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Ammonia peaks in winter: why temperature is rarely the only cause
Every autumn, many biological treatment plants face the same pattern: effluent ammonia climbs, permit limits are approached, and operators reach for the thermostat explanation. Temperature does slow nitrifying bacteria — the activity of *Nitrosomonas* and *Nitrobacter* roughly halves for every 10 °C drop — but blaming cold water alone often leads to the wrong fix.
A structured diagnostic approach identifies whether the real bottleneck is temperature, sludge age, dissolved oxygen (DO) supply, or an inhibitory compound entering with the load. Only then can the right engineering response be selected.
Key Facts: Nitrification in Cold Conditions
- Parameter
- Typical Range
- Nitrifier activity at 10 °C vs 20 °C
- 40–55 % of warm-weather rate
- Minimum sludge age for stable nitrification at 10 °C
- 15–25 days
- DO threshold for full nitrification rate
- ≥ 2 mg/l in mixed liquor
- Ammonia half-saturation constant (Ks)
- 0.5–1.0 mg/l NH₄-N
Step 1 — Confirm the temperature effect is real
Plot effluent ammonia against mixed liquor temperature over the past two winters. If ammonia rises smoothly as temperature falls and recovers in spring, temperature is a primary driver. If ammonia spikes are sudden or occur at temperatures above 12 °C, look elsewhere.
A useful cross-check: calculate the theoretical nitrification rate at the measured temperature using the Arrhenius correction (θ = 1.072 is a common value). If the measured rate is significantly below the theoretical value, an additional inhibitor or capacity constraint is present.
Step 2 — Check sludge age against the safety margin
Nitrifiers are slow-growing organisms. At 20 °C, a sludge age of 8–10 days is typically sufficient. At 10 °C, the minimum rises to 15–25 days depending on the safety factor applied. Many plants operate with a sludge age that is adequate in summer but falls below the winter minimum when wasting rates are not adjusted seasonally.
| Temperature (°C) | Minimum SRT (days) | Recommended SRT with safety factor |
|---|---|---|
| 20 | 8–10 | 12–15 |
| 15 | 10–14 | 16–20 |
| 10 | 15–20 | 22–28 |
| 8 | 20–28 | 28–35 |
Reduce wasting rates in autumn before temperatures drop. If the plant cannot hold the required sludge age without secondary clarifier overloading, biofilm carriers offer a way to retain nitrifier biomass independently of the suspended solids inventory.
Step 3 — Verify dissolved oxygen supply
Nitrification is aerobic and oxygen-demanding: oxidising 1 g of NH₄-N to NO₃-N consumes approximately 4.3 g O₂. At low temperatures, oxygen solubility increases, which helps — but aeration capacity is often reduced in winter due to blower limitations or diffuser fouling.
Check aeration before assuming a biological problem
Measure DO at multiple points in the aeration tank during peak load. A DO below 1.5 mg/l in any zone will suppress nitrification regardless of sludge age or temperature. Diffuser cleaning and blower maintenance should precede any biological upgrade.
Step 4 — Screen for inhibitory loads
Industrial discharges, cleaning agents, and process chemicals can inhibit nitrifiers at concentrations that cause no visible effect on heterotrophic BOD removal. Symptoms include sudden ammonia spikes that do not correlate with temperature or flow, and rapid recovery after the inhibitory event passes.
Batch respirometry tests on mixed liquor samples taken before and after suspected discharge events can confirm inhibition. If inhibition is confirmed, source control or equalisation upstream of the biological stage is the primary remedy.
The MBBR fix: retaining nitrifier biomass at low temperature
When diagnostics confirm that the plant genuinely lacks nitrification capacity at winter temperatures — rather than suffering from a correctable operational issue — moving bed biofilm reactor (MBBR) carriers provide a targeted solution.
When MBBR carriers are the right response
- Sludge age cannot be extended without clarifier overloading
- Temperature effect is confirmed and within expected Arrhenius range
- DO supply is adequate (≥ 2 mg/l confirmed)
- No persistent inhibitory load is present
- Civil works for a new tank are not feasible within the project timeline
Carriers are introduced into the existing aeration tank at fill ratios typically between 30 % and 60 % of the effective tank volume. The biofilm that develops on the carrier surface retains nitrifiers independently of the suspended solids wasting rate, effectively decoupling nitrifier retention time from hydraulic and solids retention times. At 10 °C, a well-established MBBR biofilm can maintain specific nitrification rates of 0.3–0.8 g NH₄-N per m² of carrier surface per day.
How quickly does an MBBR biofilm establish in cold water?+
Biofilm colonisation at temperatures below 12 °C typically takes 4–8 weeks to reach stable nitrification rates. Seeding with return sludge from a well-nitrifying plant can shorten this period. Plan carrier installation before the cold season begins.
Can MBBR carriers be added to an existing activated sludge tank without structural changes?+
In most cases, yes. The main requirements are adequate mixing energy to keep carriers in suspension (typically 5–10 W/m³), a carrier retention screen at the tank outlet, and confirmation that the existing aeration system can supply the additional oxygen demand. A hydraulic and aeration assessment should precede any installation.
What happens to effluent quality during the transition period after carrier installation?+
During biofilm establishment, the suspended biomass continues to provide nitrification at its existing (reduced) rate. Effluent quality typically does not deteriorate further, and improvement becomes measurable within 3–6 weeks as the biofilm matures. Monitoring ammonia and nitrate daily during this period is recommended.
Practical next steps
A winter nitrification problem is best addressed before the cold season arrives. The diagnostic sequence — temperature correlation, sludge age check, DO verification, inhibition screening — takes two to four weeks of targeted monitoring and costs little. The findings determine whether the solution is operational (adjust wasting, clean diffusers) or requires a capital investment such as MBBR carriers.
For sites where carriers are the right answer, early engagement with a supplier allows aeration modelling, carrier sizing, and screen specification to be completed in time for a summer installation — giving the biofilm a full warm season to establish before the next winter.
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Engineering Hub: get this sized for your plant
Describe your goal, effluent limit or sludge volume and receive a technology shortlist plus a sizing proposal – by email, no phone call required.