--- title: "Break point chlorination with sketch; factors" subject: "Water Supply Engineering" chapter: 6 key: "semester_5/water_supply_engineering:bpc-sketch-factors" source: https://www.bctengineer.com/q/semester_5/water_supply_engineering/bpc-sketch-factors site: BCT Engineering --- # Break point chlorination with sketch; factors Water Supply Engineering · Chapter 6: Water Treatment - Asked in: 2078 Kartik, 8 marks - Video: [Break point chlorination l Dr. Avani Pareek l](https://www.youtube.com/watch?v=JgTF4u93v5M) — Science by Avani - 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-sketch-factors - Whole chapter: https://www.bctengineer.com/md/semester_5/water_supply_engineering/chapter-6 ## Question Briefly describe break point chlorination with sketch. What are the affecting factors in the chlorination process? Discuss. ## Short answer Break-point chlorination adds chlorine past the break point to oxidize all demand and leave free residual. The curve shows: zone 1 no residual, zone 2 chloramine peak, zone 3 destruction to minimum, zone 4 free chlorine rise. Key factors: dose, contact time, pH 6-7, temperature, turbidity, organism type, free vs combined chlorine, mixing, and ammonia/organics. ## Answer #### Break-point chlorination **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. The dose at point **C** is the **break-point dose**. In practice the dose is chosen a little higher so that a free residual of 0.2 to 0.5 mg/l remains. The reactions with ammonia are: $$ NH_3 + HOCl \rightarrow NH_2Cl + H_2O $$ $$ 2NH_2Cl + HOCl \rightarrow N_2\uparrow + 3HCl + H_2O $$ #### Factors affecting chlorination 1. **Chlorine dose and demand**: the dose must exceed the demand (organic matter, ammonia, iron) to leave a residual. 2. **Contact time**: the longer the contact, the better the kill ($C^n t$ = constant); usually at least 30 min. 3. **pH**: hypochlorous acid (HOCl) is a much stronger disinfectant than OCl$^-$; at low pH more HOCl is present, so chlorination is most effective at pH 6 to 7. 4. **Temperature**: higher temperature speeds up the reaction and increases the kill rate; cold water needs a longer contact time or a higher dose. 5. **Turbidity and suspended matter**: particles shield bacteria and consume chlorine, so clear water is disinfected better. 6. **Kind and number of organisms**: viruses, cysts and spores are more resistant than bacteria. 7. **Nature of chlorine compound**: free chlorine is a faster disinfectant than combined chlorine (chloramines). 8. **Mixing**: proper, quick mixing ensures uniform contact. 9. **Ammonia, organic nitrogen and reducing substances** in the water, which consume chlorine. ## 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 residual falls to its minimum, after which added chlorine stays as free chlorine B. The dose at which the combined residual (chloramines) is at its peak C. The dose at which no chlorine residual is left at all D. The dose at which the free chlorine residual first appears in Zone 1 Answer: A. 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. Free chlorine HOCl, which rises in proportion to the dose B. Chloramines (NH2Cl, NHCl2) from reaction with ammonia and amines, so the combined residual rises C. Nitrogen gas, which makes the residual fall D. Nothing, because the chlorine is all consumed by reducing compounds Answer: B. 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. Chlorine is absorbed by the turbidity in the water C. Chloramines are oxidised to nitrogen gas, N2O and nitrate, destroying the combined residual D. The ammonia is being converted into more chloramines Answer: C. 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. Why is chlorination best at pH 6 to 7? A. More OCl- is present, which is stronger B. Chlorine gas escapes at low pH C. Bacteria are more sensitive at high pH D. More of the stronger disinfectant HOCl is present than OCl- Answer: D. HOCl is a much stronger disinfectant than OCl-, and low pH favours HOCl. 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