Centralized wastewater treatment at Helsinki WWTP, Finland

Solution to which problem

Since 1980, in Helsinki, there was a need for a new wastewater treatment plant, since the efficiency of the old one was not adequate anymore. At the same time, there was also a need for a centralized wastewater treatment for the entire Central-Uusimaa district, because the number of inhabitants had been growing simultaneously. In addition to that, the pressure to control indirect environmental impacts as well odours, noise and traffic increased. Small wastewater treatment plants had a high impact to the environment, so for these reasons an underground centralized plant was built in Viikinmäki.

Technical conditions

In Helsinki metropolitan area wastewater treatment is centralized into two plants: Viikinmäki and Suomenoja. Viikinmäki treats all water from Helsinki and the runoff water from downtown area, it treats also wastewaters of Järvenpää, Kerava, Sipoo, Tuusula and part of Vantaa.

The Viikinmäki wastewater treatment plant is primarily located inside rock and it treats 840 000 PE. Above ground buildings do not cause disturbance to the residential areas and the place is enough wide, there is a reserve for future expansion. The total flow rate of the treatment plant is around 270 000 m3/d.

The wastewater treatment consist of three parts: mechanical, biological treatment combined with chemical treatment and biological secondary filtration process. Phosphorus is removed by chemical precipitation with ferrous sulphate and nitrogen by nitrification denitrification process. The process is based on the activated sludge process, in which microbes feeding on organic matter form a growing biomass. Side products are sludge and biogas. The biogas is used as a source of energy (CHP) in Viikinmäki while the sludge is composted to the soil products.

The treated wastewater is led in a rock tunnel to a distance of eight kilometers from the southern point of Helsinki to the sea in front of Katajaluoto, depth of more than 20 meters.


Many benefits have been achieved by concentrating wastewater treatment on one plant and the results of the treatment have improved.

The organic matter contained by the sludge is digested. Thanks to the energy produced from biogas Viikinmäki is totally self-sufficient in terms of heating and 64% self-sufficient in terms of electricity. The plant produces about 60,000 tonnes of dried waste sludge per year. The sludge is further processed in the sludge composting field of Metsäpirtti in Sipoo. Metsäpirtti composting site has a total area of about 18 hectares. The whole area is paved with waterproof rubber bitumen asphalt. All run-off waters are collected and leaded to Viikinmäki wastewater treatment plant. This ensures that the composting process does not affect adversely surface or ground waters in the area. After composting, the product is ready to be used for landscaping and gardening.

One of the primary targets of water utility operation in Helsinki is to improve the quality of the effluent discharged into the Gulf of Finland and to make overall operations more viable environmentally. For this reason Helsinki Water invest heavily in recycling the nutrients and organic matter in wastewater sludge.

The authorities monitor the implementation of treatment requirements with the help of annual report. 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. Moreover, the treatment process’s air emissions are reviewed annually through a control programme, ventilation is necessary when a plant is built in a cave.


The Urban Waste Water Treatment Directive of the European Union defines the minimum level of the wastewater purification. However, Finland has its own higher demands for the level of the wastewater treatment (Vesioikeuden jätevesienlaskulupa 264/1961). According to the 2008 purification results the wastewater treatment plant purified phosphorus 0.08 mg/l more that the term of the license requires. The annual amount of treated wastewater is 100 million m³. This means that 8 tons less of phosphorus is getting into the Baltic Sea annually.

Composting the sludge has a lot of advantages. The composting is a flexible and reliable method that needs only simple machines and it gives back high quality products. The only negative aspects of this technology are that it has a long composting time and it needs much effort for quality monitoring.

It is a responsible waste recycling. It makes possible the recycling of 580 tons of phosphorus and 620 tons of nitrogen annually, it gives back the carbon to the ground.

Furthermore caving wastewater treatment plants are a good solutions for big cities. Environmental impacts have been very small even though plants are located in middle of residential areas.

Learn more about the case – ask and comment in the space below