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Sewer networks: how does it work?

Débordement d'égout. Crédit : La Mutuelle des municipalités du Québec

Sewer systems are invisible to us…until they aren’t! You have certainly already seen manholes in large urban centers transformed into veritable geysers circulating on social networks. What explains this strange phenomenon? And how do sewer systems work?

A drop of rain can follow several paths. It can be directly absorbed by a vegetated surface, such as a vegetable garden or a forest. In town, it often lands on an asphalt or concrete surface. By a trickling phenomenon, the drop travels on the impermeable surface until it finds an exit point. These exit points are, very often street sumps also called manholes.

Types of sewer networks: separate and unitary

Once in the sewer system, also called a pipe or conduit, two types of systems dictate the possible path of a drop of water. The drop of waste water from domestic use begins its journey here.

First there are the separative networks which provide different conduits for rainwater and wastewater. Thus, in a separative network, wastewater is transported to treatment plants while rainwater is directly discharged into our waterways.

Separative network. Credit: City of Gatineau

Secondly, there are unitary networks, also called combined, which collect both domestic wastewater and rainwater. In this way, household drips and rainwater drips are all treated as wastewater and sent to sewage treatment plants. The oldest networks are often unitary networks. To give an idea of the distribution on the island of Montreal, two thirds of the sewer network are unitary while the remaining third is separate.

Unitary network. Credit: City of Gatineau

The unitary network, ineffective in the face of climate change

With a unitary system, we frequently notice overflows in the event of heavy rain or when snow melts, because the water flow increases quite quickly. It can even happen that cesspool lids in the streets are lifted by the force of all this water, too abundant to be contained! 
Once integrated into the sewer network, all this water goes to treatment plants which, too, can be subject to overflows. It also happens that the water heads towards overflow structures and that theuntreated wastewater empties into waterways. Moreover, with climate change, the Ouranos research group predicts that it will rain more often and that the rains will be heavier. So, if more water ends up in our pipes, the unitary systems will be the scene of many overflows.

Green infrastructures to mitigate the impacts

The Biosphere has green roofs. Credit: Biosphere – Environment Museum

However, today there are alternatives to reduce the amount of water in our sewer systems by making surfaces less waterproof. They are commonly called green infrastructure, and they represent a series of initiatives such as blue roofs, rain gardens, drainage ditches and many others. 

Main photo: The Mutual of Quebec Municipalities

About the author

Coralie is a water quality field technician trainee at Fondation Rivières. With a bachelor's degree in business administration specializing in sustainable development and marketing, she is a candidate for a master's degree in environmental management at the University of Sherbrooke.


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