Process Design Scheme for Submerged Ultrafiltration Membrane in Municipal Wastewater Treatment

1.0 Design Basis and Objectives

• Treatment Source: Municipal sewage (including partially pretreated combined sewer overflow).

• Influent Quality (Typical Values):

  • CODcr: 300 - 500 mg/L

  • BOD5: 150 - 250 mg/L

  • SS: 150 - 300 mg/L

  • NH3-N: 25 - 40 mg/L

  • TN: 35 - 55 mg/L

  • TP: 4 - 8 mg/L

• Treatment Objectives: Effluent meets the Class 1A standard of the Discharge Standard of Pollutants for Municipal Wastewater Treatment Plants(GB 18918-2002) or more stringent local standards.

  • CODcr ≤ 30 mg/L

  • BOD5 ≤ 6 mg/L

  • SS ≤ 5 mg/L

  • NH3-N ≤ 1.5 mg/L

  • TN ≤ 10 mg/L

  • TP ≤ 0.3 mg/L

• Core Process: Pretreatment + Biological Treatment (AAO Process) + Submerged Ultrafiltration Membrane Separation

• System Positioning: Replaces the traditional secondary clarifier with membrane separation technology to achieve efficient solid-liquid separation, ensuring stable compliance of effluent SS and microbiological indicators, and creating favorable conditions for advanced nitrogen and phosphorus removal.

2.0 Process Flow Design

This scheme adopts the "AAO + Submerged UF Membrane" as the core process combination. Its complete process flow and material circulation paths are illustrated in the diagram below:

flowchart LR
subgraph PreTreatment[Pretreatment Section]
    A[Municipal Sewage] --> B[Coarse/Fine Screens]
    B --> C[Grit Chamber]
    C --> D[Fine Screen<br>1-3mm]
end

subgraph BioTank[AAO Bioreactor]
    D --> E[Anaerobic Zone<br>Phosphorus Release]
    E --> F[Anoxic Zone<br>Denitrification]
    F --> G[Aerobic Zone<br>Nitrification/C Oxidation]
end

subgraph MBR[Membrane Separation Zone]
    G --> H[Membrane Tank]
    H -- Mixed Liquor Recycle --> F
    H -- Waste Sludge --> I[Waste Sludge]
    I --> J[Sludge Treatment System]
end

subgraph ProductWater[Effluent & Disinfection]
    H -- Permeate --> K[Permeate Tank]
    K --> L[Disinfection Unit<br>UV/Sodium Hypochlorite]
    L --> M[Compliant Discharge/Reuse]
end

subgraph Support[Support Systems]
    N[Blower Aeration System] --> G & H
    O[Membrane Scouring Aeration System] --> H
    P[Suction & Backwash System] --> H
    Q[CIP Cleaning System] --> H
end

Step-by-Step Process Explanation:

1. Pretreatment Section:

• Purpose: Remove large debris and hard particles that could damage membrane fibers.

• Coarse/Fine Screens: Remove floatables, fibers, etc.

• Grit Chamber: Remove grit to prevent accumulation and abrasion at the bottom of the membrane tank.

• Fine Screen (Critical): Uses a drum screen or internally fed screen with 1-3mm apertures. This is the first critical barrier for protecting the UF membranes and must effectively intercept hair, fibers, and other materials prone to tangling around membrane fibers.

2. AAO Bioreactor:

• Purpose: Achieve organic matter degradation and biological nutrient removal (nitrogen and phosphorus).

• Anaerobic Zone: Phosphorus release by polyphosphate-accumulating organisms (PAOs), with concurrent fermentation of some readily biodegradable organics.

• Anoxic Zone: Utilizes nitrate recycled from the aerobic zone for denitrification, further consuming organic matter.

• Aerobic Zone: Conducts nitrification (ammonia → nitrate) and oxidation of remaining organic matter, along with luxury phosphorus uptake by PAOs. This zone is either integrated with or adjacent to the membrane tank, with mixed liquor flowing directly into it.

3. Membrane Separation Zone (Core Unit):

• Membrane Tank: Submerged ultrafiltration membrane modules are directly placed in the tank. The environment is typically aerobic, maintaining a certain level of dissolved oxygen.

• Separation Process: Under the suction (negative pressure) of extraction pumps (or by gravity), water from the mixed liquor passes through the membrane pores to become permeate (product water), while activated sludge, suspended solids, colloids, and pathogenic microorganisms are completely retained.

• Mixed Liquor Recycle: Nitrified mixed liquor is recycled from the membrane tank (end of aerobic zone) to the front of the anoxic zone for denitrification.

• Waste Sludge Discharge: Excess sludge is wasted from the membrane tank or aerobic zone to control the system's sludge retention time (SRT).

4. Effluent and Disinfection:

• Permeate: The UF membrane permeate is clear with very low SS and turbidity, directly meeting the Class 1A standard for SS.

• Disinfection: Ultraviolet (UV) or sodium hypochlorite disinfection is used to ensure compliance with fecal coliform standards.

5. Support Systems (Ensuring Stable Operation):

• Blower Aeration System: Divided into biological aeration (aerobic zone) and membrane scouring aeration (bottom of membrane tank).

• Membrane Scouring System (Key): High-intensity aeration diffusers are arranged below the membrane fiber bundles. The generated vigorous bubbles and hydraulic shear scour the membrane surface, preventing sludge floc deposition. This is the core method for controlling membrane fouling.

• Suction and Backwash System: Operates in an intermittent suction mode (e.g., 8-12 minutes suction, 1-2 minutes relaxation), with periodic backwashing using permeate to further restore membrane flux.

• Chemical Cleaning System (CIP): Includes maintenance cleaning (weekly to monthly) and recovery cleaning (quarterly to semi-annually), using chemicals like sodium hypochlorite and citric acid to remove organic and inorganic foulants.

3.0 Key Design Parameters and Equipment Selection

4.0 Scheme Advantages and Operation Management Key Points

Core Advantages:

1. Excellent and Stable Effluent Quality: Physical sieving ensures near-zero SS and turbidity, with high microbial safety, fully meeting Class 1A standards and suitable for direct reuse as reclaimed water.

2. Compact Process: Eliminates the secondary clarifier, potentially reducing footprint by ~30%, especially beneficial for space-constrained sites or upgrade projects.

3. Strong Shock Load Resistance: The high-concentration activated sludge system effectively buffers fluctuations in influent quality and quantity.

4. High Degree of Automation: Easily achieves full automation and remote monitoring, simplifying operation and management.

Key Operation Management Points:

1. Precise Aeration Control: Balance biological oxygen demand and membrane scouring demand to optimize energy consumption while ensuring effective membrane cleaning.

2. Scientific Cleaning Strategy: Develop a preventive cleaning protocol based on TMP growth trends to avoid irreversible fouling.

3. Strict Pretreatment Control: The performance of the fine screen directly impacts the long-term stability and cleaning frequency of the membrane system.

4. Sludge Property Management: Maintain good sludge settleability and activity to avoid sludge bulking or excessive aging, which increases membrane load.

Summary

The "AAO + Submerged UF Membrane" process proposed in this scheme is a mainstream and reliable technology for modern municipal wastewater treatment to achieve high-standard discharge and water reuse. The key to its successful application lies in quality membrane product selection, scientific flux design, an effective membrane scouring strategy, and meticulous operation and maintenance. During the design phase, detailed calculations and simulations should be performed based on specific wastewater quality, quantity, and site conditions. During operation, a membrane-centric maintenance management system must be established to ensure the long-term, stable, and economical performance of this process. For large-scale projects, pilot testing is recommended to determine core parameters.