A Membrane Bioreactor (MBR) integrates two distinct processes for wastewater treatment: the biological degradation of the conventional activated sludge process and a membrane-based physical separation technique. The MBR’s fundamental role is to produce high-quality treated water by effectively separating the activated sludge—the microorganisms responsible for cleaning the water—from the purified liquid stream. This integration allows for a more intense and efficient treatment compared to traditional methods.
How MBR Systems Function
MBR systems integrate a biological reactor with a physical filtration unit. Wastewater enters the bioreactor, where activated sludge consumes and breaks down organic pollutants, similar to the conventional activated sludge method.
The difference lies in the solid-liquid separation stage, where the membrane replaces the traditional secondary clarifier. Instead of relying on gravity to settle the sludge, the MBR uses microfiltration (MF) or ultrafiltration (UF) membranes to physically strain the water from the biomass. These membranes have very small pore sizes, ranging from 0.035 to 0.4 micrometers, effectively retaining nearly all suspended solids and microorganisms.
By retaining the biomass, the MBR maintains a high concentration of Mixed Liquor Suspended Solids (MLSS) within the reactor, often between 6,000 and 12,000 mg/L. This higher MLSS concentration allows the biological process to operate more efficiently and at a higher volumetric loading rate. The clarified effluent is drawn through the membrane pores, while the concentrated sludge is continuously recycled back into the reactor to maintain the high biomass level.
Essential MBR Configurations
MBR systems use two main configurations: submerged or side-stream. The submerged MBR is the most common configuration, particularly for municipal wastewater treatment. In this design, the membrane modules are placed directly inside the biological reactor tank and operated under a vacuum to draw the treated water through the membrane.
The side-stream MBR, or external MBR, positions the membrane modules in a separate unit outside the bioreactor. Here, the mixed liquor is continuously pumped from the biological tank, circulated through the external module under higher pressure, and then returned to the reactor. Submerged systems are favored for lower energy consumption, but the side-stream arrangement offers better accessibility for cleaning and maintenance.
Key Operational Advantages
MBR systems offer several operational benefits over conventional wastewater treatment methods. The superior quality of the treated water, or effluent, is a primary advantage. The fine pore size acts as an absolute barrier, ensuring complete removal of suspended solids and a high reduction of pathogens, making the water suitable for direct reuse applications like irrigation or non-potable industrial processes.
Operating the bioreactor with a high concentration of MLSS reduces the physical space required for the treatment plant. By eliminating the need for a large secondary clarifier and tertiary filtration, the MBR system achieves a smaller footprint, sometimes requiring up to four times less area than traditional systems. The MBR process also operates with a longer Sludge Retention Time (SRT) because the membranes retain slow-growing microorganisms. This improves the degradation of difficult-to-treat compounds and reduces the amount of excess sludge produced, lowering disposal costs.
Managing Membrane Fouling
Membrane fouling is the main challenge in operating MBR systems. Fouling is the accumulation of materials on the membrane surface or within its pores, which increases hydraulic resistance and impedes the flow of treated water. This requires a higher transmembrane pressure (TMP) to maintain flow, leading to increased energy consumption and more frequent cleaning. Fouling types include biological fouling from microbial growth, colloidal fouling from fine particles, and particulate fouling from larger suspended solids.
MBRs employ several strategies to control the build-up of foulants. Routine physical cleaning methods include relaxation, which briefly halts filtration, and backwashing, which forces treated water back through the membrane to dislodge material. Submerged MBRs also use coarse bubble aeration beneath the modules for continuous physical scouring of the membrane surface. When fouling is severe, chemical cleaning is performed periodically using mild oxidants or specialized enzymatic solutions to remove persistent deposits.
Primary Applications of MBR Technology
MBR technology is used in several application areas due to its ability to produce high-quality effluent within a compact space. In densely populated urban settings, municipal wastewater treatment plants use MBRs to expand capacity or upgrade existing facilities without acquiring additional land. The consistent, high-purity water is also beneficial in industrial applications, such as the food and beverage, textile, and pharmaceutical sectors, where strict discharge regulations must be met.
MBR technology is valuable in water reuse projects where treated wastewater is reclaimed for beneficial purposes. The effluent quality is high enough for non-potable uses like landscape irrigation, toilet flushing, and cooling tower makeup water. This enables communities in water-stressed regions to create a reliable, alternative water source, reducing the demand on potable water supplies.