WATER TREATMENT PLANT CHEMICAL EQUATION
1. Removal of Carbonate or Temporary Hardness:
Ca(HCO3)2 → CaCO3 + CO2 + H2O
(Soluble) (Insoluble)
Mg(HCO3)2 → Mg(OH)2 + 2CO2
Precipitates of Calcium and Magnesium can be removed by filtration
DEMINERALISATION PLANT
2.Activated Carbon Filter
C + 2Cl2 + 2H2O → 4HCl + CO2
3. Ion Exchange : Hydrogen Cycle
Strong Acid Cation:
Generation: 2 R H+ + CaCl2 → R2Ca2+ + 2HCl
2 R H+ + CaSO4 → R2Ca2+ + H2 SO4
2 R H+ + Ca(HCO3)2 → R2Ca2+ + 2H2CO3
H2CO3 → H2O + CO2
Regeneration: R2Ca2+ + 2HCl → 2 R H+ + CaCl2
R2Ca2+ + H2 SO4 → 2 R H+ + CaSO4
Strong Base Anion:
Generation: R+OH- + HCl → R+Cl- + H2O
2 R+OH- + H2 SO4 → R+2 SO42- + 2H2O
Regeneration: R+Cl- + NaOH → R+OH- + NaCl
R+2 SO42- +2 NaOH → 2R+OH- + Na2SO4
Soft water
Sodium Cycle:
Generation: 2Na+R- + Ca2+Cl2 → Ca2+R-2 + 2NaCl
Regeneration: Ca2+R-2 + 2NaCl → 2Na+R- + Ca2+Cl2
4. CHLORINATION CHEMISTRY:
Chlorine is most commonly available as chlorine gas and the hypochlorites of sodium
and calcium. In water, they hydrolyze instantaneously to hypochlorous acid:
Gas : Chlorine Cl2 + H2O → HOCl + HCl
Liquid : Sodium Hypochlorite NaOCl + H2O → HOCl + NaOH
Solid : Calcium Hypochlorite Ca(OCl)2 + 2 H2O → 2 HOCl + Ca(OH)2
Trichloroisocynuricacid C3O3N3Cl3 + 3H2O ➝ 3HOCl + C3O3N3H3
Hypochlorous acid dissociates in water to hydrogen ions and hypochlorite ions:
HOCl ↔ H+ + OCl–
HOCl <7 <---------> 8> H + OCl
At the higher pH range as the ration of OCl increase as the sterilization effect of chlorine
compound decrease.
It is practically observed that the sterilization force of OCl is approximately 20 time less as as
that of HOCl
HOCl has significantly higher effect than OCl
Chlorine kill microorganism by destroying cell wall of the microorganism with there
oxidizing forces.
TRC = FAC + CAC = FRC + CRC
The germicidal efficiency of free residual chlorine is directly related to the concentration of
undissociated HOCl. Hypochlorous acid is 100 times more effective than the hypochlorite ion OCl–.
The fraction of undissociated HOCl increases with decreasing pH.
At pH 7.5 (77°F (25°C), 40 mg/L TDS), only 50% of free residual chlorine is present as HOCl,
but 90% is present at pH 6.5. The fraction of HOCl also increases with decreasing temperature. At
41°F (5°C), the HOCl mole fraction is 62% (pH 7.5, 40 mg/L TDS). In highsalinity waters, less HOCl is
present (30% at pH 7.5, 25°C, 40,000 mg/L TDS).
5. CHLORINE DEMAND :
A part of the chlorine dosage reacts with ammonia nitrogen to combined available
chlorine in a series of stepwise reactions:
HOCl + NH3 ↔ NH2Cl (monochloramine) + H2O
HOCl + NH2Cl ↔ NHCl2 (dichloramine) + H2O
HOCl + NHCl2 ↔ NCl3 (trichloramine) + H2O
Bromide reacts rapidly with hypochlorous acid to form hypobromous acid:
Br– + HOCl → HOBr + Cl–
Thus, in chlorinated seawater the biocide is predominantly HOBr rather than HOCl.
Hypobromous acid then dissociates to hypobromite ion as follows:
HOBr ↔ OBr– + H+
BREAKPOINT CURVE
6. DECHLORINATION
Residual free chlorine can be reduced to harmless chlorides by activated carbon or
chemical reducing agents. An activated carbon bed is very effective in the dechlorination of
RO feed water according to following reaction:
C + 2Cl2 + 2H2O → 4HCl + CO2
Sodium metabisulfite (SMBS) is commonly used for removal of free chlorine and as a
biostatic. Other chemical reducing agents exist (e.g., sulfur dioxide), but they are not as cost-
effective as SMBS.
When dissolved in water, sodium bisulfite (SBS) is formed from SMBS:
Na2S2O5 + H2O → 2 NaHSO3
SBS then reduces hypochlorous acid according to:
2NaHSO3 + 2HOCl → H2SO4 + 2HCl + Na2SO4
In theory, 1.34 mg of sodium metabisulfite will remove 1.0 mg of free chlorine. In practice,
however, 3.0 mg of sodium metabisulfite is normally used to remove 1.0 mg of chlorine.
CHEMICAL REACTION OF PRETREATMENT SYSTEM
Al2 (SO4)3 + 3 Ca(HCO3)2 -----> 3 CaSO4 + 2 Al(OH)3 + 6 CO2
Na AlO2 + 2 H2O → Al (OH)3 + NaOH
FeSO4 + Mg(HCO3)2 → Fe(OH)2 + MgCO3 + CO2 + H2O
4Fe(OH)2 + O2 + H2O → 4 Fe (OH) 3
2 FeCl3 + 3Ca(OH)2 ------> 2Fe (OH) 3 + 3CaCl2
2 FeCl3 + 3 Ca (OH)2 ------> 2Fe(OH)3 + 3CaCl
2FeCl3 + 3Ca(HCO3)2 -----> 2Fe(OH)3 + 3CaCl2 + 7CO2
Ferric Hydrolysis Reactions
FeCl3 + 3H2O -----> Fe (OH)3 + 3HCl
Fe+++ + H2O <---> FeOH++ + H+
FeOH++ + H2O <----> Fe(OH)2+ + H+
Fe(OH)2+ + H2O <-----> Fe(OH)3 + H+
Fe(OH)3 + H2O <-----> Fe(OH)4- + H+
2Fe+++ + 2H2O <-----> Fe2(OH)2++++ + 2H+
3Fe+++ + 4H2O <-----> Fe3(OH)4++++ + 4H+
Alkalinity Neutralization
H+ + HCO3 ------> H2O + CO2
BOILER WATER CHEMICAL EQUATION
1. Removal of Dissolved Oxygen
2Na2SO3 + O2 → 2Na2SO4
N2 H4 + O2 → N2 + 2H2O
Na2S + 2O2 → Na2SO4
2. PHOSPHATE TREATMENT
Trisodium Phosphate: 2Na3PO4 + 3CaCO3 → Ca3(PO4)2 + 3Na2CO3
2Na3PO4 + 3CaSO4 → Ca3(PO4)2 + 3Na2SO4
2Na3PO4 + 3Ca(HCO3)2 → Ca3(PO4)2 + 6 NaHCO3
Disodium Phosphate:
2Na2HPO4 + 3CaCO3 → Ca3(PO4)2 + 2Na2CO3 + CO2 + H2O
Na2HPO4 + CaSO4 → CaHPO4 + Na2SO4
Monosodium Phosphate: This phosphate effectively reduce the alkalinity in the boiler by
two-third.
2NaH2PO4 + 3CaCO3 → Ca3(PO4)2 + Na2CO3 + 2CO2 + H2O
2NaH2PO4 + CaSO4 → Ca(H2PO4)2 + Na2SO4
Tetra sodium Pyrophosphate: Tetra sodium pyrophosphate effectively reduce the alkalinity
in the boiler by one-third. The boiler water heat hydrolyzes the tetra sodium pyrophosphate,
converting it into disodium phosphate.
Na4P2O7 + H2O + Heat → 2Na2HPO4
Na4P2O7 + 2CaSO4 + H2O → 2CaHPO4 + 2Na2SO4
Na4P2O7 + 3CaCO3 + H2O → 2Na2CO3 + Ca3(PO4)2 + H2O + CO2
Sodium Hexametaphosphate : This compound effectively reduce the alkalinity in the boiler
by two-third. The boiler water heat hydrolyzes the hexametaphosphate and converting it to
monosodium phosphate.
(NaPO3) 6 + 6H2O + Heat → 6NaH2PO4
(NaPO3) 6 + 3CaSO4 + 6H2O → 3Ca(H2PO4)2 + 3Na2SO4
(NaPO3) 6 + 9CaCO3 + 6H2O → 3Na2CO3 + 3Ca3(PO4)2 + 6H2O + 6CO2
CORROSION
Anode: Fe → Fe 2+ + 2e-
Cathode: 1/2 O2 + 2e- + H2O → 2(OH)-
Fe 2+ + 2(OH)- → Fe (OH)2
4Fe (OH)2 + O2 + 2H2O → 4Fe (OH)3
