Membrane Bioreactor (MBR) Technology: Design, Operation, and Troubleshooting

Membrane Bioreactor (MBR) Technology: Design, Operation, and Troubleshooting

Membrane bioreactors (MBRs) combine biological wastewater treatment with membrane filtration in a single process, producing high-quality effluent suitable for reuse or stringent discharge requirements. MBR technology has grown rapidly, with global installed capacity exceeding 10 million m³/day, driven by tightening discharge standards and increasing water reuse.

How MBRs Work

An MBR replaces the secondary clarifier in a conventional activated sludge process with MF or UF membranes. The membranes are either submerged in the biological reactor (submerged MBR) or located externally with mixed liquor pumped through the membrane module (sidestream MBR). The membranes retain all biomass in the reactor, allowing much higher MLSS concentrations (8,000-15,000 mg/L vs 2,000-4,000 for conventional) and producing particle-free permeate.

MBR Configurations

Submerged (Immersed) MBR

• Membranes installed directly in the aeration tank or in a separate membrane tank

• Operates under vacuum/suction (0.1-0.5 bar TMP)

• Lower energy than sidestream (0.3-0.8 kWh/m³)

• Dominant configuration for municipal wastewater (>90% of installations)

• Flat sheet or hollow fiber membranes; hollow fiber most common

Sidestream (External) MBR

• Mixed liquor pumped through external membrane modules at high crossflow velocity

• Higher TMP (1-5 bar) and energy (2-6 kWh/m³) but better fouling control

• Preferred for industrial wastewater with high fouling potential

• Tubular ceramic or polymeric membranes common

MBR Advantages

• Superior effluent quality: <1 NTU turbidity, <2 mg/L BOD, complete pathogen removal by MF/UF barrier

• Compact footprint: No secondary clarifier; 50-70% smaller than conventional systems

• High biomass concentration: MLSS 8,000-15,000 mg/L enables higher volumetric loading

• Decoupled HRT and SRT: Long SRT (15-25 days) with short HRT (4-8 hours)

• Direct reuse potential: MBR permeate is suitable feed for RO in water reuse schemes

Fouling Management

• Air scouring: Coarse bubble aeration below the membranes provides continuous cleaning and oxygen for biology

• Backwash/relaxation: Periodic backwashing (every 8-15 minutes) or membrane relaxation (stopping permeation briefly)

• Maintenance cleaning: Weekly to monthly chemical cleaning with NaOCl (200-1000 ppm) or citric acid (1-2%)

• Recovery cleaning: Intensive chemical soak (NaOCl 2000-5000 ppm + NaOH) when flux drops significantly

• MLSS control: Maintain MLSS below 15,000 mg/L to prevent excessive cake formation and poor oxygen transfer

Tech Inc. provides membrane test cells and fouling characterization equipment for MBR research, enabling evaluation of membrane fouling with activated sludge under controlled conditions.

Frequently Asked Questions

What is the energy cost of an MBR?

Submerged MBRs consume 0.4-1.0 kWh/m³ total (aeration + permeation), compared to 0.2-0.4 kWh/m³ for conventional activated sludge. The energy premium is offset by superior effluent quality and smaller footprint.

How long do MBR membranes last?

Hollow fiber MBR membranes typically last 7-10 years in municipal applications. Flat sheet membranes may last slightly longer (8-12 years). Ceramic sidestream membranes can exceed 15 years.