How water is treated to make it safe to drink
In France, two-thirds ofthe drinking water produced is drawn from groundwater, also known as aquifers. The remaining third comes from surface water (rivers, streams, lakes, reservoirs), also known as “surface water.” Once collected, the water is then transported to a water treatment plant to be treated.
Julie Mendret, University of Montpellier and Alice Schmitt, University of Montpellier
The treatment applied depends on the initial quality of the collected water, which generally depends on the location of the intake; it should be noted that surface water usually requires more extensive treatment than groundwater.
Until now, for groundwater, simple physical treatment (filtration and sedimentation) and disinfection have been sufficient in most cases.
For surface water, a more extensive process was required, combining physical and chemical treatment, disinfection, and, in 25% of cases, a so-called polishing treatment (using activated carbon or membranes) to remove residual organic matter and improve the water’s organoleptic qualities (its taste, odor, and clarity).
Raw water contains various pollutants that must be removed. Organic matter—resulting from the decomposition of living organisms (animal waste, dead animals, or decomposing plant matter)—is the primary source of pollution in water; the goal is to remove it before addressing more specific pollutants, such as pesticides or pathogens.
Start by getting rid of the largest pieces of trash
First, the water passes through screens (a process known as “screening”) to remove the largest debris. It is then filtered to remove smaller debris, such as sand.
Next, a coagulation/flocculation stage takes place, during which a coagulant is added to form flocs—that is, clusters of suspended matter—which then settle under their own weight and are removed with the sludge.
This sludge, which consists of the solid fraction produced during treatment, is mainly used in agriculture as a soil amendment through spreading.
Next, process the invisible particles
The water is then filtered through activated carbon or sand to remove any particles invisible to the naked eye that may still be present. In some cases, membranes—which act as a specialized filter that allows water to pass through while trapping suspended solids and other substances—may be used in place of activated carbon.
Only membranes that comply withthe decree of June 22, 2012—which guarantees their safety and, in particular, the compliance of the material—may be used in the production of water intended for human consumption.
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A disinfection step then takes place to eliminate pathogenic microorganisms, often through the addition of ozone. A polishing treatment (adsorption on activated carbon) may then be applied to remove residual organic matter and improve the water’s organoleptic quality.
Finally, chlorine is added for its residual effect, as it provides long-lasting disinfection, ensuring that the water remains of excellent quality while it is stored in reservoirs (water towers) and until it is distributed.
This chlorination process, which has been used in France for over a century due to its disinfecting and long-lasting effects, has been extended since 2003 to cover the majority of water systems across France; This follows a request from the authorities to protect these systems from potential terrorist attacks: in the event of intentional contamination, any toxic or biological agent introduced would thus be destroyed by the action of chlorine.
At the same time, an abnormal and rapid drop in chlorine levels resulting from malicious activity in the water system would allow the operator to detect such a potential intrusion.
What are the effects on human health?
Aluminum salts, including aluminum 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 not properly controlled during this stage, leading to aluminum leaching into tap water.
Studies in humans have identified possible links between aluminum intake and diseases of the nervous system.
In a 2003 report, ANSES stated that the available epidemiological and physiological data did not support attributing a causative role to aluminum in Alzheimer’s disease. Furthermore, exposure to aluminum through drinking water likely accounts for less than 5% of the French population’s daily dietary intake of aluminum.
The ozonation process, used to eliminate microorganisms and treat micropollutants such as pharmaceuticals, results in the production of potentially toxic byproducts. One example is bromates, which form during the ozonation of water containing bromides and are classified as probable carcinogens.
The Case of Chlorine
The use of chlorine in water treatment can also lead to the formation of byproducts suspected of being carcinogenic, such as trihalomethanes, haloacetic acids, and haloacetonitriles.
Some of these substances have been regulated for several years at the point of use—that is, at the tap—such as trihalomethanes.
The frequency of inspections ranges from once every 10 years for the smallest power plants to once a month for the largest ones.
Recently, European Directive 2020/2184 mandated the monitoring, by 2026, of chlorites, chlorates, and haloacetic acids in disinfection processes that may lead to their formation, particularly when chlorine dioxide is used.
Models have been developed to predict the formation of these compounds and to better understand how to limit or even eliminate them entirely, but the complexity of the mechanisms involved and the number of variables to be taken into account are such that significant uncertainties remain.
Until more is known about this topic, the key is to minimize the amount of residual chlorine as much as possible (while maintaining enough to ensure high-quality water all the way to the tap) in order to prevent the formation of these byproducts.
The problem with the pipes
At the outlet of the water treatment plant, with additional treatment added if necessary, the goal is to produce water that is close tocarbonate equilibrium—that is, non-corrosive and very slightly scale-forming.
Hard water causes calcium carbonate to precipitate, leading to scale buildup in household appliances. Aggressive water can corrode the metals in pipes, potentially causing contamination—such as lead contamination—during water transport. This phenomenon occurs most frequently when water sits stagnant in the pipes.
As for lead, the solution to permanently eliminate it from the water supply is to remove lead pipes. Public authorities have launched programs to replace public lead pipes in order to provide water that complies with European Directive 98/83/EC (of November 3, 1998, revised in 2013), which sets a maximum lead content at the tap of 10 µg/l.
Turning on the tap and getting drinking water—a seemingly simple daily act that actually requires a whole series of treatments beforehand.
Julie Mendret, Associate Professor, HDR, University of Montpellier and Alice Schmitt, Postdoctoral Researcher in Process Engineering, European Membrane Institute, University of Montpellier
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