Chemical Species In The Treated Water

In addition to the H+ and OH- species, a great majority of ions present in the treated water are the cations Ca2+, Mg2+, and Na+ and the anions SO4- and HCO-. Occasionally, the anions Cl- and NO- may also be present when the hardness cations in natural waters are associated with these anions.

The effluent water from the hardness removal reactor is basic and tends to deposit scales in distribution pipes. For this reason, this water should be stabilized. Stabilization is normally done using carbon dioxide, a process called recarbonation. Stabilization using carbon dioxide affects the concentration of the bicarbonate ion in the treated water. The concentrations of the SO2-, Cl- and NO- ions are not affected, however, because they do not react with carbon dioxide. Their concentrations remain the same as when they were in the influent to the treatment plant. The original cation Na+ from the influent raw water is also not affected for the same reason that it does not react with carbon dioxide. Na+ is, however, introduced with the soda ash.

Let [ SO2-]me?iinf, [Cl-]me?,inf, and [NO-]meqn be the concentrations of the indicated ions in milligram equivalents per liter in the influent to the reactor. Also, let [SO4-]meq, [Cl-]meq, and [NO-]meq be the concentrations of the ions in the treated water. Thus,

[Cl ]meq — [Cl ]meq,inf [ NO3 ]meq — [ NO3 ] meq ,inf

10.13.1 Limits of Technology

The precipitation of CaCO3 and Mg(OH)2 is never complete. At 0°C, the solubility of CaCO3 is 15 mg/L and at 25°C, it is 14 mg/L. These solubility values imply that no matter how much precipitant is applied to the water, there will always remain some calcium and carbonate ions which, in the aggregate, amounts to 14 to 15 mg/L of CaCO3 dissolved in the treated water.

For Mg(OH)2, its solubility at 0°C is 17 mg/L as CaCO3; at 18°C it is 15.5 mg/L as CaCO3. As in the case of CaCO3, some ions of magnesium and, of course, the hydroxide will always remain dissolved in the treated effluent no matter how much precipitant is employed. Operationally, let us adopt the following figures as the limit of technology: CaCO3 = 15 mg/L; Mg(OH)2 = 16 mg/L as CaCO3. This brings a total to 15 + 16 = 31 mg/L as CaCO3.

10.13.2 Concentration of Ca2+

The concentration of the calcium ion in the treated water comes from the calcium bicarbonate not precipitated, the calcium from the noncarbonate hardness of calcium not precipitated, and the calcium that results from the precipitation of the noncarbonate hardness of magnesium. Note that, as the treated water is recarbonated, the 15 mg/L of CaCO3 from the limit of technology converts to the bicarbonate form,

CaCO3 + H2CO3 (CO2 + HOH ^ H2CO3 ) ^ Ca(HCO3 )2 (10.45)

From this reaction, the number of milligram equivalents per liter of Ca2+ and HCO-are 15/(CaCO3/2) = 0.3, respectively.

Recall that MCaHCO (= f CaHCO;iMTCaHCO ) mass of calcium bicarbonate from the carbonate hardness of calcium was precipitated. Let this mass be measured in kilograms. Also, let [ Ca( HCO3 )2 ]mgnot, in mg/L, be the concentration of calcium bicarbonate not precipitated. Thus,

mgnot y

1000(1000- -1000

V is the volume of water treated in cubic meters.

Ca(HCO3)2 ionizes to Ca(HCO3)2 ^ Ca2+ + 2HCO-; thus, the equivalent mass of calcium bicarbonate is Ca(HCO3)2/2. The calcium bicarbonate not precipitated therefore produces [Ca(HCO3)2]mgnoACa(HCO3)2>2) = 0.0123( [Ca(HCO3)2]mgnot) = 0.0123[1000MTCaHCO3( 1 — f CaHCO3)/V] = 12.3[MTCaHCO3( 1 - fCaHCO3)/V] milligram equivalents per liter of Ca + and HCO3, respectively.

Now, also recall that MCa (= fCaMTCa) mass of calcium from the noncarbonate hardness of calcium precipitated. As before, let this mass be measured in kilograms. Also, let [ Ca2+]mgnot, in mg/L, be the concentration of calcium not precipitated. Thus,

MTCa - fCaMTCa

1000(1000--

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