Test: Disinfection - Civil Engineering (CE) MCQ

Treatment with chlorine:
(i) Chlorine is powerful oxidising agent, which when dissolved in water yields the following reaction:
At pH < 5, chlorine does not react with water and remains as free chlorine or unreacted chlorine.

Reaction of chlorine with water

(ii) (HOCL + OCl- and Cl2) are combinedly called freely availabe chlorine.
(iii) Out of these forms freely available chlorine hypochlorous acid HOCl is most destructive. It is 80% more effective than OCl- ion. Hence, pH of water should be maintained slightly below 7.
(iv) Moreover, chlorine will immediately react with ammonia in water to form chloramines.

 [ From the reaction it is clear that Trichloroamine is formed when pH is less than 4.4]
Additional Information
(v) The chloramines are the combined residuals. They more stable than free residuals but less effective as disinfectant and remains in water for greater duration.
(vi) In the usual chlorine treatment, in which pH is kept slightly less than 7, diachloramine (NHCl₂) is most predominant.

Concept:
When alum is added in water, it reacts with alkalinity present in water and leads to formation of sticky gelatinous precipitate of Aluminium hydroxide which attracts fine suspended impurities in water over its surface and gets easily settled in the following sedimentation process.
The following typical chemical reaction takes place:

From above chemical reaction following may be concluded:

• The carbon dioxide is formed which further reacts with and leads to formation of carbonic acid and hence, reduces the pH of water.
• The bicarbonate hardness or temporary hardness (i.e. Ca (HCO₃)₂) is removed by alum, however, non-carbonate hardness (CaSO₄) is introduced by alum.

Concept:
A disinfectant is a chemical that kills the disease-causing micro-organisms.
The most commonly used chemicals as primary disinfectants are chlorine, chlorine dioxide and ozone. Among them Chlorine is the most widely used primary disinfectant throughout the world.

Properties of a good disinfectant:
1. It should be persistent enough to prevent the regrowth of organisms in the distribution system so that the water will be safe from future recontamination.
2. It must be toxic to micro-organisms at concentrations well below the toxic thresholds of humans and higher animals so that it cannot negatively affect the animals or humans.
3. It should have a fast rate of kill of micro-organisms.
4. It should be active and stable and environmentally safe.
5. It should not leave damage and odour.

Alum is added in water in order to remove the very fine suspended particles in water by the process of coagulation and sedimentation. It also removes phosphorous from water.
Lime is added in water to lower the hardness of water. It is generally removed the temporary hardness i.e. hardness due to multivalent cations of carbonate and bicarbonate ions in water.

Lime-Soda Water Softening Process:

• Lime softening (also known as lime buttering, lime-soda treatment, or Clark's process) is a type of water treatment used for water softening, which uses the addition of limewater (calcium hydroxide) to remove hardness (deposits of calcium and magnesium salts) by precipitation.
• Soda lime is a process used in water treatment to remove Hardness from water. This process is now obsolete but was very useful for the treatment of large volumes of hard water.
• The addition of lime (CaO) and soda Na₂Co₃ to the hard water precipitates calcium as the carbonate, and magnesium as its hydroxide. The amounts of the two chemicals required are easily calculated from the analysis of the water and stoichiometry of the reactions.
• The lime-soda uses lime, Ca(OH)₂, and soda ash, Na₂Co₃ to precipitate hardness from the solution. Carbon dioxide and carbonate hardness (calcium and Magnesium bicarbonate) are complexed by lime. In this process, Calcium and Magnesium ions are precipitated by the addition of lime (Ca(OH)₂) and soda ash Na₂Co₃

Concept:
A disinfectant is a chemical that kills the disease-causing micro-organisms.
The most commonly used chemicals as primary disinfectants are chlorine, chlorine dioxide and ozone. Among them Chlorine is the most widely used primary disinfectant throughout the world.
Properties of a good disinfectant:
1. It should be persistent enough to prevent the regrowth of organisms in the distribution system so that the water will be safe from future recontamination.
2. It must be toxic to micro-organisms at concentrations well below the toxic thresholds of humans and higher animals so that it cannot negatively affect the animals or humans.
3. It should have a fast rate of kill of micro-organisms.
4. It should be active and stable and environmentally safe.
5. It should not leave damage and odour.

Alum is added in water in order to remove the very fine suspended particles in water by the process of coagulation and sedimentation. It also removes phosphorous from water.
Lime is added in water to lower the hardness of water. It is generally removed the temporary hardness i.e. hardness due to multivalent cations of carbonate and bicarbonate ions in water.

Plain Chlorination:

• If only treatment is given to water is chlorination it is termed as plain chlorination.
• It removes taste, colour, odour and micro-organism from water.
• It is not done if turbidity of water is beyond permissible limit.
• Normal dose of chlorine in this case is 0.5 mg/l.

Important Points
Types of Chlorination: 

• Plain Chlorination
• Pre-Chlorination
• Post Chlorination
• Double Chlorination
• Super Chlorination
• De-Chlorination

Disinfection  may be defined as the process of destruction or inactivation of harmful micro-organism in water either by physical process or chemical process.
Note:
pH plays an important role during disinfection

• At pH < 5, chlorine does not react with water and remains as free chlorine.
• (HOCL +OCL-) are combined called freely available chlorine. Out of these forms of freely available chlorine, HOCL is most destructive. It is 80% more effective than OCL- ion. Hence pH of water should be maintained slightly below 7
  
  For chemical disinfection of water the following disinfectants can be used:
• Chlorine (Cl₂)
• Chlorine dioxide (ClO₂)
• Hypo chlorite (OCl^-)
• Ozone (O₃)
• Halogens: bromine (Br₂), iodene (I)
• Metals: copper (Cu²⁺), silver (Ag⁺)
• (KMnO₄)
• Alcohols
• Soaps and detergents
• Hydrogen peroxide
• Several acids and bases

For physical disinfection of water the following disinfectants can be used:

• Ultraviolet light (UV)
• Electronic radiation
• Gamma rays
• Sounds
• Heat 

Concept:
In the process of chlorination chlorine reacts with water to form Hypochlorous acid as given in the below equation.

At pH < 5, chlorine does not react with water and remains in elemental or molecular chlorine form.

Other important points:
Effectivity in disinfection-
ClO₂> HOCl> Chloroamines> Hypochlorite ion (OCl^-)
Some of the dechlorinating agents are -

• Na₂S₂O₃ (Sodium thiosulphate)
• Na₂S₂O₅ (Sodium metabisulphate)
• Na₂HSO₃ (Sodium bisulphide)
• SO₂ and Activated carbon