water was possible due to the already existing backup/emergency sewer line. During autumn 2018 a dedicated pumping station was built for downtown storm water in another project (Hulevesien hallintaa kustannustehokkailla hybridiratkaisuilla / Storm water management via cost-effective hybrid solutions). The Hennala area, which was chosen to house the treatment system for the storm water conveyed from the city’s center, is part of a recently decommissioned army garrison, slated for city planning and development in the coming years. The storm water system is located at the center of this developing area. In addition to cleaning the storm water the system will serve as a recreational area for the future residents.

The storm water treatment system consists of a sedimentation basin, biofiltration field, wetland basin and a channel, which connects them (see attached overview picture).

The core of the treatment system is the biofiltration field. It is divided into three different sections, each using a different filtration material. To prevent the cross-section movement of water, partition walls made of clay are used to separate the sections. In addition, the sections have their own under drains so that water samples can be taken from each of the sections in order to be able to compare the treatment efficiency of the filtration materials. Filtration materials are basic filter sand (grain size 0,2 – 2 mm, used as a benchmark), expanded clay aggregate (Lecatm) and Filtralite P (a special clay aggregate, developed for phosphorus removal).

The treatment system is designed to receive runoff from a three-year, 10-minute design rainfall (intensity 160 l / s * ha, impact of climate change is taken into account) on the Lahti city center where the storm water is directed to the treatment system. 

Due to permit reasons, at least during the first years, the volume of storm water arriving to the treatment system is limited to 3 000 m3 per day.

Before any storm water is directed through the treatment system, vegetation planted there needs to grow for about a year. Vegetation prevents erosion, improves the permeability of the filtration structure and makes it more efficient by absorbing nutrients. 

The filtration materials for the biofiltration field were chosen based on preliminary laboratory tests carried out during the planning phase of the stormwater treatment system by the University of Helsinki as a part of the project. In the laboratory tests, the University of Helsinki tested five different filtration materials without and with plant cover on their ability to retain suspended solids, phosphorus, nitrogen and heavy metals.

Tested materials were basic filter sand, Filtralite P (a special clay aggregate, developed for phosphorus removal), expanded clay aggregate, spruce based biochar and concrete aggregate. Plant used was ribbon grass (Phalaris arundinacea). Strom water used in the tests was snow collected from heavy trafficked road in Lahti. It contained a lot of suspended solids (5g/l), which all tested materials retained effectively, 96-100 %. Strom water used in the test contained also a lot of phosphorus and metals, which were mainly absorbed in solid particles. 75-100 % of these were retained. Filtralite P and concrete aggregate were best in retaining liquid phosphorus. Filtering through these materials increased the pH to 12 while other materials kept the pH between 7-8. Seep water contained as much or more nitrogen than the incoming storm water thus the materials of the structure increased the amount. Plants helped retaining the nitrogen. Biochar retained nitrogen but phosphorus was leached through it. 


The aim of the pilot in Hennala was to develop the expertise and knowledge in storm water quality management and improve the attractiveness, healthiness and safety of the urban environment. A storm water treatment system was implemented in the Hennala area in order to improve the quality of storm water created in Lahti downtown and thus prevent the storm water from polluting Lake Vesijärvi. As a solution, part of the storm water from Lahti downtown will be directed to the Hennala area for treatment.


The system will be in operation by mid-2020. Subsequent monitoring will quantify its effectiveness in purifying storm water. Based on monitoring data collected during and before the project, transferring storm water from the city’s center for treatment in the Hennala pilot will reduce the city’s storm water load into Lake Vesijärvi by approximately 10-15 %.

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