1. Water Treatment Sources of water
1.Surface water- rivers, lakes, reservoirs etc.
2.Underground water – wells and springs
3.Rain water
4. Sea water

2. Surface water
River water – dissolved minerals
Cl-, SO42-, HCO3- of
Na+, Mg2+, Ca2+ and Fe2+
suspended impurities- Organic matter, sand, rock
composition is NOT constant – dep on the contact with soil.
Lake water: High in organic and less in minerals.
composition is constant.

3. Sea water – very impure; too saline for industrial use except cooling
Rain water – pure form
dissolved organic and inorganic particles and dissolved industrial gases CO2, NO2,SO2 etc
Underground water- free from organic impurities due to filtering action of the soil
Sea water – very impure; too saline for industrial use except cooling

4. Impurities in water Suspended impurities
inorganic (clay, sand) organic (oil,plant, and animal matter)
Colloidal impurities- finely divided silica and clay
Dissolved impurities – salts and gases
Microorganisms – bacteria, fungi and algae

5. Hardness of water Hardness prevents the lathering of soap.
due to the presence of salts of Ca, Mg, Al, Fe and Mn dissolved in it.
Soap – Na or K salts of long chain fatty acids C17H35COOH
2C17H35COONa + CaCl2 → (C17H35COO)2Ca↓ + 2NaCl
2C17H35COONa + MgSO4 → (C17H35COO)2Mg↓ + Na2SO4

6. The Cleansing Action of Soap
12.8

7. Hard Water Hard Water Soft Water Soft Water
Does not produce lather with soap
Contains Ca and Mg salts
Soap is wasted and cleaning quality is depressed
Boiling point elevated, more time and fuel for cooking
Hard Water
Does not produce lather with soap
Contains Ca and Mg salts
Soap is wasted and cleaning quality is depressed
Boiling point elevated, more time and fuel for cooking
Soft Water
Produces lather easily with soap
Does not contain dissolved Ca and Mg salts
Cleaning quality of soap not depressed.
Less fuel and time required for cooking
Soft Water
Produces lather easily with soap
Does not contain dissolved Ca and Mg salts
Cleaning quality of soap not depressed.
Less fuel and time required for cooking

8. Types of Hardness
Temporary Hardness- caused by dissolved bicarbonates of Ca and Mg
Also known as ‘alkaline or carbonate hardness’
Permanent Hardness – dissolved Cl- and SO42- of Ca, Mg, Fe and Al etc

9. Temporary Hardness caused by dissolved bicarbonates of Ca and Mg
Temporary hardness can be removed by boiling of water
Ca(HCO3)2 → CaCO3↓ + H2O + CO2↑
Mg(HCO3)2 → Mg(OH)2↓ CO2↑
Also known as ‘alkaline or carbonate hardness’
Determined by titration with HCl using methyl orange as indicator

10. Permanent Hardness Cannot be destroyed on boiling the water
CaCl2, MgCl2, CaSO4, MgSO4, FeSO4, Al2(SO4)3
Cannot be destroyed on boiling the water
Also known as non-carbonate or non alkaline hardness
non alkaline hardness = Total hardness – alkaline hardness

11. Hard Water Advantages Disadvantages Tastes better
Ca in water helps produce strong teeth and bones
Hard water coats lead pipes with layer of insoluble CaCO3, preventing any poisonous lead dissolving in drinking water
Disadvantages
no taste, produces scum with soap
Boiler feed water should be free from hardness or even explosions can occur

12. Degree of Hardness
Hardness is expressed as equivalent amount (equivalents) of CaCO3
Reason: Molar mass is exactly 100, and is the most insoluble salt that can be precipitated in water treatment.
Equvalents of CaCO3 =
( mass of hardness producing substance in mg/L) x100 / (eq.wt of substancex2)
units – mg/L = ppm
parts of CaCO3 equivalents in hardness in 106 parts of water

13. Equivalent weight Eq. wt = Molar mass/ no of charge on ion CaCO3 MM/2
NaCl MM/1
AlCl3 MM/3
Al2(SO4)3 MM/6

14. Example 1:
A water sample contains 408 mg of CaSO4 per liter. Calculate the hardness in terms of CaCO3 equivalents
Hardness = mg/L of CaSO4 x 100/MM(CaSO4 )
= 408 mg/L x 100/136
= 300 mg/L = 300 ppm

15. Example 2
How many grams of MgCO3 dissolved per liter gives 84 ppm of hardness?
Hardness = mg/L of MgCO3 x 100/MM(MgCO3)
84 ppm = ppm of MgCO3 x 100/84
ppm of MgCO3 = 84 ppm x (84/100)
= ppm
= 71 mg/L

16. Calculation of hardness caused by each ion.
Na mg/L Ca mg/L
Mg mg/L Sr mg/L
Al mg/L
Equvalents of CaCO3 =
( mass of hardness producing substance in mg/L) x100 / (eq.wt of substancex2)
Cation Eq.wt Hardness
Ca /
Mg /
Sr /
Al /
Total hardness = 82.5 ppm

17. Potable Water (Drinking water)
Colorless and odorless; good in taste
Turbidity should be less than 10 ppm
No objectionable dissolved gases like H2S
or minerals such as Pb, As , Cr, Mn salts.
Alkalinity should not be high; pH 7.0 – 8.5
Total hardness less than 500 ppm
Free of harmful microorganisms.
Cl-, F-, and SO42– less than 250, 15 and
250 ppm, respectively

18. Methods of disinfection of water
1. Bleaching powder (CaOCl2)
CaOCl2+H2O → Ca(OH)2 + HCl + HOCl
Enzymes of microorganism get deactivated by HOCl
Excess imparts bad taste and smell
Not stable during storage
Introduces Ca to water and thus increases hardness

19. 2. Chlorination Commonly used disinfectant in water
used directly as a gas or conc. solution.
It produces HOCl, a powerful germicide.
ppm chlorine is sufficient

20. 3. Disinfection by ozone O3 → O2 + O
oxygen atom is a powerful oxidizing agent.
2 – 3 ppm is injected
10 – 15 min contact time
Expensive method

21. Alkalinity The capacity of water accept H+ is called alkalinity
The basic species responsible are
HCO3- + H+ → H2O
CO H+ → HCO3-
OH- + H+ → H2O
Different from basicity; high pH
pH is an intensity factor
alkalinity is a capacity factor
1.00x10-3 M NaOH - pH=11;neutralize 1.00x10-3 mole acid
0.100 M NaHCO3 - pH = 8.34, mole acid

22. Alkalinity of water is attributed to presence of
i. caustic alkalinity (due to OH- and CO32- ions)
ii. Temporary hardness (due to HCO3- ions)
i. [OH-] + [H+] → H2O P M
ii. [CO32-] + [H+] → [HCO3- ] P M
iii. [HCO3- ] + [H+] → H2O + CO M
P = OH- + ½ CO32-
M = OH- + CO32- + HCO3-

23. Biological oxygen demand (BOD)
BOD is the quantity of dissolved O2 required by aerobic bacteria for oxidation of organic matter under aerobic conditions.
source of effluent BOD(ppm)
Domestic sewage
Cow shed sewage
Paper mill
BOD indicator of organic pollutants

24. Chemical oxygen demand (COD)
Defined as the oxygen consumed in the oxidation of organic and oxidizable inorganic matter.
Use a strong oxidizing agent like K2Cr2O7
COD > BOD (O2 is a weak oxidizing agent)
COD test does not differentiate between bio-inert and bio degradable materials