PUBLISHED: 04 December 2017



Also known as watering down, decreasing the concentration, a component of assimilative capacity. Icon showing a beaker tipping and adding water, representing dilution

What is it?

For decades, the approach to pollution management was to increase dilution. There was 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 principal of dilution is true in that the natural environment, specifically aquatic ecosystems, have capacity to absorb and degrade a certain amount 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 receive 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 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 Alberta?

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.

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. 
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. 
In Alberta 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.

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?

Wastewater system standards for performance and design
Links to websites for wastewater service providers in Alberta
Protecting surface water quality from wastewater discharges through assimilative capacity studies