Break point chlorination; residual disinfectant

Break-point chlorination in water supply

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.

In a water supply system, the chlorine dose is set at or beyond the break point. This meets the chlorine demand of ammonia, organic matter and reducing agents, kills the pathogens, removes tastes and odours, and leaves a free residual of 0.2 to 0.5 mg/l for the distribution system.

Significance of residual disinfectant

Residual chlorine is the chlorine left in water after the demand has been satisfied and the required contact time has passed. A free residual of about 0.2 to 0.5 mg/l is usually maintained at the plant.

Significance

• It is a safety margin against re-contamination in service reservoirs and distribution pipes.
• Its presence shows that the chlorine demand has been met and that disinfection is complete.
• It is easy to measure (orthotolidine or DPD test) at any point, so it gives a quick check of plant performance.
• It continues to kill bacteria that enter through leaks or back-siphonage.
• Too high a residual gives an unpleasant taste and may produce trihalomethanes, so it is limited (WHO guideline maximum 5 mg/l; Nepal's NDWQS 2062 specifies 0.1 to 0.2 mg/l in chlorinated supplies).