Break point chlorination; bleaching powder for 2 MLD

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 (Fe$^{2+}$, Mn$^{2+}$, H$_2$S, 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 (NH$_2$Cl, NHCl$_2$). 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 (N$_2$O, NCl$_3$), 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.

How the break point is obtained

Add a series of increasing chlorine doses to equal samples of the water (say 0.5, 1, 1.5, 2, ... mg/l), allow a fixed contact time (say 30 min) and measure the residual chlorine in each (DPD or orthotolidine test). Plot the residual against the dose. The residual first rises, then falls to a minimum, and then rises again. The dose at the minimum is the break point; the working dose is taken at or a little beyond it.

Bleaching powder required

The strength of the bleaching powder is not given, so the usual 30 % available chlorine is assumed. Plant flow = 2 MLD $= 2\times10^6$ l/day; dose = 0.5 ppm (mg/l).

$$\begin{aligned} \text{Chlorine} &= 0.5\ \text{mg/l} \times 2\times10^6\ \text{l/day} = 10^6\ \text{mg/day} = 1.00\ \text{kg/day} \\ \text{Bleaching powder} &= \frac{1.00}{0.30} = 3.33\ \text{kg/day} \end{aligned}$$

Answer: 3.33 kg of bleaching powder per day (assuming 30 % available chlorine).