--- title: "Break point chlorination; residual disinfectant" subject: "Water Supply Engineering" chapter: 6 key: "semester_5/water_supply_engineering:bpc-residual-disinfectant" source: https://www.bctengineer.com/q/semester_5/water_supply_engineering/bpc-residual-disinfectant site: BCT Engineering --- # Break point chlorination; residual disinfectant Water Supply Engineering · Chapter 6: Water Treatment - Asked in: 2068 Chaitra, 6 marks - Video: [Breakpoint Chlorination](https://www.youtube.com/watch?v=Auz0cpObjI8) — TheWaterSifu - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_5/water_supply_engineering/bpc-residual-disinfectant - Whole chapter: https://www.bctengineer.com/md/semester_5/water_supply_engineering/chapter-6 ## Question Explain break point chlorination in relation to water supply system. Explain the significance of residual disinfectant. ## Short answer Break-point chlorination adds chlorine beyond the break point to oxidize all demand and leave a free residual (0.2-0.5 mg/L) for distribution. Zones: 1 no residual, 2 chloramine peak, 3 destruction to minimum, 4 free chlorine rise. Residual disinfectant is a safety margin against recontamination, indicates complete disinfection, and is easily measurable. ## Answer #### 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**. ```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 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). ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Break-point chlorination: what is the break point? A. The dose at which the combined residual (chloramines) is at its peak B. The dose at which no chlorine residual is left at all C. The dose at which the free chlorine residual first appears in Zone 1 D. The dose at which the residual falls to its minimum, after which added chlorine stays as free chlorine Answer: D. Past the break point, ammonia and organics are oxidised, so extra chlorine stays as free residual that rises with dose. (Key point: Break point definition) 2. In the break-point curve, what forms in Zone 2 (up to peak a) as the dose increases? A. Chloramines (NH2Cl, NHCl2) from reaction with ammonia and amines, so the combined residual rises B. Free chlorine HOCl, which rises in proportion to the dose C. Nitrogen gas, which makes the residual fall D. Nothing, because the chlorine is all consumed by reducing compounds Answer: A. After the initial demand is met, chlorine combines with ammonia and amines to give chloramines. (Key point: Zone 2 chloramines) 3. Why does the residual fall between peak a and the break point b? A. Free chlorine is being converted back into chloride B. Chloramines are oxidised to nitrogen gas, N2O and nitrate, destroying the combined residual C. Chlorine is absorbed by the turbidity in the water D. The ammonia is being converted into more chloramines Answer: B. More chlorine oxidises the chloramines to N2 and other products, so the residual falls; tastes and odours are worst here. (Key point: Zone 3 fall in residual) 4. Where is the working dose fixed in a water supply? A. At the chloramine peak a B. In Zone 1 C. At or just beyond the break point, leaving a free residual of 0.2 to 0.5 mg/l D. At 15 mg/l always Answer: C. This meets demand, kills pathogens and leaves residual for the network. 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