--- title: "Iron and manganese: removal methods, aeration" subject: "Water Supply Engineering" chapter: 6 key: "semester_5/water_supply_engineering:iron-manganese-objectionable-aeration" source: https://www.bctengineer.com/q/semester_5/water_supply_engineering/iron-manganese-objectionable-aeration site: BCT Engineering --- # Iron and manganese: removal methods, aeration Water Supply Engineering · Chapter 6: Water Treatment - Asked in: 2076 Chaitra, 1+2+5 marks - Video: [How to Remove Iron & Manganese from Water (Complete Guide)](https://www.youtube.com/watch?v=IwXINW7F9qc) — WATERMAN - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_5/water_supply_engineering/iron-manganese-objectionable-aeration - Whole chapter: https://www.bctengineer.com/md/semester_5/water_supply_engineering/chapter-6 ## Question Why is a high content of iron and manganese objectionable in drinking water? List methods of removal of iron and manganese from water. Describe briefly the various methods commonly used for aeration of water. ## Short answer High iron and manganese impart metallic taste, stain laundry and fixtures, support slime-forming bacteria, and reduce pipe capacity. Removal methods include aeration–filtration, chemical oxidation (chlorine, permanganate, ozone) followed by filtration, greensand, lime softening, and ion exchange. Common aeration methods are cascade trays, spray nozzles, bubble diffusion, mechanical surface aerators, and packed towers. ## Answer #### Why high iron and manganese are objectionable - They give a **bitter, metallic taste** to water, tea and food. - They **stain** clothes, utensils and sanitary fittings (reddish-brown for Fe, black for Mn). - They support **iron bacteria** that form slime and clog pipes, and cause red or black water. - They reduce pipe capacity and foul zeolite softeners, boilers and industrial processes (textile, paper, food). - Recommended limits: Fe 0.3 mg/l and Mn about 0.05 to 0.1 mg/l, above which taste and staining become a problem (Nepal's NDWQS: Fe 0.3 mg/l, Mn 0.2 mg/l). #### Methods of removal 1. Aeration, sedimentation and filtration. 2. Chemical oxidation by chlorine, potassium permanganate or ozone, followed by filtration. 3. Manganese greensand filtration. 4. Lime softening at high pH. 5. Ion exchange. 6. Coagulation at suitable pH. #### Methods of aeration 1. **Gravity aerators**: water falls through the air over **cascade steps** (3 to 6 steps, each 0.2 to 0.3 m high), through **trays** of coke or stones (3 to 5 trays at 0.3 to 0.5 m spacing), or over an inclined plane, so that thin sheets and droplets are exposed to air. ```text water in \__ \__ steps / trays \__ -> collecting basin ``` 2. **Spray aerators (fountain)**: water is forced through nozzles (2.5 to 4 cm) at 70 kPa and sprayed 1 to 3 m into the air as fine droplets, which gives a very large area for gas exchange. 3. **Air diffusion (bubble) aerators**: compressed air is passed through porous plates or perforated pipes at the bottom of a tank (0.3 to 0.75 m³ air per m³ water, 10 to 30 min contact). 4. **Mechanical aerators**: paddles or turbines churn the surface to draw in air. 5. **Packed (forced draft) towers**: counter-current air and water through packing, for highest gas removal. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Which is a problem caused by high iron and manganese? A. Excess soap lather B. Rapid killing of coliform bacteria C. Lower boiler scaling D. Staining of clothes and fittings, bitter metallic taste, and iron bacteria slime that clogs pipes Answer: D. Fe stains reddish-brown and Mn black, and they feed iron bacteria. (Key point: Why Fe and Mn objectionable) 2. Which is a recognised method of removing Fe and Mn? A. Oxidation with chlorine, KMnO4 or ozone followed by filtration, or manganese greensand B. Plain sedimentation alone C. Boiling the water D. Adding more alum only at low pH Answer: A. Oxidation turns soluble Fe/Mn insoluble for filtration; greensand filters also work. (Key point: Removal methods) 3. How do gravity aerators work? A. Compressed air is blown from the bottom of a tank B. Water falls over cascades or through trays, exposing thin sheets and droplets to air C. Paddles churn the water D. Water is passed through a bed of zeolite Answer: B. Cascades, trays and inclined planes give a large water-air contact. (Key point: Gravity aerators) 4. How does a spray (fountain) aerator aerate water? A. Air is diffused through porous plates B. Water trickles down steps of 0.2 m C. Water is forced through nozzles and sprays into the air as fine droplets with a large surface area D. A turbine draws air in at the surface Answer: C. Fine droplets give very large area for gas exchange. (Key point: Spray aerators) ## Related in Water Supply Engineering - [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× - [Aeration purposes; Fe/Mn removal; residual chlorine](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Aaeration-iron-residual-chlorine) — Water Supply Engineering, chapter 6 - [Drinking water standards and WHO guidelines](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Awho-guidelines-standards) — Water Supply Engineering, chapter 4, asked 2× - [Aeration: purpose and methods](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Aaeration-purpose-methods) — Water Supply Engineering, chapter 6, asked 7× ## Related in other subjects - [Role of water; high water content](https://www.bctengineer.com/question.md?key=semester_5%2Fconcrete_technology_and_masonry_structure%3Awater-high-content) — Concrete Technology and Masonry Structure, chapter 1 - [Water quality parameters and Nepal drinking water standards](https://www.bctengineer.com/question.md?key=semester_7%2Fenvironment_and_pollution_control%3Anepal-drinking-water-quality-standards) — Environment and Pollution Control, chapter 4 - [Proposal for drinking water scheme](https://www.bctengineer.com/question.md?key=semester_5%2Fcommunication_english%3Aproposal-drinking-water-scheme) — Communication English, chapter 6 - [Pathogenic pollution in drinking water](https://www.bctengineer.com/question.md?key=semester_7%2Ftechnology_environment_and_society%3Apathogenic-pollution-drinking-water) — Technology, Environment and Society, chapter 5