New wastewater treatment plant to Blominmäki

We are building a new wastewater treatment plant in Espoo’s Blominmäki bedrock. The caves to be excavated underground will host the wastewater treatment basins and the majority of other facilities.

Blominmäki treatment plant

The new treatment plant will replace the current Suomenoja wastewater treatment plant. The capacity of the Suomenoja treatment plant is no longer sufficient to treat the entire area’s wastewater because the population and expectations for intensified wastewater treatment increase.

After its completion in 2022, the treatment plant will process the wastewater of 400,000 residents from Espoo and Kauniainen, Kirkkonummi, Siuntio and Western Vantaa.

The underground facilities of the treatment plant can be eventually expanded to cater for the wastewater of over a million residents, and so the new location of the treatment plant offers facilities for at least hundred years.

​By 2040, it is estimated that 150,000 m3 of wastewater will flow through the new treatment plant daily. The volume exceeds the current volume of wastewater flowing to the Suomenoja treatment plant by about half.


The Blominmäki wastewater treatment plant is located between Ring Road III and Turunväylä, west of the center of Espoo.

Blominmäki to become a modern wastewater treatment plant

It is our goal to build an efficient and modern wastewater treatment plant of the 2020s that treats wastewater ​​better, operates relia​bly and safely, uses its own​​ energy and blends in wi​​th the surroundings.

Treats wastewater ​​better

The resident-specific nutrient load on the Baltic Sea will be reduced from the current load. The goal is to remove from wastewater over 96% of the phosphorus and organic matter as well as over 90% of the nitrogen it contains.

The treatment goals are clearly stricter than the EU requirements and the recommendations of HELCOM (Helsinki Commission). Experiences, for example, from the Helsinki Viikinmäki treatment plant will help in reaching the challenging treatment goals cost-efficiently.

Operates reliably and safely

A large treatment plant operates more reliably than a small one because its overall management is better. Key factors in the reliability of the treatment plant include dimensioning the process while also keeping in mind maintenance situations, emergency equipment constantly on standby and ensuring the power supply. In a treatment plant excavated into bedrock, the temperature stays constant and working and maintenance conditions are better than at a plant under the open sky.

Great care will be put into the preparations for managing disturbances. Long wastewater transfer lines have their risks but these too have been taken into consideration in the plans. For example, wastewater can be temporarily dammed up in the long inlet tunnel leading to the treatment plant balancing the wastewater volume being treated at the plant and that enables stopping the treatment plant operations for maintenance. Treated wastewater will be led in the discharge tunnel first to Suomenoja and from there 7 km out to the sea. Also the reliability of the discharge tunnel will be improved.

Uses its own energy

The treatment plant produces heat energy above its own needs. Apart from the biogas power energy production, heat is recovered from treated wastewater, among others. Electricity produced from biogas generated in the anaerobic digestion facilities equates to more than the half of total electricity need of the plant. Heat generated by the electricity production and operations of large machines is recovered. It is used for instance for heating the sludge to be digested and supply air of the treatment plant.

Blends in with the surroundings

The treatment plant will be built mainly inside bedrock. The land area on top of the caves will remain unchanged for the most part. After the construction, the area can be utilised, just like today, as a green space and recreational area except for the treatment plant’s courtyard, which will be fenced in.

The treatment plant caves will be built so closely that its operations will not have any effect on the water economy of Teirinsuo or the adjacent nature conservation area. Smelling air of the treatment plant will be led via a tall chimney so high up in the outside air that it will not bother the area’s residents or people moving about in the recreational area. The machines of the plant are located indoors, mostly in the caves, so that they do not generate any noise to the surroundings.

We will invest in smooth traffic operations and in traffic safety both during and after the excavation. During the excavation, a maximum of approx. 250 loads of rock will be transported away from the caves per day. After the excavation, the volume of the site’s heavy traffic will decrease to about one tenth.

When the wastewater treatment plant is completed and in use, a few heavy vehicles take care of the plant’s sludge and chemical transports during the day. A reception site for septic tank sludge will be built near the treatment plant. In this way it is possible to remove odours, created from emptying septic tanks, together with the plant’s own odours. Vacuum tankers transporting sludge from septic tanks will be coming into the area, a few per hour at most.

Efficient wastewater treatment protects the Baltic Sea

HSY treats wastewater already better than is required at minimum by the authority regulations. When we meet the goals, nutrients contained in our wastewater strain the Gulf of Finland increasingly less.

If wastewater would flow to the Gulf of Finland as is, the rich phosphorus and nitrogen nutrients contained in it would make our coastal waters strongly eutrophic. At the moment, wastewater is treated at the Suomenoja wastewater treatment plant but its operational limits will soon be reached as the nutrient loads and population increase. Therefore, increased treatment output and capacity are needed in order to secure efficient wastewater treatment also in the future.

The goal of the Blominmäki treatment plant is to treat wastewater better than ever before. The goal is to remove from wastewater over 96% of the phosphorus and organic matter as well as over 90% of the nitrogen it contains. Even if the population and volume of wastewater to be treated increase, the resident-specific nutrient load on the Baltic Sea will decrease from the current level.

The large-scale construction project cannot be unnoticeable. In time, it will, however, transform into a wastewater treatment plant that blends in with the surroundings and is capable of excellent treatment results. With the help of the new treatment plant, we can protect our coastal waters and the Gulf of Finland far into the future.