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Mille Îles River Watersheds - Wastewater Treatment Systems Assessment

Watersheds of the Mille Îles River – Evaluation of wastewater treatment systems

A portrait of wastewater for the watersheds of the Mille Îles River

Fecal contamination and poor water quality at the mouth of the Rivière aux Chiens, one of the many tributaries of the Rivière des Milles Îles: this is what pushed the Thousand Islands Watershed Council (COBAMIL) to mandate the Fondation Rivières to evaluate the performance of municipal wastewater treatment systems on its territory. The objectives: identify issues between 2017 and 2019 and produce a summary report intended for municipal decision-makers, interest groups and the public.

Source: Rivière des Mille Îles. Credit: Parc de la Rivière-des-Mille-Îles

Zone d'étude des ouvrages municipaux de traitement des eaux usées des bassins versants de la rivière des Mille-Îles

Study area for municipal wastewater treatment works in the Mille Îles River watersheds


  • Data compilation Monitoring system for municipal water sanitation works (SOMAE)
  • Interpretation of diagrams and tracking data

Partners and collaborators

  • Thousand Islands Watershed Council (COBAMIL)



Treatment plants
Overflow structures

The raw data from Monitoring of municipal wastewater treatment works (SOMAEU) 2017-2019 were obtained from the Ministry of the Environment and the Fight against Climate Change (MELCC). The 17 wastewater treatment plants and their networks were distributed among 5 study areas corresponding to hydrographic subdivisions of the territory covered by COBAMIL (see map). An evaluation was then carried out on the basis of two main aspects: the performance of the wastewater treatment plants and the performance of the networks and overflow structures.

Duration of the study

The evaluation of wastewater treatment systems took place from June 2020 to January 2021.



The processing performance of the stations studied is generally good.


There is a high to serious frequency of overflows for 11 of the 17 health networks studied.


 2 stations studied have difficulties with the elimination of phosphorus.

Investments are necessary to maintain water quality in the watersheds of the Mille Îles River. They must be granted according to a logic of prioritization on the scale of the entire watershed.

Our mandates for evaluating wastewater treatment systems

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