Greywater Reuse - In Quebec


Greywater Reuse - In Quebec

More and more, society realizes the need to protect its environment. However, it is necessary to continue to raise public awareness on the various environmental issues. One of these crucial issues is the preservation of drinking water. Phenomenal amounts of water contaminated by fecal matter are discharged into the St. Lawrence River and into rivers in Quebec (1). There are 81 municipalities in Quebec without a treatment plant and that discharge contaminated water without treating it (1). Montreal and Laval are among the worst in Quebec with regards to this problem. However, the South Shore has the worst results of any city in Quebec. Currently it is impossible to know the volume of wastewater discharged because it is not measured (1). These contaminated waters are black water and greywater. In this article we will describe the advantages and disadvantages of the reuse of greywater in the building.

What is greywater?

Greywater comes from rain, showers and baths, laundry washing, cooking and hand washing. It can be reused for several functions that do not require drinking water, for example for toilets, car washing, irrigation, watering plants, etc. In fact, this is an opportunity to use the same water twice before it is returned to the treatment plant.

Quebec policies

As of January 1, 2030, the Ministry of the Environment will force municipalities to reduce the number of wastewater discharges into the St. Lawrence River and Quebec rivers. Unfortunately, municipalities retain permission to increase pollution in our rivers until then (2).

On the other hand, there are no subsidies for greywater recovery systems in buildings in Quebec (3). A government subsidy could influence the industrial, commercial and residential sector to want to use greywater. However, more specific regulations on greywater should be established.


Water is a resource that cannot be wasted because it is a limited resource. Municipalities invest a lot of money in treating and distributing drinking water to the population. At the same time, many companies are spending time and money developing new technology to reuse this precious water. Innovation in this area will allow these technologies to be more efficient and accessible. In short, more and more technologies that use greywater are becoming compact, require less maintenance and are more affordable.

However, there are many advantages to implement a greywater network:

  • Reduce the demand for treated drinking water in the building,
  • Reduce the energy demand for heating water through heat exchangers that recover the heat emitted by greywater,
  • Energy savings reduce greenhouse gas emissions and thereby promote environmental protection,
  • Divert rainwater from the sewers prevents this water from being contaminated and reduce the cost of water treatment by the city,
  • Save money in the long run because the widespread adoption of water-saving strategies in buildings could lead to savings of, at least 30%, in municipal water treatment,
  • Advantageous for owners of new buildings under construction. Although the design of a water system that includes the use of greywater in a new building is more expensive, the long-term savings are very beneficial,
  • Pipes carrying greywater must be clearly identified with a specific coloured tape, and must also be labelled "non-potable water" to avoid confusion with drinking water pipes (4).

There exist, however, drawbacks that weigh much less than the benefits:

  • Greywater should be used quickly, before the proliferation of bacteria that can produce putrefactive odors (4).
  • Greywater should not be sprayed, but rather applied below the surface to eliminate the danger of inhaling aerosol particles. For example, for irrigation systems, care must be taken that the water is poured and not sprayed by aerosol (4).
  • This type of system requires a large concrete retention tank in the basement and a sand filter. The equipment requires bi-weekly cleaning, and annual replacement of filtration materials (5).
  • Greywater recovery normally requires a dual piping system for the distribution of drinking water and that of the recovered water. Such a system is more expensive.
  • Need to protect the building's drinking water network with an additional backflow prevention device.

In conclusion, even if Quebec absolutely must upgrade its water purification infrastructure by taking into account toxic products that largely escape the current systems (6), the legislative component must be improved to deal with current issues. The benefits of a greywater system are many, but the protection of our resources, fauna and flora should be the primary incentive. In addition, thanks to the innovations that are made to facilitate the use of greywater, these technologies will become more affordable, and they will improve the quality of life on earth.



  1. Plus de 52 000 déversements d’eaux usées au Québec en 2020 (
  2. Déversement d’eaux usées : votre ville figure-t-elle parmi les pires au Québec? (
  3. Quels sont les types de traitements qu’on peut choisir pour décontaminer l’eau grise ? - Écohabitation (
  4. Pourquoi donc récupérer les eaux grises ? - Écohabitation (
  5. Des solutions compactes et faciles d’entretien pour le recyclage des eaux grises - Écohabitation (
  6. Le Québec exhorté à mieux traiter ses eaux usées | Le Devoir
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