An MBR, or membrane bioreactor, is a wastewater treatment system that combines biological processing with physical membrane filtration. Instead of relying on gravity settling tanks to separate clean water from sludge (the traditional approach), an MBR pushes water through a fine membrane that physically blocks bacteria, solids, and many pathogens. The result is significantly cleaner water that can often be reused directly. The global MBR market is valued at roughly $4.65 billion in 2025 and growing at nearly 12% per year, driven by tightening water regulations and increasing demand for water recycling.
How an MBR Works
A membrane bioreactor pairs two processes into one system. First, microorganisms break down organic pollutants in wastewater, just like they do in a conventional treatment plant. Second, instead of sending that water to a large settling tank where solids slowly sink to the bottom, the water passes through a membrane with extremely small pores. Microfiltration membranes have pores around 0.4 micrometers, while ultrafiltration membranes go as fine as 0.034 micrometers. For context, most bacteria are 1 to 10 micrometers across, so even the larger pore size blocks virtually all of them.
This combination means MBRs can do in a single compact unit what traditional plants need multiple stages and larger footprints to accomplish. The membrane acts as an absolute physical barrier, so the treated water quality is consistent regardless of fluctuations in the incoming wastewater.
Two Main Configurations
MBR systems come in two designs: submerged and side-stream. In a submerged (or immersed) MBR, the membrane sits directly inside the tank where the biological treatment happens. Suction draws clean water through the membrane while everything else stays in the tank. Air bubbles are blown across the membrane surface to reduce clogging. This setup uses less energy per cubic meter of water treated, making it the standard choice for large municipal wastewater plants. The tradeoff is that submerged systems need a larger membrane area and more frequent cleaning cycles.
In a side-stream MBR, the mixed wastewater is pumped out of the biological tank and through an external membrane module before being returned. This design is more compact, needs less membrane surface area, and handles stronger, harder-to-filter industrial wastewater well. It also offers more operational flexibility and is simpler to expand. The downside is significantly higher energy consumption, because pumping wastewater at high velocity through the external module takes considerable power.
Choosing Between Them
Submerged MBRs handle high-volume, lower-strength wastewater like municipal sewage and are less sensitive to flow variations. Side-stream MBRs are better suited to concentrated industrial waste with poor filterability. Side-stream systems also need less frequent backwashing, while submerged systems require it more often but use less energy overall.
How Clean Is the Output?
MBR effluent is remarkably clean. In a direct comparison study of a Greek municipal plant, an MBR reduced suspended solids from 440 mg/L down to less than 1 mg/L and biochemical oxygen demand from 400 mg/L to under 5 mg/L. Those numbers matched what a conventional plant with full tertiary treatment achieved, but the MBR did it without needing the extra treatment stage.
Nutrient removal is also strong. Advanced MBR setups that cycle water through oxygen-free and oxygen-rich zones can remove about 82% of nitrogen and over 96% of phosphorus. These are important pollutants that cause algal blooms in rivers and lakes, so high removal rates matter for environmental protection.
The ultrafiltration membranes are particularly effective at blocking viruses. A study comparing microfiltration and ultrafiltration MBR systems found that the smaller-pored ultrafiltration membranes produced water with notably fewer viral indicators, a meaningful distinction when treated water is destined for reuse.
Membrane Types: Hollow Fiber vs. Flat Sheet
The two most common membrane formats are hollow fiber and flat sheet. Hollow fiber membranes look like bundles of thin straws, and their tubular shape packs a large amount of surface area into a small space. Flat sheet membranes are exactly what they sound like: flat panels stacked in frames. Both achieve similar overall pollutant removal. In a study treating difficult landfill wastewater, hollow fiber and flat sheet membranes achieved nearly identical organic matter removal (around 92 to 93%) and phosphorus removal (roughly 79 to 87%). Flat sheet membranes showed an edge in nitrogen removal (61% vs. 49%), partly because they needed less chemical cleaning, which can interfere with the nitrogen-removing bacteria.
The Fouling Problem
The biggest operational challenge with MBRs is membrane fouling, the gradual buildup of material on and in the membrane that restricts water flow. Fouling accounts for approximately 30% of MBR operating costs. It happens in stages: first, dissolved organic compounds quickly adsorb onto the membrane surface, then particles and microbial byproducts accumulate, and eventually a biological film develops.
Biofouling is the hardest type to control because the system is, by design, full of active microorganisms in a nutrient-rich environment. The microbes produce sticky substances that bind to the membrane surface. Traditional countermeasures like surface coatings or chemical agents have limited effectiveness because they struggle to address the combined effects of microbial growth and organic matter buildup. Operators manage fouling through regular backwashing (reversing the flow to dislodge material), periodic chemical cleaning, and maintaining air scouring to keep the membrane surface turbulent.
Energy and Cost Considerations
MBRs consume between 0.4 and 1.15 kilowatt-hours per cubic meter of water treated. By comparison, conventional activated sludge systems use 0.3 to 0.64 kWh per cubic meter. The higher energy demand comes from running the membrane filtration, air scouring, and (in side-stream systems) pumping. That energy gap has narrowed considerably as membrane technology has improved, and for many applications the superior water quality and smaller physical footprint offset the extra energy cost.
Where MBRs often make economic sense is in situations where space is limited (they need roughly half the footprint of a conventional plant), where water reuse is the goal (the high-quality effluent reduces or eliminates downstream polishing steps), or where discharge regulations are strict enough that a conventional plant would need extensive tertiary treatment anyway.
Where MBRs Are Used
Municipal wastewater treatment is the largest application. Submerged MBRs treat city sewage at scale, producing water clean enough for irrigation, industrial cooling, or groundwater recharge. Industrial users include food and beverage processing, pharmaceutical manufacturing, and landfill leachate treatment, all situations involving wastewater too concentrated or variable for conventional systems to handle reliably.
The technology is growing fastest in water-scarce regions where treated wastewater is a critical resource. With the MBR market projected to reach $5.2 billion by 2026, adoption is accelerating in the Middle East, parts of Asia, and drought-prone areas of the western United States and southern Europe. The ability to produce reuse-quality water from a single, compact system is the core driver behind that growth.