Quick answer: Choose SBR when land is available and the load swings through the day. Choose MBBR when you are upgrading an existing tank and cannot add clarifiers. Choose MBR when the discharge limit is tight or the water will be reused, and you can pay for membranes and the air to scour them. All three still produce waste sludge that has to be dewatered.
The three processes in one paragraph each
SBR — sequencing batch reactor. One tank does everything in sequence. The US EPA fact sheet lists five steps: idle, fill, react, settle and draw. Aeration, settling and decanting happen in the same vessel at different times, so there is no separate clarifier and no return sludge pumping. Control is by timers and level switches.
MBBR — moving bed biofilm reactor. An aerated tank filled with small plastic carriers. Bacteria grow as a biofilm on the protected inner surface of each carrier, so the biomass stays in the tank without any sludge recycle. The carriers are kept in the reactor by a sieve at the outlet. Solids that slough off are removed downstream, usually by a clarifier or a DAF unit.
MBR — membrane bioreactor. Activated sludge with the secondary clarifier replaced by micro- or ultrafiltration membranes, either submerged in the aeration tank or in a side-stream. The EPA membrane bioreactor fact sheet describes membranes that must hold back particles of about 1 micron, which is why MBR effluent is essentially free of suspended solids and most bacteria.
What each one does well
SBR: flexibility on a small footprint
The EPA fact sheet credits SBRs with combining equalization, primary clarification, biological treatment and secondary clarification in a single vessel, with a minimal footprint and potential capital savings from eliminating clarifiers. Design values quoted in the same document give a 4-hour cycle for municipal sewage and 4–24 hours for industrial, with MLSS of 2,000–4,000 mg/L at low water level.
Because the cycle length can be changed from the control panel, an SBR can be run anywhere from a short 3.5-hour contact-stabilization mode to an 18–36-hour extended-aeration mode. That is why it suits factories whose load changes by shift or by season. BOD removal is typically 85–95%, and suppliers quoted in the fact sheet guarantee below 10 mg/L BOD and TSS.
MBBR: more capacity in the same tank
The carriers add biofilm surface without adding tank volume. The standard Kaldnes K1 carrier offers roughly 500 m² of protected surface per cubic metre of carrier, and reactors are normally filled to no more than about 67–70% of tank volume so the carriers can still circulate (Rusten et al., 2006). Fill a tank to half with carriers and the effective biomass inventory rises several times without touching the concrete.
That is the reason MBBR is the usual choice when an existing plant is overloaded and there is no room for a second aeration basin. It also handles shock loads well, because the biofilm is attached and cannot wash out the way suspended flocs can. The price is that it does not settle its own solids: a clarifier, DAF or filtration step is still required, and the sludge that reaches the dewatering stage is lighter and harder to thicken than conventional waste activated sludge.
MBR: the cleanest water and the smallest tanks
The EPA fact sheet reports full-scale plants that reduce BOD from around 150–280 mg/L to 1–2 mg/L, TSS from around 170–250 mg/L to 1–3 mg/L, and ammonia nitrogen to below 0.3 mg/L, with effluent turbidity averaging 0.3 NTU at one Georgia plant. Because the membranes hold all the biomass in the reactor, MLSS can run far higher than in a clarifier-based plant, which cuts the tank volume and hydraulic retention time.
Two cautions from the same document. Peak flow should be kept to no more than 1.5–2 times average, because membranes cannot take a hydraulic surge the way a clarifier can, so an equalization tank is usually needed. And scour air is significant: one plant averaged 1,800 kWh per million gallons on membrane scour blowers alone, roughly twice the air a conventional plant needs for aeration. Membrane guarantees quoted range from 3 to 10 years.
Side-by-side comparison
| Criterion | SBR | MBBR | MBR |
|---|---|---|---|
| Separate clarifier needed | No | Yes (or DAF) | No — membranes |
| Footprint | Small | Small; fits existing tanks | Smallest |
| Effluent TSS | <10 mg/L typical | Depends on clarifier | 1–3 mg/L |
| Tolerance to load swings | High (cycle adjustable) | High (attached biomass) | Moderate; needs equalization |
| Energy | Moderate | Moderate | Highest (membrane scour air) |
| Consumables | None | Carriers (long life) | Membranes, cleaning chemicals |
| Operator skill | Timers and valves | Low | High |
| Best fit | Variable loads, greenfield sites | Retrofits, capacity upgrades | Reuse, tight limits, no land |
How to decide in four questions
- What does the permit require? If TSS must be below 5 mg/L, or the water will be reused for cooling or process make-up, MBR is the only one of the three that gets there without a separate tertiary filter.
- Is there an existing tank? If yes and it is overloaded, MBBR carriers can be added to it. Converting to MBR is also possible but means new screens, membranes and blowers.
- How steady is the flow? Batch production, seasonal factories and food plants with washdown peaks favour SBR, because the cycle can be lengthened when the load is heavy and shortened when it is light.
- Who will run it? An MBR needs someone who understands transmembrane pressure, chemical cleaning and fouling. An MBBR needs almost nothing beyond checking the sieves and the air distribution.
The part nobody puts in the brochure: sludge
All three processes convert dissolved organics into biomass, and that biomass leaves the plant as waste sludge at roughly 0.5–1.2% solids. The EPA membrane bioreactor sheet shows waste sludge at 1–1.2% total solids; SBR and MBBR are similar or thinner.
Three things differ between them at the dewatering stage:
- MBR sludge is older and more mineralised, which usually helps cake dryness, but it comes at high MLSS and can carry fine colloids that blind cloth quickly. Test the cloth before choosing it.
- MBBR sloughed biofilm is light and often needs a higher polymer dose to flocculate. It rarely thickens well by gravity alone.
- SBR waste sludge behaves like ordinary waste activated sludge and is the easiest of the three to dewater in a chamber filter press.
For any of the three, a membrane filter press will take biological sludge from 1% solids to a 20–30% cake in one step, which is what makes haulage or landfill costs manageable. For small plants below a few tonnes of dry solids per day, a multi-disc screw press is the lower-energy option.
Related: how much sludge a wastewater plant produces, when you need dissolved air flotation, and industrial wastewater treatment equipment.
Frequently asked questions
Is MBR better than SBR?
Only if you need its effluent quality. MBR reaches 1–3 mg/L TSS and near-zero bacteria, which SBR cannot match without a tertiary filter and disinfection. SBR wins on energy, operator skill and tolerance of flow peaks, and has no membranes to replace.
Can MBBR replace an activated sludge plant?
It can take over the biological step inside the same tank, but it still needs a solids separation stage afterwards. Most retrofits keep the existing clarifier or add a DAF unit after the MBBR reactor.
What size plant suits an SBR?
The EPA fact sheet notes that SBRs are typically used at flows of 5 million gallons per day (about 19,000 m³/d) or less, and that most installations are below 2 MGD. Larger flows usually go to continuous-flow activated sludge or MBR.
How much carrier goes into an MBBR tank?
Usually between 30% and 67% of the reactor volume, and not more than about 70%, so that the carriers can still be kept in motion by the aeration. The exact fill is set from the required biofilm surface area for the design load.
Why does an MBR need an equalization tank?
Because membrane flux is limited. The EPA fact sheet recommends keeping peak design flow to no more than 1.5–2 times average; a clarifier tolerates a surge, a membrane does not.
Which of the three produces the least sludge?
MBR, in most cases, because it runs at a long sludge age and more of the biomass is consumed by endogenous decay. The difference is modest, and every one of the three still needs a dewatering step before the sludge can be hauled.
Tell us the flow, the COD or BOD load, the discharge limit and whether a tank already exists, and we will tell you which process fits and size the sludge dewatering to go with it: request a process and sizing recommendation.
References: US EPA — Wastewater Management Fact Sheet: Membrane Bioreactors (832-F-07-015), US EPA — Wastewater Technology Fact Sheet: Sequencing Batch Reactors (1999), Rusten B. et al. (2006) — Design and operations of the Kaldnes moving bed biofilm reactors, Aquacultural Engineering 34.
This article is part of our guide to wastewater sludge dewatering.