--- title: "Aeration purposes; Fe/Mn removal; residual chlorine" subject: "Water Supply Engineering" chapter: 6 key: "semester_5/water_supply_engineering:aeration-iron-residual-chlorine" source: https://www.bctengineer.com/q/semester_5/water_supply_engineering/aeration-iron-residual-chlorine site: BCT Engineering --- # Aeration purposes; Fe/Mn removal; residual chlorine Water Supply Engineering · Chapter 6: Water Treatment - Asked in: 2081 Bhadra, 8 marks - Video: [12 - Iron & Manganese](https://www.youtube.com/watch?v=BO3yoE7jJYo) — IV Water Specialists - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_5/water_supply_engineering/aeration-iron-residual-chlorine - Whole chapter: https://www.bctengineer.com/md/semester_5/water_supply_engineering/chapter-6 ## Question List the purposes of aeration of water. Discuss the removal mechanism of iron and manganese from water. Justify the significance of residual chlorine in disinfection of water. ## Short answer Aeration oxidizes iron and manganese, removes CO₂ and H₂S, reduces taste/odour, and adds oxygen. Iron and manganese are removed by oxidation to insoluble precipitates (Fe(OH)₃, MnO₂) via aeration or chemical oxidants, followed by sedimentation/filtration. Residual chlorine maintains a safety margin against recontamination, proves complete disinfection, and allows easy monitoring. ## Answer #### Purposes of aeration 1. To oxidise soluble **iron and manganese** into insoluble precipitates. 2. To remove dissolved **gases** such as CO$_2$, H$_2$S and methane, which cause corrosion, taste and odour. 3. To remove **taste and odour** from volatile and algal substances. 4. To **add oxygen** to oxygen-deficient waters and make them fresh. 5. To reduce free CO$_2$ and so raise the pH and cut corrosion. #### Removal mechanism of iron and manganese Fe and Mn are present in ground water or reservoir bottom water as soluble ferrous and manganous bicarbonates. Aeration adds oxygen and drives off CO$_2$; the oxidation then makes them insoluble: $$ 4Fe(HCO_3)_2 + O_2 + 2H_2O \rightarrow 4Fe(OH)_3\downarrow + 8CO_2 $$ $$ 2Mn(HCO_3)_2 + O_2 \rightarrow 2MnO_2\downarrow + 4CO_2 + 2H_2O\ (\text{slow, pH}>9\text{ or by catalyst}) $$ The precipitates are settled out in a tank and removed by filtration. Where aeration is not enough, **chemical oxidants** (chlorine, KMnO$_4$, ozone), **manganese greensand** filters, **lime softening** or ion exchange are used. #### 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. **Significance** - It is a **safety margin** against re-contamination in service reservoirs and distribution pipes. - Its presence proves that the chlorine demand has been met and the disinfection is complete. - It is easy to measure (orthotolidine or DPD test) at any point, so it is a quick check of the plant performance. - It continues to kill bacteria that enter through leaks or back-siphonage. - Too high a residual gives a taste and may produce trihalomethanes, so it is limited (the WHO health-based guideline is 5 mg/l, and Nepal's NDWQS requires 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. Which is NOT a purpose of aeration? A. To oxidise iron and manganese B. To kill pathogens by leaving a chlorine residual C. To remove gases like CO2 and H2S D. To add oxygen to deficient water Answer: B. Aeration deals with gases, oxygen and Fe/Mn; killing bacteria is disinfection. (Key point: Purposes of aeration) 2. How does aeration remove iron? A. Air converts insoluble iron to soluble ferric salts B. Aeration adsorbs iron on the air bubbles and carries it off as gas C. Oxygen oxidises soluble ferrous bicarbonate to insoluble Fe(OH)3, which is settled and filtered D. Aeration lowers pH so iron dissolves and escapes Answer: C. Fe(HCO3)2 plus O2 gives Fe(OH)3 precipitate, which is then removed. (Key point: Iron removal mechanism) 3. What is residual chlorine? A. The total chlorine added to water B. The chlorine consumed by organic matter C. The chlorine in the bleaching powder D. Chlorine left after the demand is satisfied and contact time has passed Answer: D. Residual = dose minus demand, kept at about 0.2 to 0.5 mg/l. (Key point: Residual chlorine meaning) 4. Why is a residual maintained in the distribution system? A. It is a safety margin that kills bacteria entering through leaks or back-siphonage B. It removes hardness in the pipes C. It removes iron from the water D. It lowers turbidity in the pipes Answer: A. A residual protects against re-contamination after treatment. (Key point: Significance of residual) ## Related in Water Supply Engineering - [Iron and manganese: removal methods, aeration](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Airon-manganese-objectionable-aeration) — Water Supply Engineering, chapter 6 - [Removal of iron and manganese](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Airon-manganese-removal) — Water Supply Engineering, chapter 6, asked 3× - [Removal mechanism of permanent hardness](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Apermanent-hardness-removal) — Water Supply Engineering, chapter 6, asked 2×