Winter nitrification failure in a sequencing batch reactor
A 4,000 m3/d sequencing batch reactor (SBR) treats municipal sewage mixed with beverage-plant effluent. The design basis was 30 mg/L influent NH4-N, but current influent averages 32 mg/L NH4-N with alkalinity of only 155 mg/L as CaCO3 and pH 7.0. The cycle is 2 h anoxic fill, 3 h aerated reaction, 1 h settle and decant; dissolved oxygen is controlled at 1.0 mg/L, solids retention time (SRT) is 6 d, and MLSS is 3,800 mg/L. Winter wastewater temperature is 13 C. Effluent NH4-N has risen from 2 to 12 mg/L, effluent NO3-N is only 2 mg/L, pH falls to 6.4, and effluent alkalinity is 20 mg/L as CaCO3. Blowers are already at 85% capacity. The utility wants to meet 5 mg/L NH4-N without new tanks, and the operations team proposes raising the DO setpoint to 2.5 mg/L.
1. Quantify the alkalinity constraint and decide whether raising DO alone can restore nitrification. Show the stoichiometric reasoning.
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Stoichiometry. Nitrification consumes about 7.14 mg alkalinity as CaCO3 per mg NH4-N oxidised; denitrification returns about 3.57 mg as CaCO3 per mg NO3-N reduced.
Current oxidation = 32 - 12 = 20 mg/L NH4-N. Alkalinity demand = 20 x 7.14 = 142.8 mg/L as CaCO3. Denitrification credit = 2 x 3.57 = 7.1 mg/L. Net demand = 135.7 mg/L. Influent 155 minus 135.7 = 19.3 mg/L predicted effluent alkalinity, which matches the observed 20 mg/L. The mass balance is consistent.
To reach 5 mg/L NH4-N, oxidation must be 27 mg/L. Demand = 27 x 7.14 = 192.8 mg/L. With only 2 mg/L denitrification, net demand is 185.7 mg/L, a deficit of about 31 mg/L as CaCO3 at the current influent. If denitrification were improved to 20 mg/L NO3-N reduced, the credit would be 71.4 mg/L and net demand would fall to 121.4 mg/L, which the existing alkalinity could absorb.
Judgement. Raising DO does not reduce the stoichiometric alkalinity demand. It may oxidise more ammonia and therefore consume more alkalinity, driving pH lower, and it can carry oxygen into the anoxic fill and suppress denitrification. DO control alone is not a fix; the limiting factors are alkalinity and SRT, with pH inhibition reinforcing both.
2. Calculate the minimum SRT at 13 C with the stated DO limitation and compare it with the installed 6 d. Use mu_max = 0.75 d-1 at 20 C, theta = 1.07, b = 0.08 d-1 at 20 C, theta_b = 1.04, and a DO half-saturation constant of 0.5 mg/L.
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Temperature correction for nitrifiers:
mu_max(13) = 0.75 x 1.07^(13-20) = 0.75 x 0.623 = 0.467 d-1
b(13) = 0.08 x 1.04^(-7) = 0.061 d-1
DO limitation: mu_eff = mu_max x DO/(Ks + DO).
At DO = 1.0 mg/L: mu_eff = 0.467 x 1.0/1.5 = 0.311 d-1. SRT_min = 1/(0.311 - 0.061) = 4.0 d. With a safety factor of 2.5, the design SRT is about 10 d.
At DO = 2.5 mg/L: mu_eff = 0.467 x 2.5/3.0 = 0.389 d-1. SRT_min = 1/(0.389 - 0.061) = 3.05 d. With the same safety factor, the design SRT is about 7.6 d.
The installed SRT of 6 d is below even the 7.6 d value at elevated DO and well below the 10 d value at the actual DO. In addition, pH 6.4 and low alkalinity reduce the effective growth rate further, so the real safety factor is less than 1.5. The biomass is close to washout. Raising DO to 2.5 mg/L helps kinetics but does not make 6 d adequate, and the blowers may not sustain it.
3. Recommend a practical recovery plan, including an alkalinity dose and the SRT and denitrification changes needed. Distinguish the case where denitrification is improved from the case where it is not.
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Priority order. Stabilise pH and alkalinity first, then increase SRT, then improve denitrification. Do not simply raise DO.
Alkalinity dose. If denitrification is improved to about 20 mg/L NO3-N reduced, net demand is 121 mg/L and the existing 155 mg/L leaves about 34 mg/L residual. To hold a 50 mg/L residual, dose about 16 mg/L as CaCO3, i.e. 16 x 4,000/1,000 = 64 kg/d as CaCO3. As sodium bicarbonate this is 64 x 84/50 = 108 kg/d.
If denitrification remains at 2 mg/L, net demand is 186 mg/L. Holding 50 mg/L residual requires 236 mg/L influent equivalent, a dose of about 81 mg/L as CaCO3, i.e. 324 kg/d as CaCO3 or about 544 kg/d NaHCO3. Sodium bicarbonate is preferred over lime because it avoids a local pH spike and calcium carbonate scaling; dose it in the anoxic or early aerated phase with mixing.
SRT and denitrification. Reduce wasting to raise SRT from 6 d to 10-12 d. Keep the anoxic fill mixed but unaerated so it receives nitrate from the previous cycle; avoid carrying DO into it. Consider step-feed or a short post-anoxic phase. Equalise or neutralise the caustic cleaning discharge. Monitor NH4-N, NO3-N, alkalinity and pH each cycle, and raise DO only to 1.5-2.0 mg/L, not 2.5 mg/L, to protect the blowers and denitrification.