What an MBR Actually Is
A membrane bioreactor (MBR) combines activated-sludge biology with microfiltration membranes—typically with a 1 μm pore size—within a single process train. Wastewater contacts microorganisms that break down organics, then the mixed liquor is filtered through submerged or external membranes; clean water passes as permeate while biomass and solids are retained. According to the U.S. EPA fact sheet, MBR effluent typically contains very low BOD, TSS, ammonia-nitrogen, and turbidity, often approaching analytical detection limits.
The EPA fact sheet frames the technology as a single unit operation rather than biology followed by a membrane add-on: "the membrane filtration system in effect can replace the secondary clarifier and sand filters in a typical activated sludge treatment system." That substitution is the heart of the design—the membrane performs the work of gravity settling and sand filtration, and the biology runs at a higher mixed-liquor concentration than a clarifier can tolerate. Two historical configurations coexist in the market: immersed (submerged) membranes sitting directly in the aeration tank, shown in EPA Figure 3, and external (side-stream) membranes housed in a separate vessel through which mixed liquor is circulated, shown in EPA Figure 4.
The Process Flow, Step by Step
Five distinct unit operations comprise an MBR process flow when read from left to right. Each has a defensible parameter range an engineer can put on a P&ID.
- Influent screening. The EPA fact sheet states that "all MBR systems require 1- to 3-mm-cutoff fine screens immediately before the membranes, depending on the MBR manufacturer." Coarse debris that would shred a fiber bundle or lodge in a plate spacer must be removed; primary settling is optional in small to medium plants but common above roughly 5 mgd.
- Biological zone. Screened influent flows into an anoxic tank for denitrification, then an aerobic tank where heterotrophs and nitrifiers consume carbonaceous BOD and ammonia-N. The EPA schematic (Figure 3) shows the mixed anoxic/aerobic layout with sludge recycle from the membrane tank back to the anoxic zone to keep the recycle loop denitrifying the nitrate produced upstream.
- Membrane separation. Mixed liquor contacts submerged or external membranes under a vacuum (most common in MBRs) or low pressure. Clean water passes through the membrane wall as permeate; the concentrated stream that does not pass is the retentate and is returned to the bioreactor, building biomass concentration over time.
- Permeate polishing. Because TSS in the permeate is already very low, downstream treatment requirements are minimal. The EPA fact sheet notes that "disinfection is easy and might not be required, depending on permit requirements"—for many reuse applications, a simple UV bank is sufficient.
- Sludge wasting. Periodic wasting of mixed liquor controls SRT; the wasted stream goes to standard thickening and dewatering unit operations, covered in this filter press vs decanter centrifuge comparison.
How the Membrane Does the Filtering
MBR membranes are constructed of cellulose or polymer—PVDF is the modern standard because of its chemical tolerance during clean-in-place cycles—with a maximum pore size set during the manufacturing process. The EPA fact sheet specifies the design intent: "the membranes prevent passage of particles the size of microorganisms, or about 1 micron (0.001 millimeters), so that they remain in the system." A 1 μm cutoff physically retains bacteria, most colloids, and essentially all TSS in a single barrier, which is why MBR effluent turbidity is so much lower than clarifier effluent.
Terminology for the fluid streams is standard: water that passes through the membrane wall is the permeate; the stream that does not pass and is recycled is the concentrate or retentate. MBRs almost always draw a vacuum—ambient pressure on the mixed-liquor side, suction on the permeate side—rather than pressurizing the feed. The EPA fact sheet notes the trade-off: "the advantage of the vacuum is that it is gentler to the membranes; the advantage of the pressure is that throughput can be controlled." Vacuum operation is the default in submerged designs and is the reason submerged MBRs use roughly an order of magnitude less energy than external cross-flow systems.
Why MBR Effluent Is Cleaner Than Conventional Activated Sludge
Three coupled parameters explain the effluent quality difference. First, biomass is not lost over a clarifier weir, so MBR mixed-liquor suspended solids (MLSS) run far higher than in conventional activated sludge. Second, those higher concentrations allow longer SRTs, which the EPA fact sheet ties directly to lower sludge yield: "MBRs have often been operated with longer solids residence times (SRTs), which results in lower sludge production." Third, higher MLSS permits higher volumetric loading and therefore a lower hydraulic retention time, which the same fact sheet summarizes as "MBRs operate at higher volumetric loading rates which result in lower hydraulic retention times." The combined effect is a smaller biological volume producing better effluent.
| Parameter | Conventional Activated Sludge | Membrane Bioreactor (MBR) |
|---|---|---|
| Footprint (relative) | 1.0× baseline | ≈0.4–0.6× of conventional (per EPA fact sheet, smaller bioreactors) |
| MLSS in aeration tank | 2,000–4,000 mg/L | 8,000–12,000 mg/L (typical MBR operating range) |
| SRT | 5–15 days | 20–60 days (per EPA fact sheet) |
| HRT | 6–12 h | 2–5 h (lower due to higher volumetric loading) |
| Effluent TSS | 10–30 mg/L | <1–5 mg/L (frequently <1 mg/L) |
| Effluent BOD | 10–30 mg/L | <5 mg/L (often at detection limit) |
| Sludge yield | Baseline | Lower (longer SRT reduces observed yield) |
Hollow-Fiber vs Flat-Plate Membrane Modules
Engineers must choose between two primary module geometries during MBR procurement. Hollow-fiber systems (GE/Zenon ZeeWeed, Siemens/U.S. Filter Memcor and Memjet, GE/Ionics) bundle thousands of fine fibers between two manifolds; a cassette holds the bundles and slides into a rack. Per the EPA fact sheet, hollow-fiber systems require 1–2 mm pre-screening to keep hair, fibers, and grit from snagging on the bundle. Flat-plate systems (Kubota/Enviroquip and the DF series flat-sheet MBR module) mount membranes on rigid plates with a thin spacer channel; the same source notes these require 2–3 mm pre-screening because the flow path is more tolerant of small debris.
Fouling control is the operational reality behind both geometries. All major MBR systems use continuous air scour—coarse-bubble aeration directed across the membrane surface to keep solids in suspension. GE/Zenon systems add a back-pulse step in which permeate is pumped back through the membrane on a timer; the EPA fact sheet puts the duty cycle at "1 to 5 percent of the total operating time." Flat-plate submerged modules can use 10–20× less energy than external cross-flow systems because no recirculation pump is required—the mixed liquor stays in the tank and only the permeate is pumped out, which is the key reason most new municipal MBRs specify submerged flat-plate or hollow-fiber designs over external loops.
Design and Operating Parameters Engineers Specify
The EPA fact sheet provides a flow constraint that drives much of the design: "peak design flows should be no more than 1.5 to 2 times the average design flow." When peak flow exceeds that envelope, the engineer has two choices: add membrane area to handle the peak, or add equalization. Equalization can be external (a separate basin upstream) or internal (holding the aeration and membrane tanks at higher water levels and drawing them down to absorb peaks). Internal equalization is cost-effective but reduces the volume available for biology during the diurnal low.
Redundancy is specified by the N+1 rule: "the installation include one additional membrane tank/unit beyond what the design would nominally call for" (EPA fact sheet, citing Wallis-Lage et al. 2006). The extra unit lets the operator take a cassette offline for service without losing permit compliance. On the biological side, MBR design uses MLSS in the 8,000–12,000 mg/L range and SRTs from 20 to 60 days—both well above conventional activated sludge—and the higher oxygen demand of that biomass means oxygen-transfer capacity, not SRT, often sets the aerobic tank volume (per Crawford et al. 2000, cited in the EPA fact sheet). For a worked example of a complete factory-integrated skid, see the HydropureWater integrated MBR system, which combines screening, anoxic/aerobic zones, and submerged PVDF membranes in a single packaged unit sized from 10 to 2,000 m³/day. For a wider procurement context including downstream solids handling and reuse targets, the effluent treatment plant buyer's guide walks through matching equipment to the permit envelope.
Real Performance Data from Operating Plants
Two operating plants referenced in the EPA fact sheet anchor the technology in measured numbers. Calls Creek, Georgia, retrofitted a Siemens/U.S. Filter Memcor system onto an existing Orbal oxidation ditch at 0.35 mgd average flow and 0.67 mgd design flow, with a 2-mm fine screen just before the membranes. Cauley Creek, Fulton County, Georgia, runs a 5-mgd GE/Zenon ZeeWeed system with biological phosphorus removal, mixed-liquor surface wasting, sludge thickening to minimize aerobic digester volume, and UV disinfection.
| Parameter | Calls Creek — Influent (avg) | Calls Creek — Effluent (avg) | Cauley Creek — Effluent |
|---|---|---|---|
| Flow (mgd) | 0.35 (max month 0.44, min month 0.26) | — | 5 (design) |
| BOD (mg/L) | 145 | ≈ detection limit (111 in min-month tabulation; field data, 2005) | Very low (consistent permit compliance) |
| TSS (mg/L) | 248 | ≈ detection limit (111 in min-month tabulation; field data, 2005) | Very low |
| Ammonia-N (mg/L) | 14.8 | 0.21 (max 0.72, min 0.10; field data, 2005) | Consistently low |
| Phosphorus (mg/L) | — | 0.88 avg (max 0.55, min 0.12; field data, 2005) | Low (bio-P removal) |
| Fecal coliforms (/100 mL) | — | 14.2 (max 20, min 0; field data, 2005) | Low (UV polished) |
| Turbidity (NTU) | — | 0.30 avg (max 1.31, min 0.01; field data, 2005) | Very low |
Both plants demonstrate the primary benefits of MBR technology: ammonia-N around 0.2 mg/L, turbidity around 0.3 NTU, and TSS and BOD at or below the analytical detection limit—numbers that allow for simplified downstream disinfection.
Frequently Asked Questions
What pore size is used in MBR membranes?
Around 1 μm (0.001 mm). Per the EPA fact sheet, the membrane is designed to "prevent passage of particles the size of microorganisms, or about 1 micron," so bacteria, TSS, and most colloids are physically retained in a single barrier.
How is MBR different from conventional activated sludge?
The membrane replaces both the secondary clarifier and the downstream sand filter. That substitution allows the bioreactor to run at much higher MLSS (typically 8,000–12,000 mg/L) and longer SRT (typically 20–60 days), resulting in a smaller footprint, lower sludge yield, and effluent TSS and BOD that are frequently below the analytical detection limit.
Frequently Asked Questions
What is a membrane bioreactor and how does it work?
A membrane bioreactor (MBR) is a wastewater treatment process that combines biological degradation with membrane filtration. In this system, microorganisms decompose organic pollutants in an aeration tank, and instead of using a gravity-based secondary clarifier, the treated water is extracted through semi-permeable membranes. This physical barrier ensures that solids, including bacteria and suspended particles, remain within the bioreactor, resulting in a high-quality effluent suitable for reuse.
What is the difference between MBR and conventional activated sludge?
The primary difference lies in the solid-liquid separation method and the resulting biomass concentration. Conventional activated sludge (CAS) systems rely on secondary clarifiers, which are limited by sludge settling characteristics and typically operate at mixed liquor suspended solids (MLSS) concentrations of 2,000 to 5,000 mg/L. MBR systems, by contrast, decouple hydraulic retention time from solids retention time, allowing for much higher MLSS levels ranging from 8,000 to 15,000 mg/L, which enables a significantly smaller footprint and superior effluent clarity.
What pore size do MBR membranes use?
MBR membranes typically utilize microfiltration or ultrafiltration technology, with pore sizes generally ranging from 0.01 to 0.4 micrometers. This range is specifically engineered to retain all suspended solids, bacteria, and many viruses, while allowing water and dissolved minerals to pass through. Most commercial MBR membranes are rated at approximately 0.04 to 0.1 micrometers to ensure effective pathogen removal and consistent flux rates.
How often do you have to clean an MBR membrane?
Membrane cleaning is categorized into maintenance cleans and recovery cleans. Maintenance cleans, which involve a low-concentration chemical backpulse, are typically performed on a daily or weekly basis to prevent irreversible fouling. Recovery cleans, which involve soaking the membranes in higher concentrations of chemicals like sodium hypochlorite or citric acid, are generally conducted every 3 to 6 months depending on the feed water quality and the rate of trans-membrane pressure (TMP) increase.
How long do MBR membranes last?
The operational lifespan of MBR membranes typically ranges from 7 to 10 years, depending on the feed water characteristics, operating flux, and adherence to established cleaning protocols. While the physical integrity of the membrane fibers can often last longer, the effective service life is reached when fouling becomes irreversible or when fiber breakage leads to a decline in effluent quality that can no longer be restored through chemical recovery cycles.
