Running MBBR reactors in series means wastewater flows through one reactor after another, each reducing the pollutant concentration further. The first reactor sees the highest load and develops a biofilm dominated by fast-growing organisms; the last reactor sees the lowest load and supports a thin, specialised biofilm. This plug-flow-like behaviour is inherently more efficient than a single completely mixed reactor of the same total volume.
Benefits of series operation
In a single completely mixed reactor, inlet water is instantly diluted, so the biofilm always sees a moderate concentration. In series, the first reactor sees the full-strength feed and removes the easy fraction quickly, while downstream reactors progressively tackle harder-to-degrade residuals. This concentration gradient drives higher overall removal efficiency for the same total volume.
Series operation also naturally separates biofilm communities: the high-load first reactor favours heterotrophs; the polishing reactor favours nitrifiers. Each reactor can be independently sized, aerated and monitored, giving the operator more control over the process.
Design factors
Series MBBR design (indicative — confirm per project)
| Parameter | Typical Value | Unit | Notes |
|---|---|---|---|
| Concentration gradient | First = highest load | — | Each reactor removes a defined fraction. |
| Biofilm community | Shifts reactor to reactor | — | Heterotrophs early; nitrifiers later. |
| Efficiency | Higher than single CSTR | — | Plug-flow advantage for same total volume. |
| Carrier retention | Screens between reactors | — | Keeps media in assigned reactor. |
Frequently Asked Questions
Is series MBBR more efficient than a single large reactor?
How are carriers kept in each reactor?
How many reactors in series are typical?
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