Where does tap water come from?

June 14, 2016 – Nicolas Pelletier, Ecoloodi Volunteer

Tap water has two sources: surface water drawn from a river, lake, dam, reservoir, etc., and groundwater found underground. Before arriving in our home, tap water passes through three steps: capture, water treatment and distribution.

Surface water: What are the three main steps?

1. Catchment

Because surface water is easily accessible, the structures used are generally very simple:

  • Drains (usually plastic pipes through which water flows using gravity).
  • Water intakes that collect water from the surface.

Figure 1
*Source: Veolia

2. Treatment

After the catchment phase, the water is routed towards a treatment plant. The goal of this phase is to eliminate any elements that could be harmful to human health, whether chemical, physical, or biological.

To reach this goal, several types of treatment are followed:

  • Screening: Filters with different sizes for large and smaller waste
  • Coagulation, flocculation, sedimentation: This treatment involves adding a coagulant to the water that binds to waste particles, forming heavier flakes that sink to the bottom. This forms a “slop” that is then discarded
  • Ozonation: Disinfects the water
  • Activated carbon filter: Removes micropollutants such as pesticides or drugs
  • Chlorination: Prevents the emergence of new bacteria


Une fois traitée, l’eau part dans les réseaux de distribution.

3. Distribution

Distribution is the final step before arriving at the tap. Water can be stored in a water tower, a structure tens of metres high. Pumps at the end of the pipeline network raise the water to the top of the tower. It is then redistributed to our homes through gravity flow.

Figure 2
Source: Azur Environnement

Groundwater: What are the differences?

Before reaching our taps, groundwater goes through the same three stages as surface water. However, extracting groundwater requires the construction of more complex structures. Indeed, these structures must extend to the water table, which could be tens of metres underground. For this, a collector is built. There is usually a pump as well as strainers that allow the water to pass inside the structure.

Source: BRGM

Once the water is collected, treatment is slightly different from that of surface water, although the principle remains the same. Being underground, there is not as much large waste. Problems with chemicals are more common. This is why we find water treatment stations to deal with iron, phosphate, etc., before arriving at chlorination.

*Source : Jousse S.A.S.

After treatment, the distribution of groundwater and surface water are the same.

*Source: SYDED


Whether groundwater or surface water, three steps are necessary for water to arrive at our taps safely. The treatment phase is fairly complex, depending on the amount of pollution, usually anthropogenic (caused by humans). Protecting our water resources is very important. To join our movement to help protect our water resources, start simple: 1) Sort your waste and dispose of it in designated containers, to keep it from ending up in bodies of water, and 2) Reduce the amount of water you use, to limit the need for expensive water treatment.

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