--- title: "Super vs break point chlorination; residual" subject: "Water Supply Engineering" chapter: 6 key: "semester_5/water_supply_engineering:super-vs-break-point-chlorination" source: https://www.bctengineer.com/q/semester_5/water_supply_engineering/super-vs-break-point-chlorination site: BCT Engineering --- # Super vs break point chlorination; residual Water Supply Engineering · Chapter 6: Water Treatment - Asked in: 2070 Chaitra, 8 marks - Video: [Breakpoint chlorination/Disinfection method in drinking water treatment](https://www.youtube.com/watch?v=LqHrxoGHbNk) — Revathi Purushothaman - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_5/water_supply_engineering/super-vs-break-point-chlorination - Whole chapter: https://www.bctengineer.com/md/semester_5/water_supply_engineering/chapter-6 ## Question 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? ## 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. ## Answer #### Super-chlorination and break-point chlorination | Point | Super-chlorination | Break-point chlorination | |---|---|---| | Dose | Very high (3 to 15 mg/l), much above the demand | Just enough to pass the break point (typically 1 to 3 mg/l) | | Aim | Quick kill in an emergency or with short contact time | Destroy ammonia, organics, taste and odour, and obtain a free residual | | Residual | Large; the excess is removed by de-chlorination | Free residual of 0.2 to 0.5 mg/l | | De-chlorination | Always required | Not required | | Use | Epidemics, new or repaired mains, very poor water | Routine for water with ammonia or organic load | | Cost | Higher (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**. ```text 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 (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 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). ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. How do the doses of the two methods differ? A. Break-point uses 3 to 15 mg/l; super uses 1 to 3 mg/l B. Both use identical doses C. Super uses no chlorine D. Super-chlorination uses a very high dose (3 to 15 mg/l); break-point just passes the break point (1 to 3 mg/l) Answer: D. Super-chlorination far exceeds the demand. (Key point: Dose difference) 2. Which one always needs de-chlorination? A. Super-chlorination, to remove the excess chlorine B. Break-point chlorination C. Neither D. Both equally Answer: A. Super-chlorination leaves a large residual removed with SO2 or thiosulphate. (Key point: De-chlorination) 3. Break-point chlorination: which form is present beyond b? A. Chloramines at their peak B. Free available chlorine (HOCl and OCl-), rising with dose C. No residual at all D. Nitrogen gas as residual Answer: B. After the break point all demand is met and added chlorine stays free. (Key point: Forms in zones) 4. What is a key significance of residual chlorine? A. It softens water B. It removes taste C. It acts as a safety margin against re-contamination and shows disinfection is complete D. It replaces the need for filtration Answer: C. A measurable residual proves demand was met and continues protecting the network. (Key point: Significance of residual) ## Related in Water Supply Engineering - [Super/break point chlorination; bleaching 20 MLD](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Ableaching-a76) — Water Supply Engineering, chapter 6 - [Break point chlorination; residual disinfectant](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Abpc-residual-disinfectant) — Water Supply Engineering, chapter 6 - [Break point chlorination; bleaching powder 500000 L](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Ableaching-c71) — Water Supply Engineering, chapter 6 - [Break point chlorination; bleaching powder for 2 MLD](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Ableaching-c69) — Water Supply Engineering, chapter 6 - [Break point chlorination; pH and temperature effects](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Abpc-ph-temperature-hocl) — Water Supply Engineering, chapter 6, asked 2× - [Break point chlorination with sketch; factors](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Abpc-sketch-factors) — Water Supply Engineering, chapter 6