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Break point chlorination; bleaching powder 500000 L

Short answer

Break-point chlorination adds sufficient chlorine to oxidize all chlorine-demanding substances and leave a free residual; the break point is the dose where residual reaches its minimum before rising. For 500,000 L/day at 2.00 mg/L dose with 35% available chlorine, chlorine needed = 1.00 kg/day, so bleaching powder = 2.86 kg/day.

Explain break point chlorination. Calculate the required quantity of commercial bleaching powder for the disinfection of water in rural water supply schemes: (i) Chlorine content in the commercial bleaching powder = 35%; (ii) Dose of chlorine = 2.00 mg/l; (iii) Water demand per day = 500000 liters.

Break-point chlorination

Break-point chlorination is the addition of chlorine to water in a quantity large enough to oxidise all the chlorine-demanding substances (reducing agents, ammonia, organic matter) so that a free residual chlorine is left. The dose at which the residual falls to its minimum and then starts to rise again as free chlorine is the break point.

 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

  1. Zone 1 (O to A): the chlorine first reacts with reducing compounds (Fe2+^{2+}, Mn2+^{2+}, H2_2S, organic matter) and is used up. No residual appears.
  2. Zone 2 (A to B): once this demand is met, chlorine reacts with ammonia and amines to form chloramines (NH2_2Cl, NHCl2_2). The combined residual rises to a peak at B.
  3. Zone 3 (B to C): further chlorine oxidises the chloramines to nitrogen gas and other products (N2_2O, NCl3_3), so the residual falls to a minimum at C. Tastes and odours are most noticeable in this zone.
  4. Zone 4 (beyond C): all the ammonia and oxidisable organic matter have been destroyed; the added chlorine remains as free chlorine (HOCl and OCl−^-), and the residual rises in direct proportion to the dose.

Quantity of bleaching powder

Water demand = 500 000 l/day =0.5= 0.5 ML/day =500= 500 m³/day. Dose = 2.00 mg/l. Available chlorine = 35 %.

Chlorine required=2.0 mg/l×500 000 l/day=1 000 000 mg/day=1.00 kg/dayBleaching powder=1.000.35=2.86 kg/day\begin{aligned} \text{Chlorine required} &= 2.0\ \text{mg/l} \times 500\,000\ \text{l/day} = 1\,000\,000\ \text{mg/day} = 1.00\ \text{kg/day} \\ \text{Bleaching powder} &= \frac{1.00}{0.35} = 2.86\ \text{kg/day} \end{aligned}

Answer: 2.86 kg of bleaching powder per day (about 2.9 kg/day).

✓ Answer checked by Claude Opus 5.5 (high effort)

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