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Super/break point chlorination; bleaching 20 MLD

Short answer

Break-point chlorination uses just enough chlorine to pass the break point and leave free residual without dechlorination, unlike super-chlorination’s high dose and required dechlorination. Residual chlorine provides a safety margin against recontamination. For20 MLD at0.5 mg/L and35% chlorine, daily bleach=10.0/0.35=28.57 kg.

Differentiate between super chlorination and break point chlorination. What is the significance of residual chlorine? Calculate the daily quantity of bleaching powder required in a treatment plant of capacity 20 million liters per day. Consider chlorine dose of 0.5 mg/l assuming 35% chlorine available in bleaching powder.

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

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).

Daily quantity of bleaching powder

Flow = 20 MLD =20×106= 20\times10^6 l/day; dose = 0.5 mg/l; available chlorine = 35 %.

Chlorine=0.5×20×106 mg/day=10.0 kg/dayBleaching powder=10.00.35=28.57 kg/day\begin{aligned} \text{Chlorine} &= 0.5\times20\times10^6\ \text{mg/day} = 10.0\ \text{kg/day} \\ \text{Bleaching powder} &= \frac{10.0}{0.35} = 28.57\ \text{kg/day} \end{aligned}

Answer: 28.57 kg of bleaching powder per day.

✓ Answer checked by Claude Opus 5.5 (high effort)

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