How to design MBR Membrane System

Design Scheme and Key Technology Analysis of Membrane Bioreactor (MBR) System

The following is an original article on the design scheme of the Membrane Bioreactor (MBR) system, covering technical principles, key design elements, and engineering practice suggestions.

I. Introduction

As a revolutionary breakthrough in the water treatment field of the 21st century, the Membrane Bioreactor (MBR) technology has comprehensively upgraded the traditional activated sludge process by integrating biodegradation and membrane separation technologies. Its effluent water quality can reach the Class-IV surface water standard, and the occupied area is reduced by over 40%. Thus, it has become the core solution for municipal sewage treatment, industrial wastewater treatment, and reclaimed water reuse. This article will systematically elaborate on the key elements and optimization strategies of MBR process design.

II. Composition and Technical Principles of the MBR System

1. Composition of Core Components

The MBR system achieves efficient sewage treatment through the synergistic effect of biodegradation and membrane separation. Its core architecture consists of the following functional units:

(1) Bioreactor Unit

Process Type: The modified activated sludge process is predominantly adopted, such as the A²O (Anaerobic – Anoxic – Oxic) – MBR combined process, enabling synchronous nitrogen and phosphorus removal.
- Anaerobic Zone: Phosphorus – releasing bacteria decompose organic matter and release phosphorus. The hydraulic retention time (HRT) here is 1 – 2 hours.
- Anoxic Zone: Denitrifying bacteria utilize carbon sources for nitrate reduction, with an HRT of 2 – 3 hours.
- Aerobic Zone: Nitrification and organic matter oxidation take place, and the HRT is 4 – 6 hours.
Sludge Concentration Regulation: The mixed liquor suspended solids (MLSS) are maintained at 8 – 12 g/L (3 – 4 times that of the traditional process). The activity of the microbial community is adjusted via the sludge return ratio (100 – 300%).
Aeration System Design: A combination of microporous aeration disks and swirl aerators is employed to boost the oxygen utilization rate to 25 – 35%. The dissolved oxygen is precisely controlled in different zones (dissolved oxygen in the anaerobic zone is less than 0.2 mg/L, while in the aerobic zone, it is 2 – 4 mg/L).

(2) Membrane Separation Module

Membrane Module Type:
- Hollow Fiber Membrane: It features a large specific surface area (500 – 1000 m²/m³) and a relatively low cost. However, it is prone to filament breakage and is thus suitable for municipal sewage treatment.
- Flat – Sheet Membrane: With high mechanical strength and excellent anti – fouling properties, it is particularly suitable for industrial wastewater containing high levels of suspended solids.
Membrane Material Selection: The mainstream material is PVDF (polyvinylidene fluoride). Through hydrophilic modification (such as grafting PEG), the contact angle can be decreased to less than 50°, resulting in a 20 – 30% increase in flux.
Operating Parameters:
- Operating Flux: For municipal sewage, it ranges from 15 – 25 LMH (L/m²·h). For industrial wastewater, it is reduced to 8 – 15 LMH depending on the water quality.
- Transmembrane Pressure Difference (TMP): It is controlled within the range of – 0.01 – 0.05 MPa. Once it exceeds 0.03 MPa, the cleaning procedure is activated.
- Suction Mode: It operates intermittently (for instance, suction for 8 minutes and stop for 2 minutes) to mitigate the formation of the concentration polarization layer on the membrane surface.

(3) Integrated Control System

Membrane Fouling Monitoring System: Parameters such as TMP, permeability (Flux), and sludge viscosity (\mu) are monitored in real – time. The degree of fouling is determined by the formula \[ J = \frac{TMP}{\mu \cdot R} \] (J represents the flux, and R denotes the fouling resistance).
Intelligent Cleaning Strategy:
- Online Maintenance Cleaning: Daily backwashing is carried out using sodium hypochlorite (200 – 500 mg/L) or citric acid (pH = 2) for 30 minutes.
- Restorative Cleaning: Monthly off – line immersion cleaning is conducted using a composite agent of NaOH (pH = 10 – 12) + NaClO.
Precise Aeration Control: Based on the feedback from the DO sensor, the frequency of the blower is dynamically adjusted through the PID algorithm, reducing energy consumption by 15 – 25%.

(4) Auxiliary Systems

Pretreatment Unit: A 1 – mm fine screen and a cyclone grit chamber are utilized to remove fibrous substances and sand grains (with a particle size greater than 0.2 mm), thereby preventing membrane filament abrasion.
Sludge Treatment System: The excess sludge is processed by a belt filter press to a moisture content of less than 80%. Compared with the traditional process, the sludge yield is reduced by 30%.
Chemical Dosing System: It encompasses automatic dosing devices for carbon sources (sodium acetate), phosphorus – removing agents (PAC), and membrane cleaning agents ($NaClO$/citric acid).

2. In – Depth Analysis of Technical Principles

The core technical advantages of the MBR system stem from the synergistic mechanism of biological reaction and membrane separation:

(1) Biological Enhancement Mechanism

Microbial Retention Effect: Ultra – filtration membranes (with a pore size of 0.01 – 0.1 μm) completely retain microorganisms. This allows the sludge retention time (SRT) to be independently extended to 20 – 100 days, facilitating the enrichment of nitrifying bacteria (whose generation time is 3 – 5 days).
Simultaneous Nitrification and Denitrification (SND): High MLSS creates a dissolved oxygen gradient, generating an anoxic micro – environment within the aerobic zone. This enables a single – tank nitrogen removal rate of over 70%.

(2) Membrane Separation Kinetics

Sieving Effect: The membrane pores retain suspended solids, colloids, and macromolecular organic matter (with a molecular weight greater than 100 kDa), ensuring that the effluent turbidity is less than 0.5 NTU.
Formation of Dynamic Filtration Layer: Activated sludge forms a bio – gel layer (with a thickness of 50 – 200 μm) on the membrane surface. Through bridging, this layer enhances the interception of small molecules.

(3) Energy Transfer Optimization

Gas – Water Two – Phase Flow Design: Aeration at the bottom of the membrane tank generates rising bubbles (with a flow velocity of 0.3 – 0.5 m/s), creating a shear force exceeding 3 Pa. This scours the membrane surface and reduces the fouling rate.
Cross – Flow Filtration Mode: The mixed liquor flows tangentially along the membrane surface (with a flow velocity of 0.5 – 1.5 m/s), minimizing pollutant deposition.

3. System Synergistic Operation Mechanism

The MBR system achieves efficient and stable operation through the following coupling effects:

Sludge Concentration – Oxygen Mass Transfer Balance: High MLSS requires a corresponding high – intensity aeration (0.3 – 0.6 Nm³/(m²·h)). The oxygen transfer efficiency is evaluated by the α factor (\alpha=\frac{K_{La} sewage}{K_{La} clean water}), usually with \alpha ranging from 0.4 to 0.6.
Membrane Flux – Fouling Rate Dynamic Balance: The designed flux should adhere to the critical flux theory (< J_{critical}) to avoid irreversible fouling. The membrane performance is restored through periodic backwashing (backwashing for 30 seconds every 15 minutes).
Carbon Source – Nitrogen and Phosphorus Ratio Regulation: When the influent C/N ratio is less than 4, sodium acetate (with a COD equivalent of 200 – 300 mg/L) needs to be added to enhance denitrification.

4. Quantitative Analysis of Technical Advantages

Index	Traditional Activated Sludge Process	MBR System	Improvement Effect
Effluent SS	20 – 30 mg/L	< 5 mg/L	Reduced by 83%
Occupied Area	Benchmark Value	Reduced by 40 – 60%	Compact Layout
Nitrogen Removal Efficiency	60 – 70%	85 – 95%	Enhanced Biological Nitrogen Removal
Impact Load Resistance	Low	COD Fluctuation < 10%	Sludge Buffer Effect

Through the in – depth integration of the above – mentioned technical principles, the MBR system not only guarantees the effluent water quality but also achieves a dual breakthrough in treatment efficiency and operation stability. In practical projects, the membrane type and cleaning strategy should be adjusted according to the influent characteristics (such as the salinity and oil content of industrial wastewater) to optimize the technical and economic performance.

III. Key Parameters for Process Design

1. Design of the Biological Treatment Unit

Parameter	Design Range	Optimization Direction
Hydraulic Retention Time	4 – 8 h	Coupling with Anaerobic/Anoxic Zones
Sludge Retention Time (SRT)	15 – 30 d	Dynamically Adjusting Nitrogen Removal Efficiency
Dissolved Oxygen (DO)	2 – 4 mg/L	Hierarchical Aeration Control

2. Design Specifications for the Membrane System

Flux Selection: 15 – 25 LMH for municipal sewage and 8 – 15 LMH for industrial wastewater.
Membrane Area Redundancy: It is configured based on 120% of the peak flow rate, taking into account membrane fouling attenuation.
Backwashing Strategy: Backwash for 30 seconds every 15 minutes to maintain \Delta P below 0.03 MPa.

IV. Innovative Design Practices

1. Intelligent Control System

Establish a multi – parameter feedback system: MLSS, TMP, DO, and pH are linked and adjusted in a coordinated manner.
Apply machine learning algorithms to predict membrane fouling trends and optimize the cleaning cycle.

2. Energy – Saving and Consumption – Reduction Design

Adopt the air – scouring + intermittent suction mode (with an on/off ratio of 8:2).
Install energy recovery devices (such as hydraulic turbine generators).

3. Anti – Fouling Membrane Technology

Modified PVDF membrane (with a contact angle less than 50°) combined with a nano – TiO₂ coating.
Employ pulse aeration technology (with a frequency of 0.5 – 2 Hz) to mitigate concentration polarization.

V. Engineering Case Analysis

1. Project Background

The upgrading and renovation of a 100,000 – ton/day municipal sewage treatment plant.

2. Design Scheme

The plant is divided into 8 membrane tanks, with each tank equipped with 1500 flat – sheet membranes.
The membrane flux is set at 22 LMH, and the aeration intensity is 0.35 Nm³/(m²·h).
A deep nitrogen – removal module (post – denitrification filter) is integrated.

3. Operation Results

The COD removal rate exceeds 95%, and the TN content is less than 10 mg/L.
The operating energy consumption is reduced to 0.6 kWh/m³.
The membrane life is extended to 7 years.

VI. Challenges and Countermeasures

1. Membrane Fouling Control

Strengthen pretreatment (using a 1 – mm fine screen and cyclone grit removal).
Develop self – cleaning membrane materials (such as conductive membranes).

2. Cost Optimization

Implement modular design to cut down civil engineering costs.
Opt for domestic – made membrane components as substitutes, which can reduce costs by 40%.

3. System Stability

Adopt a dual – power redundancy design.
Set up emergency bypass pipelines.

VII. Future Development Trends

1. Smart MBR System

Integrate digital twin technology to realize predictive maintenance.

2. Low – Carbon Path

Generate electricity from biogas and adopt photovoltaic complementary energy supply.

3. Resource Recovery

Simultaneously recover high – value substances such as phosphorus and alginate.

Conclusion

The optimized design of the MBR system requires a comprehensive consideration of water quality characteristics, operation economy, and technical advancement. Through modular design, intelligent control, and the application of new materials, the new – generation MBR technology will play an even greater role in the field of sewage resource utilization. It is recommended to conduct pilot – scale verification during the design stage and establish a dynamic process model to ensure the efficient operation of the system throughout its life cycle. This scheme incorporates the cutting – edge technologies in the current MBR field and can be customized according to the specific water quality characteristics, treatment scale, and investment budget of the project. For in – depth design in specific scenarios, detailed calculation sheets and 3D layout plans can be further provided.