Chlorination and De-chlorination Process

Ø  Raw water stored in several days  in storage tanks due to microbial growth occurred.

Ø  Raw water basic treatment  is chlorination,Chlorination is most commonly used.

Ø   Chlorination process used chemical sodium hypochlorite (Naocl).

                     NaOCl + H2O → HOCl + NaOH

Ø  When sodium chlorite Dissolved in water is gives to free chlorine that forms hypochlorous acid .

Ø  Hypochlorous acid dissociates  in water to hydrogen ions and hypochlorite ions.

                               HOCL →H⁺ + OCL^-

Ø  Hypochlorous acid  kill the microbial contaminants by disinfection process.

Ø  Chlorination effective against many pathogenic bacteria.

Ø  This prevents the transmission of disease.

Ø  Chlorine is a strong Oxidizing agent.

Ø  Hypochlorous acid easily penetrate  into the negatively charged pathogen cell wall. Due to micro organisms are dies or microorganisms inhibited or can not reproduced.

Ø  Chlorination process “time and concentration of water”plys a significant role in water disinfection .

Ø  5 ppm  concentration of chlorine 15-30 minutes effectively kill the microorganisms.

Volume of Sodium Hypochlorite =

 Volume of water X required concentration in ppm / 1000000 X %age of chlorine in Hypo

= Ex : 100000 liters X 5 ppm / 1000000 X 5.8

= 500000/ 5800000 = 0.09 liters (90 ml)

De-Chlorination :

Ø  Purified water should not contain more than 3 ppm of free chlorine.

Ø  So removel of remains chlorine is necessary to water treatment  is called dechlorination.

Ø  Sodium Thio Sulfate (STS) (Na2 S2O3)  or  Sodium Meta Bi Sulfate (SMBS) (Na2 S2O5) mainly u sed for dechlorination process.

Na₂ S2O₃   + 4 HOCL  + H₂O → 2NaHSO₄ + 4 HCL

Na₂ S2O₅   + 2 HOCL  + H₂O → 2NaHSO₄ + 2 HCL

Ø  Sodium meta bi sulfate (SMBS) widely used for dechlorination treatment.

Ø  It produced to Sodium bi sulfate and hydrochloric acid.