A common assumption in MBBR systems is:

“The more biofilm on the media, the better the treatment performance.”

But is that really true?

Not always.

Biofilm is the heart of an MBBR process, but what matters is not simply the amount of biofilm—it is the quality, thickness, activity, and stability of the biofilm.

1. Too little biofilm

With insufficient biofilm, the available surface area for microorganisms is not fully utilized.

This may result in:

• Lower biological treatment capacity
• Insufficient nitrification
• Longer start-up periods
• Unstable treatment performance

So, obviously, having an established biofilm is essential.

2. But thicker doesn't always mean better

As biofilm continues to grow, the outer layer may consume oxygen and substrates before they can reach the deeper layers.

Eventually, the inner part of a very thick biofilm can become oxygen-limited or substrate-limited.

In some cases, microorganisms in the inner layer may lose activity or detach.

So simply adding more biofilm does not necessarily mean more effective biomass.

3. Shear force matters

One of the advantages of an MBBR system is the continuous movement of the carriers.

Aeration and mixing create shear forces that help control biofilm thickness.

A healthy MBBR system therefore aims for a dynamic balance:

Biofilm growth → Shear → Detachment → New growth

This continuous cycle helps maintain an active biofilm layer.

4. Thin and active can outperform thick and inactive

A relatively thin, well-aerated and highly active biofilm can sometimes provide better biological performance than a very thick biofilm.

This is especially important for nitrification, where maintaining favorable conditions for nitrifying microorganisms is critical.

The objective should therefore be:

Not maximum biofilm.
But maximum effective biofilm.

5. What determines effective biofilm?

Several factors work together:

✔ MBBR media geometry and protected surface area
✔ Media filling ratio
✔ Dissolved oxygen
✔ Mixing intensity
✔ Organic loading
✔ Ammonia loading
✔ Temperature
✔ Hydraulic conditions
✔ Biofilm thickness and detachment

This is why selecting MBBR media should not be based only on a single number such as surface area (m²/m³).

The real question is:

How much of that surface can actually support stable and active biofilm under operating conditions?

The Bottom Line

In MBBR, more biofilm is not necessarily better.

The goal is to maintain a healthy, active and continuously renewed biofilm that provides efficient contact between microorganisms, wastewater and oxygen.

Good MBBR performance is not about growing the thickest biofilm possible.

It is about finding the right balance between growth and detachment.

What has been your experience?

Do you prefer a thicker biofilm or a more active and stable biofilm?

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