From the source to the tap: how does water become drinkable?

What exactly is “drinking water” ? Contrary to what we might think, it is not simply clear or odourless water. It must comply with a hundred or so very specific criteria, which are defined by the French Public Health Code, in accordance with European standards and the recommendations of the World Health Organisation (WHO).

In France, drinking water comes primarily from two major sources:

Groundwater (68%): drawn from phreatic water tables, it is naturally filtered through layers of sand and rock. Its composition varies depending on the geological characteristics of the area: in limestone regions, it contains more mineral salts; in granitic soils, the water is more acidic and contains fewer minerals. However, it is important to note that shallow water tables, which are close to the surface, are more vulnerable to agricultural or industrial pollution.

Surface water (32%): this includes rivers, lakes and dams. As they are more exposed to urban pollution (wastewater discharges), industrial pollution (heavy metals and solvents) and agricultural pollution (nitrates and pesticides), they require more intensive treatment.

Quantitative example: In 2023, 68% of the French population relied on groundwater for their water supply, but this water is not exempt from monitoring, as some sources naturally contain iron, manganese or even arsenic, which means that specific treatment is required.

Before reaching our tap, the water goes through a series of stages in specialised treatment plants:

Catchment
The water is collected at source: by drilling into water tables, or by pumping it from rivers or lakes. To minimise the risk of pollution, protection zones are established around water catchment areas, where certain activities are prohibited (spreading of manure, construction, storage of chemicals).

Pre-filtration
When it reaches the plant, the raw water passes through screens, filters and settling tanks that trap the larger particles: branches, leaves, sand, oil and floating debris. This is a crucial mechanical stage to avoid damaging the more intricate equipment.

Clarification
This stage removes the smaller particles, which are invisible to the naked eye. Reagents (aluminium or iron salts) are used to bind the fine particles into “flocs”, which then settle to the bottom (decantation) or rise to the surface. These physico-chemical techniques are used to clarify the water before it is filtered.

Filtration
The water passes through sand or activated carbon filters. The sand traps the residual particles; the highly absorbent activated carbon captures organic substances and certain pesticides, and improves the taste and smell.

Disinfection
This is the key stage to eliminate viruses, bacteria and parasites. Either ozone, which is a powerful disinfectant, is used, or more commonly, chlorine, which has the advantage of remaining active during distribution. Chlorine levels are strictly controlled to prevent an unpleasant taste whilst providing protection right up to the tap.

Storage et distribution
Once it has been treated, the water is stored in tanks or water towers, which help to maintain pressure in the network and provide a constant supply, even at times of high demand.

Close-up: The French water network comprises over 900,000 km of pipes used to deliver drinking water to households. That’s the equivalent of 22 times the circumference of the Earth!

Techniques tailored to specific requirements

When we talk about water, it is essential to fully understand the difference between water purification and sanitation. Although both of these activities involve treating water, they serve very different purposes and involve very different processes.

Water purification: turning raw water into drinking water

Sanitation: treating water after use

The challenges facing drinking water in the future

Safeguarding access to high-quality drinking water is an ongoing challenge. Even though in France we are fortunate enough to have a relatively efficient network and water that is closely monitored, the demands placed on this vital resource are becoming increasingly prevalent and a cause for concern. The main challenges surrounding drinking water are as follows:

Agricultural pollution: a major challenge

Intensive agricultural practices are one of the main sources of water pollution, particularly due to:

Pesticides, which are used to protect crops but can seep into the water table or run off into rivers.

Nitrates, originating from fertilizers, which cause imbalances in aquatic ecosystems (algal blooms and oxygen depletion).

Key figure: according to IFEN, the French Institute for the Environment, approximately 70% of the nitrates found in groundwater in France come from agriculture.

Industrial pollution: heavy metals and solvents

Industrial activities sometimes release highly problematic substances, such as:

Heavy metals (lead, mercury or cadmium) that can accumulate in sediments and living organisms.

Organic solvents and chemical residues, which contaminate surface water.

Such pollution is often more localised, but it requires specific and costly treatment to be eliminated.

Urbanisation: strain on networks

As urbanisation increases, new challenges are emerging:

Soil sealing (caused by roads, car parks and buildings) reduces natural water infiltration and increases the risk of flooding.

Drinking water and wastewater networks are getting old: in France, nearly 50% of the network is over 40 years old, and leaks account for approximately 1 in 5 litres lost before the water even reaches the tap
(source: Observatoire des services publics d'eau - Observatory on public water and sanitation services in France).

Climate change: from droughts to floods

Global warming is seriously disrupting the water cycle:

Prolonged droughts are depleting available water supplies, particularly groundwater reserves.

Heavy rainfall is increasing surface run off and washing pollutants into watercourses.

Rising temperatures are encouraging the growth of algae and harmful microorganisms.

A striking example: In 2022, 700 French municipalities had to restrict the use of drinking water or rely on water tankers due to the drought (source: French Ministry for Ecological Transition).

Restoring a polluted resource is a long and costly process. For example, cleaning up a groundwater table can take several decades and cost millions of Euros, with no guarantee that the water will return to its original quality. That is why preventing pollution at source and safeguarding the quality of the natural environment remains the most effective strategy: preventing pollutants from reaching natural resources rather than having to eliminate them afterwards.

In the face of these challenges, each and every one of us has a part to play:

Local authorities: by modernising networks, adapting urban planning policies and protecting water catchment areas.

Businesses: by reducing pollutant emissions, improving industrial processes and investing in clean technologies.

Citizens: by limiting the use of chemicals (weedkillers and cleaning products), saving water in day-to-day activities (taking shorter showers, eliminating leaks) and supporting local initiatives.

Did you know? A standard toilet flush uses between 6 and 12 litres per flush, whereas a dripping tap can waste up to 35,000 litres a year. So even the smallest actions can have a huge impact!

To rise to the challenges faced by everyone, the Claire Group is stepping in at every key stage in the water cycle to ensure the sustainable preservation of this resource: