4.1.1 Aluminium sulfate in water treatment

The Group 1 and Group 2 metals form carbonates, with those of Group 2 being insoluble in water. Aluminium carbonate, Al₂(CO₃)₃, however, cannot be prepared; if aluminium sulfate is added to a solution containing carbonate or hydrogen carbonate ions, Al(OH)₃ is precipitated and carbon dioxide is evolved:

Figure 17 shows the stages in a typical water treatment process for river water. In stages 1 and 2 chemicals are added and mixed. Then in stage 3 flocculation or aggregation occurs involving large amounts of aluminium sulfate to clear the water of fine suspensions that are difficult to filter off, such as clay particles. The tiny particles usually carry surface negative charges, which repel each other and so remain in suspension.

The positively charged aluminium ions get between the negative particles, counteracting the repulsion and encouraging flocculation. Then, when Reaction 42 occurs because of the HCO₃⁻ ions usually present in natural waters, the particles sediment with the precipitate of aluminium hydroxide that forms in the sedimentation tank in stage 4. The water is filtered in stage 5 before disinfection in stage 6 via chlorination.

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Figure 17 Diagram of a typical water treatment process.

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Figure 17 A river meanders its way down the left hand side of the diagram. A tube goes to the right from the river. The left hand end of the tube is labelled 'inlet screen'. The tube leads to a larger area labelled 'preliminary settlement tank or storage reservoir'. From here a tube goes to the right, then up, then right again. An arrow labelled '(1) chemicals added' points to this tube.

The tube turns down, and there is a cross. A vertical line goes up from the cross to a grey square. Above the square it says '(2) mixer'.

The tube now goes horizontally to the right where it joins a larger horizontal tank, which has two vertical divisions towards the left hand end. Each of these divisions contains a cross, and from each cross a vertical line goes up to a grey square. Above these it says '(3) flocculation tank'.

There is a '(4)' above the right hand half of the large tank, which is labelled 'sedimentation tank'. A tube labelled 'sludge removal' comes out the bottom of the tank, while from the right hand side another tube goes first right, then up, then right again, to a box labelled '(5) filtration'.

A further pipe leads to the right to another wider box, labelled '(6) chlorination'. From the right hand side of this box, a tube labelled 'to distribution network' goes to the right.

The river is coloured dark blue and as we move to the right across the diagram this lightens as we pass through each section.

Figure 17 Diagram of a typical water treatment process.

Then, Equation 43 becomes:

The positively charged alumnium ions get between the negative particles, counteracting the repulsion and encouraging flocculation. Then, when Reaction 42 occurs because of the HCO₃ ions usually present in natural waters, the particles sediment with the precipitate of aluminium hydroxide that forms in the sedimentation tank in stage 4. The water is filtered in stage 5 before disinfection in stage 6 via chlorination.

Natural water generally requires disinfection before drinking to kill pathogenic or disease-causing microorganisms. Ultraviolet irradiation is one disinfection method and the major chemical disinfectants are ozone and chlorine. These compounds are thought to kill microorganisms by rupturing the cell membrane and reacting with proteins and enzymes within the cells. Once the chemical structures of proteins and enzymes have been altered they may either fall apart or adopt an unnatural state. Consequently, they fail to perform their roles and so the cell or bacteria dies.

A more obvious sign of the acidic character of [Al(H₂O)₆]³⁺ is the fact that aqueous solutions of aluminium sulfate are acidic, unlike those of, say, sodium sulfate:

Again, a water molecule coordinated to Al³⁺ is transformed into an OH⁻ ligand; at the same time, hydrated protons, H₃O⁺, are generated.