Nitrification Slips on a Low-Alkalinity BNR Basin in Winter
A 30,000 m³/d activated sludge plant has four passes: a first anoxic zone with internal mixed liquor recycle, then three aerated passes with fine-bubble diffusers. Permit limit is 3.0 mg/L NH3-N monthly average. Influent TKN averages 30 mg/L and influent alkalinity 160 mg/L as CaCO3. In January the mixed liquor is 13 °C, SRT is held at 6 days by the wasting schedule, MLSS is 2,600 mg/L, and aerobic-zone DO is 1.3 mg/L even though the setpoint is 2.0 mg/L and both blowers run at maximum. Effluent NH3-N is 8.0 mg/L, effluent NO3-N is 6.0 mg/L, effluent alkalinity is 12 mg/L, and pH is 6.5 and drifting down. The secondary clarifiers have a normal blanket and the return rate is unchanged.
1. Is a 6-day SRT defensible for nitrification at 13 °C in this basin? Show your reasoning with the growth-rate factors actually present here.
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No - and the margin is worse than the number suggests.
Minimum SRT. For nitrifiers at 13 °C the minimum solids retention time is roughly 4–5 days depending on the growth expression used. Design practice applies a safety factor of 2–3 for the lag between load changes and biomass response and for clarifier losses. That puts the practical target near 10–12 days, not 6.
Two factors cut the actual growth rate below the design curve:
- DO. Nitrifier DO half-saturation is about 0.3–0.5 mg/L. At 1.3 mg/L the limitation factor is 1.3 / (1.3 + 0.5) ≈ 0.72, i.e. roughly a 28% reduction in specific growth rate. The effective minimum SRT rises accordingly.
- pH. At 6.5 you are at the lower edge of the nitrifier working range; below about 6.5 the rate falls off steeply, which feeds back into the SRT requirement.
Also verify the SRT calculation itself. SRT must be computed over the whole biomass inventory - all four passes plus the clarifier blanket - divided by the actual solids leaving in waste sludge and in the effluent. If the wasting flow is estimated rather than measured, or if the effluent TSS is climbing, the calculated 6 days may be optimistic on paper as well as in reality.
The diagnosis is a plant operating below its required SRT with two additional rate suppressants, not a mystery organism problem.
2. You decide to raise the SRT to fix the ammonia. Work through the alkalinity balance and explain why that plan could make the situation worse if you do it carelessly.
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The stoichiometry. Nitrification consumes about 7.14 mg of alkalinity as CaCO3 per mg of NH4-N oxidized. Denitrification recovers about 3.57 mg as CaCO3 per mg of NO3-N reduced - half as much per unit of nitrogen, so recovery never fully pays back the nitrification cost.
The arithmetic. Currently about 22 mg/L of the 30 mg/L TKN is being nitrified (30 in, 8 out as ammonia).
- Alkalinity consumed ≈ 7.14 × 22 = 157 mg/L
- Alkalinity recovered at 16 mg/L denitrified ≈ 3.57 × 16 = 57 mg/L
- Net consumption ≈ 100 mg/L against 160 mg/L influent, which should leave about 60 mg/L - but you measure 12 mg/L.
That gap does not close, and that is information: look for another acid load (industrial discharge, high influent VFAs, ferric or alum addition, or a nitrite residual that exerts a fast chlorine-style demand on your analytical results) before assuming the averages are right.
The trap. If you complete nitrification, the demand rises to 7.14 × 30 ≈ 214 mg/L. Recovery does not scale up proportionally, because denitrification is limited by how much nitrate the internal recycle actually returns to the single anoxic pass (about a quarter of the basin volume) and by oxygen carried back in that recycle. You could drive pH below 6.3, stall the nitrifiers, and end up worse off than you started.
So the SRT increase must be paired with maximizing denitrification, and with a plan to add alkalinity if the balance cannot be held.
3. Rank your corrective actions for the next two weeks, and note what you would deliberately not do.
Read the worked answer
1. Verify the data before acting (day 1). Calibrate the DO probes in the aerobic pass and the anoxic zone; a probe reading 1.3 when the true value is 2.0 changes everything. Pull a fresh effluent ammonia and nitrate, and confirm the waste sludge flow meter and concentration used in the SRT calculation. Confirm the mixed liquor temperature at more than one point.
2. Fix the air delivery (days 1–3). Both blowers at maximum with DO stuck at 1.3 mg/L is an air-side problem, not a setpoint problem. Check diffuser fouling and headloss (old fine-bubble diffusers foul and the blowers run out of pressure), check the DO control valve and any dissolved-oxygen trim, and check for a failed blower inlet filter. Restoring DO toward 2.0 mg/L is the cheapest single gain in nitrifier growth rate.
3. Raise SRT deliberately (days 1–14). Reduce wasting in steps, e.g. target 8 days, then 10–12, watching the clarifier blanket and effluent TSS at each step. Do not slash wasting in one move - you risk a blanket rise and a solids washout.
4. Maximize denitrification (ongoing). Confirm the internal recycle pump is delivering its design ratio, keep anoxic-zone DO below about 0.2 mg/L, verify the anoxic mixer is running, and consider feeding a supplemental carbon source to the anoxic zone only if the internal recycle is already at capacity.
5. Manage alkalinity. Sodium bicarbonate is the safe choice - it raises alkalinity without a sharp pH spike. Lime is faster but can scale and drive pH swings, and caustic belongs nowhere near the anoxic zone.
6. Regulatory. Model the monthly average now. A 30-day average gives you time, but only if you notify early.
What not to do: do not shorten SRT to improve settling, do not dose a commercial nitrifier or biostimulation product in place of fixing SRT and DO, and do not chase pH with caustic while the underlying alkalinity deficit is unresolved.