Membrane Bioreactor (MBR) Systems: How They Work and Why They Matter

Membrane Bioreactor (MBR) Systems: How They Work and Why They Matter

Membrane bioreactors (MBRs) combine biological wastewater treatment with membrane filtration to produce high-quality effluent suitable for reuse or stringent discharge requirements. By replacing the conventional gravity-settling clarifier with a membrane separation step, MBRs achieve superior effluent quality in a significantly smaller footprint. This article explains how MBR systems work, their advantages, configurations, and key design considerations.

How Does an MBR Work?

An MBR integrates two fundamental processes: biological degradation by activated sludge, and physical separation by membrane filtration. Microorganisms in the bioreactor break down organic pollutants (BOD, COD) and nutrients (nitrogen, phosphorus) through aerobic or anaerobic metabolic processes. The membrane module then separates the treated water from the biomass, producing clarified, disinfected effluent.

Unlike conventional activated sludge (CAS) systems that rely on gravity settling to separate sludge from treated water, MBRs use microfiltration (0.1-0.4 μm) or ultrafiltration (0.01-0.1 μm) membranes. This provides an absolute physical barrier that retains virtually all suspended solids, bacteria, and most viruses, regardless of sludge settling characteristics.

MBR Configurations

Submerged (Immersed) MBR

In submerged MBR systems, membrane modules are placed directly inside the bioreactor tank or in a separate membrane tank connected to the bioreactor. A vacuum or low suction pressure (typically 0.1-0.5 bar) draws treated water through the membranes while retaining biomass. Coarse bubble aeration provides membrane scouring to control fouling.

Submerged MBRs are the most common configuration due to their lower energy consumption compared to sidestream systems. They use hollow fiber or flat sheet membrane modules from manufacturers such as GE/Suez, Kubota, and Mitsubishi.

Sidestream (External) MBR

In sidestream MBR systems, mixed liquor is pumped from the bioreactor to an external membrane module (typically tubular or multichannel ceramic) at high crossflow velocity. The higher pressure and velocity provide better fouling control but at higher energy cost.

Sidestream MBRs are preferred for industrial wastewater treatment where high pollutant concentrations, extreme pH, or high temperatures preclude the use of submerged polymeric membranes.

Advantages of MBR Over Conventional Treatment

• Superior effluent quality: BOD < 5 mg/L, TSS < 1 mg/L, turbidity < 0.5 NTU, near-complete removal of bacteria and protozoa

• Smaller footprint: 30-50% less space than conventional systems due to elimination of secondary clarifiers and higher MLSS concentrations

• Higher sludge retention time (SRT): Enables complete membrane retention of slow-growing organisms for enhanced nutrient removal

• Consistent effluent quality: Independent of sludge settling properties, eliminating issues with filamentous bulking

• Water reuse capability: Effluent quality suitable for non-potable reuse or as pretreatment for RO-based potable reuse

• Modular expansion: Easy to add capacity by installing additional membrane modules

Membrane Fouling in MBRs

Fouling is the primary operational challenge in MBR systems. Fouling mechanisms include:

• Cake layer formation: Accumulation of biomass and particles on the membrane surface; managed by air scouring and periodic backwashing

• Pore blocking: Internal clogging of membrane pores by dissolved organic matter and fine colloids

• Biofouling: Growth of biofilm on membrane surfaces; controlled by cleaning chemicals and operating conditions

• Inorganic scaling: Precipitation of calcium carbonate, struvite, or other minerals; managed by pH control and chemical cleaning

Fouling management strategies include optimizing aeration intensity, controlling MLSS concentration (typically 8,000-12,000 mg/L), regular maintenance cleaning (weekly with NaOCl), and periodic recovery cleaning (monthly with acid and alkali).

MBR Membranes: Materials and Formats

• Hollow fiber PVDF: Most common for submerged MBRs. High packing density and flexible module design. Typical pore size 0.04-0.4 μm

• Flat sheet PVDF/PE: Used by Kubota and similar manufacturers. Easier to clean and replace but lower packing density

• Tubular ceramic: Preferred for industrial sidestream MBRs. Superior chemical and thermal resistance for challenging wastewaters

Lab-Scale MBR Testing

Researchers developing new MBR membranes or optimizing operating conditions need reliable testing equipment. Key components include membrane test cells for evaluating membrane performance under MBR-like conditions, as well as bench-scale bioreactor systems.

Tech Inc. manufactures membrane test cells suitable for evaluating MBR membrane materials. Our crossflow and submerged test cell configurations enable researchers to simulate both sidestream and submerged MBR conditions at laboratory scale.

Frequently Asked Questions

How much does an MBR system cost?

MBR capital costs are typically 1.5-2x higher than conventional activated sludge systems, primarily due to membrane costs. However, smaller footprint, fewer unit operations, and higher effluent quality can offset the higher capital cost, especially for water reuse applications.

What is the typical membrane life in an MBR?

Polymeric MBR membranes typically last 7-10 years with proper operation and maintenance. Ceramic membranes in sidestream MBRs can last 15-20+ years.

Can MBR effluent be used for potable reuse?

MBR effluent serves as excellent pretreatment for potable reuse systems. The typical treatment train for indirect potable reuse is MBR → RO → UV/AOP. MBR provides consistent, low-fouling feed water for the downstream RO system.

What is the energy consumption of an MBR?

Submerged MBRs typically consume 0.4-0.8 kWh/m³ for aeration and 0.02-0.1 kWh/m³ for permeate extraction. Sidestream MBRs consume more energy (1-3 kWh/m³) due to high crossflow pumping requirements.