The water comes from Päijänne

​​Päijänne Tunnel leads raw water from Lake Päijänne in central Finland to Helsinki’s water treatment plants in Pitkäkoski and Vanhakaupunki. It secures the access to sufficient, high-quality raw water for around a million residents. This longest connected rock tunnel in the world travels 30 to 100 metres below the ground. The length of the tunnel is 120 kilometres.

The tunnel produces el​​​ectricity

The tunnel starts from the southern end of Lake Päijänne, Asikkalanselkä, where the water intake spot is located 25 metres below the surface. This ensures that the water entering the tunnel remains cool (0.5-11 degrees) all year round. The tunnel ends in the Silvola artificial lake located between Helsinki and Vantaa. On the southern side of this lake is the Pitkäkoski water treatment plant.

With free fall, the largest volume of the water flowing in through the tunnel would be 10 m3/s, but currently the average flow rate of water is kept at 3.1 m3/s. In addition to the water treatment plant, the water is also utilised in Keravanjoki and Rusutjärvi in Tuusula, into which water is led in order to improve their water quality. Around one percent of the average flow rate in to Kyminjoki is led to the tunnel.

The height difference between Lake Päijänne and the Silvola artificial lake is utilised to produce around seven million kilowatt-hours of electricity from the flowing water in the Kalliomäki hydro power plant annually.