Landfill Leachate Treatment: MBR Core Process Design Scheme

1.0 Design Basis and Objectives

• Influent Quality:Based on the characteristics of typical medium-to-late-stage landfill leachate.

  • CODcr: 5,000 - 20,000 mg/L

  • BOD5: 2,000 - 8,000 mg/L

  • NH3-N: 1,000 - 3,000 mg/L

  • SS: 500 - 2,000 mg/L

• Treatment Objectives:Ensure MBR effluent meets the feed water requirements for subsequent advanced treatment units (e.g., NF/RO).

  • CODcr Removal Rate > 90%

  • NH3-N Removal Rate > 98%

  • SS: Nearly complete removal

  • SDI15 < 3 (Providing high-quality feed for Reverse Osmosis)

• Core Function:Replaces the traditional secondary clarifier, achieving efficient biochemical degradation, nitrification/denitrification, and complete solid-liquid separation.

2.0 MBR Core Process Flow Design

The MBR system is essentially a highly integrated combination of a "bioreactor" and a "membrane separation unit." Its core principle is using the membrane's retention capability to control the biochemical reaction under optimal conditions. The flowchart below clearly illustrates the two typical internal processes of an MBR system and its connection to external systems.

Step-by-Step Process Explanation:

1. Biochemical Reaction Unit (Pollutant Degradation)

• A/O (Anoxic/Oxic) Process:The MBR bioreactor is typically divided into an anoxic zone and an aerobic zone.

  • Anoxic Zone:Conducts denitrification. Nitrate (NO₃⁻) from the recycled mixed liquor from the aerobic zone is reduced to nitrogen gas (N₂), which escapes, achieving Total Nitrogen (TN) removal. Simultaneously, refractory organic matter undergoes hydrolysis and acidification here, improving its biodegradability.

  • Aerobic Zone:Conducts carbonaceous BOD removal (carbonation) and nitrification of ammonia nitrogen (conversion of NH₃-N to NO₃⁻). A high concentration of Mixed Liquor Suspended Solids (MLSS)is maintained in the tank, typically controlled at 12,000 - 18,000 mg/L(3-4 times that of traditional activated sludge processes). This enables extremely high resistance to shock loads and high pollution degradation efficiency.

2. Membrane Separation Unit (Core of Solid-Liquid Separation)

• Configuration Selection:

  • Submerged (Integrated) MBR:The membrane modules are directly submerged in the end of the aerobic zone. Intensive aerationat the bottom of the membrane tank creates vigorous scouring to prevent pollutant deposition on the membrane surface. A suction pumpuses negative pressure to draw purified water through the membrane pores. This is the most mainstreamprocess for leachate treatment, offering a compact structure and relatively low energy consumption.

  • External (Side-Stream) MBR:The membrane unit is separate from the bioreactor. Mixed liquor is circulated at high velocity across the membrane surface via a recirculation pump. The high cross-flow velocity (typically >3 m/s) offers strong anti-fouling capability, suitable for ultra-high concentration or scaling-prone leachate, but at a higher energy cost.

• Separation Process:The membrane pore size is typically in the range of 0.01 - 0.1 µm (Ultrafiltration), which absolutely retains all suspended solids, colloids, bacteria, and macromolecular organics, producing clear effluent.

3. Key Auxiliary Systems (Ensuring Stable Operation)

• Aeration System:Divided into biological aeration and membrane scouring aeration. Biological aeration provides oxygen for microorganisms; membrane scouring aeration is critical for preventing membrane fouling and requires strong airflow to scour the membrane surface.

• Recirculation System:Includes nitrified mixed liquor recirculation (from the aerobic zone to the anoxic zone) and sludge recirculation (from the membrane zone to the front end of the bioreactor), used to maintain biochemical reaction balance and high biomass.

• Cleaning-In-Place (CIP) System:Performs regular maintenance cleaning and recovery cleaning using sodium hypochlorite (removes organic/biofouling) and citric or oxalic acid (removes inorganic scaling) to restore membrane flux.

3.0 Core Design Parameters and Equipment Selection

4.0 Scheme Advantages and Key Considerations

Advantages:

1. High Efficiency and Stability:Completely resolves issues like sludge bulking and washout common in traditional secondary clarifiers when treating leachate, producing consistently high-quality effluent.

2. High Volumetric Loading Rate:High MLSS allows for significantly smaller reactor volumes, reducing footprint.

3. Excellent Nitrogen Removal:Long sludge age provides a favorable environment for nitrifying bacteria, resulting in very high ammonia removal rates.

4. Superior Effluent Quality:Provides ideal pre-treated feed (low SDI, low SS) for subsequent NF/RO advanced treatment.

Key Considerations (Challenges & Countermeasures):

1. Membrane Fouling Control:This is the core challenge for stable MBR operation.

   • Countermeasures:Optimize aeration intensity, employ intermittent suction mode (e.g., 8 minutes on, 2 minutes off), and perform regular CIP.

2. Impact of High Salinity:High salt content in leachate can affect microbial activity.

   • Countermeasures:Acclimatize salt-tolerant microbial strains through long SRT operation to maintain system stability.

3. Operational Costs:Aeration energy consumption and membrane replacement are major cost components.

   • Countermeasures:Select high-efficiency diffusers and energy-efficient blowers, optimize operational protocols, and extend membrane lifespan.

Summary

The MBR process is an indispensable core unit in modern landfill leachate treatment plants. Its successful application relies on appropriate membrane selection, precise aeration control, and strict membrane fouling prevention strategies. This scheme provides a professional technical framework for building an efficient, stable, and reliable leachate MBR system. In practical engineering, pilot testing based on specific water quality is recommended to optimize the various design parameters.