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Kakolanmäki Wastewater Treatment Plant

Turun seudun puhdistamo Oy is a wastewater treatment service provider that is owned by 14 municipalities and offers high-quality wastewater treatment services to its owners. The company is responsible for the operation and treatment results of the Kakolanmäki wastewater treatment plant, with the aim of providing an optimal purification performance. The plant processes the wastewater of almost 300,000 residents in the Turku region, in addition to the industrial wastewater of the region.

Turun seudun puhdistamo Oy commenced its production operations at the beginning of 2009. The Kakolanmäki wastewater treatment plant processes all of the municipal wastewater produced in the Turku region.

If you have any questions regarding wastewater treatment operations in your area, please contact the wastewater treatment service provider of your municipality.

Area Of Operation

The map of western Finland shows the area of operation of Turun seudun puhdistamo Oy.

The map also shows municipalities whose prospects for joining the company’s area of operation are currently under review.

The Environment

The hygienic conditions of the sea areas surrounding Turku underwent a significant change between 2008 and 2011. A study carried out in 2008 concluded that the hygienic quality of the discharge areas of the Raisio, Turku and Kaarina wastewater treatment plants’ was poor and that the quality of the surrounding areas was passable. When another study was carried out in 2011, the state of the sea areas had improved considerably. In the discharge area of the Kakolanmäki wastewater treatment plant, the hygienic quality of the sea was poor, while in the surrounding areas, the quality was good. In the discharge areas of the wastewater treatment plants of Raisio and Kaarina, wastewater no longer had a discernible effect on the hygienic quality of the sea.

During its operation, the Kakolanmäki wastewater treatment plant has met all of the requirements of its environmental permit regarding concentrations and treatment efficiency. According to Turun Seudun Puhdistamo Oy’s strategy, the plant aims for better results than those required by the environmental permit. So far, this goal has been met.


The Kakolanmäki wastewater treatment plant is a 4-lane wastewater treatment plant, based on mechanical, chemical and biological treatment processes. The purification process is very efficient: the plant removes up to 99% of organic matter, phosphorous and solids (requirement 95%) and 79% of nitrogen (requirement 70%) from processed water.

Wastewater enters the plant’s intake pumping station, via tunnels from two different directions. From the intake pumping station, the water is led to screening and sand separation, where coarse matter is mechanically removed from the water. In order to facilitate the treatment process, air is also added to the wastewater during sand separation, since wastewater usually becomes anoxic as it passes through sewers.

Before moving on to the primary clarification phase, a chemical is added to the water, in order to remove phosphorous. In primary clarification, some of the solids in the water are removed by allowing them to settle at the bottom of the treatment basin. These solids contain high amounts of phosphorous. The raw mixed sludge that is collected into the clarification basin’s sludge pockets during the clarification process is pumped out to an intermediate storage and then onwards to sludge centrifugation.

In the aeration phase, microbes are used to remove organic matter and nitrogen from the water. Different microbes are used to remove different substances, and each microbe requires specific conditions, which is why the water passes though both aerobic and anaerobic compartments during the aeration process. In the aerobic compartments, air is injected into the water from the bottom of the basin, while in the anaerobic compartments, the wastewater is mixed with large mixers. Nitrogen is removed in both aerobic and anaerobic compartments, while organic matter is only removed in aerobic conditions.

In the secondary clarification phase, the microbe mass that is mixed in with the wastewater during aeration is removed by allowing it to settle at the bottom of the treatment basin. Most of the phosphorous in the wastewater is also removed through this settled sludge, while the now clear water moves on to the next stage of the purification process. The removed sludge is conducted as return sludge back to the aeration phase, where surplus sludge is removed from the process by way of primary clarification.

The final stage of the treatment process is sand filtration, where the water is filtered through a layer of sand.

In order to increase the reliability of the process, the plant also utilises a two-line by-pass water treatment unit, which is used during peak flow times to chemically process wastewater after it has passed through primary clarification.

Once the plant’s treatment process is complete, the purified water is discharged into the Linnanaukko harbour basin in Turku.

The sludge that is removed from wastewater during the treatment process is dried using centrifuges and stored in silos. The centrifugation that takes place at the plant is only a small part of the overall sludge treatment process. The dried sludge is transported by tank trucks to Gasum Oy’s biogas plant at the Topinoja waste treatment centre, where the actual sludge processing takes place, using anaerobic digestion and post-composting.

The Kakolanmäki wastewater treatment plant also features a heat pumping station. Oy Turku Energia uses the heat pumping station to extract some of the thermal energy from the wastewater to produce heat for district heating purposes. The water that is cooled down by the pumping station is also used for district cooling.

Outlet pipe

More outlet pipe capacity, less environmental impact for the sea

The outlet pipe project of Turun seudun puhdistamo Oy helps mitigate climate change and updates the company’s purification processes and resilience. Started in 2018, the project is set to be completed in the spring of 2023.

The Kakolanmäki waste water purification plant’s waste water outfall system has insufficient capacity and it overloads during heavy rain. The plant also lacks a backup outfall system, which makes it ill-prepared for emergencies. Turku gains some 2,000 new citizens annually, which increases the amount of waste water in the sewer network. Meanwhile, climate change will bring more rain and increase the frequency of exceptionally heavy rain, adding more storm water. The sea level is also expected to rise, which limits the current outfall system’s capacity.

Once the outlet pipe project is completed, the plant will be able to operate at full capacity during exceptionally heavy rain and high sea levels. The purification plant’s resilience will also be improved with the old outfall system remaining as a backup for handling maintenance and disruptions. A new UV disinfection facility will also be built at the plant. The project will significantly improve the sanitary quality of the waste water released into the sea in the Turku area. Furthermore, the plant’s turbine will generate eco-friendly energy from the waste water flow.

Excavation and tunneling were chosen as the working methods. Tunnels only require open cuts for their start and end and cause the least nuisance for both traffic and residents. The technology’s carbon footprint and energy consumption are also considerably lower than those of fully open excavations. Another benefit is that the water purification plant can operate normally during construction.

A cut-and-cover excavation was dug from the plant, continuing into a tunnel that terminates in the port basin. Excavation for the UV facility and outlet tunnel began in October 2019 and was completed in March 2021. The work was carried out from inside the plant. Tunnelling began in February 2020 with the excavation of the start and end cuts. The pipe’s jacking, micro tunneling, was carried out between July and August in 2020, and the jacking contract was completed in April 2021. The construction of the UV facility in a cavern dug out inside the plant started in May 2021. The full-scale operation of the outlet pipe started in early July in 2022 and the full-scale operation of the UV facility started in March 2023. The project is set to be completed in the spring of 2023.

The project was granted Blue Project-certified funding by the Nordic Investment Bank. The jacked section of the project has received two prestigious awards: the project was issued the Vuoden työ 2021 award by the Finnish Society for Trenchless Technology and it shared second place in the RIL-Palkinto 2021 competition. In addition, the project was among the finalists in the International Ground Engineering Awards 2022 competition, in the International Project of the Year series.

Turun seudun puhdistamo Oy sends out regular communications about the project and organises press conferences, as necessary.

  • Project management and monitoring: Fimpec PMO Oy
  • Design and planning: AFRY Finland Oy
  • Procurement legal: Procurement Partners Ltd
  • Excavation contractor: Kalliorakennus-Yhtiöt Oy
  • Excavation vibration consultants: Forcit Consulting Oy
  • Jacking contractor: Ward & Burke Construction Limited
  • Equipment supplier: Hyxo Oy (UV)
  • Equipment supplier: Saahkarin Kone Ky (turbine)
  • Equipment supplier: Tecalemit Flow Oy (diversion channel floodgates, stop dams, valves, lateral adjustment flanges, flexible couplings, and pipe clamps)
  • Equipment supplier: Oy Lining Ab (turbine sluice gates)
  • Worksite services: Ramirent Ltd
  • UV plant contractor: Skanska Infra Oy
  • Final excavation restoration: V-S Asfaltti Tiimi Oy

Turun seudun puhdistamo Oy advocates for a cleaner Baltic sea as part of the Baltic Sea Action Plan 2019–2023 of the cities of Helsinki and Turku.


Mirva Levomäki, tel. +358 40 712 6241
Managing director, Turun seudun puhdistamo Oy

Jarno Arfman, tel. +358 40 712 6231
Technical manager, project lead, Turun seudun puhdistamo Oy

Our email addresses are

Personnel contact details

Mirva Levomäki
+358 40 712 6241

On-call duty and control room
+358 40 712 6240

Quality and Environmental Manager
Jarkko Laanti
+358 40 712 6230

Our e-mail addresses are titled as

Technical Manager
Jarno Arfman
+358 40 712 6231

Office Administrator
Mari Laaksoharju
+358 40 712 6239