La mise à l'équilibre des eaux tropicales : conséquences sur la déferrisation Equilibrium of tropical waters : its consequences on the iron removal

Résumé

L'étude de l'équilibre calco-carbonique d'une eau souterraine de l'Afrique de l'Ouest par la méthode de LEGRAND et POIRIER, a conduit à la détermination des conditions de traitement pour neutraliser l'agressivité naturelle de l'eau, due essentiellement à une forte teneur en acide carbonique et réduire ainsi son caractère corrosif. le problème de l'évolution du fer présent en relation avec la mise à l'équilibre a été également étudiée.

L'application réalisée sur un pilote de laboratoire a permis d'optimiser les principales étapes du traitement, c'est-à-dire l'addition d'une base forte, la précipitation et la filtration des hydroxydes ferriques qui se forment à la suite de la neutralisation de l'acide carbonique.

Mots-clés:

• Eaux souterraines tropicales,
• eaux potables,
• agressivité,
• déferrisation

Abstract

Studies on calcium carbonate equilibrium of West African ground water, a sample of potable water distributed in Lomé (Togo), were carried out using the method described by LEGRAND and POIRIER. It is a standard graphic method in which calculations include all basic (Ca⁺⁺, HCO₃^-, CO₃^--, CO₂, H₂O⁺, OH^-), and characteristic (Mg⁺⁺, Na⁺, K⁺, SO₄^--, Cl^- ...) chemical parameters of natural water. Processing of a computer program for calculations enabled the equilibrium and saturation curve [CO₂] = f[Ca⁺⁺], to be obtained. The aggressivity to the lime deposit character, different conditions of treatment, aeration, and addition of an appropriate base to neutralise the nature aggressivity of water due to its high dissolved carbon dioxide content could also be defined.

The problem of the presence of iron was studied; high concentration of iron in natural ground water lead to various problems : developing of tastes, staining and discoloration of clothes, and growth of iron bacteria in the distribution system. Its removal from water is accomplished by means of the oxidation of iron (II) to iron (III) followed by the precipitation of relatively insoluble ferric hydroxide. The oxidation rate is highly dependent on pH. Thus, the process of removal of carbon dioxide by stripping or neutralisation raises the pH of the water. If the pH is raised high enough, an insoluble form of iron will precipitate. Another important aspect of the chemical behaviour of iron is the formation of complex ions with inorganic as well as organic ligands; silica in water forms a relatively stable complex with iron (III). The difficulty in removing iron from water is often associated with this phenomenon.

Continuous flow studies were carried out to improve important steps of treatment such as addition of a strong base, and precipitation and filtration of ferric hydroxide produced during carbonic acid neutralization. They show the weak stability of silicato-iron (III) complex at high pH value and prove that practically complete iron removal may be obtained.

Keywords:

• Tropical underground waters,
• drinking waters,
• agressivity iron removal