Process Technology
Membrane Bioreactor (MBR)
Combining biological treatment with membrane filtration. Eliminates secondary clarifiers and produces high-quality effluent in a compact footprint.
Process Description
A Membrane Bioreactor combines a conventional activated sludge process with membrane filtration (microfiltration or ultrafiltration). The membranes replace the secondary clarifier, retaining all suspended solids within the bioreactor.
This allows operation at significantly higher MLSS concentrations (8,000–15,000 mg/L) compared to conventional activated sludge (2,000–4,500 mg/L), resulting in a smaller bioreactor footprint and higher volumetric treatment capacity.
Key Design Parameters
MLSS
Typical MBR range
Flux
Design net flux
Pore Size
MF/UF membranes
Footprint Reduction
vs. conventional AS
Advantages
No secondary clarifier needed — significant footprint reduction
Complete solids retention allows higher MLSS and longer SRT
Superior effluent quality (typically <1 NTU turbidity)
Decoupled HRT from SRT — allows compact design
Effluent suitable for reuse applications
Well-suited for space-constrained sites and retrofits
Energy Considerations
MBR systems typically consume 0.4–1.0 kWh/m³ of treated water, compared to 0.2–0.5 kWh/m³ for conventional activated sludge. The additional energy is primarily for membrane aeration (scouring) and permeate suction.
Engineering Note
When to Consider MBR
| Condition | Potential Issue | What to Investigate |
|---|---|---|
| Limited footprint | No space for clarifiers | MBR eliminates need for secondary clarifiers |
| Strict effluent limits | TSS/turbidity requirements | MBR provides <1 NTU, near-complete pathogen removal |
| Water reuse target | Need RO-quality feed | MBR effluent suitable as RO pretreatment |
| High MLSS needed | Increase biological capacity | MBR supports 8,000–15,000 mg/L MLSS |
Frequently Asked Questions
An MBR combines biological treatment with membrane filtration (micro- or ultrafiltration) instead of a secondary clarifier. The membranes retain all biomass, allowing operation at high MLSS and producing a very high-quality, low-solids effluent suitable for reuse.
MBR delivers excellent effluent quality, a small footprint (thanks to high MLSS of roughly 8,000–15,000 mg/L) and complete biomass retention independent of settling. This makes it attractive where space is limited, effluent standards are stringent, or water reuse is a goal.
MBR typically has higher aeration and energy demand, requires membrane cleaning and eventual replacement, and needs effective pre-treatment (fine screening) to protect the membranes. These operating factors should be weighed against its effluent quality and footprint benefits in an engineering assessment.
Engineering Assessment
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