Fighting pollution and restoring water quality with a nature-based filtration system in Finland

Key Learnings

About the Region

Climate Threats

Polluted storm water, human activity and wastewater reduce water quality, especially in the Helsinki area. Climate change, with a rising frequency and intensity of extreme weather such as heavy rain and snow, adds pressure to the problem of already polluted waters. The high percentage of sealed surfaces prevents water from soaking into the soil, causing rapid runoff that leads to flash floods. These sudden, intense floods overwhelm drainage systems, which, when overloaded, can carry high levels of pollutants – like solids, nutrients, and oils – from heavily trafficked and paved areas into water bodies. For example, rainwater and meltwater entering the Haaganpuro brook are often contaminated, harming aquatic life such as fish stocks. This pollution exacerbates the challenges posed by flooding and erosion. Rising sea levels also significantly increase the risk of flooding and landslides by reducing the stability of slopes.

As large parts of the Helsinki area are subject to significant industrial activity and intense agricultural use, water and soil pollution have become a challenge for the local environment. The Baltic Sea is one of the most polluted brackish water bodies in the world. High nutrient loading from municipal wastewater, fertilisers in agriculture, scattered settlements, heavy traffic, and industry and energy production are among the most serious threats which severely threaten aquatic biodiversity. They disrupt habitats, reduce species diversity, and endanger vulnerable marine and coastal organisms. Protecting biodiversity in this region requires addressing these pollution sources to maintain ecosystem health and resilience.

Heavy rain and uncontrolled stormwater runoff occasionally overwhelm the drainage system in Helsinki. Asphalt covers most of the drainage area, so drainage system overflows lead to blockages and allow rain and meltwater with high levels of nutrients and oil to seep into local aquifers. The pollution from water runoff can contaminate the environment and harm the local fish stocks (e.g. trout) and other organisms in the brook ecosystem.

A biofiltration system for climate adaptation and a clean drainage system

Figure 1: Biofiltration system at Maunulanpuisto Park. Source: Urban Nature Atlas. Image Credit: Kajsa Rosqvist (City of Helsinki/CITYWATER project), retrieved 08/15/2018).

Combining water treatment with biodiversity preservation – an effective symbiosis

The biofiltration system consists of two filtration stages:

First, the polluted water runoff (from rain and snow) flows into a sedimentation basin. Here, the solid pollutants settle to the bottom. The water then moves into a bio-filtration area, where special plants successively bind nutrients and heavy metals contained in polluted water. The plants and their roots enable the water to gradually permeate the sandy filtering layers beneath the vegetation, capturing and absorbing harmful pollutants. The water then slowly percolates through the sandy layers beneath the vegetation, where microbes absorb pollutants.  Finally, the purified water discharges into the ditch via a drainpipe and into the Haaganpuro stream.

By collecting and treating polluted water runoff, the biofiltration system reduces the pressure on the urban drainage system and mitigates the flood risk. The sedimentation process, the plants, and the microbes improve water quality and preserve the local water habitat and ecosystem, which benefits fauna and flora. Fish stocks, such as economically significant trout and other organisms, benefit from this water-purifying Nature-based Solution in the Haaganpuro brook, where trout and salmon stocks breed.

In autumn 2018, the number of plant seedlings in the area of Kauppalanpuisto was estimated to have risen about ten times compared to estimations in 2015. The renovations in the lower parts of the brook seemed very successful.

[…] the recovery of trout in Haaganpuro is already a significant economic and nature conservation success story.

The Haaganpuro association (translated from Finnish, Source: Haaganpuro, 2022)

Nature-based Solution Maintenance

The sedimentation basin and the biofiltration area (Figure 2) require annual performance checks, which the responsible city authorities carry out. Vegetation maintenance comprises annual cuttings in August or September. Every five years, the sediment deposited in the riverbed requires removal from the dredges. Every ten years, the biofiltration layers need replacement.

Figure 2: Biofiltration system under contruction at Maunulanpuisto Park.Source: Urban Nature Atlas. Image Credit: Kajsa Rosqvist (City of Helsinki/CITYWATER project), retrieved 08/15/2018.

Success factors

Environmentally friendly and cost-effective water protection measures in the Baltic Sea region (Finland and Estonia) improve the quality of local water aquifers. Communication and knowledge exchange improved working procedures, enabling more effective adaptation measures. The Baltic Sea Challenge, a network connecting more than 170 actors from various backgrounds, principles supported the measures. Assessing the environmental impact and cost-benefit of water protection measures and synthesising the results in a handbook enables access to a collection of effective water protection measures for cities.

The project also raised awareness about the importance of protecting local flora and fauna. The Haaganpuro association, a voluntary association working to preserve trout stocks in the Haaganpuro stream, highlighted this impact.

Governance and funding

The project CITYWATER cost € 1.16 million in total, of which about half (€ 578,909) stems from the European Union’s LIFE+ programme. The Finnish Ministry of the Environment contributed the rest of the budget, which also led the project management and collaboration process with local stakeholders and partners such as the Baltic Sea Challenge and the city authorities of Helsinki. The City of Helsinki contracted the Finnish Consulting Group to plan and implement the biofiltration system at Maunulanpuisto Park. The local costs of the biofiltration measures implemented here amounted € 220,000.

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