How water is treated to make it drinkable

In France, two-thirds of thedrinking water we produce comes from groundwater. The remaining third comes from surface waters (rivers, lakes, dams). Once captured, the water is transported to a water purification plant for treatment.

Julie Mendret, University of Montpellier and Alice Schmitt, University of Montpellier

The treatment applied depends on the initial quality of the water collected, which in turn generally depends on the location of the catchment; it should be noted that surface water usually requires more extensive treatment than groundwater.

Until now, for groundwater, simple physical treatment (filtration and decantation) and disinfection were sufficient in most cases.

For surface water, a more advanced process was required, combining physical and chemical treatment, disinfection and, in 25% of cases, a so-called refining treatment (with activated carbons or membranes) to eliminate residual organic matter and improve the water's organoleptic qualities (taste, odor, clarity).

Raw water contains various pollutants that need to be eliminated. Organic matter, resulting from the decomposition of living matter (animal droppings, dead animals or decomposing plant fragments, etc.), is the main pollutant present in water, and is eliminated before more specific pollutants, such as pesticides or pathogens.

Eliminate the biggest waste first

The water first passes through screens (known as "screenings") to remove larger particles. It then undergoes a sieving process to remove smaller particles, such as sand.

This is followed by a coagulation/flocculation stage, in which a coagulant is added to create flocs, i.e. groupings of suspended matter which then fall by their own weight and are eliminated in the sludge.

This sludge, which corresponds to the solid part produced during treatment, is mainly used in agriculture as a soil improver.

Then process the invisible particles

Activated carbon or sand filtration then takes place to remove any particles that may still be present, invisible to the naked eye. In some cases, membranes acting as a specific filter, allowing water to pass through and retaining suspended solids and other substances, can be used instead of activated carbon.

Only membranes complying with thedecree of June 22, 2012, which guarantees their safety and in particular the conformity of the material, can be used in the production of water intended for human consumption.

[Nearly 80,000 readers trust The Conversation newsletter to better understand the world's major issues. Subscribe today]

A disinfection stage then takes place to eliminate pathogenic micro-organisms, often by adding ozone. A refining treatment (adsorption on activated carbon) may then be added to eliminate residual organic matter and improve the organoleptic quality of the water.

Finally, chlorine is added for its long-lasting disinfectant effect, maintaining excellent water quality from storage in reservoirs (water towers) right through to distribution.

Chlorination, which has been used in France for over a century for its disinfectant and remanent effect, was extended to most water networks in France in 2003, in response to a request from the authorities to protect these networks from potential terrorist attacks: in the event of intentional pollution, the toxic or biological agent introduced would be destroyed by the action of the chlorine.

At the same time, an abnormal and rapid drop in chlorine levels resulting from malicious action in the water network would enable the operator to detect this possible intrusion.

What are the effects on human health?

Aluminum salts, including alumina sulfate, are sometimes used in surface water treatment as flocculants and clarifiers to remove dissolved organic matter and suspended particles. Occasionally, the pH of the water is poorly controlled during this stage, leading to aluminum leaks into the tap water.

Human studies have shown possible links between aluminum ingestion and nervous system disorders.

In a 2003 report, Anses stated that the epidemiological and physiological data available did not allow us to attribute an etiological role to aluminum in Alzheimer's disease. What's more, exposure to aluminum through drinking water probably accounts for less than 5% of the French population's daily dietary intake of aluminum.

The ozonation stage, used to eliminate micro-organisms and act on micropollutants such as pharmaceuticals, leads to the production of potentially toxic by-products. One example is bromates, formed during the ozonation of water containing bromides, which have been identified as probable carcinogens.

The case of chlorine

The use of chlorine in water treatment can also lead to the production of suspected carcinogenic by-products, such as trihalomethanes, haloacetic acids and haloacetonitriles.

Some, such as trihalomethanes, have been regulated for several years at the point of use, i.e. at the tap.

Their inspection frequency varies from once every 10 years for the smallest production plants to once a month for the largest.

Recently, European directive 2020/2184 requires the monitoring of chlorites, chlorates or haloacetic acids by 2026 in the case of disinfection treatments that may lead to their production, notably when chlorine dioxide is used.

Models have been developed to anticipate the production of these compounds and better understand how to limit or even eliminate them altogether, but the complexity of the mechanisms involved and the number of variables to be taken into account are such that they still present major uncertainties.

Until more is known on this subject, the watchword is to keep chlorine residuals to a minimum (while retaining enough to ensure top-quality water right up to the point of distribution), in order to avoid the appearance of these by-products.

The piping problem

At the output of the drinking water treatment plant, with the addition of further treatment if necessary, the aim is to obtain water close tocalcocarbonic balance, i.e. non-aggressive and very slightly scaling.

Scaling water precipitates limescale, leading to the scaling of household appliances. Aggressive water can corrode the metals that make up pipes, leading to lead contamination, for example, during water transport. This phenomenon occurs mainly when water stagnates in the pipes.

As far as lead is concerned, the solution to eliminating it permanently from water is to remove lead pipes. Public authorities have launched programs to replace lead pipes in order to deliver compliant water, in line with European Directive 98/83/EC (November 3, 1998, revised in 2013), which sets a maximum lead content in taps at 10 µg/l.

Turning on the tap and obtaining drinking water: a seemingly innocuous daily gesture that requires a whole series of upstream treatments.

Julie Mendret, Senior Lecturer, HDR, University of Montpellier and Alice Schmitt, Post-doctoral fellow in process engineering, European Membrane Institute, University of Montpellier

This article is republished from The Conversation under a Creative Commons license. Read theoriginal article.