FAQ: List and map of wastewater spills in Quebec

Each treatment plant has a maximum treatment capacity which is calculated in m3 of water per day (1 m3 = 1,000 L). These stations are supplied by a sewer network interspersed with various structures, including pumping stations and overflow structures. The function of the pumping station is to push the water towards the station, while the overflow structures are the equivalent of circuit breakers: they are there to protect the station and the sewer network against water overloads.

When there is too much water, in case of heavy rains for example, the flow exceeds what the network is able to take and this causes a discharge of wastewater directly into the rivers. Spill, overflow and overflow are all synonyms.

Quebec had 884 stations and 4,667 structures in 2022.

Wastewater outfalls can affect both the environment and public health. 

Untreated wastewater contains many contaminants (suspended solids, oxygen demand, nutrients including nitrogen and phosphorus, microbes, oils and greases, pharmaceuticals, micro-plastics, etc.) that can affect the aquatic fauna. These contaminants can deprive affected organisms of dissolved oxygen, expose them to viruses or bacteria, alter their biochemistry or degrade the quality of their breeding or feeding environments. 

The quality of aquatic environments can also be affected by overflows. Overflows can cause the release of non-biodegradable solids, accelerated eutrophication due to nutrient inputs, blue-green algae blooms, decreased dissolved oxygen, decreased water clarity, or other changes in the physical and chemical characteristics of the water.

Outfalls can also pose a public health risk by compromising the quality of the water, making it unsuitable for recreational use (swimming, canoeing, kayaking, etc.). Finally, they have the effect of affecting the drinking water treatment costsand even to limit the production of drinking water in the short term.

While we consider that all forms of overflow should be avoided, we are aware that the majority of overflow is caused by a liability of infrastructure built at a time when the common practice was to discharge stormwater through the sewer system without treatment and as quickly as possible into the environment. We also recognize that the dilution capacity of a receiving environment can reduce the impacts of outfalls on it. That being said, we are convinced that Quebec can aspire to better results in reducing the frequency and intensity of overflows into its waterways.

It is very difficult to measure the amount of sewage that escapes into rivers in the event of an outfall. These data do not exist. However, we do know the size of each of the structures that overflow. It is also known that the larger the structure, the greater the volume of wastewater that is likely to be released during an outfall.

For this reason, we developed the Spill Intensity Index, an index that takes into account the design flow of the treatment plant, the size of the structure that overflowed, and the duration of each outfall. It is assumed that the larger a facility is, the larger the outfall will be. Add the duration of the outfall and you get an order of magnitude of the amount of water that could potentially have overflowed from the facility.

Since these facilities of comparable size are found in all Quebec municipalities, it can be assumed that a medium-sized facility that overflows in Trois-Rivières will discharge the same amount of wastewater into the environment as a facility of the same size in La Tuque or Rimouski.

This index allows everyone, citizens and elected officials, to compare the probable impact of outfalls in their municipality with other municipalities in Quebec and to see where it is necessary to intervene as a priority to plug the gaps.

We know that this benchmark is effective, as we have validated it by applying it to 50 of the wastewater systems we have studied in recent years. It has substantiated all of the priority cases we identified in our analyses.

The index is reported over 1440 minutes (24 hours) because overflow events are measured in minutes and are considered daily (between 00:00:00 and 23:59:59) by the Ministry of the Environment (MELCCFP).

The intensity index is not a measure of the volumes of wastewater released to the environment from a facility when it overflows. Rather, it is the volume of wastewater that can theoretically pass through an overflow structure for the duration of an event without causing an overflow. This index still makes it possible to bring the capacities of overflow structures onto a common scale for comparison purposes.

The concept behind this index is that the higher the capacity of a structure, the larger its catchment area will be and therefore, the more likely it is to receive a significant water surcharge during overflow events.

The calculation of outfall intensity meets the following limitations:

• Without a spill duration, the calculation of the intensity of an event is based on a duration of 24 hours, which could lead to an overestimation of the duration of the event. This adjustment to our methodology is intended to avoid wrongly favoring certain municipalities with non-compliant practices, which would otherwise have experienced spill intensities equal to zero. This adjustment applies from the 2022 rankings and has led to a revision of the history of the rankings.
• It happens that the actual flow rates arriving at a treatment plant are different from its design flow rate. If the actual flow is much lower than the design flow, the index could be overestimated. On the contrary, if the actual flow is much higher than the design flow, the index could be underestimated.
• It happens that the percentage of flow passing through a structure is not entered correctly in its monitoring form.

Using data provided by the Ministry of the Environment (MELCCFP), we can associate each spill event with a specific overflow structure which is itself associated with a single wastewater treatment plant. Each treatment plant is ultimately associated with the municipalities connected to its sewer network.

The vast majority of municipalities operate their own wastewater treatment plants and therefore the method is not biased. This is not the case when a treatment plant serves more than one municipality, as the available data does not always allow us to distinguish which facilities belong to Municipality A and which belong to Municipality B. All results related to the treatment plant are associated equally with both municipalities, even though one of the two may be the primary source of the outfalls.

Why do this? Quite simply because almost half of the overflow structures in Quebec are not geolocated. If all the structures were geolocated, we would have been able to accurately associate the overflows with the municipalities. If you believe that your municipality is penalized by this method of calculation, insist that it transmit the geographical coordinates of its works to the MELCCFP. We can then eliminate this bias.

It is very easy to check if a given municipality is connected to a station outside its territory and if there may be such a bias, as shown in the following illustration:

A wastewater spill in Laval, for example, is always larger than a spill in La Pocatière, simply because it is a larger city. How can cities be compared to each other? By normalizing the data by the number of inhabitants of each municipality.

In doing so, municipalities can be accurately compared to each other and even a ranking of the best and worst performing cities in terms of municipal wastewater treatment can be established.

This is the default consultation mode that we offer:

• Per capita outfall intensity
• The number of spills
• Duration of spills

It is preferable to compare municipalities with each other according to their size. We have defined 3 categories based on the population:

• Small: less than 10,000 citizens
• Average: between 10,000 and 100,000 citizens
• Large: over 100,000 citizens

Measurement quality is an indicator we developed to assess a municipality's ability to accurately measure its overflow events. 

Electronic overflow recorders (EED) are an essential component for proper monitoring of the performance of overflow structures. These make it possible to precisely measure the numbers, durations and times of wastewater overflow from the structures where they are installed. This measurement method is recommended over that of the visual reference. As the calculation of the intensity of spills is dependent on the durations of overflows that are reported, a place where there are gaps, even partial, in the EED installations presents an overflow intensity that is not faithful to reality.

The data made available by the Ministry of the Environment (MELCCFP) reports, for each of Quebec's overflow structures, its capacity and whether or not there is an EED installed. Rather than simply judging the number of structures with or without EED, we chose to consider the relative importance of structures with or without EED for a given network, as well as their overflow frequencies. 

Concretely, the quality of the measurement is obtained from the ratio (in percentage) between the summation of overflow intensities calculated from the reported durations and the summation of overflow intensities calculated with the adjusted method (where overflows without duration are considered to have lasted 24 hours). This percentage is then converted to a letter grade using the following scales: 

A+: [100%]
A :    [90% to 100%[
B :    [75% to 90%[
C :    [60% to 75%[
D :    [45% to 60%[
E :    [25% to 45%[
F :    [0% to 25%[

To measure the performance of municipalities, they are ranked in ascending order of the number, duration or intensity of their outfalls. Each of these lists is then broken down into five 20% bands to produce what statisticians call quintiles. These are five equal parts that include an equal number of municipalities. Values that are zero or equal to zero are isolated and classified as "Null or Inapplicable".

The "Very Low" category includes the 20% of municipalities that reported the smallest values greater than zero, and the "Very High" category includes the 20% of municipalities that reported the largest values for the given situation (the number of outfalls in 2019, for example). This method of categorization means that the thresholds between these categories (called quantiles) will change from year to year, as the categorized values also change from year to year.

As an example, here are the intensity and per capita thresholds for the years 2017 and 2019:

Prior to 2017, the number and duration of outfalls were measured by the "visual benchmark" method: an employee would visit the plant and observe an outfall (or not) and note the estimated duration of the outfall. Thus, data from before 2017 are of poorer quality than those obtained from 2017 onward.
Today, all municipalities are held by thearticle 9 of Regulation respecting municipal wastewater treatment works (ROMAEU) to have an electronic overflow recorder (EED) which automatically measures the duration of each spill on structures that have experienced spills in the past. This regulation has been in force since January 2014.

In practice, 10 % of the works are still not equipped with EED. To find out what we are doing so as not to disadvantage municipalities that comply with the regulation, refer to the question How is the intensity of outfalls calculated?