Where are pre- and post-chlorination applied?

Pre before filters or sedimentation; post after filtration before the clear water tank

Pre after filtration; post before sedimentation

What follows super-chlorination?

De-chlorination with SO2, sodium thiosulphate, sodium sulphite or activated carbon

What does the chloramine process give?

A more lasting but weaker residual, suited to long distribution systems

A stronger, quicker kill than free chlorine

When is plain chlorination used?

For already good or well-treated water, with dose 0.5 to 1 mg/l and at least 30 min contact

For highly polluted water in epidemics

2070 Asar · 8 marks

Various forms of chlorination

Short answer

Forms include plain chlorination (chlorine only, dose 0.5-1 mg/L), pre-chlorination before filters, post-chlorination after filtration leaving residual, double (pre+post), break-point to achieve free residual, super-chlorination with high dose and dechlorination, de-chlorination to remove excess, and chloramine process using combined residual for long distribution.

Describe in detail the various forms of chlorination.

Chlorination is the addition of chlorine, as gas, bleaching powder (CaOCl $_2$ , 25 to 35 % available Cl), high-test hypochlorite (HTH) or sodium hypochlorite, to destroy pathogenic organisms. The following forms are used.

Plain chlorination: chlorine is the only treatment given to the water. It is used for fairly clear raw water from protected sources (for example lakes or springs with low turbidity). Dose about 0.5 to 1 mg/l.
Pre-chlorination: chlorine is added before the filters (or before sedimentation) to reduce the load of organisms and algae, control taste and odour, oxidise iron, and help coagulation.
Post-chlorination: the normal disinfection step, applied after filtration, just before the water goes to the clear water tank. The dose is set to leave a residual of 0.2 to 0.5 mg/l.
Double chlorination: pre- and post-chlorination used together; also called two-stage chlorination.
Break-point chlorination: chlorine is added beyond the break point so that a free residual is present (see the curve below).
Super-chlorination: the dose is much higher than the demand (3 to 15 mg/l or more), followed by de-chlorination (using sulphur dioxide, sodium thiosulphate, sodium sulphite or activated carbon). Used during epidemics, after repair of mains, when the contact time is short or when the water is of very poor quality.
De-chlorination: removal of the excess chlorine after super-chlorination.
Chloramine process: ammonia is added with the chlorine to form chloramines, which give a longer-lasting but weaker residual; used in long distribution systems.

Reaction in water:

Cl_2 + H_2O \rightarrow HOCl + HCl, \qquad HOCl \rightleftharpoons H^+ + OCl^-

Break-point curve:

 Residual
 chlorine
   ^
   |                  B
   |                 / \                     /
   |                /   \                  /
   |               /     \               /    free residual
   |              /       \            /      (HOCl + OCl-)
   | combined    /         \         /
   | residual   /           \      /
   |           /             \   /
   |          /               \/
   +----------+-------+-------+----------------->  Chlorine dose
   O          A       B       C
   |  Zone 1  | Zone 2| Zone 3|     Zone 4

A = demand of reducing agents met, B = peak of combined residual (chloramines), C = break point

✓ Answer checked by Claude Opus 5.5 (high effort)

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