Chemical precipitation depends upon the use of an added reagent which combines with the impurity to be removed to give an insoluble product which can then be removed by sedimentation, preceded by flocculation if necessary.
Impurity + reagent → precipitate + byproduct
It is clearly essential that any byproduct of the reaction does not itself have undesirable properties in relation to the eventual use of the water or wastewater. It is also important to remember that chemical precipitation processes produce sludge containing the impurities and that the cost of handling and disposing of these sludge in a safe manner can be significant.
To illustrate the way in which chemical precipitation may be used, it is convenient to consider the softening of water. Hardness in water is due to the presence of calcium and/or magnesium which may be associated with carbonates, bicarbonates, sulphates and chlorides. The metallic cations actually cause the effect of hardness (scale formation in hot water systems and reduced efficiency of soap) but the anions with which they are associated control the behaviour of the hardness. It is perhaps worth noting that although there are often economic reasons for reducing hardness, there is evidence to suggest that hard waters have a beneficial effect in relation to some heart diseases. Many natural waters contain material which has entered by dissolution of insoluble rocks under the action of water and biologically produced carbon dioxide:
CaCO3 + H2O + CO2 → Ca(HCO3)2
and reversal of this process can be used to remove the hardness.
A convenient way to set out the steps in chemical precipitation softening is to use a bar diagram which expresses the composition of the water in terms of calcium carbonate. This is obtained from the water analysis by multiplying the concentration of each constituent by the ratio:
Equivalent weight of CaCO3/equivalent weight of constituent. Thus
55 mg/l Ca2+ = 55 X (100/2) / (40/2) = 137.5 mg/l as CaCO3.
125 mg/l HCO3– = 125 X (100/2) / 61 = 102.5 mg/l as CaCO3.
40.2 mg/l Cl– = 40.2 X (100/2) / 35.5 = 56.7 mg/l as CaCO3.
Lime softening utilizes the reaction which occurs when lime is added to a water containing calcium hardness associated with bicarbonates:
Ca(HCO3)2 + Ca(OH)2 → CaCO3 + 2H2O
If calcium hardness of both carbonate and non-carbonate forms is present, the lime soda process must be used. Here the first stage is as for lime softening but this is followed by the addition of sodium carbonate to allow further precipitation of calcium carbonate:
CaSO4 + Na2CO3 → CaCO3 + Na2SO4
When magnesium hardness is present, the softening process is complicated because magnesium carbonate is soluble:
Mg(HCO3)2 + Ca(OH)2 → CaCO3 + MgCO3 + 2H2O
However, by raising the pH to 11, magnesium hydroxide is precipitated with a solubility of 10 mg/l:
MgCO3 + Ca(OH)2 → Mg(OH)2 + CaCO3
It will be noted that the process is fairly complicated and that it produces large volumes of sludge. When both calcium and magnesium carbonate and non-carbonate forms of hardness are present, the even more complex excess lime soda process can be used, although ion exchange treatment may be more economic in such cases. Softened waters should be stabilized to prevent further precipitation of calcium carbonate scale by the addition of carbon dioxide, which converts the carbonate back into soluble bicarbonate, or of polyphosphates, which keep carbonate in suspension as a floe. This stabilization is important, since otherwise the distribution system carrying the softened water can have its capacity seriously reduced by calcium carbonate scale.
Another application of chemical precipitation is for the treatment of industrial wastewaters containing metals such as chromium. These wastewaters arise from metal-finishing operations like chromium plating and can cause serious damage to sewage treatment processes if not subjected to effective pretreatment before discharge to the sewer. Chromic acid and Chromate wastes can be treated by reduction of the Chromate using ferrous sulphate and adding lime to cause precipitation of chromium as the hydroxide. This can then be removed by sedimentation. The basic equations for the reactions when the chromium is present in acid solution, which is usually the case, are:
2CrO3 + 6FeSO4.7H2O + 6H2SO4 →
3Fe2(SO4)3 + Cr2(SO4)3 + 13H2O + 12Ca(OH)2 →
6Fe(OH)3 + 2Cr(OH)3 + 12CaSO4