The Viikinmäki Wastewater Treatment Plant, built mainly inside the rock, was commissioned in 1994 and is the largest wastewater treatment plant in Finland and the Nordic countries.

The Viikinmäki Wastewater Treatment Plant process is based on the activated sludge method, in which the organic matter from the sludge is digested, producing energy.

Rockplan has been involved in both the original design of the Viikinmäki Wastewater Treatment Plant as well as in its later expansion phase. In the expansion, space was excavated for a new biological post-filter and a new treatment line, keeping the treatment plant is in operation throughout the excavation. The expansion increased the plant’s nitrogen removal capacity to the level of 70%, as required by the authorities.