Treatment for Domestic Use

Treatment for domestic use

Requirements for drinking water #

1. Clear and Odourless and devoid of unpleasant taste.
2. Devoid of heavy metals and arsenic
3. Free from pathogenic microorganisms
4. pH~8
5. Suspended matter<10ppm
6. Dissolved salts<500ppm

Natural water treated for domestic use #

1. Aeration of raw water: passing compressed air to remove odour
2. Sedimentation
   1. Settling in a large tang
   2. Adding coagulants (in the form of solution) for colloidal particles
      1. Alum: K₂SO₄
      2. Green vitriol: FeSO₄
3. Precipitates contain
   1. Aluminium, ferrous, ferric hydroxides
   2. Bacteria
4. Partially clarified water is filtered through sand gravity filters: rectangular tanks which contain fine layers of sand

Sterilization #

1. By adding bleaching powder #

1kg/1000kl of powder and treated water should contain <0.2ppm Cl

• CaOCl₂+H₂O → Ca(OH)₂ + Cl₂
• Cl₂ + H₂O → HCl + HOCl HOCl kills the bacteria

Advantage #

1. Effective, economical
2. Does not make water hard
3. Required little storage

Disadvantage #

1. Imparts unpleasant taste and odour
2. Lowers pH below 6.5 and causes irritation to mucus membrane

2. Breakpoint chlorination #

When liquid chlorine is added or gas is passed

1. Available Cl decreases: consumption of it in oxidizing some organic matter
2. Increases due to decomposition of chloro-organic compounds Free chlorine: destruction of bacteria. Breakpoint: point at which residual chlorine begins to appear

Advantages #

1. Organic matter is oxidized⇒Odour-free
2. Pathogenic bacteria are destroyed
3. Prevents growth of weed in water

3. Chloramine #

Cl+NH₃ mixed in 2:1 by volume

• Cl₂ + NH₃ → ClNH₂ + HCl
• ClNH₂ + H₂O → HOCl + NH₃
• HOCl → HCl + [O]

[O] nascent oxygen also has bactericidal properties.

4. Ozonation #

Ozone is unstable and gives [O]

Advantage #

1. Sterilization, bleaching, decolorization and deodourazation take place at the same time
2. Does not change pH
3. Time of contact is only 10-15min

Disadvantage #

1. Costly

Desalination of Brackish Water #

Water containing dissolved salts and having a very salty taste is called brackish water. Eg: sea water

1. Electrodialysis #

By applying current and employing pair of plastic membranes through which ions can pass

• Cathode(-): Attracts +ve
• Anode(+): Attracts -ve

2. Reverse Osmosis #

Osmosis: Flow of solvent from dilute to concentrated solution through a semipermeable membrane. Reverse osmosis: flow of solvent in oppositie direction. (Done by applying pressure)

Advantages #

1. Removal of ionic, non-ionic, colloidal solutes
2. Renegeneration is easy: replacement of semipermeable membrane
3. Economical
4. Continuous supply of large volumes of water

Usually used for purification of sea water for domestic use.