An MBBR polishing stage is a low-loaded reactor placed at the end of the treatment train to bring residual pollutant concentrations down to stringent discharge limits. The biofilm is thin and fully active because the remaining load is small, and the carrier surface is used efficiently. Polishing is often the critical step for meeting tight ammonia or total-nitrogen limits.
When polishing is needed
Any time discharge limits demand very low concentrations — single-digit mg/L for BOD, sub-mg/L for ammonia, or tight total-nitrogen caps — a polishing stage can be cost-effective. Without it, achieving those limits in a single reactor means oversizing the entire process for the last few percent of removal. A dedicated polishing reactor operates at low loading so the biofilm is thin, well-oxygenated and highly active, converting the residual efficiently.
Downstream solids separation (clarifier, DAF or filter) must be matched to the polishing reactor, because the biofilm-origin solids leaving the MBBR determine the final effluent quality. Even an excellent biofilm stage can fail to meet limits if the downstream separation is undersized.
Design considerations
MBBR polishing design (indicative — confirm per project)
| Parameter | Typical Value | Unit | Notes |
|---|---|---|---|
| Loading | Low | g/m²·d | Only the residual reaches this stage. |
| Biofilm | Thin and active | — | Efficient use of surface. |
| Downstream | Solids separation critical | — | Sets final effluent quality. |
| Typical target | Sub-mg/L NH₄ or low TN | — | Stringent discharge limits. |
Frequently Asked Questions
What does MBBR polishing remove?
Is polishing the same as tertiary treatment?
Can the polishing stage use a different carrier?
Related Pages
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