FAQ about the background of the project

Why not implement a treatment plant that is in​ part excavated into bedrock and in part covered? 

When considering the implementation options of the wastewater treatment plant, a treatment plant that is in part excavated into bedrock and in part covered was considered a poor compromise where the benefits of an underground treatment plant would have been lost. Inside bedrock, the working and maintenance conditions improve and the temperature and flow of water stabilise compared with an aboveground plant. This improves the wastewater treatment process. Water can be temporarily dammed up in the long inlet tunnel, which balances the flow to be treated and brings added security to managing disturbances.

A partial solution would prevent using the land, for example, for recreation in almost the entire plant area. In addition, this kind of solution would be more expensive to implement in terms of rock and constructional engineering than a plant excavated into bedrock. It would also be a more impractical solution in view of operations and expandability than a plant excavated into bedrock.

​Why is treated wastewater​ led out to the sea? 

The quality of inland waters and coastal waters is improved by leading the treated wastewater to open sea or far from the coastline. The treated wastewater is led in a tunnel 7 km away from the shore of Suomenoja. In a possible plant disturbance, it must be possible to lead untreated or partly treated wastewater to the sea instead of letting it flood, for example, residential areas.

​How is overflow ​​​prevented? 

The transfer of untreated and treated wastewater involves minor technical risks, such as disturbances at pumping stations and sewer clogs. The collection network of wastewater remains mainly unchanged just like the risk of minor fugitive discharges related to technical issues. These cannot be prevented entirely. Instead, securing the internal operations of the plant and the large volume of the sewer tunnel ensure that no major overflow is created. The plant’s environmental permit regulates to include all wastewater discharges in the treatment result to be notified to the authorities. 

Where is the water di​scharged to in case of overflow? 

The plant’s inlet sewer tunnel has an overflow to the discharge tunnel leading from the plant out to the sea. The overflow connection in question will probably be used less than once in ten years. The discharge tunnel has an overflow connection to Nuottalahti at the point where it connects to the current Suomenoja treatment plant’s discharge tunnel. This connection too will probably be used less than once in ten years. The water volumes to be discharged in Nuottalahti are minor, except when the current Suomenoja treatment plant’s discharge tunnel needs possible repairs, in which case all treated wastewater would be led there for the duration of a few months. 

A secondary overflow connection to Espoonjoki River will be built in the discharge tunnel. In case we have to resort to discharging to Espoonjoki River (probability less than once in 50 years), the wastewater treatment will be further intensified (e.g. increasing oxygen content and disinfecting) and the discharge time, duration and water volume are strictly limited and supervised by the authorities. 

Why have the treatment plants in the bedrock become such a trend? 

Locating a wastewater treatment plant in the bedrock is especially popular in the Nordic countries where almost all new large treatment plants are located in the bedrock. Most of the treatment plants in the bedrock are located in rather densely populated areas. At many sites, such as Viikinmäki, Turku, Oslo and Stockholm, there is housing on top of treatment plants excavated into the bedrock.

​​A treatment plant located into bedrock causes far fewer disturbances to housing and recreational uses than an aboveground treatment plant. In addition, the working and maintenance conditions of an underground treatment plant stay more stable than in aboveground treatment plants. 

Why is the treatmen​​t plant not constructed in an industrial area or even further away from settlements? 

Unfortunately, Espoo does not have any suitable industrial area available for locating the treatment plant. The further away the treatment plant is located from Suomenoja, the more expensive it will become and the greater the pumping-related operating costs will be. Every kilometre of transferring the plant further away introduces at least an added investment cost of 5 million euros in sewer tunnels and the transfer also increases operating costs. Pumping from the inlet sewer tunnel to the treatment plant should be increased by one metre because the sewer tunnel slants one metre per kilometre.

​Why is the treatment plant not constr​​​ucted closer to Suomenoja? 

During the earlier stages of the project, a wide spectrum of alternative locations was examined. The decision to locate the treatment plant in Blominmäki was made already back in 2009. Since then, all further studies and further planning has been focused to regard this location option only. The new treatment plant needs to be commissioned as quickly as possible and changing the location at this point would mean an additional delay of at least 5 to 10 for its completion. In such a case, HSY would no longer be able to ensure a good treatment result with the current treatment plant. 

What kinds of risks are involved in locatin​​​g a large wastewater treatment plant in one location? 

Large treatment plants introduce an improved treatment result in comparison to smaller ones, the same as the reliability is better with large plants. In a large treatment plant, wastewater transfer sewers are long and introduce their own risks. In terms of overall risks, there is no significant difference between processing wastewater in one large treatment plant or several smaller ones. However, it is usually easier to control a large plant than several smaller ones. 

What is the share of Espoo’s wastewater in the wastewater coming to the Suomenoja treatment plant? What is the share of wastewater coming from other municipalities? 

Currently, the Suomenoja treatment plant receives wastewater from Espoo, Kauniainen, Kirkkonummi and Western Vantaa. According to the annual average measured in 2005–2006, the distribution of wastewater is as follows: Espoo 74%, Vantaa 17%, Kirkkonummi 6% and Kauniainen 3%. The predicted wastewater distribution of 2030 is: Espoo 68%, Vantaa 19%, Kirkkonummi 10% and Kauniainen 3%. 

​​How will the treatment plant’s sewer and work tunn​els be placed in view of the residential areas?

The treatment plant’s inlet and discharge tunnels have been aligned based on investigations of the rock foundation as linearly as possible via the forced points set by the rock face height. Between the forced points, additional tunnel metres cause a cost of at least 5,000 euros/m. A large transfer or a transfer at the forced points would also increase the pumping costs because the tunnel would have to be excavated deeper.

​The tunnels will be inside the bedrock and will not disturb housing during their use. The work tunnel is a route from the ground surface to sewer inlet tunnels that can be driven by a lorry. The placement options of the work tunnels are very limited, i.e. they are in practice forced points on the line. 

Rock waste to be removed from the sewer tunnels will be transported via the work tunnels. During the operation of the treatment plant in the bedrock, there is normally no traffic in the work tunnels. It is only used for occasional inspection visits and maintenance, as necessary. The maintenance need of the sewer tunnels is minor. 

​How much heavy-vehicle​​​​ traffic does the Blominmäki treatment plant’s construction cause in the neighbouring area? How much traffic will there be after the construction? 

Excavation of the caves during the construction process will cause the most traffic. It has been calculated that during the excavation of the treatment plant 250 rock waste loads per day will be transported from the caves. The transports will be distributed between the interchanges of Mikkelänkallio–Espoonväylä and Blominmetsäntie–Myntti. The dumping location for the rock waste has not yet been confirmed. Rock crushing has been planned for the top of Blominmäki Hill where part of the rock waste can be refined into aggregate that is better suited to construction. Good noise protection can be arranged on top of the former artificially filled hill along with a rather wide storage area for the waste rock and aggregate types. 

​Crushing in connection with the site reduces the total traffic volume. After the excavation, the volume of the site’s heavy traffic will decrease to about one tenth. Special attention will be paid to smooth traffic operations and traffic safety in designing the implementation of the treatment plant. 

​When the treatment plant is completed and in operation, a few heavy vehicles per day will handle the plant’s sludge and chemical transports. A reception point for septic tank sludge has been planned in connection with the treatment plant and a few tanker-lorries per hour may drive there.