Chemistry of feed water treatment in membrane plants

14 CHEMISTRY OF FEED WATER TREATMENT IN MEMBRANE PLANTS 

barium, magnesium and strontium can co-precipitate with CaCOa- The principal 

Calcium carbonate (CaCOs) is a common scale component in desalination plants because of its very low solubihty 18 mg/1 in pure water at 15°C. The solubility product of CaCOs is 8.7 x 10 at 15°C, which is one of the lowest among sparingly soluble salts. See Table 6.9. 

Total hardness comprises aU divalent salt ions in solution (mainly calcium and magnesium), while carbonate hardness is the fraction of total hardness that relates to dissolved CO2. The actual concentration of dissolved Ca (or Mg ) in water depends on the availability of CO2, which solubilises CaC03 (MgC03) according to the equation  

A concentration of CO2 corresponding to its partial pressure yields a carbonate hardness of about 100 mg/1 (2 meq/1). Scale inhibitors such as polymeric phosphates can sustain a concentration of250 mg/1. WHO hardness recommendation for drinking water is 2 meq/1 (max.) and a permissible total hardness of 10 meq/1 (max.). Carbonate hardness is reduced by the addition of lime, Ca(OH2), which precipitates CaC03, as discussed under lime softening in Chapter 2. 

---