1. Water and its treatment (3)
Prepared By
Dr. Biswajit Saha

2. Softening of water
The process whereby remove or reduce the hardness of water,
irrespective of whether it is temporary or permanent is termed as
softening of water
External treatment:
Internal treatment:
External treatment: Carry out before its entry into the boiler.
Lime-soda,
Zeolite and
ion-exchange process
Internal treatment: Suitable chemicals are added to the boiler water either to precipitate or to convert scale in to compounds.

3. Internal Treatment Method
1. Colloidal Conditioning:
-Using kerosine, tannin, starch etc
-Get coated over the scale forming particles
-Removed by Blow down Process
2. Phosphate Conditioning:
- Small amount of phosphate ions are added to precipitate Ca ions.
- Chosen depending on the pH conditions of boiler
3CaCl2 + 2Na3PO4 → Ca3(PO4)2 + 6NaCl
Removed by Blow down Process
3. EDTA method

4. Na2 [Na4 (PO3)6]→ 2Na+ + [Na4P6O18]2-
4. Carbonate conditioning:
Na2CO3 is added to precipitate Ca salts as CaCO3, (concentration of CO32- increases and becomes greater than Ksp)
Removed by Blow down Process, Used in low pressure boilers
CaSO4 + Na2CO3→ CaCO3 + Na2SO4
5. Calgon Conditioning:
Scale forming salts are converted into soluble complex.
E.g. Sodium Hexameta Phosphate (Na2PO3)6 is added…reacts with Ca and forms Calcium Hexameta Phosphate (Ca2PO3)6
Prevents Scale formation
Na2 [Na4 (PO3)6]→ 2Na+ + [Na4P6O18]2-
2CaSO4 + [Na4P6O18]2- → [Ca2P6O18]2- + 2Na2SO4
soluble complex

5. External Treatment Method for Water softening
Lime-soda process:
Principle
Chemically converted all the soluble hardness causing impurities into
insoluble Precipitates which may be removed by settling and filtration.
For this purpose, a suspension of calculated amount of lime, Ca(OH)2,
and soda Na2CO3 are added.
Ca(HCO3)2 + Ca(OH)2 = 2 CaCO H2O
Mg (HCO3)2 + 2 Ca(OH)2 = 2 CaCO Mg(OH)2 + 2H2O
(MgCl2 or MgSO4 )+ Ca(OH)2 = Mg (OH)2 + (CaCl2 or CaSO4 )
CaSO4 + Na2CO3 = 2NaSO4 + 2 CaCO3

6. Types of Lime-soda process: Cold Lime –soda process
At the room temperature, the ppt formed are very fine, do not settle
down. So, small amount of Coagulants like [K2SO4.Al2(SO4)3.24H2O],
[NaAlO2] to make coarse ppt. They hydrolysis to Hydroxide and
entraps the fine ppt like CaCO3, Mg(OH)2.
NaAlO2 +2H2O= NaOH + Al(OH)3
Types of Lime-soda process:
Cold Lime –soda process
Hot lime –soda Process
What are the differences between them??

7. Advantages of soda lime process:
It increases the pH of the softened water thereby reducing corrosion of feed and distribution pipes.
The process is economical.
The microbial load particularly that of pathogenic organisms, is considerably reduced in the softened water.
DISADVANTAGES:
Inability to soften water to less than ppm.
Generation of large amount of sludge
Requires well trained and skilled operators.

8. External Treatment Method: Water softening Method
Zeolite (or Permutit) Process: In this process, the raw (hard) water is allowed to percolate through a bed of zeolite (Na2Z) packed in a vertical cylindrical tank. The zeolite bed retains the Ca2+ and Mg2+ ions from hard water by exchanging with Na+ ions thereby the out flowing water contains sodium salts. Zeolite is Hydrated sodium alumino Silicate Na2O. Al2O3 XSiO2 YH 2O (X= 2-10, Y= 2-6 )
Na2Z + (Ca2+/Mg2+)X = (Ca2+/Mg2+) Z + Na2X

9. Process:
Higher exchange capacity and porous in Nature.
Consist of Steel Tank
Having Thick Layer of Zeolite
When water pass through it hardness causing ions (Ca, Mg) are retained by Zeolite.

10. Chemical Reaction:
Ca(HCO3) 2 + Na2 Ze → CaZe + 2 NaHCO3
CaSO4 + Na2Ze → CaZe + Na2SO4
MgCl 2 + Na2Ze → MgZe + 2NaCl
Mg (NO3)2 + Na2Ze → MgZe + 2NaNO3
Exchange of Na ions continues until Na ions are exhausted
Regeneration:
CaZe + 2NaCl → Na2Ze + CaCl 2
MgZe + 2NaCl → Na2Ze + MgCl 2
CaCl 2 and MgCl 2 led to drain and Na2Ze can be reused.

11. Advantages
Hardness can be completely removed
Process can be made automatically
Easy operation. No experts required
Less time and sludge
Small area requires
Disadvantages
Only Ca+ and Mg+ ions can be removed
Large amount of Na ions present in treated water.
Leaves other acids which causes corrosion
Fe 2+ and Mn 2+ containing water can not be treated because Fe and Mn Zeolite can not be regenerated
Water should be free from suspended impurities to prevent clogging on Zeolite beds.

12. B. Ion Exchange (or) Demineralization (or) Deionization Process
Complex organic compound is called resins.
Resins act as ion-exchangers and removed all cations and anions from hard water.
Resins are cross link polymers
Cation exchange resins: exchange cations present in hard water with H+. They possess acidic groups such as –COOH, -SO3H etc
Anion exchange resins: exchange anions present in hard water with OH-. They possess basic groups such as OH-, NH2- etc

15. Advantage:
Produces soft water
Can treat highly acidic or alkaline water
Regeneration of ion resins are possible
Disadvantage:
Cost of equipment is high
Highly turbid water can not be treated
Expensive chemicals are required.

16. Desalination
Processes that remove some amount of salt and other minerals from
Water.
Brackish Water: contain 3.5 % salts & salty taste. Unfit for drinking
Separation of salts from water by evaporation followed by condensation.
Commonly used methods:
Electrodialysis
Reverse Osmosis

17. Semi permeable membranes are placed
A. Electrodialysis:
Method of separation of ions from salt water by passing electric current.
Semi permeable membranes are placed
Consist of three compartments containing
1. Sea Water 2. Pair of electrodes Semi permeable membrane

18. As current applied Na+ ions moves towards cathode and Cl- moves towards anode
As result brine concentration decreases in the middle compartment.
Pure water is removed from the central compartment.
Conc. Brines are replaced by fresh brine water.
Much more effective separation Ion selective membranes are used

19. Reverse Osmosis (or) Super Filtration (or) Hyper Filtration
Movement of solvent molecule from concentrated side to diluted side.
If only pressure higher than that of osmotic pressure
Membranes:
Cellulose Acetate, Polymide,
Polymethylmethaacrylate
Thickness:
to µm
Advantages:
Removes both ionic and non ionic and colloidal matters, SiO2
Maintenance cost is low
Membrane replacement can be done with in few minutes.
Raisin