المحاضرة 3-4

Lime :

When lime alone is added as a precipitatnt, the principles of clarification are explained by the following reactions for the carbonic acid and the alkalinity:

44 (as CO2) 56 (as CaO) 100 2 x 18

H2CO3 + Ca (OH) 2 CaCO3 + 2H2O

Carbonic Calcium Calcium
acid hydroxide carbonate
(soluble) ( slightly soluble) (somewhat soluble)


100 (as CaCO3) 56 (as CaO) 2 x 100 2 x 18

Ca (HCO3)2 + Ca (OH) 2 2 CaCO3 + 2H2O

Calcium Calcium Calcium
bicarbonate hydroxide carbonate
(soluble) ( slightly soluble) (somewhat soluble)

Ferrous Sulfate and Lime :

In most cases, ferrous sulfate cannot be used alone as a precipitant because lime be added at the same time to form a precipitate. When ferrous sulfate alone is added to wastewater, the following reactions occur:

278 100 (as CaCO3) 178 136 7 x 18

FeSO4. 7H2O + Ca (HCO3)2 Fe (HCO3)2 + CaSO4 + 7H2O

Ferrous Calcium Ferrous Calcium
sulfate Bicarbonate Bicarbonate sulfate
(soluble) (soluble) (soluble) (soluble)


178 89.9 2 x 44

Fe (HCO3)2 Fe (OH) 2 + 2CO2

Ferrous Ferrous Carbon
bicarbonate hydroxide dioxide
(soluble) (Very slightly Soluble) ( soluble)


If sufficient alkalinity is not available, lime is often added in excess in conjunction with ferrous sulfate. The resulting reaction is:


178 2 x 56 (as CaO) 89.9 6 x 44

Fe (HCO3)2 + 2Ca (OH) 2 Fe (OH) 2 + 6CO2

Ferrous Calcium Ferrous Carbon
bicarbonate hydroxide hydroxide dioxide
(soluble) (slightly Soluble) (Very slightly Soluble) ( soluble)


The ferrous hydroxide can be oxidized to ferric hydroxide, the final form desired, by oxygen dissolved in the wastewater. The reaction is:

89.9 1/4 x 32 1/2 x 18 106.9

Fe (OH) 2 + 1/4 O2 + 1/2 H2O Fe(OH)3

Ferrous Oxygen Ferric
hydroxide (soluble) hydroxide
(Very slightly Soluble) (insoluble)


The insoluble ferric hydroxide is formed as a bulky, gelatinous floc similar to the alum floc. The alkalinity required for a 10 mg/L dosage of ferrous sulfate is:
( 10 mg/L)(( (100 g/mole))/(278 g/mole)))=3.6 mg/L
The lime required is:
( 10 mg/L)((2 (56 g/mole))/(278 g/mole)))=4 mg/L
The oxygen required is:
( 10 mg/L)(((1/4) (32 g/mole))/( (278 g/mole)))=0.29 mg/L
Because the formation of ferric hydroxide is dependent on the presence of dissolved oxygen, the reaction given cannot be completed in most wastewater, and, as a result, ferrous sulfate is not used commonly in wastewater.