Break point chlorination with sketch; factors

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.

The dose at point C is the break-point dose. In practice the dose is chosen a little higher so that a free residual of 0.2 to 0.5 mg/l remains. The reactions with ammonia are:

$$NH_3 + HOCl \rightarrow NH_2Cl + H_2O$$ $$2NH_2Cl + HOCl \rightarrow N_2\uparrow + 3HCl + H_2O$$

Factors affecting chlorination

1. Chlorine dose and demand: the dose must exceed the demand (organic matter, ammonia, iron) to leave a residual.
2. Contact time: the longer the contact, the better the kill ($C^n t$ = constant); usually at least 30 min.
3. pH: hypochlorous acid (HOCl) is a much stronger disinfectant than OCl$^-$; at low pH more HOCl is present, so chlorination is most effective at pH 6 to 7.
4. Temperature: higher temperature speeds up the reaction and increases the kill rate; cold water needs a longer contact time or a higher dose.
5. Turbidity and suspended matter: particles shield bacteria and consume chlorine, so clear water is disinfected better.
6. Kind and number of organisms: viruses, cysts and spores are more resistant than bacteria.
7. Nature of chlorine compound: free chlorine is a faster disinfectant than combined chlorine (chloramines).
8. Mixing: proper, quick mixing ensures uniform contact.
9. Ammonia, organic nitrogen and reducing substances in the water, which consume chlorine.