1. Particle separation: Chemical precipitation
TVM 4145 Vannrenseprosesser / unit processes
Particle separation:
Chemical precipitation
Prof. TorOve Leiknes

2. Definition:

3. General expression: aA + bB  cC + dD A+ + B-  AB(s)
Equilibrium:
aA + bB  cC + dD
Solubility:
A+ + B-  AB(s)
AxBy(s)  xAy+ + yBx-
Precipitation with metals (M) the following general equation applies:
MAx(S)  Mx+ + xA-
For A- = OH-

4. Example: excess lime process
Removal of hardness (Ca+2, Mg+2)
Carbonate equations with addition of excess lime (for pH increase) and resulting precipitation:

5. Classical process configuration for excess-lime / soda ash softening:8
sludge
1. Dosage and mixing of lime
2. Precipitation of CaCO3 and Mg(HO)2
3. Dosage and mixing of CO2 and soda ash
4. Precipitation of CaCO3
5. pH adjustment and precipitation of residual CaCO3

6. Carbonate system determines the waters stability:
Measured with respect to the carbonate systems equilibrium – expressed as Langlier’s saturation index
Rearranged:
Index:
Positive – precipitation of CaCO3
Negative – dissolution of CaCO3

7. Chemical precipitation of inorganic compounds:
Iron (Fe) and manganese (Mn)
Problem:
precipitation / sludging
colouring of water
growth of “iron bacteria”
odour and taste issues
Methods for removal:
chemical precipitation
oxidation and precipitation
ion exchange
biological oxidation
Drinking water standards (NOR):

8. Reduced form – Fe2+, Mn2+ Oxidation by: oxygen (air) ozone
potassium permanganate
chlorine dioxide
Iron removal:
(Ksp = , 25C)
Manganese removal:
not so readily oxidized by oxygen (air)
precipitation is more pH dependent

9. Basics of iron oxidation:
Stoichiometric balance:
1 mg Fe/L  0.14 mg/L O2 requires
0.04 mg/L H+ produced
1.80 mg/L alk as CaCO3 used
Kinetics of oxidation:
Where: concentration is in mol/L
pO2 is partial pressure in atm
k= L2/(min)(atm)(mol)2 at 25C
Pseudo 1.order expression commonly used:
K = / [H+] min-1 at PO2 = 0.21 atm

10. Phosphate (P) Coagulation and precipitation by:
- aluminum salts : AI2(S04) H20
- ferric salts : Fe(CL)3, Fe(S04)3
Stoichiometric balance Al:
Al2(SO4)3 ·18H2O + PO43-  2AlPO4 + 3SO H2O