Five acronyms, two questions. Where the biology lives, and how the solids get separated. Get the second one wrong and you are comparing a complete plant against half of one.
A procurement document lands with five acronyms in it and no explanation. Somebody says the neighbouring township went MBBR. Somebody else says MBR is the modern choice. A third person points out the existing plant is an RBC and asks whether it can simply be upgraded.
These are not five competing brands of the same thing. They answer two separate questions, and most confused conversations about wastewater technology come from mixing the two up.
This is a general explanation of how the processes differ. It is not advice on any particular facility.
Every biological treatment process relies on the same basic mechanism. Microorganisms consume the organic matter in wastewater. The processes differ in where those microorganisms are kept.
Suspended growth keeps the biomass floating freely in the water as mixed liquor, held in suspension by aeration and mixing. Conventional activated sludge, SBR and MBR all work this way.
Attached growth, also called fixed film, grows the biomass as a biofilm on a solid surface and passes the water across it. RBC and MBBR belong here, as does the older trickling filter.
IFAS does both at once, which is the whole point of it.
The practical difference is how the system responds to being upset. Suspended-growth biomass can be washed out if flow surges or a toxic slug arrives, and rebuilding it takes weeks. Biofilm is anchored to a surface and is generally harder to lose. Against that, suspended-growth systems are easier to control deliberately, because the operator can adjust the biomass concentration by wasting sludge.
This is the question that actually distinguishes the processes, and it is the one that gets least attention in early conversations.
Treated water has to be separated from biomass before it can be discharged. There are three ways to do it.
Separation in space. The mixed liquor flows to a separate tank and gravity does the work. Conventional activated sludge, MBBR and RBC all need a clarifier downstream.
Separation in time. Everything happens in one tank, but at different points in a cycle. Aeration stops, solids settle, clear water is drawn off. This is the SBR.
Separation by barrier. A membrane physically retains the solids regardless of how well they settle. This is the MBR.
Note what this means for MBBR specifically. An MBBR is not a complete treatment plant. It is a biological reactor that still needs solids separation downstream. Comparing "MBBR versus MBR" without accounting for the clarifier, its footprint and its civil works is comparing incomplete things.
| Process | Biomass | Solids separation | Recirculation |
|---|---|---|---|
| Conventional activated sludge | Suspended | Separate clarifier | Return activated sludge |
| SBR (sequencing batch reactor) | Suspended | Same tank, by cycle phase | None in principle |
| MBR (membrane bioreactor) | Suspended | Membrane barrier | Membrane tank recirculation |
| MBBR (moving bed biofilm reactor) | Attached, on free-moving carriers | Separate clarifier or filter | None |
| RBC (rotating biological contactor) | Attached, on rotating discs | Separate clarifier | None |
| IFAS (integrated fixed-film activated sludge) | Both | Separate clarifier | Return activated sludge |
The reference case. Aeration tank, clarifier, sludge returned to the front of the process. It typically runs at mixed liquor suspended solids concentrations somewhere in the range of 1,500 to 5,000 mg/L, commonly 2,000 to 4,000.
It is well understood, it is what most operators are trained on, and the equipment is available from many suppliers. It also needs the most land of any option here, and it is the most sensitive to settling problems. When the sludge does not settle, effluent quality follows it out of the clarifier.
An SBR is activated sludge rearranged in time rather than space. A single tank cycles through five phases: fill, react, settle, draw, and idle. Aeration and mixing happen during react, the tank goes quiescent during settle, and a decanter draws off the clear supernatant during draw. Cycle times commonly run somewhere between four and twelve hours, and the draw phase alone can take over 30 percent of the cycle.
The attraction is that it eliminates the secondary settling tank and, in principle, sludge recirculation. Fewer structures, less pumping and piping, less civil work. It also tolerates variation in flow and load well, and the controlled anoxic conditions during settling tend to produce good sludge settling characteristics.
The cost is control complexity. The process is run by timers and level sensors, and it depends on them working. Historically most SBR plants were built for small flows, though the technology now scales to large municipalities.
Activated sludge with the clarifier replaced by a membrane. Because separation no longer depends on the sludge settling, the biomass can be held at much higher concentration. Membrane tanks typically operate around 10 to 12 g/L for immersed hollow fibre and 12 to 15 g/L for immersed flat sheet, with the biological tank running perhaps 20 to 25 percent lower, since recirculation typically runs at four to five times the permeation rate.
Higher biomass concentration means a smaller tank for the same treatment, which is where the footprint advantage comes from. The membrane also acts as an absolute barrier, so effluent quality does not degrade when the sludge settles poorly.
Two honest caveats, both worth stating plainly.
First, the footprint advantage is not automatic. Below roughly 6 to 8 g/L there is no substantial footprint benefit over conventional activated sludge, which can itself run up to about 4 g/L. An MBR designed conservatively at low solids concentration gives up much of the reason for choosing it.
Second, aeration efficiency falls as mixed liquor concentration rises. Thicker mixed liquor transfers oxygen less readily, so some of what is gained in tank volume is paid back in air. Membranes also require periodic chemical cleaning and eventual replacement, and finer upstream screening than a conventional plant needs.
Free-moving plastic carriers circulate in an aerated tank, and biofilm grows in their protected internal surfaces. The technology was developed in Norway in the late 1980s. Carriers are usually high-density polyethylene, with specific surface areas commonly in the range of 400 to 500 m²/m³ and some commercial designs above 600.
The carrier fill fraction is the main design lever, generally between 30 and 70 percent of the reactor volume, and it can be increased later to add capacity. That upgrade path is a genuine advantage: adding treatment capacity can mean adding carriers rather than building a tank. Very high fill fractions reduce mixing effectiveness as carriers collide more, so it is not unlimited.
Operationally it is simple. There is no return activated sludge stream, and no mixed liquor concentration or food-to-microorganism ratio to maintain. The biofilm sloughs and regenerates on its own in response to load.
The constraint is the one noted above: solids separation is still required downstream.
Discs on a horizontal shaft, roughly 40 percent submerged, turning slowly at around one to two revolutions per minute. Each patch of biofilm alternates between the wastewater and the air, so oxygen transfer happens through rotation rather than through blowers. That is the main attraction: low energy, few moving parts, minimal operator intervention.
RBCs earned a poor reputation in earlier decades through shaft failures, where the structural load of accumulated wet biofilm was underestimated. Modern designs address this through shaft design and load monitoring, but shaft alignment remains the component to ask about.
They dislike shock loads and high fat, oil and grease. They do not scale gracefully, since adding capacity means adding disc banks and shafts rather than simply enlarging a tank. And they need a clarifier downstream.
Carriers added into an activated sludge tank, so suspended and attached biomass work together. Unlike MBBR, IFAS retains the return activated sludge stream and the clarifier.
Its main use is upgrading. An existing activated sludge plant that needs more capacity, or needs to nitrify reliably in cold weather, can often get there by adding carriers to tanks that already exist. For a community with an aging plant and no land to expand onto, that is worth understanding before assuming replacement is the only route.
Ask about solids separation first. Any quotation that names a biological process without stating how solids are separated, and what that separation costs in footprint and civil works, is describing part of a plant.
Match the process to the site's real constraints. Abundant land, stable flow, and a certified operator on site favour simpler options. Tight footprint, variable load, stringent effluent requirements, or no full-time operator push toward processes that separate solids without depending on settling.
Be suspicious of technology-first arguments, including ours. A supplier's preferred process is the one they sell. The federal Regulations, as covered in our piece on what secondary treatment actually means, set numerical limits and do not name a technology. The right question is not which process is best in general, but which will meet those numbers at that site, with the operators and budget actually available.
General technical overview, accurate as at 9 September 2026 · Process selection depends on site-specific conditions not addressed here
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Published for general information. It is not engineering advice and does not address any specific facility.