Skip to main content

Super vs break point chlorination; residual

Short answer

Super chlorination uses a very high dose (3-15 mg/L) requiring dechlorination for emergencies; break-point chlorination doses just beyond break point (1-3 mg/L) to leave free residual 0.2-0.5 mg/L. Break-point curve zones: 1 no residual, 2 chloramines, 3 destruction at break point, 4 free chlorine. Residual chlorine provides a safety margin against recontamination and indicates complete disinfection.

Differentiate between super chlorination and break-point chlorination. Explain break-point chlorination with a neat sketch explaining the types of chlorine forms available in the various stages of the break-point curve. What is the significance of residual chlorine?

Super-chlorination and break-point chlorination

PointSuper-chlorinationBreak-point chlorination
DoseVery high (3 to 15 mg/l), much above the demandJust enough to pass the break point (typically 1 to 3 mg/l)
AimQuick kill in an emergency or with short contact timeDestroy ammonia, organics, taste and odour, and obtain a free residual
ResidualLarge; the excess is removed by de-chlorinationFree residual of 0.2 to 0.5 mg/l
De-chlorinationAlways requiredNot required
UseEpidemics, new or repaired mains, very poor waterRoutine for water with ammonia or organic load
CostHigher (extra chemicals)Moderate

Break-point chlorination with sketch

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

Chlorine forms at each stage

  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.

In short: in Zone 1 there is no residual; in Zone 2 the residual is mainly combined chlorine (mono- and dichloramines); in Zone 3 the chloramines are destroyed and the residual falls; and beyond C, the free available chlorine (HOCl + OCl−^-) increases with the dose.

Significance of residual chlorine

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).

✓ Answer checked by Claude Opus 5.5 (high effort)

Related questions

All Water Supply Engineering chapter 6 questions →