Winter Nitrification Failure at a 20 Mgal/d Activated Sludge Plant
A 20 Mgal/d (75,700 m³/d) activated sludge plant must meet a monthly average effluent ammonia limit of 1.5 mg/L year-round. In winter the mixed liquor temperature falls to 13 °C. Current operation is: solids retention time 4.5 days, MLSS 2,800 mg/L, dissolved oxygen 1.6 mg/L in the last aeration pass, influent TKN 38 mg/L, influent alkalinity 200 mg/L as CaCO3, effluent ammonia 3.8 mg/L and effluent pH 6.5. The sludge volume index is 180 mL/g and the secondary clarifier sludge blanket sits at 3.2 m in a 4.0 m sidewater depth. Aeration blowers are running at 85 % of capacity and the diffusers are nine years old. The permit also sets a total residual chlorine limit of 0.02 mg/L, and the outfall discharges to a cold-water fishery, so chlorination-dechlorination is under scrutiny.
1. Determine whether 4.5 days SRT is adequate at 13 °C given the measured DO and pH, showing the calculation.
Read the worked answer
Use the nitrifier growth and decay temperature corrections with the minimum-SRT criterion, SRT_min = 1/(μ − b), where μ is the net-adjusted specific growth rate.
Temperature: μ_max at 13 °C = 0.47 · e^{0.098(13−15)} = 0.47 × 0.822 = 0.386 /d. Decay: b = 0.05 × 1.04^{(13−20)} = 0.05 × 0.760 = 0.038 /d.
DO limitation: with a half-saturation constant near 0.5 mg/L, the factor at 1.6 mg/L is 1.6/(0.5 + 1.6) = 0.76. μ = 0.386 × 0.76 = 0.293 /d → SRT_min = 1/(0.293 − 0.038) = 3.9 d.
pH limitation: at pH 6.5, nitrifier growth is strongly suppressed; a factor of roughly 0.65 is consistent with published pH response curves below 7.0. μ = 0.386 × 0.76 × 0.65 = 0.191 /d → SRT_min = 1/(0.191 − 0.038) = 6.5 d.
Conclusion: the actual SRT of 4.5 d is below the pH-and-DO-adjusted minimum of 6.5 d, so washout is occurring and the 3.8 mg/L effluent ammonia is the expected result, not an upset. Even on the most favourable reading (ignoring pH), 4.5 d is only 1.15× the minimum - far below the 2–3× margin that a nitrifying design needs for winter reliability. The fix is not a mystery; the constraint is that both SRT and pH must be corrected, not just one.
2. Explain the low effluent pH and quantify the alkalinity constraint.
Read the worked answer
Nitrification consumes alkalinity: about 7.14 mg of alkalinity as CaCO3 per mg of ammonia nitrogen oxidised.
If the full 38 mg/L of influent TKN were nitrified, the demand would be: 38 × 7.14 ≈ 271 mg/L as CaCO3.
Only about 200 mg/L is available in the influent. With 3.8 mg/L ammonia still leaving in the effluent, roughly 34 mg/L of nitrogen has actually been oxidised, consuming about 34 × 7.14 ≈ 243 mg/L of alkalinity - more than the influent supplies, with the balance drawn from the pH buffer. That is exactly why the effluent sits at pH 6.5.
This is the trap in the current operation: the plant is partially nitrifying, and that partial nitrification is destroying the alkalinity and the pH that the nitrifiers need to finish the job. Below about pH 6.8, nitrifier growth falls off sharply, so the system has settled into a self-limiting state - as more ammonia is oxidised, pH falls, growth slows, and the residual ammonia stays high.
Options:
- Chemical alkalinity addition at the head of aeration: caustic soda, soda ash or hydrated lime to hold the last pass above pH 7.0. Feed rate follows the ammonia load and should be controlled on pH with a setpoint around 7.2.
- Denitrification: recovering 3.57 mg alkalinity per mg nitrate reduced roughly halves the net demand, and also reduces aeration energy and the chlorine demand downstream.
- Confirm the NPDES pH limits before dosing; if the permit bottom limit is 6.5, the plant is already at the edge and this is a compliance issue, not just a process one.
3. Given limited blower capacity, a 3.2 m sludge blanket and a nine-year-old diffuser grid, rank the corrective actions you would take.
Read the worked answer
Rank by speed, cost and whether it addresses a binding constraint now.
- Start alkalinity addition (days, low cost). It is the fastest way to lift pH above 6.8 and immediately restores nitrifier growth rate. Without it, raising SRT or DO will not deliver the required performance.
- Protect the clarifier before raising MLSS (weeks). With SVI 180 mL/g and a blanket at 3.2 m of 4.0 m depth, there is 0.8 m of freeboard and the sludge is bulking. Raising SRT by holding more solids will push the blanket over the weir and cause a solids washout - which is worse than an ammonia exceedance. Before any MLSS increase: check RAS rates and flow splitting, consider polymer to lower SVI, and if necessary remove a train from service to reduce hydraulic loading.
- Then raise SRT toward 8–10 days by reducing wasting, in steps, with daily blanket and SVI checks and a defined trigger to back off.
- Improve oxygen transfer (months, capital). At 1.6 mg/L in the last pass the DO factor is only 0.76. Replacing the nine-year-old diffusers typically recovers 15–25 % of transfer efficiency, which creates DO headroom without new blowers - a far cheaper route than adding blower capacity.
- Consider a winter nitrification upgrade (next capital cycle). If the load cannot be met at 13 °C with the existing tank volume, IFAS or MBBR media in the aeration tanks is the standard capacity fix and is compatible with the existing clarifiers.
Sequencing rationale: fix the chemistry first, fix clarification second, then increase the biomass inventory, and buy oxygen transfer efficiency before buying blowers.