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Carrier Fill Ratio

How much media to add to a reactor — and why more is not always better.

The carrier fill ratio is the fraction of the reactor volume occupied by media at rest, expressed as a percentage. It directly sets how much biofilm surface is available per cubic metre of reactor, but it is capped by the need to keep the carriers moving freely. In practice the filling is kept at or below roughly two-thirds of the reactor volume.

Why fill ratio is limited

Carriers must circulate freely so that biofilm is oxygenated, substrate reaches every carrier, and collisions keep the media self-cleaning. If the reactor is overfilled, carriers jam, mixing energy rises steeply and dead zones form. For this reason a practical upper limit of about 60–67% fill is widely applied, leaving headroom for the media to fluidise. As biofilm grows and effective carrier volume increases, this headroom is what keeps the bed moving.

The fill ratio is also the lever that converts a carrier's surface area into reactor-based area: the surface area per m³ of reactor equals the carrier's protected surface area multiplied by the fill ratio. Choosing a fill ratio is therefore part of sizing the reactor, not an afterthought.

Typical fill-ratio logic

Values below are indicative literature ranges to illustrate the trade-offs — always confirm per project with the carrier supplier.

Fill-ratio guidance (indicative — confirm per project)

ParameterTypical ValueUnitNotes
Practical maximum fill~60–67%Leaves headroom for free carrier movement.
Common design rangeBelow the maximum%Chosen to balance area, mixing energy and future headroom.
Area per m³ reactorProtected SSA × fill ratiom²/m³How filling converts to usable area.
Over-filling riskPoor mixing—Jamming, dead zones and higher energy demand.

Frequently Asked Questions

What is the maximum MBBR fill ratio?
A practical upper limit of roughly 60–67% of the reactor volume is widely applied so that carriers can still move and fluidise freely. The exact maximum depends on the carrier and reactor geometry and should be confirmed with the supplier.
Can I increase capacity by adding more carriers later?
Up to the fill-ratio limit, yes — one of the advantages of MBBR is that capacity can be increased by adding media to an existing tank. Beyond the practical maximum, however, mixing suffers and adding more carriers becomes counterproductive.
How does fill ratio affect reactor size?
Usable biofilm area per cubic metre of reactor equals the carrier protected surface area multiplied by the fill ratio. A higher fill ratio packs more area into a given tank, but the movement limit caps how far this can be pushed.

Related Pages

Protected Surface Area

The area the fill ratio multiplies.

Media Movement

Why free circulation limits filling.

MBBR Carrier Filling

Filling within the wider design context.

MBBR Filling Ratio

Filling fundamentals on the pillar.

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