Aeration Tank Design for Small-Scale Water Plants

1.0 Design Basis and Objectives

• Treatment Objectives:Primarily remove ironand manganesefrom groundwater or surface water, as well as partial ammonia nitrogenand volatile organic compounds(e.g., hydrogen sulfide odor).

• Source Water Quality:Assumed to be groundwater or reservoir bottom water.

  • Iron (Fe²⁺): 2 - 5 mg/L

  • Manganese (Mn²⁺): 0.5 - 1.5 mg/L

  • Ammonia Nitrogen (NH3-N): 0.5 - 2.0 mg/L

  • Dissolved Oxygen (DO): < 2 mg/L

• Effluent Quality:Meets the pre-treatment requirements of the "Standards for Drinking Water Quality" (GB 5749-2022).

  • Iron < 0.3 mg/L, Manganese < 0.1 mg/L

  • Dissolved Oxygen increased to near saturation level, creating conditions for subsequent filtration.

• Core Principle:Through aeration, oxygen is dissolved into the water to achieve:

  1. Oxidation:Oxidize soluble ferrous iron (Fe²⁺) into insoluble ferric iron (Fe³⁺), forming flocs; and oxidize divalent manganese (Mn²⁺) into manganese dioxide (MnO₂)precipitate.

  2. Stripping:Remove gases like CO₂ and H₂S from the water, increasing the pH value and promoting oxidation reactions.

  3. Biological Action:Provide oxygen for microorganisms in subsequent filters responsible for removing iron, manganese, and ammonia nitrogen.

2.0 Recommended Aeration Process Options

For small-scale plants, two economical and practical aeration processes are recommended:

Option 1: Perforated Pipe Aeration + Cascade Aeration (Contact Oxidation Method)

This is the most classic, reliable, and lowest-investment option, especially suitable for small plants focused primarily on iron and manganese removal.

The flowchart below illustrates the complete process of this combined approach:

flowchart TD
    A[Raw Water] --> B[Feed Pump];
    B --> C[Cascade Aeration Tower<br>Preliminary Aeration/Stripping];
    C --> D[Perforated Pipe Aeration Tank<br>Main Oxidation Reaction Zone];
    D --> E[Flocculation Tank<br>PAC/PAM];
    E --> F[Sedimentation Tank];
    F --> G[Rapid Filter<br>Intercepts Precipitates];
    G --> H[Clear Water Tank<br>Disinfection];

Step-by-Step Process Explanation:

1. Cascade Aeration Tower:

   • Design:Raw water is pumped to an elevated tower and then free-falls via a weir into a lower basin, with a typical drop of 0.5-1.5 meters.

   • Function:Utilizes intense water-air contact during the fall for preliminary oxygenation and strips out gases like CO₂ and H₂S. Simple structure, no power required.

2. Perforated Pipe Aeration Tank (Core Unit):

   • Design:Aeration laterals and mains are laid on the tank bottom; the laterals have densely spaced 3-5mm diameter holes. Air is supplied by a blowerand released as fine bubbles through these holes.

   • Parameters:

     • Aeration Time:15 - 30 minutes (determined by iron/manganese content).

     • Air-to-Water Ratio:0.5:1 - 1.5:1 (cubic meters of air per cubic meter of water).

     • Orifice Velocity:10 - 15 m/s, ensuring fine bubbles.

   • Function:The large number of fine bubbles provides a vast gas-liquid contact area, efficiently dissolving oxygen to complete the primary oxidation process for iron and manganese.

3. Flocculation and Sedimentation:

   • After aeration, the oxidized Fe(OH)₃ is colloidal. A small amount of coagulant (e.g., PAC)is added to form larger flocs.

   • Most flocs are removed in a sedimentation tank (e.g., inclined tube settler) to reduce the load on subsequent filters.

4. Filtration:

   • A rapid filter(with quartz sand media) intercepts remaining fine precipitates. The effluent is disinfected with chlorine before entering the clear water tank.

Option 2: Spray Aeration (Surface Mechanical Aeration)

• Application Scenario:When higher oxygenation efficiency is required, or the raw water has high iron/manganese content.

• Design:Install spray nozzlesor surface aeratorsabove the aeration tank.

  • Spray Aeration:Water is sprayed upwards through nozzles into fine droplets for full air contact.

  • Mechanical Aeration:A high-speed rotating impeller breaks water into spray and draws in air.

• Characteristics:Higher oxygenation efficiency than perforated pipes, but equipment is slightly more complex and consumes power.

3.0 Key Design Parameters and Equipment Selection

4.0 Key Design Considerations and Notes

1. pH Control:The oxidation rate of iron and manganese is highly influenced by pH. Iron oxidizes rapidly when pH > 7.0; manganese oxidation requires pH > 9.0 for a faster rate, often relying on subsequent biological manganese removalin filters. If the raw water pH is low (<6.8), consider adding limeor sodium hydroxideafter aeration to raise the pH.

2. Integration with Subsequent Processes:The aeration tank must be closely coordinated with the filter. If the contact oxidation processis adopted, the sedimentation tank can be omitted, allowing water containing iron/manganese precipitates to flow directly into the filter for deep oxidation and interception via the catalytic film and microorganisms on the filter media, simplifying the process.

3. Sludge Removal Design:Sludge hoppersand drain pipes are needed at the bottom of the aeration and sedimentation tanks for periodic manual or automatic sludge removal.

4. Ventilation and Safety:The aeration room requires good ventilation to prevent moisture damage to electrical equipment. The blower room requires soundproofing treatment.

5.0 Scheme Characteristics Summary

1. Mature and Reliable Technology:The process route is classic, operation and management are simple, making it very suitable for small-scale plants with relatively limited technical resources.

2. Low Investment and Operating Costs:The main equipment is the blower, which has low energy consumption and is easy to maintain.

3. Stable Treatment Effect:Effectively resolves iron and manganese exceedances in groundwater, improving water color and taste.

4. High Flexibility:Aeration volume can be flexibly adjusted based on changes in raw water quality (e.g., seasonal variations).

Note:This is a generic design. For actual projects, process calculations and equipment selection must be based on a detailed full analysis report of the raw water quality. For special water qualities (e.g., very high iron/manganese, high coexisting organics), pilot testingmay be necessary to determine the optimal process parameters.