Why Membrane Bioreactors Are Reshaping Wastewater Treatment in 2025

BLOG August 1, 2025

When we examine how membrane bioreactors (MBRs) work, we can see they offer significant advantages over conventional treatment methods. Specifically, next-generation membranes are addressing the limitations of older designs, with technologies like nanofiltration and ultrafiltration enabling unprecedented water purity levels. Additionally, these systems demonstrate high separation efficiency, making treated water suitable for various applications including agricultural and industrial uses.

The impact of these membrane innovations extends beyond performance improvements. Consequently, we’re witnessing an expansion of water reuse and recycling opportunities. Furthermore, advanced treatment processes such as membrane bioreactors are becoming more efficient and cost-effective, ensuring high-quality effluent that meets stringent regulatory standards.

Throughout this article, we’ll explore the different types of membrane bioreactors, examine how they function in modern treatment plants, and highlight the efficiency gains that make them crucial to wastewater management in 2025.

How Membrane Bioreactors (MBRs) Work in Modern Treatment Plants?

Membrane bioreactors (MBRs) combine biological processes with membrane separation technology, representing a significant advancement over conventional activated sludge systems. These systems essentially replace secondary clarifiers with membrane filtration units, enabling superior solid-liquid separation ¹.

Biological and membrane filtration integration

MBRs integrate microfiltration or ultrafiltration membranes with suspended growth bioreactors to treat wastewater more effectively than traditional methods ². Rather than relying on gravity settling for solids separation, MBRs employ semipermeable membranes that retain biomass while producing clean permeate. This integration creates two primary configurations: submerged (immersed) and sidestream MBRs ³.

In submerged configurations, membranes are placed directly inside the bioreactor and operated under vacuum, with air scouring to reduce fouling ⁴. Sidestream configurations, however, position membranes externally with wastewater pumped through at elevated pressures (2-3 bar) in crossflow mode ³. Despite higher energy demands, sidestream MBRs offer operational flexibility and easier maintenance ⁵.

Activated sludge process in MBRs

Unlike conventional activated sludge systems, MBRs operate with dramatically different parameters. They maintain much higher mixed liquor suspended solids (MLSS) concentrations of 5,000-20,000 mg/L compared to approximately 2,000 mg/L in conventional systems ⁵. Moreover, MBRs typically operate with extended solid retention times (SRT) of 20-30 days versus 5-20 days in conventional processes ⁵.

The activated sludge in MBRs flows through distinct treatment zones. Initially, wastewater enters a denitrification tank where it’s mixed with a stirrer, then passes to an aeration tank containing diffusers before reaching the membrane tank ¹. This process produces exceptionally high-quality effluent with BOD and total nitrogen below 5 mg/L and suspended solids under 1 mg/L ¹.

Hollow fiber vs flat sheet membrane configurations

Two predominant membrane configurations exist in modern MBR systems:

Hollow fiber membranes: Consist of bundles of thin, straw-like fibers with a hollow core, offering superior packing density (up to 10,000 m² per cubic meter) ⁶. They permit easier backwashing and require less tank volume, although they’re more susceptible to internal pore blocking ⁶.
Flat sheet membranes: Planar membranes arranged in cassettes with uniform thickness ⁷. Though they provide lower surface area per volume, they offer simpler maintenance through surface flushing and manual cleaning ⁶.

Both configurations achieve high removal rates for pollutants, bacteria, and suspended solids, though they differ in energy consumption, cleaning protocols, and application suitability ⁶.

Types of Membrane Bioreactors and Their Use Cases

Various types of membrane bioreactors have evolved to address specific treatment needs across different sectors. Each configuration offers unique advantages based on the target application and wastewater characteristics.

Submerged MBRs for municipal wastewater

Submerged MBRs represent the dominant configuration for large-scale municipal wastewater treatment since their commercialization in the 1990s ⁸. These systems feature membrane modules immersed directly within the bioreactor tank, where air sparging along the membrane surface creates turbulence that cleans and scrubs the membrane ⁹. Notably, submerged MBRs operate at significantly lower water velocity and flux compared to other configurations, making them ideal for treating high volumes of municipal wastewater due to their reduced energy requirements ⁹.

The performance metrics of submerged MBRs are impressive: they achieve over 95% COD reduction and even higher BOD removal ⁹. The turbidity in treated effluent consistently remains below 0.1 NTU, with COD and BOD reduction exceeding 97% ⁹. Moreover, these systems can effectively remove total suspended solids (TSS) to concentrations below 1 mg/L, demonstrating excellent solids separation ⁸.

Side-stream MBRs in industrial applications

Side-stream configurations position membrane modules outside the bioreactor tank, primarily serving industrial wastewater treatment needs since the mid-1990s ¹⁰. This arrangement facilitates easier membrane cleaning and replacement compared to submerged systems ¹¹. Side-stream MBRs generate high cross-flow velocity and shear stress across membrane surfaces, effectively reducing fouling rates ¹².

In industrial settings, side-stream MBRs typically process challenging effluents with low filterability ¹³. These systems handle wastewater from food processing, beverage production, and other industrial sectors ¹⁰. According to research, side-stream MBRs require smaller membrane areas yet function effectively with strong sewage and low filterability wastewater ¹³.

Hybrid MBR systems with anaerobic digestion

Anaerobic membrane bioreactors (AnMBRs) combine anaerobic digestion with membrane filtration, offering substantial energy benefits through biogas generation. These systems remove over 97% of chemical oxygen demand (COD) while producing biogas equivalent to 2.4 kWh per kg of COD in the feed daily ⁴. Additionally, they convert nutrients into chemically-available forms like ammonia and phosphate, facilitating potential nutrient recovery ¹⁴.

Hybrid configurations frequently incorporate anaerobic processes with other treatment stages. For instance, moving bed biofilm membrane reactors (MBBMR) combine MBR and moving bed biofilm reactor technologies, resulting in improved removal efficiency, less sludge generation, and reduced biofouling ¹⁵. Similarly, hybrid systems utilizing AnMBR with microalgal membrane reactors demonstrate multiple benefits: high effluent quality, nutrient recycling, and renewable biomass production ⁴.

Efficiency Gains in 2025: Energy, Footprint, and Water Quality

In 2025, membrane bioreactors stand out primarily for their remarkable efficiency metrics across three critical dimensions: space utilization, energy consumption, and effluent quality.

30% smaller footprint vs conventional systems

MBR systems require 30-50% less space than conventional activated sludge processes ¹⁶. This dramatic footprint reduction occurs because MBRs maintain higher mixed liquor suspended solids concentrations (8,000-12,000 mg/L), allowing smaller reactor volumes ¹⁶. Indeed, the integration of biological treatment with membrane filtration eliminates the need for separate secondary clarifiers, further minimizing spatial requirements ¹⁷. For facilities with limited expansion options, particularly in densely populated regions, this compact design proves invaluable ¹⁸.

Energy recovery through biogas in aerobic MBRs

Anaerobic MBRs (AnMBRs) have emerged as especially promising, generating energy-rich biogas alongside treatment. These systems typically produce 4.7±0.15 L/day of biogas with 64% methane content ¹. This equals approximately 0.3–0.5 L biogas per gram of COD removed, translating to an energy production rate of 2.4 kWh/kg COD daily ¹. Subsequently, this biogas generation offsets operational costs, pushing facilities closer to energy self-sufficiency ¹⁹.

Effluent quality: <5 mg/L BOD and TSS

MBRs consistently deliver exceptional water quality with:

BOD and TSS levels below 5 mg/L ²⁰
COD removal efficiencies exceeding 95% ²¹
TSS removal approaching 99.5% ²¹

These superior metrics make MBR-treated water immediately suitable for irrigation, cooling towers, and other reuse applications without additional processing ²⁰.

MBRs in Action: Real-World Deployments and Results

Real-world implementations of membrane bioreactors across diverse settings demonstrate their adaptability and effectiveness in modern wastewater treatment.

Case study: MBR retrofit in Singapore’s NEWater plant

Singapore’s national water agency PUB has strategically implemented MBR technology to enhance water reclamation capabilities ²². The Changi Water Reclamation Plant underwent a significant retrofit, transforming four existing conventional bioreactors into an MBR facility without increasing the footprint yet expanding treatment capacity from 800,000m³/d to 920,000m³/d ²³. Likewise, Jurong Water Reclamation Plant’s MBR facility currently treats 68,000m³/d of wastewater ²³, producing industrial-grade water for nearby Jurong Island’s manufacturing sector.

Decentralized MBRs in rural India

Compact, containerized MBR systems effectively address water scarcity in underserved areas. A residential community installation by Almasa demonstrates this potential, with their MBR consistently producing high-quality effluent suitable for irrigation and landscaping, substantially reducing freshwater dependence ²⁴. These small-scale systems prove particularly valuable where centralized infrastructure remains impractical.

Zero liquid discharge (ZLD) with MBR pre-treatment

MBR technology serves as a vital pre-treatment component in zero liquid discharge systems. In India, a tannery effluent facility services 130+ member tanneries via a common treatment plant where MBR provides superior feed quality to reverse osmosis units ²⁵. This arrangement enables RO recovery rates up to 95% ⁷, minimizing liquid waste while producing reusable water. ZLD systems recover salts and water ⁶, creating closed-loop processes that eliminate environmental discharge entirely.

Conclusion

Membrane bioreactors have undoubtedly reshaped wastewater treatment landscapes in 2025, offering unmatched advantages over conventional systems. Their combination of biological processes with membrane filtration technology represents a significant leap forward for water reclamation worldwide. Throughout this article, we examined how these systems deliver exceptional water quality with BOD and TSS levels consistently below 5 mg/L while requiring 30-50% less space than traditional activated sludge processes.

The various MBR configurations discussed—submerged, side-stream, and hybrid systems—provide adaptable solutions for different treatment scenarios. Municipal facilities benefit primarily from submerged systems, whereas industrial applications often favor side-stream configurations for their maintenance advantages and ability to handle challenging effluents. Additionally, anaerobic MBRs generate valuable biogas, producing approximately 2.4 kWh per kilogram of COD daily, thus offsetting operational energy demands.

Real-world applications further demonstrate MBR versatility. Singapore’s NEWater plants showcase large-scale implementation, while decentralized systems in rural India prove the technology’s scalability. Zero liquid discharge facilities highlight MBR effectiveness as pre-treatment for advanced water recovery systems.

The superior contaminant removal capabilities, smaller footprint requirements, and energy recovery potential make membrane bioreactors a cornerstone technology for modern water management challenges. These systems will certainly continue to evolve, addressing growing water scarcity concerns and enabling expanded water reuse opportunities across agricultural, industrial, and municipal sectors. MBRs stand as testament to how technological innovation can transform essential infrastructure while delivering environmental and economic benefits simultaneously.

FAQs

Q1. How do membrane bioreactors (MBRs) improve wastewater treatment? Membrane bioreactors combine biological treatment with membrane filtration, resulting in superior contaminant removal and higher quality effluent. They maintain higher concentrations of microorganisms, eliminate the need for secondary clarifiers, and produce water suitable for various reuse applications.
Q2. What are the key advantages of MBRs over conventional treatment systems? MBRs offer several benefits, including a 30-50% smaller footprint, superior effluent quality with BOD and TSS levels below 5 mg/L, and the potential for energy recovery through biogas production in anaerobic configurations. They also provide better pathogen removal and can handle challenging industrial effluents effectively.
Q3. How do MBRs contribute to water reuse and conservation efforts? MBRs produce high-quality effluent that’s immediately suitable for irrigation, cooling towers, and other reuse applications without additional processing. This capability expands water recycling opportunities, reducing the demand for freshwater sources and supporting water conservation initiatives in various sectors.
Q4. Are there different types of MBRs for various applications? Yes, there are several MBR configurations. Submerged MBRs are commonly used for large-scale municipal wastewater treatment, while side-stream MBRs are preferred for industrial applications. Hybrid systems, such as anaerobic MBRs, offer additional benefits like biogas production and nutrient recovery.
Q5. How effective are MBRs in removing contaminants from wastewater? MBRs demonstrate exceptional contaminant removal capabilities. They consistently achieve over 95% COD reduction, BOD removal exceeding 97%, and can reduce total suspended solids (TSS) to concentrations below 1 mg/L. The treated effluent typically has a turbidity below 0.1 NTU, indicating excellent water clarity.

References

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