Unevenly distributed on Earth, fresh water is also often misused. Processes exist to remedy these difficulties of access, offering promising results, as in Senegal.
The water on our blue planet is 97,2% salt; it is found in the oceans, the seas, but also in certain groundwater. Fresh water represents only 2,8% of the world's total water. Polar glaciers contain 2,1%.
As for accessible fresh water, it only corresponds to 0,7% of the total that must be distributed between agriculture, its biggest consumer (~70% of the water withdrawn), industry (~20%) and domestic use (~10%).
Not only is fresh water geographically badly distributed on Earth, but it is also, and often, very badly used.
This problem, serious in our temperate countries, becomes extremely serious elsewhere, where the financial and/or technological means are lacking: 11% of the world's population thus has no access to drinking water and almost 30% of this population do not have access to safe drinking water in their homes.
Ancestral or innovative water purification processes
Transforming water that was initially not drinkable into drinkable water has become child's play… or almost. Technologies – some inspired by ancestral processes (distillation, bed of adsorbents, etc.) and others very innovative – have become commonplace, offering a panoply of solutions that can be adapted to almost all situations.
Two major families of technologies can be identified here: those based on distillation, which therefore consume heat; those using membranes and operating mainly with electrical energy. We will speak for example of “multiflash distillation”, “vapour compression distillation” for the first family; of “reverse osmosis”, “nanofiltration”, “electrodialysis” for the second.
The major drawback of these techniques is that they are very sophisticated and require heavy investments, impossible to ensure by countries without significant financial or technical means, associated with robust and well-branched distribution infrastructures.
Is it possible to offer less expensive solutions in this area?
The example of fluorine-laden water
Fluoride is a trace element present in very small quantities (~2 grams) in the human body. In low doses, it is very useful to prevent dental caries; it helps in the mineralization of bones, in the same way as calcium and phosphorus.
But when the dose of fluoride becomes large, it can cause dental fluorosis; and, in very high doses, skeletal fluorosis. These two diseases are common in Africa where drinking water comes from groundwater with a high fluoride content (more than 1,5 mgF-/L for dental fluorosis, and more than 4 mgF-/L for bone fluorosis) .
Dental fluorosis results in the appearance of white spots on the teeth; they evolve with age to become brown, which can even lead to calcification of the teeth. Bone fluorosis, characterized by massive bone fixation of fluorine often of hydrotelluric origin, results in blockages in the joints, even severe motor handicaps.
Groundwater defluoridation in Senegal
In Senegal, a very old technique, recently revisited as part of the project to improve and strengthen water points in the groundnut basin, consists of fixing fluorine ions by adsorption on calcined bones.
The groundwater in this basin (Kaolack, Diourbel and Fatick) is indeed known for its high fluoride content, often greater than 5 mg F-/L. This project led to the design and production of family defluoridators.
The animal bones collected in approved slaughterhouses are calcined, crushed, sieved and put in the form of a column, by associating other types of materials (gravel, coal). Well water, rich in F-, then passes through this column which will fix a good part of the F- by adsorption on the fine grains of calcined bone.
This technique makes it possible to treat a large volume of water (fluorine concentration <1,5 mgF-/L) at a cost of 780 to 2500 CFA francs/m3 of treated water (i.e. €1,20/m3 at €3,80/m3).
However, its widespread use has not been possible due to taste and odor problems observed during processing.
It is the reverse osmosis technique that has been promoted so far by local authorities, with some fixed installations in the largest cities. Admittedly, this technique makes it possible to have better quality water, but at a very high price, around 8 €/m3 ; this is extremely costly for the population.
A promising new process
Within the Paris-Est Institute of Chemistry and Materials, we have developed another technique. This is a very simple membrane technique, accessible and much less risky in terms of health, but with a cost price very comparable to that of adsorption on calcined bones.
This technique, described in the figure opposite, is called cross-ionic dialysis. An anion exchange membrane (MEA) is used which allows only negative ions to pass. It consists of a sheet of a special polymer with a thickness of ~150 µm, placed between two compartments; one (denoted F) supplied with water to be treated, the other (denoted C), containing a solution made up of the same water enriched with cooking salt (NaCl) at a concentration of 5 g NaCl/L .
Under the effect of their difference in concentration, the Cl- ions cross the MEA. Since positive sodium ions cannot cross the MEA, an equivalent quantity of F- anions must pass from compartment F to C to balance the electrical charges.
Thus, the water is depleted in F- and enriched in Cl-, an anion highly tolerated by the body as long as its concentration in drinking water is less than ~250 mgCl-/L (European Directive 98 /83 of November 3, 1998).
To circulate, at a very low flow rate, the solutions of compartments F and C, a little low-power electricity is sufficient here to activate aquarium pumps. In the absence of an electrical network, these pumps working in direct current can be powered by photovoltaic panels. We can also simply use gravity to make the water to be treated flow towards compartment F.
Thirty liters of water every night
Laboratory tests using reconstituted water proved to be very conclusive and made it possible to optimize the process parameters. These tests are confirmed by tests with real water on an A4 format pilot.
This format makes it possible to produce enough water overnight for the daily consumption of a family of around ten people, ie around thirty liters per night. The cost price remains quite low since there is no significant energy expenditure and the membrane used has proven to be quite efficient.
However, as with any installation, the ionic dialyzer requires fortnightly maintenance. This involves washing with fairly dilute solutions of citric acid or vinegar, followed by washing with soda or lime.
Fully ready at the technical level, the project is currently awaiting funding to distribute these ionic dialyzers to users.