11.4 Membrane Bioreactors
Key Takeaways
- An MBR replaces the secondary clarifier with membrane filtration, so solids separation no longer depends on how well the sludge settles.
- Because separation is physical, MBRs run at mixed liquor concentrations of 8,000 to 12,000 mg/L or higher, roughly three to four times conventional activated sludge.
- Transmembrane pressure is the primary fouling indicator, and a rising TMP at constant flux means the membranes need cleaning.
- Fine screening at 1 to 3 mm ahead of the bioreactor is mandatory, because hair and fibrous material braid onto membrane fibers and cause irreversible damage.
- Maintenance cleaning is a frequent short chemical soak, while recovery cleaning is an infrequent intensive clean that restores permeability after fouling has accumulated.
11.4 Membrane Bioreactors
A membrane bioreactor (MBR) is activated sludge in which the secondary clarifier has been replaced by membrane filtration. That single substitution changes almost everything about how the process is operated.
Why Removing the Clarifier Matters
In conventional activated sludge, everything downstream depends on whether the sludge settles. Bulking, poor settleability, and rising sludge limit how much mixed liquor you can carry, because the clarifier must be able to separate it.
An MBR separates solids by physical exclusion through a membrane, so settleability becomes irrelevant to compliance. The consequences:
| Consequence | Detail |
|---|---|
| High MLSS | 8,000 to 12,000 mg/L and above, versus 2,000 to 4,000 mg/L conventional |
| Small footprint | Higher biomass per unit volume plus no clarifier |
| Complete solids removal | Effluent TSS essentially non-detectable; turbidity below 0.2 NTU |
| Disinfection benefit | Membranes physically remove bacteria and protozoa, providing substantial log removal credit |
| Long SRT decoupled from HRT | Sludge age is set by wasting alone, independent of settling |
| Reliable nitrification | Long achievable sludge age retains nitrifiers |
| Reuse quality | Effluent is well suited to reclaimed water production |
Trade-offs are real: higher energy consumption, membrane replacement cost every several years, greater operational complexity, and sensitivity to fouling.
Configurations
- Submerged (immersed): membrane modules sit directly in the bioreactor or in a separate membrane tank. Permeate is drawn through under vacuum. The dominant configuration.
- Sidestream (external): mixed liquor is pumped through membrane modules outside the tank under pressure. Higher energy; used for industrial applications.
Membrane formats are hollow fiber and flat sheet, generally in the microfiltration to ultrafiltration range, with pore sizes around 0.04 to 0.4 micron.
Core Operating Parameters
Flux
Expressed in gallons per square foot per day (gfd) or liters per square meter per hour (LMH). Operating above the sustainable flux accelerates fouling disproportionately.
Transmembrane Pressure
TMP is the pressure difference driving water across the membrane. It is the primary fouling indicator.
[!IMPORTANT] Rising TMP at constant flux means fouling. Equivalently, falling permeability means fouling. Trend TMP daily. A slow rise is normal and is addressed by routine maintenance cleaning; a rapid rise signals a specific problem such as a screening failure, a solids excursion, or a sludge quality change.
Relaxation and Backpulsing
Membranes are operated cyclically, not continuously — for example nine minutes of filtration followed by one minute of relaxation (permeate pump off while air scour continues), or brief backpulsing in which permeate is pushed backward through the membrane. Both dislodge the cake layer and are essential to sustained operation.
Air Scour
Coarse-bubble air is directed along the membrane surface to scour accumulated solids. This is separate from the process aeration that supplies oxygen to the biology, and it is a major share of an MBR's energy consumption.
Fouling and Cleaning
| Fouling type | Cause | Removed by |
|---|---|---|
| Cake layer | Solids accumulating on the surface | Air scour, relaxation, backpulse |
| Organic/biofouling | Extracellular polymeric substances, biofilm | Sodium hypochlorite cleaning |
| Inorganic scaling | Calcium carbonate, struvite, iron | Citric or oxalic acid cleaning |
| Irreversible | Pore plugging, physical damage | Not recoverable; requires replacement |
Two Levels of Chemical Cleaning
| Maintenance cleaning | Recovery cleaning | |
|---|---|---|
| Frequency | Weekly to monthly | Every 6 to 12 months, or when permeability drops below a threshold |
| Duration | Short, often under an hour | Extended soak, several hours |
| Concentration | Low | High |
| Location | Usually in place | In place or in a separate tank |
| Purpose | Prevent fouling accumulation | Restore lost permeability |
Both typically use hypochlorite for organic fouling and an acid for scaling. Never mix them — sequence them with a rinse between, since hypochlorite and acid together liberate chlorine gas.
Protecting the Membranes
Fine Screening Is Mandatory
1 to 3 mm fine screening ahead of the bioreactor is not optional. Hair, lint, and fibrous material braid around hollow fibers, forming ropes that abrade and eventually sever them. This damage is irreversible, and a screening bypass during a high-flow event can cost a membrane cassette.
Other Protections
- Fats, oils, and grease coat membranes and are poorly removed by cleaning; upstream FOG control matters more at an MBR than at a conventional plant.
- Foaming is aggravated by high MLSS; control with defoamer, sludge age adjustment, and by addressing filamentous causes.
- Membrane integrity testing — typically a pressure decay test — verifies that no fibers are broken, which matters because integrity underpins the pathogen removal credit.
- Do not let membranes dry out. A membrane allowed to dry may suffer permanent permeability loss. Follow the manufacturer's preservation procedure for any extended outage.
Aeration at High MLSS
Oxygen transfer efficiency declines as mixed liquor concentration rises, because the thicker mixed liquor is more viscous and resists bubble dispersion. The correction factor applied to oxygen transfer, commonly called alpha, drops significantly at MBR solids concentrations. Operators therefore cannot assume that the blower capacity adequate for a 3,000 mg/L conventional basin will deliver equivalent oxygen at 10,000 mg/L, and dissolved oxygen must be monitored directly rather than inferred from airflow.
An MBR operating at constant permeate flux shows transmembrane pressure rising steadily from 3 psi to 9 psi over three weeks. What does this indicate?
Why is 1 to 3 mm fine screening ahead of a membrane bioreactor considered mandatory rather than optional?
An MBR operates at 10,000 mg/L mixed liquor suspended solids. Compared with a conventional activated sludge basin at 3,000 mg/L, what should the operator expect regarding aeration?