The Viikinmäki wastewater treatment plant located in Helsinki is the largest treatment plant in Finland and the Nordic Countries. The Viikinmäki bedrock treatment plant processes the wastewaters of residents in Helsinki, middle and east Vantaa, Kerava, Tuusula, Järvenpää and Sipoo. In total, it processes the wastewaters of around 800,000 residents as well as the wastewaters of the region’s industry.
The plant was mostly build inside the bedrock, and it was taken into use in 1994. Now, together with the open sea tunnel, it replaces the separate small-scale water treatment plants.
The total flow rate of the treatment plant is around 270,000 cubic metres in a day, and an average of 100 million cubic metres of wastewater is treated at the plant every year. The treated wastewater is conducted through a rock tunnel into the sea in front of Katajaluoto, eight kilometres away from the southernmost tip of Helsinki, into a depth of 20 metres.
Around 85 percent of the water conducted to the treatment plant is domestic wastewater and the remaining 15 percent is industrial wastewater.
The treatment process in Viikinmäki wastewater treatment plant is based on an activated sludge method and it has three phases: mechanic, biological and chemical treatment. The efficiency of nitrogen removal has been increased with a biological filter, which is based on the activity of denitrification bacteria.
Viikinmäki wastewater treatment process
The organic matter in the sludge created during the wastewater treatment process is utilised by digesting the sludge. The biogas generated during this digestion process is recovered. With the help of energy produced with biogas, the plant produces all its heating and around 50 percent of its own electricity. The plant produces around 60,000 tonnes of dried wastewater sludge a year.
The sludge is processed further in Sipoo, on the composting field of Metsäpirtti. After composting, the product is ready to be used in green construction.
Treatment results and control
In Viikinmäki wastewater treatment plant, 95 percent of solid and oxygen-consuming matter and phosphorus are removed from the wastewater as well as 90 percent of nitrogen. The load conducted to open waters is, in its entirety, the most significant environmental indicator of Viikinmäki wastewater treatment plant.
The water frame directive by EU defines the minimum level of wastewater treatment. However, Finland has its own stricter regulations concerning the treatment level, which are based on national legislation (Water Act: license to conduct wastewaters by Water Court 264/1961) A plant-specific environmental licence defines the treatment requirements for Viikinmäki wastewater treatment plant and it also sets the boundary conditions for waste conducted to air and solid waste. The legal treatment obligations of the wastewater treatment plant are the plant’s direct, concrete goals.
The authorities monitor the implementation of treatment requirements with the help of annual and quarterly reports. Additionally, the Viikinmäki wastewater treatment plant is obligated to meet the quality goals set by the City of Helsinki’s City Council in its Baltic Sea strategy.
In addition to the load conducted to open waters, the treatment process’s air emissions are reviewed annually through a control programme. Viikinmäki wastewater treatment plant is also obligated to report the emissions of certain, separately defined compounds to air and water for the Finland’s national PRTR register.