Elevated effluent ammonia is the most frequently reported MBBR performance issue, particularly in nitrifying systems. The root cause is always the same: the nitrifying biofilm is not converting ammonia fast enough. The diagnostic task is to determine why — is it oxygen, pH, temperature, toxicity, load, or biofilm condition? A structured checklist works through these in order of likelihood.
Diagnostic checklist
1. Check DO: Is the reactor DO consistently ≥3 mg/L? Low DO is the single most common cause of poor nitrification in MBBR. Verify the DO probe is calibrated and not fouled. If DO is low, check aeration (blower, diffusers, VFD).
2. Check pH and alkalinity: Is pH above 6.8? Is there residual alkalinity (≥50 mg/L CaCO₃) in the reactor? Nitrification consumes alkalinity and drops pH; if the influent alkalinity cannot keep up, pH falls and nitrification rate crashes.
3. Check temperature: Has temperature dropped? A 5 °C drop can reduce nitrification rate by 35–40 %. If the reactor was sized for summer conditions, winter ammonia exceedance is a design shortfall, not an operational failure.
4. Check load: Has the influent ammonia concentration or flow increased? If the surface-area loading (g NH₄-N/m²·d) exceeds the design rate at the current temperature, the biofilm cannot keep up regardless of other conditions.
5. Check for toxicity: Has there been an industrial discharge, a chemical spill, or a slug of chlorinated water? Nitrifiers are sensitive to many toxicants including heavy metals, chlorine, and some organic compounds. A sudden loss of nitrification after a stable period suggests a toxic event.
6. Check biofilm condition: Pull carriers and inspect. Thin or patchy biofilm indicates biomass loss (toxic event, excessive shear). Very thick, dark biofilm suggests oxygen limitation within the biofilm — the inner layers are anaerobic and only the outer thin layer is nitrifying.
Cause–remedy matrix
High ammonia: common causes and actions
| Parameter | Typical Value | Unit | Notes |
|---|---|---|---|
| Low DO (<3 mg/L) | Increase aeration | — | Check blower, diffusers, probe. |
| Low pH (<6.8) | Add alkalinity | — | NaOH or NaHCO₃. |
| Low temperature | Raise DO setpoint | — | Partial compensation; may need more area. |
| Overloading | Reduce feed / add carriers | — | Check SARR vs design. |
| Toxic event | Identify & isolate source | — | Recovery: weeks for nitrifiers. |
| Thin/lost biofilm | Reduce shear; allow regrowth | — | Lower aeration to minimum mixing. |
Frequently Asked Questions
How quickly can nitrification recover after an upset?
Should I increase aeration as the first response?
Is high effluent ammonia always a nitrification problem?
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