Fine screening is the single most important pretreatment step for an MBBR. Rags, fibres, hair and other stringy material must be removed before they reach the reactor, because any debris that passes through will accumulate on the carrier retention screens at the outlet, causing blinding, head-loss build-up and ultimately flow restriction.
Sizing the inlet screen
The aperture of the inlet (headworks) screen must be smaller than the aperture of the carrier retention screen in the reactor. If it is not, debris that passes the inlet will be caught at the retention screen instead, which is far harder to clean during operation. The exact aperture depends on the carrier and should be coordinated with the MBBR and screen suppliers.
Self-cleaning drum screens, step screens and band screens are all used ahead of MBBR systems. The key requirements are reliable automatic cleaning, redundancy (at least N+1 units or a bypass channel) and the capacity to handle peak flow. In industrial applications where the wastewater may not contain rags, the screening requirement can be relaxed — but should still be evaluated per project.
Screen design factors
Inlet screening for MBBR (indicative — confirm per project)
| Parameter | Typical Value | Unit | Notes |
|---|---|---|---|
| Aperture | < retention screen | — | Must catch what would blind the reactor screen. |
| Cleaning | Automatic | — | Self-cleaning drum, step or band screen. |
| Redundancy | N+1 or bypass | — | A blocked screen must not stop the plant. |
| Industrial | Case-by-case | — | May be relaxed if no fibrous material. |
Frequently Asked Questions
What screen aperture is needed before an MBBR?
What happens if the headworks screen fails?
Do industrial MBBRs need fine screening too?
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