What is a Membrane bioreactor (MBR)

A membrane bioreactor, commonly abbreviated as MBR, is a modern wastewater treatment process that combines conventional biological treatment with advanced membrane filtration. The technology merges the principles of activated sludge systems with microfiltration or ultrafiltration membranes to achieve high-quality effluent suitable for discharge or reuse.

MBR systems represent a significant evolution in wastewater treatment, offering compact design, excellent pollutant removal, and reliable performance. They are widely used in municipal, industrial, and decentralised treatment applications, particularly where space is limited or high effluent standards are required.

The concept and working principle of MBR technology

The membrane bioreactor process integrates two essential stages of wastewater treatment within a single system: biological degradation and solid-liquid separation.

In the biological stage, microorganisms in an aerated tank break down organic pollutants, ammonia, and other contaminants in the wastewater. This process is similar to that of a conventional activated sludge system. However, instead of using a secondary clarifier to separate treated water from biomass, MBRs use a membrane module that physically filters the mixed liquor.

The membranes act as a barrier, allowing clean water (permeate) to pass through while retaining suspended solids, bacteria, and most viruses. This results in effluent with very low turbidity and high clarity. The membranes typically have pore sizes ranging from 0.1 to 0.4 micrometres for microfiltration or even smaller for ultrafiltration.

Components of a membrane bioreactor system

A typical MBR system consists of several key components that work together to treat wastewater efficiently and maintain membrane performance.

  • Bioreactor (aeration tank): The main biological treatment unit where microorganisms degrade organic matter under aerobic conditions. Air diffusers provide oxygen and maintain mixing.

  • Membrane module: The core component responsible for filtration. Membranes can be arranged in flat-sheet, hollow-fibre, or tubular configurations, depending on the system design.

  • Permeate extraction system: A suction pump draws treated water through the membrane under a controlled vacuum or pressure.

  • Blowers and aeration system: Supply air for both biological treatment and scouring of the membrane surface to prevent fouling.

  • Control system: Monitors key parameters such as transmembrane pressure (TMP), dissolved oxygen, sludge concentration, and flow rates to optimise performance.

  • Cleaning and backwash system: Periodically cleans the membranes using physical or chemical methods to maintain permeability.

In some cases, MBR systems also include pre-treatment equipment such as screens or grit chambers to remove large particles that could damage the membranes.

Types of MBR configurations

Membrane bioreactors are categorised based on how the membrane module is integrated into the system. The two main configurations are:

1. Submerged (immersed) MBR

In submerged systems, the membrane modules are placed directly inside the bioreactor tank or in a separate membrane tank that receives mixed liquor. Permeate is drawn through the membranes using suction pressure. Air bubbles from diffusers beneath the membranes provide aeration and help minimise fouling.

Submerged MBRs are energy-efficient, compact, and commonly used for municipal and medium-scale applications due to their low operating costs and simplicity.

2. Side-stream (external) MBR

In side-stream systems, the mixed liquor is pumped from the bioreactor through external membrane modules and then returned. These systems operate under higher pressure and crossflow velocity, which helps control fouling but increases energy consumption.

Side-stream MBRs are preferred for industrial wastewater with high organic loads, as they can handle more variable conditions and allow easier membrane maintenance.

The biological process in MBRs

The biological process in an MBR is similar to that of a conventional activated sludge system but operates at much higher mixed liquor suspended solids (MLSS) concentrations, typically between 8,000 and 15,000 mg/L. This allows for a greater population of microorganisms and enhanced treatment efficiency.

Because the membranes completely retain biomass, the sludge age (also known as solids retention time, or SRT) can be extended significantly. This enables better breakdown of complex organic compounds, improved nitrification, and reduced sludge production.

The combination of a stable biological environment and effective solids separation results in superior treatment performance, even with fluctuating inflows or pollutant concentrations.

Membrane separation and performance

The membrane component is responsible for separating treated water from biomass and suspended solids. Depending on the design, membranes can be made from polymeric materials such as polyvinylidene fluoride (PVDF) or polyethylene (PE), or from ceramic materials for high-resilience applications.

The membranes operate under low pressure, typically between 0.1 and 0.5 bar in submerged systems, or higher in side-stream configurations. The driving force for filtration is the pressure difference across the membrane surface, known as the transmembrane pressure (TMP).

Over time, fouling occurs as organic and inorganic matter accumulates on the membrane surface. To maintain performance, MBR systems use:

  • Air scouring: Continuous aeration to remove deposits and enhance mixing.

  • Backwashing: Reversing flow periodically to dislodge fouling material.

  • Chemical cleaning: Using mild chemicals such as sodium hypochlorite or citric acid to dissolve organic or mineral fouling layers.

Careful control of these cleaning strategies is essential to extend membrane lifespan and maintain stable permeability.

Advantages of membrane bioreactors

Membrane bioreactors offer several key advantages compared to conventional activated sludge and tertiary treatment systems:

  • High effluent quality: Produces water with very low suspended solids, turbidity, and pathogens, often suitable for reuse in irrigation or industrial applications.

  • Compact footprint: Eliminates the need for secondary clarifiers and tertiary filters, allowing installation in limited spaces.

  • Stable operation: Resistant to hydraulic and organic shock loads, providing consistent performance.

  • Reduced sludge production: Longer solids retention time minimises waste sludge volume.

  • Automation and control: Advanced sensors and PLCs enable efficient, largely unmanned operation.

  • Effluent reuse potential: Treated water can meet strict discharge standards or be reused for non-potable applications.

These benefits make MBR systems ideal for locations where land is scarce or where stringent environmental regulations apply.

Limitations and challenges

Despite their many advantages, membrane bioreactors also face certain challenges:

  • High capital cost: The cost of membranes and associated equipment is significantly higher than conventional systems.

  • Energy consumption: Aeration and membrane cleaning increase operational energy use.

  • Membrane fouling: Although manageable, fouling remains a major operational concern, requiring regular cleaning and maintenance.

  • Skilled operation: Proper system management requires trained personnel and sophisticated monitoring equipment.

  • Replacement costs: Membranes have a finite lifespan, typically 5–10 years, and replacement can be expensive.

Advances in membrane materials, energy-efficient aeration, and automated control systems continue to address these challenges, improving the economic and environmental viability of MBR technology.

Applications of MBR technology

Membrane bioreactors are used across a broad range of applications in both municipal and industrial sectors.

In municipal wastewater treatment, MBRs are often used for:

  • Decentralised treatment plants serving small communities or housing developments.

  • Upgrading existing treatment works to meet stricter effluent standards.

  • Wastewater reuse schemes for irrigation, toilet flushing, or industrial supply.

In industrial applications, MBRs are widely adopted in:

  • Food and beverage manufacturing.

  • Pharmaceutical and chemical production.

  • Textile and paper industries.

  • Landfill leachate treatment.

Their ability to handle variable and complex wastewater compositions makes MBRs highly versatile and reliable across different sectors.

Effluent quality and reuse potential

One of the most significant benefits of membrane bioreactor technology is the quality of effluent it produces. The membranes act as an effective barrier against pathogens, suspended solids, and even some micro-pollutants. Effluent typically meets or exceeds tertiary treatment standards without additional clarification or filtration.

The treated water can often be reused for:

  • Agricultural irrigation.

  • Industrial cooling or process water.

  • Landscape irrigation in urban developments.

  • Groundwater recharge, subject to regulatory approval.

This reuse potential supports sustainable water management and helps reduce pressure on freshwater resources, particularly in regions facing water scarcity.

Maintenance and operational strategies

To ensure long-term efficiency, MBR systems require regular monitoring and maintenance. Operators track parameters such as dissolved oxygen, MLSS, TMP, and membrane flux to identify fouling trends or process imbalances.

Best practices for maintenance include:

  • Periodic backwashing and air scouring to control fouling.

  • Routine chemical cleaning cycles at scheduled intervals.

  • Maintaining optimal aeration levels for both biological and membrane processes.

  • Balancing solids concentration to prevent excessive viscosity or clogging.

  • Monitoring pH and nutrient levels to sustain microbial health.

Automation and remote monitoring have made MBR operation more reliable, allowing real-time adjustments and predictive maintenance to reduce downtime.

Environmental and economic considerations

From an environmental perspective, MBRs support sustainable wastewater treatment by reducing pollution, enabling water reuse, and minimising sludge production. Their compact design reduces land requirements and allows retrofitting of existing facilities without expanding their footprint.

Economically, while MBRs have higher initial costs, their long-term benefits include lower sludge handling expenses, consistent compliance with discharge standards, and opportunities for water recycling. As membrane prices continue to decrease and energy-efficient technologies evolve, the cost gap compared to conventional systems continues to narrow.

Future developments in MBR technology

Research and innovation continue to advance MBR technology, focusing on reducing energy consumption, improving fouling control, and enhancing performance. Emerging developments include:

  • Next-generation membranes: Using nano-materials or hydrophilic coatings to resist fouling and improve permeability.

  • Hybrid systems: Combining MBRs with reverse osmosis or advanced oxidation for high-grade water reuse.

  • Energy recovery systems: Capturing biogas or heat from sludge for energy production.

  • Digital optimisation: Using artificial intelligence and machine learning to predict maintenance needs and optimise performance.

These innovations are expanding the role of MBRs in sustainable wastewater management and helping cities and industries move toward circular water use.

Conclusion

A membrane bioreactor (MBR) is a cutting-edge wastewater treatment process that combines biological degradation with membrane filtration to produce exceptionally clean effluent. It represents a significant advancement over conventional systems, offering compact design, superior treatment quality, and potential for water reuse.

While MBR systems require careful operation and investment, their environmental benefits, operational reliability, and adaptability make them one of the most promising technologies for modern wastewater management. As global demand for clean water and sustainable solutions grows, membrane bioreactors will continue to play an increasingly important role in the future of water treatment and resource recovery.