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Iron and manganese: removal methods, aeration

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.

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.

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.
      water in
         \__
            \__  steps / trays
               \__ -> collecting basin
  1. 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.
  2. 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).
  3. Mechanical aerators: paddles or turbines churn the surface to draw in air.
  4. Packed (forced draft) towers: counter-current air and water through packing, for highest gas removal.

✓ Answer checked by Claude Opus 5.5 (high effort)

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