The Suomenoja wastewater treatment plant started its operations in 1963 as a pond treatment plant and it is the second largest wastewater treatment plant in Finland. Before the central treatment plant was built, wastewaters were treated at the small treatments plants of each village. Now, Suomenoja treats the wastewaters of over 310,000 residents from Espoo, Kauniainen, western parts of Vantaa and Kirkkonummi.
Around 35 million cubic metres of wastewater is treated annually by the plant and the average flow rate in a day is around 100,000 cubic metres. The share of industrial waste waters of all the water treated by the plant is around eight percent. Biologically and chemically treated wastewaters are conducted to the sea, in front of Gåsgrundet island, in a 7.5-kilometre long discharge tunnel, which was taken into use already in 1974.
The wastewater treatment plant is an activated sludge plant. The current treatment process of this plant, originally intended as a mechanical treatment plant, has three phases: mechanic, chemical and biological treatment.
In 1997, the plant also launched nitrogen removal, which is done through a process based on pre-denitrification. The water coming in to be treated includes a large amount of nitrogen and phosphorus nutrients. Inflow and infiltration from Ämmässuo and other landfill sites removed from use, which are conducted to Suomenoja wastewater treatment plant, also increase the nitrogen load. Ferrous sulphate is used for processing phosphorus. The total delay of water at the treatment plant is around 24 hours.
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. Thanks to biogas, the plant produces 90 percent of its own heat and 40 percent of its electricity.
Suomenoja produces around 25,000 tonnes of sludge a year. The wastewater sludge from Suomenoja is processed further in Vapo Oy’s composting plant in Nurmijärvi. The compost soil is currently utilised fully as the covering material of Ämmässuo Waste Treatment Centre.
Treatment results and control
A plant-specific environmental licence (26/2007/1) defines the treatment requirements of Suomenoja wastewater treatment plant. The former Uusimaa Environment Centre approved the Suomenoja treatment plant’s control plan, which determines the plant's license terms. The legal treatment obligations of the wastewater treatment plant are the plant’s direct, concrete goals.
The wastewater discharge permit and other environmental requirements of Suomenoja wastewater treatment plant are based on a decision given by Environmental License Department of Western Finland. Control in marine areas is performed by the City of Helsinki’s Environment Centre.