Combined Sewer Overflows in Quebec

Written by Rachel Laplante | July, 2022


In October 2015, during the now famous Flushgate, the city of Montreal discharged eight billion liters of wastewater into the St. Lawrence River in order to carry out repairs on the sewer system. This highly-publicized spill drew a lot of attention to the phenomenon of overflows that occur in a large number of waterways in Quebec. Through this article, I intend to provide answers to several questions concerning overflows, such as: “What are overflows?” and “What is their real impact on waterways in Quebec?”


Sewer Systems


In order to understand the causes of overflows, we must first know how our sewer systems work. First, water collected by the sewer system is divided into two main categories: rainwater and wastewater. Wastewater is water which has been affected by human activities, whether at home (e.g. showering, washing) or in an industrial setting (e.g. factory). The sewer system is responsible for directing this wastewater to a sewage treatment plant in order to clean it before it is discharged into the environment. However, it is not always possible to direct all this water to the treatment plant due to the limited capacity of sewer systems, as well as the type of sewer system, which differs from one city to another.

In a combined sewer system, wastewater and rainwater are sent in the same pipe to a treatment plant where they will be cleaned. In Quebec, the majority of the sewer system was built between 1950 and 1960, a period during which the combined sewer system was preferred. Since 1965, separate sewer systems have been favored since they make it possible to separate rainwater from wastewater. The wastewater is then channeled to a treatment plant and the rainwater is sent directly into the environment.



Causes of Overflows


The vast majority of overflows occur in combined sewer systems when the system is at capacity and it is impossible to treat all the water being circulated. Instead of letting the water flow back into the basements of buildings, this untreated water is discharged into a receiving environment – a river or a lake – via a diversion structure, usually called an overflow structure. These overflows can occur in different contexts, such as rain and melting snow, or during planned works such as Flushgate. In general, rain is the primary reason for overflows, followed by snowmelt. For more information on overflows by region, an interactive map has been created by the Rivers Foundation.




Effects of Overflows on the Environment


Since they contain wastewater, overflows are often composed of substances that are harmful to the environment and to human health. These substances can include, but are not limited to, waste (e.g. toilet paper), chemicals (e.g. de-icing salt), nutrients (e.g. phosphorus and nitrogen) and pathogens (read our article, Exploring pathogens in urban recreational water).

In terms of human health, overflows often contain a large quantity of feces and, therefore, several pathogens, such as the bacterium Escherichia coli. These pathogens can cause diseases, such as gastroenteritis. Thus, when a significant risk of overflows is suspected – for example, after a period of heavy rainfall – restrictions on recreational activities are put in place. These limitations include access to beaches or the collection of molluscs, since molluscs can be a vector for bacterial transmission to humans.

Overflows also have an impact on the fauna and flora of waterways. An intense supply of water and a gradual accumulation of contaminants and pollutants can deteriorate habitats and affect populations. The supply of nutrients by overflows can also favor certain species to the detriment of others. This is the case for cyanobacteria, also known as blue-green algae, which proliferate when there is a surplus of phosphorus in the water and which can have multiple consequences on the receiving environment. Certain chemicals can also disrupt the endocrine system of fish and decrease their ability to reproduce. Finally, overflows can also lead to a drop in oxygen in the water, which is essential for the survival of different species.


As we have just seen, the impact of an overflow on the environment depends on the substances released, their concentration, and their persistence in the environment. Although little is known about the composition of overflows, their volume and frequency can be used to estimate their impact. The characteristics of the receiving lake or river are also important to consider, such as the ability of the lake or river to dilute the overflow. Indeed, the impact of an overflow in the St. Lawrence River will be less significant than in a small watercourse. Finally, the location of the overflow structures and their density must be taken into account, because even if a single structure has little impact on a watercourse, the cumulative effect of all the structures can be significant.



Wastewater discharge affects the quality of our waterways and our use of that water. In Quebec, it is very likely that the problem of overflows will be exacerbated by climate change due to the increase in the intensity and frequency of precipitation. This could lead to more frequent flooding and backups, and have greater impacts on lakes and rivers. Fortunately, there is hope, as several actions can be taken to limit the effects of overflows at both the individual and municipal level. These are the solutions that I will cover in my next article.




Evaluation of a Predictive Water Quality Model

Evaluation of a Predictive Water Quality Model

Identifying times when water is unsafe for recreation, for drinking, or for aquatic life is a major challenge. Traditionally, sampling has been the preferred means of determining whether water is safe. Predictive modeling based on artificial intelligence (AI) is an approach that is becoming more and more popular.