Membrane Bioreactor (MBR) Process
The Membrane Bioreactor, almost always shortened to MBR, combines the biological treatment stage of a conventional activated sludge plant with a membrane filtration step that replaces the secondary clarifier. Instead of relying on gravity to settle biological floc out of the mixed liquor, an MBR pulls the treated water through microfiltration or ultrafiltration membranes with pore sizes small enough to physically block bacteria, most viruses, and suspended solids. The result is an effluent that is consistently clear and low in solids, produced from a much smaller physical footprint than an equivalent conventional plant.
How a Membrane Bioreactor Works
An MBR system still has a biological stage that looks familiar to anyone who has studied the activated sludge process: wastewater enters an aeration basin where a suspended culture of microorganisms consumes organic matter and, in most designs, carries out nitrification. What differs is what happens next.
Biological Stage
The aeration basin is typically arranged as a series of zones (anoxic, anaerobic, and aerobic) depending on whether the plant is also targeting nitrogen and phosphorus removal. Because the membrane step is so effective at retaining biomass, MBR bioreactors can be operated at a much higher mixed liquor suspended solids (MLSS) concentration than a conventional plant, commonly cited in the range of roughly 8,000 to 12,000 mg/L or higher, compared with roughly 2,000 to 4,000 mg/L in a conventional activated sludge basin. Actual operating concentrations vary widely by design and should be set by the plant’s own process engineer, not treated as a fixed target.
Membrane Separation
The membranes themselves are usually hollow-fiber or flat-sheet modules with nominal pore sizes in the microfiltration to ultrafiltration range (roughly 0.03 to 0.4 microns). Water is drawn through the membrane wall under a light vacuum while solids, bacteria, and most pathogens are rejected and stay in the bioreactor. Because there is no need for a separate clarifier or, in many designs, a downstream sand filter, the overall plant footprint is significantly smaller than a conventional secondary treatment train.
Submerged vs. Side-Stream Configurations
MBR systems are generally built one of two ways:
- Submerged (immersed) MBR — membrane modules sit directly inside the aeration basin (or an adjacent membrane tank fed by the basin), with permeate drawn out under vacuum. Coarse-bubble aeration is supplied at the base of the membrane modules to scour the membrane surface and limit fouling. This is the more common configuration for municipal plants because it is more energy-efficient than the alternative.
- Side-stream (external) MBR — mixed liquor is pumped out of the bioreactor, across the membrane surface under pressure, and the concentrate is returned to the tank. Cross-flow velocity does the fouling control instead of aeration. Side-stream systems tend to use more pumping energy but can be easier to maintain and replace membrane modules on, which suits some industrial applications.
Design and Operating Parameters
A few parameters define how an MBR is sized and operated. As with any biological process, the numbers below are typical or commonly cited figures, not universal design rules — actual values depend on influent characteristics, effluent targets, membrane manufacturer guidance, climate, and the responsible engineer’s judgment.
| Parameter | Typical Range Often Cited |
|---|---|
| MLSS in bioreactor | 8,000–12,000 mg/L (can be higher) |
| Solids retention time (SRT) | 10–30 days |
| Hydraulic retention time (HRT) | 4–8 hours |
| Membrane flux | 15–35 L/m²·h, design-dependent |
| Membrane pore size | 0.03–0.4 microns (MF/UF range) |
Because SRT and HRT are effectively decoupled in an MBR — the membrane can hold solids back almost indefinitely regardless of hydraulic flow — designers have much more freedom to run a long SRT (favoring nitrification and lower sludge production) in a smaller tank volume than a conventional plant would need.
Membrane Fouling and Cleaning
Fouling is the central operating challenge of any MBR. Biological floc, extracellular polymeric substances, colloids, and inorganic scale can all build up on and inside the membrane, reducing permeability and driving up the transmembrane pressure needed to maintain flow. Well-run plants manage fouling with a layered strategy:
- Coarse-bubble aeration or cross-flow scouring to continuously sweep solids off the membrane surface during normal operation.
- Relaxation or backwashing cycles, where permeate flow is paused or briefly reversed at set intervals to let material release from the membrane before it becomes attached fouling.
- Chemical maintenance cleaning, typically a periodic soak or backwash with dilute sodium hypochlorite or citric acid, done in place on a routine schedule.
- Chemical recovery cleaning, a more intensive off-line soak used when fouling has progressed further and normal maintenance cleaning is no longer restoring performance.
Upstream fine screening (commonly 1–3 mm) ahead of the bioreactor is essential in almost every MBR design, since hair, fibers, and debris that a conventional plant would tolerate can quickly damage or blind hollow-fiber membranes.
Advantages Over Conventional Activated Sludge
- Consistently high effluent quality — low turbidity, low suspended solids, and significant pathogen removal, often good enough for direct water reuse applications without additional filtration.
- Smaller footprint, since clarifiers and often tertiary filters are eliminated.
- Greater tolerance to influent variability because of the high biomass concentration and long SRT.
- Easier compliance with strict nutrient or reuse-quality discharge limits.
Limitations and Challenges
- Higher capital cost for membrane modules and the finer preliminary screening they require.
- Higher energy demand than conventional activated sludge, largely from the aeration needed for membrane scouring on top of the biological aeration demand.
- Membranes have a finite service life and represent an ongoing replacement cost.
- Operators need more specialized training to manage fouling, chemical cleaning cycles, and membrane integrity testing than a conventional secondary clarifier requires.
Where MBR Fits
MBR technology has become common for facilities where land is limited, effluent quality standards are strict, or the treated water is destined for reuse — municipal plants in dense urban areas, decentralized or packaged plants serving developments and resorts, and industrial wastewater applications where variable or difficult influent makes a robust, compact process attractive. It is generally not the automatic choice for every application; conventional activated sludge, oxidation ditches, or sequencing batch reactors can still be the more economical option where footprint and effluent quality targets are less demanding.
As membrane costs continue to fall and reuse regulations tighten in many regions, MBR has moved from a specialty technology to a mainstream option that every wastewater engineer should understand alongside the more traditional activated sludge variants.
