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Overflow: an overview



By Caroline Larocque, Project Manager at Réseau des femmes en environnement and coordinator of the «Diminuer les rejets toxiques dans l'eau: j'agis aujourd'hui» project.


Pour lire l'article en français, cliquez ici.


In 2019, the city of Montreal discharged 8 billion litres of non treated wastewaters in the Saint-Laurence River. Since, regional and local media have covered time and again this event. Although this was not a unique situation, the public is increasingly interested and has had questions and concerns about water management. Why do some cities in Quebec discharge wastewater directly into bodies of water? What are the consequences of such actions? What are the possible solutions?


What is an overflow?

Every water treatment plant has a maximal capacity of water that can be treated daily. Often, the surplus of water collected during the day is stored in overflow structures. These are basins set up upstream of a water treatment plant to contain the surplus of wastewater until the plant has the capacity to treat it. An overflow occurs when the water collection network, including the treatment plant and overflow structures, receive an amount of water that exceeds its treatment and storage capacity. To avoid sewage backflow and other damages to the city infrastructure, water surplus are discharged without treatment into the closest body of water, often the Saint-Laurence River. Overflows are thus occasional.

Typical flow diagram of an overflow infrastructure

Typical flow diagram of an overflow infrastructure. Credits: City of Montreal


According to Fondation Rivières, a non-profit that supports conservation, restoration and showcasing of the natural and ecological value of rivers, more than 52,000 discharges have occurred in 2020. The organisation created an interactive map of spill events that happen on a yearly basis.

Many different situations can lead to an overflow. It mostly happens in the event of heavy rainfall or when snow melts in spring. The debit of water then exceeds what the network can support and the infrastructure overflows. Emergency situations such as breakage, power outage and cleaning needs might also explain some overflow events. From one year to the next, the cases of overflows varies as they are linked to climate and unforeseen issues.

Overflow in dry weather is also possible. This occurs when water treatment plants aren't capable of treating the totality of wastewater generated by users. Users include citizens, stores, businesses, and factories located on the treatment plant's territory


Fondation Rivières

Overflow map (in French), Fondation Rivières


Are overflows regulated?

Water treatment plants are obligated to monitor and compile all untreated wastewater overflows. They must also identify the cause of overflow (rainfall, snowmelt, emergency situation, etc.). Overflow structures must meet minimal requirements set by the Ministère du Développement durable, de l’Environnement et des Parcs (MDDEP) depending on which type of activities occur in the surrounding environment.

The only type of overflow that are not tolerated are the ones that occur in dry weather. However, in an ideal situation, no overflows would ever occur, except in time of emergency. 


What kind of issues are generated by overflows?

Water that overflows is neither filtrated nor treated. This means that waste and contaminants, including fecal coliforms and toxins, are discharged directly into natural streams. Many studies show the consequences of such discharges on aquatic species and soils. For instance, wastewater effluents may contain chemicals that can alter growth and development, as well as reproduction of aquatic species and thus lead to a decrease in population or changes in biodiversity of rivers and lakes (Government of Canada, 2013). The effects of overflows are greater in small urban bodies of water where dilution is minimal. In the long run, stream morphology can change drastically.

As bacterial quality of water is affected, overflows can also be harmful to populations along the stream that would use the bodies of water for leisure purposes such as swimming.


Improving the infrastructure

A number of municipalities across the province of Québec have added basins or overflow structures as an answer to increasingly high amount of water transiting through water treatment plants. This allows to better control surplus of water and avoid overflow. However, many infrastructure are obsolete and need to be modernised and improved to adequately cater to a growing population as well as to adapt to a changing climate. This improvement needs financing and the processes have been very slow.

Governments and industries need to contribute and act swiftly in order to reduce the amount of overflows. Until all infrastructure is modernised, other solutions can be put forth by cities and populations.


Creative urban planning

The first thing that can be done to reduce overflows' consequences is to reduce the amount of water to be treated. This can be done by focusing on surface runoff water which is rainwater that runs off the pavement to be collected by sewer systems instead of being absorbed into the ground. The key is to make sure this water doesn't end up in sewer systems.

Green infrastructures such as green roofs and walls, as well as rain gardens allow for better infiltration of water into the ground which lessens the pressure on sewage systems. Urban planning strategies put forth by cities can lead to better soil infiltration and fewer overflows of untreated wastewater. Draining trenches, permeable paving and water collection tanks are other ways to divert rainwater from sewer systems. Green infrastructures can not only reduce the amount of overflows, they are also cheaper than overflow structures. Furthermore, green infrastructures reduce heat island effect, enhance air quality and refill underground water sources (Eau Secours, 2009).


Social responsibility

Bearing in mind the objective of reducing the amount of wastewater to be treated at any given moment, citizens can also play a central role within their homes. Many daily actions can help reduce water consumption as well as foster good water quality. Here are a few examples:

  • Use low water consumption appliances and dual-flush toilets
  • Make sure the washing machine is full before doing laundry
  • Get a water collection tank in your garden
  • Avoid or reduce lawn watering
  • Avoid letting the water run when brushing your teeth
  • Avoid throwing pharmaceuticals and cosmetics in the sink or toilet

Although it is important to reduce water consumption in all situations, it is even more important to do so in heavy rainfalls as water collection systems might be saturated. The population can thus avoid taking showers or using washing machines or dishwashers during heavy rainfall to help release the pressure on water treatment plants.

You can also sign the charter and commit to reduce toxic discharges in your home on a daily basis.  

There's a variety of solutions! Populations, cities, industries, and governments can act to reduce the amount of overflows and to lessen their impact on the environment. 


To learn more about overflows, read the following resources: