R+F

Carrier Retention Screens

The sieves that hold the media in the reactor — and why their design makes or breaks an MBBR.

Retention screens are the sieves at the reactor outlet that let treated water pass while keeping the carriers inside. They are one of the most critical mechanical elements of an MBBR: if a screen blinds, flow is restricted; if it fails, carriers escape and are lost. Screen aperture, area and geometry must all be matched to the media and the flow.

How screens are designed

The aperture must be smaller than the smallest carrier so media cannot pass, yet large enough to let water and suspended solids through without blinding. Screen area is sized so the approach velocity stays low; if water is forced through too fast, carriers are pinned against the screen, biofilm and solids accumulate, and head loss rises. Common arrangements include cylindrical or flat mesh screens, and some designs use aeration or mechanical cleaning at the screen to keep it clear.

Because smaller carriers need finer apertures and hence more screen area, screen design is tightly coupled to carrier size selection. A carrier chosen purely for high surface area can become impractical if it forces an oversized or blinding-prone screen. Screen materials must also resist corrosion in the wastewater environment. All screen dimensions and velocities should be confirmed per project.

Screen design considerations

The considerations below are indicative and must be confirmed against media and flow data per project.

Retention screen considerations (indicative — confirm per project)

ParameterTypical ValueUnitNotes
ApertureBelow carrier size—Retains the smallest media reliably.
Screen areaSized for low velocity—Prevents carriers pinning and blinding.
CleaningAir / mechanical—Keeps the screen clear during operation.
MaterialCorrosion resistant—Suited to the wastewater environment.

Frequently Asked Questions

What are MBBR retention screens for?
They keep the plastic carriers inside the reactor while allowing treated water to flow out. Without reliable screens, carriers would wash out with the effluent and be lost, steadily reducing treatment capacity.
Why do retention screens blind or clog?
Blinding usually happens when the approach velocity is too high, pinning carriers and solids against the screen, or when the screen area is too small. Adequate screen area and cleaning (air scour or mechanical) prevent this.
How does carrier size affect screen design?
Screen apertures must be smaller than the carriers, so smaller media require finer screens and more screen area. This is why carrier size and screen design must be considered together rather than separately.

Related Pages

Carrier Size

Size that sets screen aperture.

Media Movement

Flow and mixing around screens.

MBBR Media Types

Carriers the screens must retain.

MBBR Plant Design

Screens in the overall plant.

Need MBBR Design Support?

Submit your wastewater data for a preliminary MBBR engineering assessment. Our team reviews every submission within 2 business days.

Start MBBR Assessment