Stormwater sewerage

HSY takes care of stormwater sewerage

We take care of stormwater sewerage in the Helsinki Metropolitan Area. We provide services for stormwater sewerage in the stormwater sewerage area approved by the cities. Our responsibilities include the construction and maintenance of the main drain network for stormwater. The service pipe up to the main pipelines falls under the responsibilities of the owner or holder of a property. 

The property must be connected to the HSY’s stormwater sewer if it is located in the stormwater sewerage area. If property’s stormwater is treated entirely on its own property or it is conveyed to the city-run stormwater system (e.g. open trench), an exemption from the connection obligation can be applied for from the environmental protection authority.

Our clients pay a stormwater fee enabling us to maintain and construct the stormwater network. The stormwater fee is collected from those properties that are located in the stormwater sewerage area.

Flow capacity of a stormwater sewer

The conveyance capacity of HSY stormwater sewers has been designed so that stormwater resulting from normal rainfall can be led off from built-up areas. However, during exceptionally heavy rainfall the conveyance capacity of the system may not be sufficient, in which case different sorts of terrestrial stormwater conveyance systems, i.e. flood routes, are necessary. For example, streets or pavement gutters, trenches or green area bioswales can function as flood routes. Construction and maintenance of flood routes are the city’s duty. 

Infiltration and detention of stormwater on the property decrease the loading of the stormwater network. 

Stormwater must not run into a wastewater sewer

The residential areas in the Helsinki Metropolitan Area have a separate sewerage system, except for downtown Helsinki. This means that stormwater and wastewater have their separate sewers.

A property must be connected to a stormwater sewer if such is available in the area. Also the drainage water from building foundations, i.e. underdrain water, is conveyed to the stormwater network. 

It is forbidden to convey stormwater to the wastewater sewer. If stormwater is conveyed to the wastewater sewer, so much water may suddenly enter the sewer network during heavy rainfall that the wastewater pumping stations cannot pass it any further. The water amount might momentarily grow exponentially compared to the normal situation. This means that the risk of an overflow in the wastewater sewer network increases and untreated, diluted wastewater as such might get into the water body. Stormwater also cools the wastewater and reduces the treatment efficiency at the wastewater treatment plant.

Why do I have to pay a stormwater fee?

A basic fee for stormwater is charged from the property that is located in the stormwater sewerage area and also a connection fee when joining the stormwater sewer network. The basic stormwater fee is included in the water invoice. The fee serves for covering the maintenance and construction costs of the stormwater sewerage system. In the future, the stormwater fee will be identified more clearly as a separate item in the water invoice. Its basis for payment will be changed to better match the actual costs. This change is based on the amendment of the Water Services Act (681/2014).

In which cases it is possible to be exempted from getting connected to a stormwater sewer?

The property can apply for an exemption from the obligatory stormwater sewer connection from the municipal environmental protection authority.  Possible grounds for granting an exemption can include for example, the following reasons:

  • Infiltration of stormwater and other treatment on the property so that the treatment does not cause any harm to the neighbours.

  • Conveying stormwater into a stream or an open trench in a manner that the management of a property’s stormwater does not exploit the HSY stormwater network at all.

If the authority grants the exemption, the property is also exempted from the connection fee for a stormwater sewer. However, the property still has to pay a stormwater basic fee in the stormwater sewerage area because the sewerage system serves all properties in the area. For instance, during heavy rains stormwater might be released from the property to the stormwater sewer network although at other times, the property would absorb its stormwater or convey it for instance to an open trench.

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