The lack of dissolved minerals in the high purity waters produced by desalination processes raises some problems. High purity water tends to be highly reactive and unless treated, it can create severe corrosion difficulties during its transport in conventional pipelines. For example, the cement mortar lining of water pipes deteriorates by the corrosive attack of soft waters.
Also, untreated desalinated water cannot be used directly as a source of drinking water. A certain degree of remineralisation is necessary in order to make the water palatable and for re-introducing some essential ions required from health considerations. Distilled waters or highly soft waters produced by desalination plants cannot be directly used as they are unpalatable & corrosive. Remineralization is necessary in order to overcome these problems.
A commonly used operation in the remineralization process is to contact CO2 acidified desalinated water with a bed of domestic limestone. Limestone dissolution provides two essential ingredients to the water – bicarbonate alkalinity and calcium content:
CaCO3 + CO2 + H2O = Ca+2 + 2HCO3-1
The main processes for the remineralization of desalinated water are as follows:
A. Dosage of Chemical Solutions (based on calcium chloride and sodium bicarbonate)
Large scale preparation and dosage of such mineralizing solutions is costly and impractical. This remineralization method is a viable option only for small capacity plants.
B. Lime Dissolution by Carbon Dioxide
This process involves treatment of milk of lime with CO2 acidified desalinated water. The reaction involved is:
Ca (OH)2 + 2CO2 = Ca+2 + 2HCO3-1
C. Limestone Dissolution by Carbon Dioxide
Contacting limestone with CO2 acidified desalinated water mineralizes the solution according to:
CaCO3 + CO2 + H2O = Ca +2 + 2HCO3 -1
Limestone dissolution is the simplest and most widely used process. Limestone is cheaper than lime and half the CO2 amount is consumed in the formation of the same minerals. Moreover, the equipment for handling limestone is much cheaper compared with the system required for preparing and dosing lime slurries. The only advantage of the lime process is that the reaction proceeds almost to completion whereas in the limestone process, the reaction is much slower and does not reach completion so that residual excess CO2 has to be neutralized by addition of NaOH or Na2CO3. In large capacity plants, it is more economical to recover the excess CO2 by degasification.
Lime contactors are either upflow units or downflow Cells for very large plants. Upflow contactors are preferred for small flows. Irrespective of the type the basic principle of contact time remains the same.