Stormwater sewers

In stormwater sewers, stormwater flows towards streams, rivers, lakes or the sea. Stormwater is conducted into water bodies or the sea either directly or through open ditches as well as detention and other structures belonging to the municipality’s stormwater system. As a rule, stormwater is not treated, but rather discharged into water bodies without treatment.

Customer responsibilities and duties

The owner or occupier of the property is responsible for the management of stormwater generated on the property. For example, stormwater can be detained and absorbed on the lot as well as conducted into HSY’s stormwater sewer or the municipality’s stormwater system in a controlled manner. Stormwater must not be conducted to someone else’s lot or to the street. Stormwater or its management arrangements must not cause detriment to neighbours.

Properties located in the stormwater sewerage area must be connected to the stormwater sewer network, even if some of the stormwater is absorbed on the lot or conducted, for example, into an open ditch. For properties connecting to the stormwater sewer network, we indicate a connection point through which the stormwater from the property is conducted to HSY’s stormwater sewer network. The owner or occupier of the property is responsible for the construction of a lot sewer for stormwater from the lot to the connection point indicated by HSY.

In the combined sewerage area, the property conducts wastewater and stormwater to the combined sewers connection point in separate pipes. The wastewater sewer pipe and stormwater sewer pipe are combined before the connection point and connected to HSY’s combined sewers as a single pipe. The property’s stormwater sewer must be built at a higher elevation level corresponding to the marked combined sewers pipe diameter. This makes it possible to separate the property’s wastewater and stormwater when constructing the stormwater sewer for the street.

Connecting to the stormwater sewer network

We provide stormwater sewerage services in the stormwater sewerage area. The location of your own property in the stormwater sewerage area can be checked, for example, from HSY’s map service. When the property is located in the stormwater sewerage area, the customer requests a connection point statement from HSY.

Connecting to a stormwater sewer may also become topical when a stormwater sewer network is built in the area. In this case, too, the customer requests a connection point statement from HSY to connect to the stormwater sewer.

Properties outside the stormwater sewerage area conduct their stormwater to the municipality’s stormwater system, such as into open ditches in the area.

More information

Exemption from connecting to the stormwater sewer network

If you do not want to connect your property located in the stormwater sewerage area to the stormwater sewer network, you must apply for exemption from the connecting obligation. Exemption from the connecting obligation is sought from the environmental protection authority of your municipality.

Exempted properties do not need to be connected to the stormwater sewer network and will not be charged a connection fee for connecting to the stormwater sewer network. However, the basic rate for stormwater sewerage is charged from all properties located in the stormwater sewerage area, including those exempted from the connecting obligation.

Dimensioning of stormwater sewers

HSY’s stormwater sewers make it possible to conduct stormwater caused by normal rainfall away from built-up areas. The current stormwater sewers have been dimensioned based on the amount and intensity of heavy rains that occur every few years. The capacity of the stormwater sewers may not be sufficient during exceptionally heavy rainfall. In this case, various stormwater backup routes located on the ground, called flood routes, are needed. Flood routes may include, for example, streets or gutters, ditches or depressions in green areas. The planning, construction and maintenance of flood routes are the responsibility of cities.

Property owners and occupiers can influence the operation and capacity of the stormwater sewer network during heavy rains. Absorbing and detaining stormwater on the lot before it is conducted into the stormwater sewer network reduces the load on the stormwater network and the likelihood of flooding.

Stormwater does not belong in the wastewater sewer

Residential areas have separate sewerage in the territory of HSY’s member municipalities, i.e. the cities of Helsinki, Espoo, Kauniainen, and Vantaa. This means that stormwater and wastewater pass through different sewers. An exception is downtown Helsinki, where combined sewerage is still in use. In the combined sewerage area, wastewater and stormwater are discharged into the same sewer, through which they are conducted to be treated at the Viikinmäki wastewater treatment plant.

Stormwater must not be conducted into a wastewater sewer. The ban also applies to drainage water from the foundations of buildings. Properties located in a stormwater sewerage area must be connected to the stormwater sewer network to remove stormwater. In other areas, stormwater is conducted into the municipality’s stormwater system, such as a network of open ditches, or treated on the lot, for example, by absorption.

Stormwater must not be conducted into a wastewater sewer, as this increases the risk of overflows in the wastewater network. During overflows, untreated, dilute wastewater may flow into a water body as such. If stormwater is conducted into a wastewater sewer, there may suddenly be so much water entering the sewer network during heavy rainfall that wastewater pumping stations are unable to pass it on, causing an overflow. The amount of water can momentarily increase to be many times over normal. In addition, stormwater cools down wastewater and reduces the treatment efficiency at the wastewater treatment plant.