Core Logic of MBR – Biological Treatment + Membrane Separation
The Essence of Replacing the Secondary Sedimentation Tank
In traditional wastewater treatment processes, after the biological treatment tank removes COD and ammonia nitrogen, a secondary sedimentation tank is required to separate sludge and water. Activated sludge settles by gravity, and the clarified supernatant is discharged as effluent.
However, the separation efficiency of the secondary sedimentation tank is highly dependent on sludge settling performance. If sludge bulking or deflocculation occurs, it can lead to sludge carryover and excessive suspended solids (SS) in the effluent. In addition, secondary sedimentation tanks require a large footprint and have limited separation efficiency.
The core improvement of the MBR process is the replacement of secondary sedimentation with membrane separation. The overall process can be summarized as:
Wastewater → Pretreatment (screening, grit removal) → Biological treatment tank (aerobic/anaerobic degradation) → Membrane module (solid–liquid separation) → Effluent
The biological treatment stage is identical to that in traditional systems. It relies on microorganisms in activated sludge to degrade pollutants such as COD, ammonia nitrogen, and total phosphorus. The metabolic mechanisms and nutrient requirements remain unchanged.
The membrane separation stage is the defining feature of MBR. It replaces gravity settling with physical filtration. Regardless of sludge settling characteristics, the membrane retains activated sludge and suspended particles, allowing only clarified water to pass through.
In simple terms, MBR replaces "gravity settling" with "physical retention," eliminating dependence on sludge settling performance, improving separation efficiency, and reducing footprint.
Key On-Site Characteristics of MBR
Equipped with aeration systems similar to conventional biological tanks
Contains membrane modules (flat-sheet or hollow fiber)
No large secondary sedimentation tank
Effluent is clear, with near-zero SS
Membranes require regular cleaning to prevent fouling
Core Advantages of MBR
The adoption of MBR is driven by practical benefits rather than trends. It is particularly suitable for applications requiring high effluent quality and compact design.
1. Superior Sludge–Water Separation and Stable Effluent Quality
Secondary sedimentation tanks rely on gravity settling, which is unstable under poor sludge conditions. In contrast, membrane modules provide high-precision filtration, retaining nearly all sludge and suspended solids.
Effluent SS can be consistently maintained below 10 mg/L, while COD and ammonia nitrogen levels remain stable, easily meeting stringent discharge standards.
2. Reduced Footprint
Secondary sedimentation tanks typically occupy 30–40% of the total plant area. MBR systems eliminate this requirement, reducing overall footprint to approximately 50–70% of conventional systems.
This makes MBR ideal for space-constrained applications such as urban plants and industrial facilities.
3. Higher Sludge Concentration and Improved Treatment Efficiency
Traditional systems operate at MLSS levels of 2,000–4,000 mg/L due to settling limitations. MBR systems can maintain MLSS at 8,000–12,000 mg/L.
This higher biomass concentration enhances pollutant degradation efficiency, making MBR especially suitable for high-strength or refractory wastewater.
4. Prevention of Sludge Loss
Conventional systems are prone to sludge washout during hydraulic shocks or poor sludge management. MBR membranes effectively retain biomass, significantly improving process stability.
5. Simplified Process Operation
Eliminating the secondary sedimentation tank removes the need for sludge blanket control and scraper operation. Maintenance is focused on the biological tank and membrane system.
Although membrane cleaning is required, overall operation is more manageable.
Key Points for MBR Operation and Maintenance
The core principle is:
Stable biological system + effective membrane maintenance
Both are essential-biological treatment removes pollutants, while membranes ensure solid–liquid separation.
Biological System Operation and Maintenance
The operational logic is consistent with conventional activated sludge systems, with emphasis on maintaining microbial activity.
1. Nutrient Ratio Control
Maintain C:N:P ≈ 100:5:1 to support microbial growth. Due to higher MLSS in MBR systems, nutrient consumption is faster, requiring regular monitoring and supplementation.
2. Dissolved Oxygen (DO) Control
Maintain DO at 2.0–3.0 mg/L in aerobic zones to ensure proper degradation of COD and ammonia nitrogen.
Avoid excessive aeration, which may:
Break sludge flocs
Increase membrane fouling risk
3. Sludge Age (SRT) Control
SRT should be maintained at 10–20 days, longer than in conventional systems.
No need for frequent sludge wasting
Perform periodic low-volume sludge discharge
Maintain MLSS at 8,000–12,000 mg/L
Membrane Module Operation and Maintenance
The membrane system is the core component of MBR. Fouling can lead to reduced flux, increased energy consumption, and potential damage.
1. Routine Online Cleaning (TMP Monitoring)
Includes:
Air scouring: removes surface sludge
Backwashing: clears membrane pores
Monitor transmembrane pressure (TMP):
Increase cleaning frequency when TMP exceeds 0.1 MPa
2. Periodic Offline Chemical Cleaning
Performed every 3–6 months using:
Sodium hypochlorite (organic fouling removal)
Citric acid (inorganic scaling removal)
3. Influent Quality Control
Ensure effective pretreatment:
Remove grit, hair, and large solids
Control oil and refractory organics
This reduces membrane fouling risk.
4. Membrane Protection
Keep membranes continuously submerged
Prevent drying (avoids pore shrinkage and damage)
Avoid mechanical damage from sharp objects
Damaged membranes must be replaced immediately.
Common Problems and Solutions
Problem 1: Membrane Fouling (Reduced Flux)
Solutions:
Increase air and water cleaning frequency
Perform chemical cleaning if TMP continues rising
Improve pretreatment
Optimize sludge condition
Problem 2: Effluent COD and Ammonia Exceed Limits
Solutions:
Ensure DO ≥ 2.0 mg/L
Supplement nutrients
Reduce influent load if necessary
Adjust sludge age and discharge rate
Problem 3: Membrane Damage (High Effluent SS)
Solutions:
Check effluent SS for sudden increases
Identify and replace damaged modules
Improve pretreatment
Adjust aeration intensity
Problem 4: Rapid Sludge Accumulation on Membrane
Solutions:
Increase air scouring intensity
Improve sludge characteristics
Reduce MLSS via controlled sludge wasting
Add coagulants if necessary to improve floc structure