--- title: "Break point chlorination; bleaching powder 500000 L" subject: "Water Supply Engineering" chapter: 6 key: "semester_5/water_supply_engineering:bleaching-c71" source: https://www.bctengineer.com/q/semester_5/water_supply_engineering/bleaching-c71 site: BCT Engineering --- # Break point chlorination; bleaching powder 500000 L Water Supply Engineering · Chapter 6: Water Treatment - Asked in: 2071 Chaitra, 8 marks - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_5/water_supply_engineering/bleaching-c71 - Whole chapter: https://www.bctengineer.com/md/semester_5/water_supply_engineering/chapter-6 ## Question Explain break point chlorination. Calculate the required quantity of commercial bleaching powder for the disinfection of water in rural water supply schemes: (i) Chlorine content in the commercial bleaching powder = 35%; (ii) Dose of chlorine = 2.00 mg/l; (iii) Water demand per day = 500000 liters. ## Short answer Break-point chlorination adds sufficient chlorine to oxidize all chlorine-demanding substances and leave a free residual; the break point is the dose where residual reaches its minimum before rising. For 500,000 L/day at 2.00 mg/L dose with 35% available chlorine, chlorine needed = 1.00 kg/day, so bleaching powder = 2.86 kg/day. ## 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. #### Quantity of bleaching powder Water demand = 500 000 l/day $= 0.5$ ML/day $= 500$ m³/day. Dose = 2.00 mg/l. Available chlorine = 35 %. $$ \begin{aligned} \text{Chlorine required} &= 2.0\ \text{mg/l} \times 500\,000\ \text{l/day} = 1\,000\,000\ \text{mg/day} = 1.00\ \text{kg/day} \\ \text{Bleaching powder} &= \frac{1.00}{0.35} = 2.86\ \text{kg/day} \end{aligned} $$ **Answer: 2.86 kg of bleaching powder per day (about 2.9 kg/day).** ## 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. What is the daily chlorine needed? A. 10 kg/day B. 2 mg/l x 500,000 l = 1.0 kg/day C. 0.1 kg/day D. 2.86 kg/day Answer: B. 1,000,000 mg = 1 kg. (Key point: Chlorine required) 4. What is the bleaching powder per day at 35 % chlorine? A. 1.0 x 0.35 = 0.35 kg/day B. 1.0 / 35 = 0.029 kg/day C. 1.0 / 0.35 = 2.86 kg/day D. 3.5 kg/day Answer: C. Divide the chlorine mass by the available fraction. 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