Ammonia breakthrough at a municipal activated-sludge plant
A 25,000 m³/d municipal activated-sludge plant with anoxic/aerobic tanks and secondary clarifiers must meet an effluent ammonia limit of 5 mg/L. Influent ammonia has risen from 25 to 35 mg/L NH3-N, and influent alkalinity is 190 mg/L as CaCO3. The aeration basin is operated at SRT 8 days, MLSS 3,500 mg/L and dissolved oxygen 2.0 mg/L; winter mixed-liquor temperature has fallen to 14°C. Final effluent ammonia has drifted up from 1–2 mg/L to 8–9 mg/L over three weeks, with occasional pH excursions below 6.8 in the aerobic zone. The operations team wants to increase the sludge waste rate to control MLSS and has already ordered polymer to improve clarifier settling. The client is due to renew its discharge licence in two months and asks whether a short-term permit limit relaxation should be requested. You are the engineer asked to review the data and advise.
1. Using the operating data, identify the most likely cause or causes of the ammonia breakthrough and explain why increasing sludge wasting is likely to make it worse.
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Diagnosis. The key signal is the alkalinity and pH data, not the clarifier. Nitrification consumes about 7.14 mg alkalinity as CaCO3 per mg NH3-N oxidised. At 35 mg/L influent NH3-N, the stoichiometric demand is about 250 mg/L as CaCO3, against only 190 mg/L available. Even allowing for nitrogen assimilated into sludge and ammonia leaving in the effluent, the plant is alkalinity-deficient; the aerobic-zone pH falling below 6.8 is consistent with this. Nitrification is pH-sensitive, so a downward spiral is plausible: lower pH slows nitrification, ammonia rises, and the alkalinity demand per unit of ammonia oxidised remains. Temperature at 14°C also reduces nitrifier growth, and an SRT of 8 days leaves little safety margin for a 5 mg/L limit. Increasing sludge wasting reduces SRT further and washes out slow-growing nitrifiers, so it will worsen ammonia breakthrough. Polymer may improve settling but does not address nitrification. Before concluding, check the dissolved-oxygen profile: 2.0 mg/L at the probe may mask low DO inside the flocs. Also confirm there is no inhibitory trade discharge.
2. Set out the operational changes you would make over the next four weeks and the parameters you would monitor to confirm the diagnosis.
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Actions over four weeks. First, stop increasing wasting and raise SRT to 12–15 days by reducing waste sludge flow, provided the clarifiers can handle the resulting MLSS. Target MLSS 4,000–4,500 mg/L only if the sludge blanket and SVI allow; do not chase a number. Second, correct alkalinity: dose sodium bicarbonate or another suitable source to maintain aerobic-zone pH 7.0–7.5 and a residual alkalinity of at least 50–80 mg/L as CaCO3. Base the initial dose on 7.14 × the ammonia actually oxidised, then trim daily. Avoid lime over-dosing, which can cause scaling and pH spikes. Third, improve aeration control: maintain DO 2.0–3.0 mg/L in the aerobic zone and not less than 1.0–1.5 mg/L at the outlet; check diffusers and blowers. Fourth, equalise load where possible and review trade discharges. Monitoring: daily influent and effluent NH3-N, NO3-N, pH, alkalinity, DO, temperature and MLSS; at least twice weekly SVI, clarifier blanket and microscopic examination; weekly SRT calculation. Falling effluent ammonia with rising nitrate and stable pH confirms the diagnosis. If ammonia does not respond within two SRTs, sample for inhibition and review the aeration basin hydraulics.
3. The client asks you to support a licence relaxation application. What is your professional position?
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Position on licence relaxation. I would not support a relaxation as the first response. A licence variation may be appropriate only if the client can show that the exceedance is temporary, that the cause is understood, that practicable corrective measures are being implemented, and that the receiving-water impact is acceptable. The engineer's role is to give the regulator accurate information, not to present an operational failure as an unavoidable constraint. I would advise the client to notify the authority promptly of the non-compliance, submit the alkalinity and SRT diagnosis, set out the corrective plan and a realistic timeline, and propose tighter monitoring. If a short temporary relaxation is sought, it should be time-limited, supported by receiving-water data and agreed with the regulator. I would not sign a statement that the plant cannot meet the limit without a proper options assessment. I would also record my advice and the client's decision.