R+F

Poor Biofilm Growth on MBBR Carriers

When carriers remain bare or patchy — diagnosing the colonisation failure.

Healthy biofilm on carriers is the foundation of MBBR performance. When carriers are bare, patchy or developing only thin, easily-detached biofilm, the reactor cannot achieve design removal rates. Poor biofilm growth can occur during initial start-up (slow colonisation) or during operation (biofilm loss after a disturbance). The causes differ and so do the remedies.

Causes during start-up

New (virgin) carriers have a smooth surface that bacteria must colonise from scratch. This takes time — one to three weeks for heterotrophs, longer for nitrifiers. Factors that slow start-up colonisation include: cold temperature (biological growth is slower), intermittent or low wastewater feed (not enough substrate to sustain growth), and excessive aeration (high shear strips the fledgling biofilm before it can anchor). Patience and gentle conditions during start-up are key.

Causes during operation

If an established biofilm deteriorates, the most common causes are: toxic event (industrial discharge, chlorination, heavy metals) that kills the biofilm organisms; excessive shear from over-aeration or mechanical damage; prolonged starvation (no feed for extended periods, e.g. seasonal shutdown); and extreme pH or temperature excursions.

Cause–remedy matrix

Poor biofilm growth: causes and actions

ParameterTypical ValueUnitNotes
Cold start-up (<12 °C)Allow more time; seed if possible—Start in warm months if feasible.
Excessive shear (over-aeration)Reduce airflow to minimum mixing—Protect fledgling biofilm.
Toxic inhibitionIdentify & isolate source—Recovery may take weeks.
Insufficient substrateIncrease feed rate gradually—Biofilm needs BOD to grow.
Nutrient deficiencyDose N, P if unbalanced—Industrial WW may lack nutrients.
Carrier surface issueRare; check carrier specification—Some coatings inhibit attachment.

Frequently Asked Questions

Can I seed new carriers with biofilm from another MBBR?
Yes. Transferring a portion of mature carriers from an operating MBBR into the new reactor provides an immediate inoculum and can shorten colonisation by one to two weeks. Alternatively, adding activated sludge from a nearby plant provides bacteria that will colonise the carriers, though this is less direct.
How do I know if a toxic event caused the biofilm loss?
A sudden, significant drop in performance (COD or ammonia removal) after a period of stable operation strongly suggests toxicity. Correlate with upstream events — industrial discharges, chemical deliveries, cleaning operations. If possible, test the influent for inhibitory substances (heavy metals, solvents, biocides).
Should I increase organic loading to promote biofilm growth?
Within reason, yes — the biofilm grows in response to available substrate. But increasing load too fast during start-up overwhelms the small amount of biofilm present and degrades effluent quality. A gradual ramp-up (30–50 % of design initially, increasing as performance improves) is the safest approach.

Related Pages

MBBR Start-Up

Biofilm acclimation during start-up.

Biofilm Control

Managing established biofilm.

MBBR Biofilm

Biofilm fundamentals.

Excessive Biofilm Growth

The opposite problem.

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