Problem Diagnosis
Poor Nitrification
Incomplete ammonia oxidation in the biological process. Systematic diagnosis of the limiting factors.
Executive Answer
Poor nitrification means nitrifying bacteria are not converting ammonia at the required rate. This is fundamentally a capacity problem: either there are not enough nitrifiers (low SRT), they are not active enough (low DO, low temperature, inhibition), or they are being outcompeted (high organic load consuming available oxygen).
Minimum SRT
At 15°C for reliable nitrification
SRT at 10°C
Cold weather requirement
DO Requirement
Throughout aeration basin
Alkalinity
Consumed per mg NH₄-N
Nitrification Failure Analysis
| Condition | Potential Issue | What to Investigate |
|---|---|---|
| Effluent NH₄-N = Influent NH₄-N | Complete nitrification failure | Toxic event, extreme SRT loss, total aeration failure |
| Partial NH₄-N removal (30–70%) | Capacity limitation | SRT marginally low, partial DO coverage, temperature effect |
| Seasonal pattern (winter failure) | Temperature-related | SRT design for minimum temperature, consider MBBR retrofit |
| Ammonia breakthrough during peak flow | Hydraulic washout of nitrifiers | SRT drops below critical during peaks; add equalization or carriers |
| Nitrite accumulation detected | Partial nitrification | Nitrobacter inhibited — check free ammonia, DO, temperature |
| Good SRT but poor nitrification | Inhibition or dead zones | Check mixing, toxic inputs, DO distribution across basin |
Engineering Note
Frequently Asked Questions
Common causes are insufficient SRT (especially in cold weather), low dissolved oxygen, depleted alkalinity or low pH, and inhibition by toxic industrial inputs. Because nitrifiers are slow-growing, any of these can cause incomplete nitrification and elevated effluent ammonia.
Check the SRT against the temperature-dependent requirement, review the dissolved oxygen profile in the aeration basin, monitor alkalinity and pH, and look for nitrite accumulation that can indicate partial nitrification or Nitrobacter inhibition. These point to the limiting factor.
Corrective measures depend on the cause: increase SRT by reducing wasting, raise dissolved oxygen through aeration, restore alkalinity, or add biological capacity (for example MBBR/IFAS) where the plant is genuinely undersized for the required nitrification.
Engineering Assessment
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