PUBLISHED: 07 September 2023


Icon showing a beaker tipping and adding water, representing dilution

Also known as “watering down”, “decreasing the concentration”, a component of assimilative capacity.

What is it?

For decades, the approach to contaminants, also referred to as pollution in wastewater management, was to increase dilution. There is even a saying: “The solution to pollution is dilution.” Adding water (diluting) and decreasing the concentration of a pollutant was thought to reduce negative impacts on the environment.

The principle of dilution is true in that the natural environment, specifically aquatic ecosystems, have capacity to absorb and degrade certain types of pollutants. For a river or lake this is referred to as the assimilative capacity; but it takes much more than simple dilution of the pollutant stream to decrease its concentration. The water body’s capacity to deal with pollutants depends on many other processes (e.g. sedimentation, volatilization, chemical breakdown, microbial breakdown/transformation, and uptake by aquatic plants).

However, not all contaminants can be degraded quickly through natural processes and humans are now producing more waste in concentrated areas than can be adequately diluted and assimilated by the environment. Wastewater from communities and homes needs to go through a treatment process before it is released to the environment. Additionally, treating wastewater can be used to produce by-products from heat and electricity.

Although dilution is an important factor in wastewater management, dilution is not the solution to pollution.

Is dilution regulated in Canada?

Yes, there are regulations for the necessary dilution ratio of effluent released into surface waters. The type of treatment process and level of treatment used to treat the wastewater also changes the required dilution. However, in general, the ratio of effluent volume to surface water volume is 1:10 but this may vary by municipalities’ and provincial (e.g. [i]) and territorial legislation.

Why is dilution important?

Even though wastewater treatment plants remove many contaminant materials, there are usually remaining organic matter, microorganisms, and other contaminants in the water being returned to the environment.

Downstream water users and downstream environments can be protected through dilution or additional water treatment. Additional water treatment can be expensive, so having adequate dilution in a receiving water body reduces water treatment costs for communities.

What are the impacts on human health?

Many contaminants are not harmful when they are present in low concentrations, so even when there may be a source of pollution upstream, downstream water quality will be improved through the process of dilution. Also, some contaminants may be consumed by aquatic species which are then consumed by other species higher up in the food chain such as fish. If too concentrated these contaminates could be toxic to consumers such as wildlife, birds and even humans [ii].

Conversely, in a lake or pond that evaporates over time, some types of contaminants will become more concentrated, making it unsafe for swimming.

What are the impacts on the environment?

Dilution is an important factor in effectively protecting aquatic ecosystems.

If there is not enough dilution (e.g. if the river level is too low in late summer), aquatic organisms can be overwhelmed by the concentration of contaminants [iii].

In Canada during the winter, dilution is a particularly important factor when rivers and streams freeze over. Because ice prevents most of the oxygen from passing into the water, the decomposition of organic matter in the water can use all the remaining oxygen, leading to fish and other organisms dying [iv].

How does dilution happen?

When tributary streams join a river, or two rivers join (confluence), the water mixes and dilutes. Similarly, when adding treated wastewater from a community into a large river, the wastewater is diluted into the full volume of the river.

What can we do about dilution?

Think about what goes down the drain in your home, garden or property.

As the population grows, human communities produce more waste (including emerging contaminants), which dilution may not be able to deal with.

Where can I find more information?


[i] Government of Alberta, n.d., Wastewater and storm water management – Overview.  Accessed 2023-04-27.

[ii] Government of Canada, 2013, Mercury: fish consumption advisories.   Accessed 2023-04-27.

[iii] Government of Prince Edward Island, 2020, Fish Kill Information and Statistics.   Accessed 2023-04-27.

[iv] Government of Alberta, 2023, Summer and Winter Kill.   Accessed 2023-04-27.