Karjaa Pohja Wastewater Treatment Plant, Finland

Project Introduction

The Karjaa Pohja wastewater treatment plant, located in Åminnefors in Pohja municipality, treats wastewater from Karjaa city and the wider Pohja area. The plant superseded two old treatment plants that had reached the end of their design lifespan and could no longer meet the latest Finnish effluent standards. The old plants would have been expensive to upgrade and it was decided to replace them with a single state-of-the-art wastewater treatment plant.

Skanska designed and constructed the Karjaa Pohja wastewater treatment plant for Karjaa Water as part of a turnkey project worth US$ 12 million. Construction work began in January 2006 and the project was completed in August 2007. The turnkey plant was handed over to the client in December 2007, one month earlier than planned, following a period of thorough testing by Skanska to ensure that the plant met the design effluent and energy consumption criteria. Skanska also trained personnel during the testing period to ensure that they were capable of operating, managing and maintaining the plant themselves. The plant uses microorganisms and aerobic treatment processes to remove contaminants from wastewater and includes four processing areas: pre-treatment with screening and sand separation, activated sludge processing, sand filtration as tertiary treatment and sludge treatment. Phosphorus is removed by simultaneously adding precipitation chemicals to the process. The pre-treatment and activated sludge aeration processes involve two treatment lines, and the secondary clarification process comprises of four. The sand filters are continuous-backwash upflow filters. The Karjaa Pohja wastewater treatment plant has a daily average design flow of 7,400 m3, which is within the parameters of projected future population growth in the Pohja municipality.

Treated water from the Karjaa Pohja plant is released into the Pohjanpitäjä Bay, which contains sensitive aquatic ecosystems and endangered clam species. The plant was consequently designed to optimize treatment efficiency and effluent quality. Particular species that are sensitive to water quality include the freshwater pearl mussel (Margaritifera margaritifera) and the thick-shelled river mussel (Unio crassus), which are both in decline throughout Europe. The Pohjanpitäjä Bay is one of the few bodies of water in the world populated by thick-shelled river mussels, which are included in the International Union for Conservation of Nature’s (IUCN) Red List of Threatened Species.

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