The flow of water through the soil is difficult to observe in the field, but an understanding of how it behaves through the different layers of material that make up a treatment or leaching unit could help us make better decisions when preparing plans and specifications of a septic system, all the while complying with applicable local laws and regulations.


Following our recent collaboration with Alexandre Cabral, P. Eng., Ph. D., Université de Sherbrooke professor in the Engineering Department and an expert in soil mechanics and environmental geotechnics, we are sharing with you some of our observations from the research conducted on the flow of water through different types of soil, within the context of decentralized wastewater treatment with infiltration.

The flow of water through the soil - DBO))Clic 1
Contrary to what one might believe, a material’s water permeability is not a constant, but rather a function of suction.

Hydraulic Loading Rate

The hydraulic loading rate of soil is a very important parameter when designing a septic system with infiltration. When soil is saturated, it has its greatest water permeability. Contrary to what one might believe, a material’s water permeability is not a constant, but rather a function of the suction in the material, also called “capillarity”.


The capillarity is caused by the surface tension that develops between materials when humidity is present (Holtz et al.). Forces of attraction are present and create capillary bridges between the grains of the material. It is thanks to this phenomenon that humid sand can maintain a specific structure. This allows, among other things, for the creation of sand castles!


However, when this same material becomes completely inundated or completely dry, these forces of attraction become negligible, resulting in a material that has no capacity to maintain a structure; the sand castle crumbles.


It is this phenomenon that makes a material retain or release water, according to its water content. This is why a material’s hydraulic loading rate is not a constant, but varies according to its saturation level.

to conclude

The hydraulic loading rate of the material used is often calculated in saturated conditions, while what happens in reality is quite different. Although these concepts are not generally applied in designing septic systems, understanding how water flows is a fundamental element in defining standards, regulations and best practices. This information can also help guide decisions when special situations are encountered in the field.



For a more in-depth look at this subject, please consult the technical document below.

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