Application Design of MBR Process in Upgrading Municipal Wastewater Treatment Plants

I. Background of Upgrading and Advantages of MBR Process

1.1 Background of Retrofit and Core Requirements

With the continuous elevation of the Pollutant Discharge Standard for Urban Wastewater Treatment Plantsand the increasing urgency for water resource recovery, a large number of existing municipal wastewater treatment plants (WWTPs) face the challenge of upgrading from the Class 1B or Class 1A standard of GB 18918-2002 to stricter standards, such as Quasi-Surface Water Class IV or even higher. Traditional activated sludge processes and their derivatives (e.g., A²/O, Oxidation Ditch) encounter the following core challenges during upgrades:

• Bottleneck in Effluent Quality: Traditional processes have limited capability in removing suspended solids, microorganisms (especially viruses), and some refractory organics, making it difficult to consistently meet high standards.

• Space Limitations: Upgrades often require adding advanced treatment units (e.g., filtration, advanced oxidation), but many existing plant sites are land-constrained, making expansion difficult.

• Pressure on Nitrogen and Phosphorus Removal: Stricter standards demand stringent limits for total nitrogen and total phosphorus. The efficiency of biological nitrogen and phosphorus removal in traditional processes is limited by sludge settleability and insufficient carbon sources.

• Operational Stability: The solid-liquid separation performance of secondary clarifiers is significantly affected by shock loads, sludge bulking, etc., leading to relatively large fluctuations in effluent quality.

1.2 Core Advantages and Application Positioning of MBR Process

The Membrane Bioreactor (MBR) process combines membrane separation technology with a biological treatment unit, using membrane modules (typically microfiltration/ultrafiltration membranes) to replace the secondary clarifier. It demonstrates irreplaceable advantages in the upgrading of municipal WWTPs:

• Excellent and Stable Effluent Quality: Membrane retention yields effluent with near-zero suspended solids, low turbidity, and good clarity; high retention of bacteria and viruses reduces disinfection burden. The effluent can directly meet high-quality reuse (e.g., landscape, municipal miscellaneous) or high-standard discharge requirements.

• Enhanced Biological Treatment Efficiency: Maintains very high mixed liquor suspended solids (MLSS) concentration (8-15 g/L) in the reactor, increasing volumetric load and enhancing nitrification and degradation of refractory organics; achieves complete solid-liquid separation, preventing sludge loss, which is beneficial for enriching slow-growing nitrifying bacteria.

• Space Saving: Eliminates the need for secondary clarifiers, and the biological tank can operate at higher sludge concentrations, reducing tank volume. This is particularly suitable for in-situ upgrades at land-constrained sites.

• Strong Shock Load Resistance: High sludge concentration and large biomass volume provide strong resistance to fluctuations in water quality and quantity.

• Facilitates Automated Management: Simple process flow, easy to achieve automated control, reducing manual intervention.

Application Positioning in Upgrades: The MBR process is a core process option to address the three major challenges of high-standard effluent, space constraints, and operational stability. It is especially suitable for retrofit scenarios aiming to directly upgrade from Class 1B/A standards to Quasi-Class IV or similar standards, while also achieving wastewater resource recovery.

II. MBR Upgrading Technical Route and Scheme Design

Depending on the existing process and upgrade targets, MBR can be integrated in various ways. The mainstream approach is to retrofit the existing biological system into an A/O-MBR or A²/O-MBR system, while enhancing pre-treatment and post-disinfection.

2.1 Typical Retrofit Route and Full-Process Design

The diagram below illustrates a typical technical route for MBR retrofitting based on a traditional A²/O process, clearly showing how the MBR unit replaces the secondary clarifier and synergizes with existing facilities.

III. Key Points for Core Retrofit Unit Design

3.1 Biological System Retrofit and Optimization

• Retrofit Objective: To meet the high MLSS operational requirements of the MBR system and ensure efficient nitrogen and phosphorus removal under high standards.

• Key Measures:

  1. Tank Volume and Zone Adjustment: Evaluate the volume of the existing biological tank (e.g., A²/O tank). Assess whether the sludge loading rate and nitrification/denitrification volumes are sufficient when operating at increased MLSS (e.g., 8-10 g/L). Increase or adjust the volume ratio of anaerobic, anoxic, and oxic zones if necessary.

  2. Aeration System Retrofit:

     • Biological Tank Aeration: Replace with high-efficiency, clog-resistant fine bubble diffusers (e.g., tubular, disc type) to meet higher oxygen transfer rate demands. Recalculate air demand; blowers may need capacity increase or replacement.

     • Membrane Scouring Aeration: This is unique to MBR. A high-flow, low-shear aeration system must be installed at the bottom of the membrane tank to scour the membrane surface and prevent fouling. This is critical for stable membrane operation, consuming approximately 50-70% of the MBR's total electricity.

  3. Recycle System Adjustment: Optimize sludge recycle paths. Typically, set up sludge recycle from the membrane tank to the front end of the biological tank, and mixed liquor internal recycle from the end of the oxic zone to the anoxic zone to enhance nitrogen removal.

3.2 Membrane System Design (Core)

• Membrane Tank Construction:

  • New Membrane Tank: The most common approach. Constructed beside the secondary clarifier or using available space. Requires careful design for membrane module layout, feed water distribution, scouring aeration, permeate suction, and