MBR stands for membrane bioreactor, a wastewater treatment technology that combines biological treatment with membrane filtration to produce high-quality treated water. Instead of relying on gravity settling tanks to separate solids from water (the traditional approach), an MBR uses fine physical membranes to filter out contaminants, bacteria, and suspended solids. The result is cleaner water in a smaller physical footprint, which is why MBR systems have become one of the most widely adopted technologies for both municipal and industrial wastewater treatment.
How an MBR System Works
An MBR pairs two processes that would normally happen separately: biological breakdown and physical filtration. In the biological stage, microorganisms in aerated tanks consume organic pollutants in the wastewater, just like in a conventional treatment plant. The key difference is what happens next. Rather than sending the water to a large settling tank where solids slowly sink to the bottom, the mixed water passes through a membrane with pores fine enough to physically block bacteria, suspended particles, and sludge.
These membranes use either microfiltration or ultrafiltration, meaning their pores are small enough to catch particles down to fractions of a micron. A typical MBR plant includes a mechanical screen for initial pretreatment, anoxic and aerobic tanks for biological treatment, air blowers for aeration, a sludge recirculation system, chemical dosing equipment, and a cleaning tank for membrane backwashing.
Two Main Configurations
MBR systems come in two primary designs: submerged and sidestream. In a submerged MBR, the membrane modules sit directly inside the biological treatment tank, immersed in the mixed liquor. In a sidestream MBR, the membranes are housed in a separate unit outside the main tank, and the wastewater is pumped through them externally.
Each design handles fouling (the gradual clogging of membrane pores) differently. Submerged systems tend to run longer before needing cleaning. In comparative testing, submerged configurations operated steadily for 61 days, while sidestream setups lasted about 39 days before fouling became a problem. On the other hand, sidestream membranes develop less biological buildup on their surfaces because the environment around the external module is nutrient-poor, making it harder for microorganisms to form a sticky biofilm layer. The choice between the two often depends on the scale of the plant and the type of wastewater being treated.
How MBR Compares to Conventional Treatment
The biggest advantage of MBR over traditional activated sludge systems is the elimination of settling tanks. In a conventional plant, treated water flows into large clarifiers where gravity slowly separates the solids. These tanks take up significant space and don’t always produce consistently clean water. MBR membranes replace that entire step, which dramatically reduces the physical footprint of the plant.
The water quality difference is measurable. MBR systems produce lower concentrations of organic pollutants and suspended solids in the treated water compared to conventional activated sludge processes. They’re also faster. A submerged aerobic MBR can remove 90 to 95 percent of organic contaminants in as little as 4 hours of hydraulic retention time (the amount of time water stays in the system). Conventional plants typically need much longer retention times to achieve comparable results. This speed and efficiency make MBR especially useful in dense urban areas or industrial sites where space is limited and discharge standards are strict.
The Fouling Problem
The main operational challenge with MBR systems is membrane fouling. Over time, biological material builds up on and inside the membrane pores, reducing the flow of treated water and increasing energy costs. The primary culprits are substances produced by the microorganisms themselves: sticky polymers they secrete outside their cells. These compounds decrease the membrane’s ability to filter water and, over time, cause a type of deep fouling that ordinary cleaning can’t fully reverse.
Operators manage fouling through a combination of routine backwashing (reversing the flow to dislodge surface buildup) and periodic chemical cleaning, typically with a sodium hypochlorite solution. The goal is to extend the time between intensive cleanings, since each chemical wash temporarily takes the membrane offline and shortens its overall lifespan.
One promising biological approach involves cultivating specific microorganisms that store carbon inside their cells rather than secreting it externally. When less sticky material is released into the water, the membranes stay cleaner for longer, and routine backwashing becomes more effective. This reduces the need for harsh chemical cleanings and lowers long-term maintenance costs.
Anaerobic MBR and Energy Recovery
Most MBR systems are aerobic, meaning they require constant aeration to keep the biological process running. This makes them energy-intensive. A newer variation, the anaerobic membrane bioreactor, flips the energy equation. Instead of consuming large amounts of electricity to pump air into the tanks, anaerobic systems treat wastewater without oxygen and produce biogas (methane) as a byproduct, which can be captured and used for energy.
A pilot system tested by the California Energy Commission processed about 24,000 gallons per day and met U.S. secondary effluent standards in a more compact footprint than typical aerobic systems. The most striking finding: at full scale, this type of anaerobic MBR could generate a renewable energy surplus of about 0.35 kilowatt-hours per cubic meter of water treated, while cutting biosolids production (the sludge that needs disposal) by roughly 90 percent. That means wastewater treatment plants could shift from being large electricity consumers to net energy producers, a significant change for an industry that accounts for a notable share of municipal energy budgets.